ETHAN SIEGEL: I am Ethan Siegel, theoretical astrophysicist and science communicator, author of the James Webb Space Telescope book, "Infinite Cosmos," and writer of the science blog "Starts With a Bang."

- [Host] The origins of the universe with Ethan Siegel. Why did you become a science communicator?

- I like to think about what it was like when I was a small child, and I first started to wonder about things. Maybe the most profound question that I knew how to ask was, what is all of this? You know, the planet and everything beyond the planet. What is all of this, and where did it come from? And if I was being a little more detailed, I'd ask, and how does it get to be that way? And for centuries, millennia, probably for all of human history, and before, people would make up stories. We would tell each other stories that were comforting, that made sense, that attempted to bring order to this thing we didn't understand, right? Where did the universe come from? Where did our planet come from? Where did human beings come from? All of that was in the realm of poetry, of philosophy, of theology for millennia, and probably a whole lot longer than that. And the 20th century began to change all of that. All of a sudden, these big existential questions that had baffled and puzzled and mystified human beings since time immemorial, we actually began to ask questions of the universe itself that started to reveal answers to that. And that's why talking about and asking the question of "Where did the entire universe come from?" is no longer a question for poets and theologians and philosophers. This is a question for scientists, and we have some amazing scientific answers to this question that have totally, in many ways, defied even the wildest of our expectations. Pause for dramatic effect. It took years and decades of study, of hard work, of solving equations, and learning how to ask the right questions and figure out the answers before I was able to learn those answers for myself. And now that I know them, I don't think it should be restricted to those few 1,000 people who go through all the steps that I went through. I think these answers should be available to anyone in the world who has a curiosity for them. So when I put forth the efforts to communicate what I've spent a lifetime learning to the general public, I feel like I'm not only doing a good service for anyone who wants to know these answers for myself, I'm also sating a need that a younger version of myself would've loved to have the answers at their fingertips to back when I was young. And hopefully, there are some people out there will say, "I still have these questions, I still have these curiosities. I still wanna know the answers." And even though I didn't spend a lifetime working to become an expert in this particular realm of theoretical astrophysics, when I tell them the answers with no equations and no jargon, but still at that very high level of understanding, they'll be able to come along on the journey with me just as I hoped when I was young, that someone would take me along on the journey with them. You know, there are challenges that you face when you try and communicate things. Sometimes you'll make the mistake of, "Ooh, I went too deeply too fast," and I didn't understand the gaps in the audience's foundations that I was talking to. And so you learn the lesson: well, next time, I'll start at this more universally understood place, and I'll take additional steps to get towards where we wanna wind up eventually. Other times, it's really been a challenge of how do I reach the audience that I'm attempting to reach? How do I break through this chamber of noise that people are surrounded by? We have a lot of, I'll be generous and call it partially correct, information that's out there. And how can I get people to pay attention to what's actually true and unlearn the misconceptions that they've picked up along the way? And I found that if you start at the right place where everyone is on board initially, and you take just one step at a time, where people can follow you from where you were to where you're headed, if you keep them along with you for that ride, you can wind up pretty much anywhere you want to take them. And so that's my goal today.

- [Host] What are the origins of the big bang theory?

- We have this idea of where our universe came from that most people have heard of called the big bang. And the way we came about upon this idea was we weren't actually looking for the beginning of the universe. We were looking at some objects in the sky, these faint fuzzy objects that appeared through telescopes that looked like little spirals in the sky. Some of them were fuzzier, some of them were seen at angles, some of them weren't spirals at all but still had that same faint fuzzy structure. And we were wondering, what are these things? Because back in, say, 1920, we didn't know what the Milky Way was or how big it was. There were some people, maybe even the majority of astronomers at the time, who thought that the Milky Way itself represented the entirety of the universe and that anything we saw, including the spiral and elliptical nebulae, must have been located within the Milky Way. And there were other people who thought otherwise, who said, "No, no, no, the Milky Way is its own..." We would call it a galaxy today. Back then, they didn't have that word. They called it an island universe. And they thought maybe these spirals and these ellipticals were also island universes. Maybe these faint fuzzy blobs that we're seeing are their own island universes way outside of the Milky Way. And that's great. People argued over it. They argued over why. They said, "Oh, well, Vesto Slipher saw that these things are moving really fast relative to the Milky Way, and that's faster than all the stars. And since I know how gravity works, these things have to be completely outside of the Milky Way or they would escape from the Milky Way's gravity. Other people said, "No, no, no. These are just things like maybe protostars, where you have gas collapsing and an object at the center that's heating up, and it's gonna be a star, and maybe it'll form planets around it like our own star did. But these are surely within the Milky Way itself." And for as long as we were identifying these objects, we didn't know. And then some critical observations came in. It started in 1923. This astronomer named Edwin Hubble, who's very famous for the Hubble Constant, the Hubble Space Telescope bears his name, Edwin Hubble had access to what was then the largest, newest, and most powerful telescope in history. And he was looking at the largest of the spirals that we see in the night sky, which today we know as the Andromeda Galaxy. And what Hubble was looking at was saying, "Hey, when I look at the Andromeda Nebula through my telescope, here's what I'm seeing. That, you know, it looks like it's this faint extended structure. But within it, with my new telescope, I can see that there are these bright flares that go off, something appears to brighten, and then it appears to fade away." And we know of objects like that in astronomy. We knew even 100 years ago of objects like this in astronomy. We said these are what we call a nova. And we think a nova is what happens when a dead corpse of a star called a white dwarf starts to build up matter on its surface. Matter like hydrogen and helium, these light gases, maybe it siphons it off from a companion, maybe it accretes it from its surroundings, but it builds up matter on its surface. And when enough matter builds up, something happens at the interface of the white dwarf, and this matter, and you get a burst of nuclear fusion. We didn't know about nuclear fusion, but you get a burst of something that causes it to brighten. And they said, "Okay, maybe these things that we see are novae," or, you know, the plural of nova, "happening in Andromeda." So he saw one, and he marked it down. And he saw a second, and he marked it down. And he saw a third, and he marked it down. And then, when he came back the next night, he saw a fourth one, but it was in the same exact spot as the first one. And he said, "Well, goodness, a nova can't repeat itself in just a day or two. Novae take years, decades, centuries, or more to recharge. So if this isn't a nova, what could it be?" And he realized, "Oh, this is a variable star." This is a star that intrinsically brightens and faintens and brightens and faintens and brightens and faintens periodically over time. And these classes of stars that do this brightening and faintening over time had already been studied for decades by a woman named Henrietta Leavitt, who said, "Oh, these are what we call Cepheid variable stars, and there's a relationship between how quickly they brighten and fainten and how intrinsically bright they are." So this is kind of brilliant, just like if you have a 100-watt incandescent light bulb, you know how intrinsically bright it is. You know, "Oh, if I have a 100-watt light bulb and it's this, you know, I pump this much power through it, here's how bright it's gonna look." So when you observe that light bulb, you can tell just from how bright it appears; just from how bright you see it, you can know how far away it is. And that was what Hubble did is he said, "Well, if I see this variable star and I see how it's brightening and faintening, then from Henrietta Leavitt's work, I know how intrinsically bright it is. And through my new powerful telescope, if I observe this star over time, I can figure out how distant it has to be." And it was by doing that that Edwin Hubble first said, "Oh, my goodness, this nebula in Andromeda, it can't be within the Milky Way because it's not light years away, or hundreds of light years away, or thousands of light years away. It's something more like a million light-years away. Today, we know it's more like 2.5 million. But he knew, "Oh, my goodness, it has to be very, very far away." So that solved the puzzle people were thinking about: Are these nebulae within our Milky Way or are they their own, what we would now call, extragalactic objects? And we figured that out. But this is one of my favorite things about science is once you make a new discovery, you're compelled to use what you learn to go out and learn even more about the universe. So Hubble took his same telescope and, with his assistant, Milton Humason, went out and said, "Well, I'm gonna start looking at all of these faint fuzzy nebulae in the sky, and I'm gonna see if I can find variable stars within them with this period-luminosity relationship, with this pulsing. And then I can say, 'Oh, here's how intrinsically bright these stars are, and here's how faint they appear through my telescope.' So I can say, 'Now, here's how far away each of these objects are.'" And what he found was that, in fact, the Andromeda Galaxy was one of the closest ones to us and that he was finding galaxies that were not just a million or so light-years away, but several million, tens of millions, and quickly thereafter, even over 100 million light-years away. And he was able to say, "Well, now, I also have these observations from this older guy, Vesto Slipher," who was measuring how fast these objects appear to be moving away. When an object emits its light to you, I think about the ice cream truck. Have you ever been a kid listening to the ice cream truck? And, you know, it plays its ice cream tune. You know... ♪ I hope this is in the public domain ♪ ♪ That's the ice cream truck ♪ Right? And so you hear it. But if you're a kid and you listen to it, you can know if you hear this sound becomes higher pitched, "Ooh, the ice cream truck is coming towards me." Now, it's gonna be time for me to get ice cream, and I can tell from the pitch that the truck is coming towards me. Just like if it's lower pitched, you can hear it moving away from you. You might more commonly hear this as an adult with emergency sirens: police cars, ambulances, firetrucks. But I always think of the ice cream truck as a kid when I start thinking about this. Light, just like sound, is a wave. If a light source like a galaxy is moving towards us, the light that it makes is gonna have a wavelength. And if it moves towards you, that wavelength gets compressed, or what we call in astronomy blueshifted. It gets compressed to shorter wavelengths, which means the light appears bluer when you observe it. But the opposite is also true. If the object is moving away from you, the wavelength lengthens, and the light becomes redshifted or shifted to longer wavelengths. And because we have the same atoms, things like hydrogen atoms and helium atoms all throughout the universe, we can see by measuring those spectral lines that come from these atoms whether that light from a distant galaxy is blueshifted or redshifted and by how much. And this was great because Hubble had all these observations that told us the distances to these galaxies. But Vesto Slipher had all of these observations that told us how much is the light from each of these, now we know they're galaxies, either blueshifted or redshifted with respect to us. So it's true, Hubble put this together in the late 1920s and early 1930s. But even before him, we had other people put these two pieces of information together. The first one to do it was in 1927, a Belgian Catholic priest named Georges Lemaitre put them together, and he said, "Look at what's happening. The farther away a galaxy is from us, right? We see them at greater and greater and greater distances. Then when I go and I measure, is the light from them redshifted or blueshifted? I see that the more distant a galaxy is from us, the more its light is redshifted, and that the farther away I look, the bigger the redshift is that I see." And so Lemaitre said, "Well, what does this mean? What does it mean that the farther away a galaxy is, the more its light looks redshifted?" Well, the way I like to think about it is to think about baking. Have you ever had a ball of dough? And I don't want you to imagine plain, boring bread dough. Let's make it a little exciting, and let's put some raisins in there. Let's make a nice loaf of raisin bread, right? What happens: I make my dough. It's got the raisins all embedded throughout it. I'm gonna put it down. I'm gonna cover it with plastic wrap, and I'm gonna let the dough do what it does, react with the yeast and the sugars, and start to leaven. As the dough starts to leaven, what happens to the raisins within the dough? As the dough leavens, the raisins move along with the dough. They get carried by the dough. So that from the perspective of any raisin within the dough, it looks like the other raisins are moving away from it. And the farther away a raisin is from it, the faster it looks like it's being pushed away from the raisin that you are. And I say that "you are," because in this analogy of the expanding universe, the raisins are galaxies, and the dough is this fabric of space itself that comes up in Einstein's general theory of relativity. In general relativity, space and time are woven together into a fabric, but this fabric is not constant. This fabric can evolve with time. And it wasn't actually Einstein who figured out how the fabric evolves. It was a Soviet scientist from 1922 named Alexander Friedman. Friedman said, "Look, if you start with a universe that obeys the laws of general relativity, but you fill it uniformly with something like matter or radiation or any form of energy you imagine, that universe cannot be static and stable. Instead, what's going to happen is that universe is going to either expand or contract; it has to do one of those, otherwise it won't be stable. Now, it's sort of like if I tell you, "Hey, I've got a number, and this number squared is four, what is the number I'm thinking of?" right? You might quickly jump in and say, "I know. Two squared is four. So if you're saying this number squared is four, that number has to be two." Is it two, though? Couldn't it also be negative two? What's the universe doing? Is it expanding or is it contracting? These solutions are both mathematically possible. The way you learn which one represents our universe, this is the big difference between math and physics, is you have to look at the universe itself, take the critical observations, and say, "Which one is it doing?" Lemaitre was the first to put all of this together: Einstein's equations, Friedman's solutions, Vesto Slipher's redshift observations, and Hubble and Humason's distance observations. He put them all together and said, "Oh my goodness, look what this means. The universe is expanding." And he said, "Well, hang on. If the universe today is expanding, that means it's getting bigger and its density is lower than it was yesterday. Density today is lower than it was yesterday. So let's think about yesterday. Let's go back in time. If my universe is expanding, then in the past it was smaller and everything was closer together, and it was more dense. And I can go back further to when the universe was smaller and more dense and smaller and smaller." And he said, "And there's no limit. I can go as far back as I want, as far back as I can imagine, until all of space and time was compressed into a single tiny point." And he called this point the cosmic egg. And this was our very first conception of what we now call the big bang. That all of space and time and all of the matter and energy within the universe was once compressed into this tiny indivisible point. Something that today we might call a singularity. And this is where the big idea of the big bang first came from.

- [Host] What is the difference between singularity and hot big bang?

- So there were two aspects to this thought that Lemaitre had that other people expanded on later. One aspect is if we think about the universe being small and hot and dense, that's something we can consider as the big bang, right? This hot, dense, expanding state that evolves into this star and galaxy-rich expanding universe we see today. But the big bang can also mean let's go all the way back to this singular state, to this moment, to this event where space and time itself, as well as all of the matter and energy within it, emerged in this moment from this singularity state. So we have two meanings for the big bang. We have the hot, dense, rapidly expanding state, which I will call the hot big bang. And we also have the singular birth of space and time with all the matter and energy in it, which I will call the big bang singularity. And for many, many decades, these two terms were used interchangeably and synonymously as this is both part of the big bang. So here's an important thing to keep in mind when it comes to these two definitions of the big bang. The hot, dense, rapidly expanding state is going to lead to observable consequences. It's going to lead to things, "Well, when the universe was a certain temperature, it was too hot to make neutral atoms, so we should be able to see something about when neutral atoms first formed." It, at some point, it was too hot to make atomic nuclei, so there should be some early signature leftover of, "Ooh, what happened when we first formed protons and neutrons bound together?" And because the universe also gravitates as it expands, we expect that there's also some first time where stars and galaxies form and turn on and that they should merge and grow and collect more mass over time. So we should see that galaxies farther away earlier on are more primitive and smaller and maybe are forming stars more rapidly than the later-evolved galaxies we have today. Those are all observable consequences of that hot, dense early state. But would there be any observable consequences from the singularity part? From that birth of space and time from when all of matter and energy was compressed into a single state? That one is much more complicated. So there's a difference between thinking about the hot, dense, rapidly expanding state that's going to leave clear imprints that we can look for on the face of the universe versus the singularity, which is a bit more of an extrapolation that maybe we should be a little skeptical of until we have evidence that says, "Yes, you can go all the way back there, not just theoretically, but observationally. So when Lemaitre says the cosmic egg, or later, people say the primeval atom, that's talking about the singularity. That's the assumption that it does go all the way back to something that birthed the universe. Is that necessarily true? Maybe not. We can imagine something that goes back, back, back, and it's hot and dense and expanding, but maybe something other than a singularity happened there.

- [Host] What are the three big predictions of the hot big bang?

- So, we started today where our universe is full of these rich, evolved galaxies that are all expanding away from each other. If our universe was expanding the whole time from a hotter, denser, and more uniform state, 'cause gravity over time is going to draw things in and make them clump and cluster together, then that means as we look back in time, we should see a few different things. We should see that earlier galaxies as we look farther and farther away, we're looking back in time because coming back to an initial state, the universe had a beginning in space and time. So if we look back a certain distance away, we're looking back in time 'cause it takes all those millions of light-years for the light to travel intergalactic distances and reach our eyes. So millions of light years away means millions of years back in time, and billions of light years away means billions of years back in time. So we look farther and farther and farther away; we should see galaxies are smaller, less evolved, should have different populations of stars in them than galaxies today do. And we do. We do see that. We see that the ways that galaxies cluster and evolve change over time. So that's plus one for the hot big bang. We also, say, go back farther and farther, and there should have been a time where the universe was small enough that it was so hot, the wavelength of light was so short, that you couldn't form neutral atoms. That anytime you had an electron and a proton bind together to make hydrogen, the most abundant element in the universe, a photon would come along with enough energy to kick that electron off. But at some point, the universe expands, the light's wavelength gets long enough that you can form neutral atoms. So there should be a leftover background of this light that finally couldn't ionize atoms anymore. And in all the billions of years that have passed since, that light is no longer ultraviolet or even visible or infrared. It has stretched so much to such long wavelengths that it should be microwave wavelength light by the time we get to today. And you can go back even further and say, "Well, it was so hot at some point and your wavelength of light was so short that you couldn't even make atomic nuclei. Everything would get blasted apart into just protons and neutrons." So at some point, you must have made the first elements in the universe. Maybe hydrogen is some of it, but not just hydrogen: deuterium, helium-3, helium-4, lithium, maybe a little bit of beryllium, boron, carbon, nitrogen, oxygen. How high can you go before the expanding universe stops you from fusing elements? So, these three big predictions, that galaxies should cluster and evolve over time, that we should have a leftover glow of radiation that shifted to very low temperatures into these microwave wavelengths, and we should see evidence for not just hydrogen but an abundance of different elements that exist even in pristine regions that have never formed stars. These are all predictions of that hot big bang. They don't say anything about the singularity if you were curious about that. But they do all give us ways to go out and test for whether the hot big bang happened. Let's go a little further. So you say, "Okay, what do we see?" Well, the galaxy one is easy. As long as you can see far enough away, as long as you can probe to those billions of light-years away, you're gonna see that galaxies do evolve over time. They were smaller, less massive, the stars within them were bluer in color, and they had higher star formation rates as you look back in time. We first started to reveal this in the '60s and '70s. The light elements, even in pristine gas clouds, as pristine as we can find them, it's never just hydrogen. They have helium and these other abundances of elements like helium-3, deuterium, lithium. We see them everywhere we look. We know that some sort of nuclear reactions must have occurred even outside of stars, but we didn't really discover that until the '70s or '80s. The smoking gun, evidence, for where the hot big bang occurred came in the 1960s. These two scientists, Arno Penzias and Bob Wilson, were working for the Navy, and they had a new radar dish. It was called the Holmdel Horn Antenna and it was located in New Jersey. And they were calibrating it. And they said, "What we're gonna do is we're gonna take this dish and we're gonna look at the sky and we're gonna try and see... Like, okay, we're gonna try and get it calibrated so that there's no noise." And it was weird. No matter where they pointed their telescope in the sky, they heard this noise. Obviously, don't point it at the Sun. You point it at the Sun, you'll see signals of the Sun. You point it at the plane of the Milky Way, and you'll get signals from the plane of the Milky Way. But everywhere else, you still got this noise, except if you pointed it down. If you pointed it at the ground, you didn't get the noise. "Why are we getting this noise? Why are we getting this noise everywhere we look in the antenna?" And they could not figure it out. There were nests of birds living in the antenna. So they got mops out and cleaned it out and removed the nest, and still, the noise remained. It was a mystery for many months until someone put them in contact with the team from Princeton that was actually building a device known as a radiometer that they wanted to fly up to the highest layers of Earth's atmosphere to go and look for this predicted microwave wavelength leftover radiation from the hot big bang. Back at that time, they didn't have a name like the cosmic microwave background. They called this expected radiation the primeval fireball because it should be leftover radiation from this primeval state that expanded maybe from a primeval atom or a singularity. And as it cooled below a certain threshold, you form neutral atoms, but that radiation keeps traveling, and it keeps expanding and lengthening. And they realized that's what they had found. They had found the leftover glow of the big bang. And that was the smoking gun for science to say, "Oh, my goodness, this is where our universe came from." The hot big bang, that hot, dense uniform, rapidly expanding state was real, and we can still see the evidence for it today.

- [Host] How was the cosmic inflation theory discovered? So, we've got this picture of the universe, right? We're measuring the cosmic microwave background, and we're measuring it at different wavelengths, and we know it has the right properties that agree with predictions. We start seeing the light elements everywhere, and we know this phase of what we call big bang nucleosynthesis must have happened. And we're seeing not only the expanding universe but galaxies evolving within it, growing, changing their stellar populations, merging together, becoming more massive. All of these cornerstones of the big bang are now in place, but people are still asking questions. So you start thinking about, well, we go back, galaxies evolve, and we see that happening as the universe expands. And we go back, and we see the evidence for the cosmic microwave background and this hot, dense state where you couldn't form neutral atoms. And we see these gas clouds with these elements in them that tell us, "Oh, we must have gone back to a place where you couldn't even make atomic nuclei, where we were that hot and dense. So why not go all the way back to a singularity? And the same people who were looking at that smoking gun evidence of the cosmic microwave background were now looking at, hey, the universe has some properties that tell us maybe this idea of a singularity isn't quite right. Because if you start from a singularity and everything expands away from that singularity, you're gonna have a region of space on one side of the universe versus the other side of the universe that have never had time to talk or exchange information or what we call in thermodynamics thermalize and come to thermal equilibrium with each other. So why, when we look at the leftover glow from the big bang, at this cosmic microwave background, why are they the same exact temperature? It's sort of like having a heater on one side of your room and not having had enough time pass for those molecules to distribute themselves over the whole room and asking, "Why is my room the same temperature everywhere if I've got a heater on one side and there hasn't been enough time for it to reach equilibrium?" We also say, look, we can have the universe expanding and filled with matter and energy, and it's going to expand at a certain rate, and the expansion rate is going to change at a certain rate as the universe gets less dense. Why did the initial expansion and the amount of matter and energy within it balance each other out so perfectly that we didn't either collapse 'cause there was too much matter for the expansion rate, or we didn't expand into oblivion 'cause there was too much expansion for the matter in there? Why is there that balance? And furthermore, we know that new physics must exist at very, very high energies. We know that there's more to the universe than what we know of now in the standard model. So if we reach those very, very, very high energy states, where's the evidence of that? Where are these leftover high-energy relics that should be there? In physics, we call these the horizon problem for why is it the same temperature everywhere, the flatness problem for why do the expansion and the matter and energy density balance so perfectly, and the monopole problem of where are all these leftover high energy relics? Because if you had a singularity at the start of the hot big bang, you would not expect to see the universe that we get. So what is the solution to these three puzzles? Our first inkling of that came about when a spectacular realization was had in late 1979 by a young scientist named Alan Guth, and that's called cosmic inflation.

- [Host] What is cosmic inflation?

- All right, so I wanna start by going back to a name we brought up way earlier called Alexander Friedman, right? Alexander Friedman was the first one to say, "Hey, if you have a universe that's uniformly filled with any type of matter or energy, it's going to either expand or contract." And Friedman doesn't just say that it's going to expand or contract, he tells you how. His equations tell you how something that's filled with matter and energy is going to expand or contract. So you can say let's consider a few different cases. So you can say let's imagine we have a universe that's filled with matter, right? We know what matter is; it's made out of particles. And these particles generally move pretty slow compared to the speed of light. So if I have a universe that's full of matter, it's going to expand at a rate that's proportional to the square root of its energy density. So for normal matter, what is its energy density? It is how many mass of particles you have in a given region of space. And as the universe expands, right, in all three dimensions, expands in height, expands in length, and expands in depth, as your universe gets bigger in all three dimensions, the density is gonna go down, it's gonna dilute as the volume of the universe increases. What about if a universe is made of radiation? Well, radiation is like matter. It's made out of particles. But these particles all move at the speed of light and their energy is defined by their wavelength. So, yes, as you double and double and double the size of your universe, its volume will also increase while the number of particles stays the same, but the wavelength of each little quantum of radiation is going to stretch also and lose energy. So a universe with radiation in it, as opposed to a universe with matter in it is gonna dilute more rapidly. Its energy density is gonna drop faster, and that means its expansion rate is gonna drop at a different rate. But what if the universe wasn't filled with matter or radiation? What if it was filled with some type of energy that was inherent or intrinsic to space itself? Sounds weird, but this is a possibility. Einstein didn't even know it, but he first put forth this possibility way back when he first put forth the general theory of relativity. This is what he called a cosmological constant. It behaves as though it's energy that's intrinsic to space itself. So if the universe expands and expands and expands in all three dimensions, the energy density stays constant even though the volume is increasing because the same amount of energy density is present everywhere, the energy density doesn't drop. And in this case, we get a universe that expands not just rapidly, like a matter or radiation-filled universe will do early on, and then decrease. It's gonna expand at this rapid rate, and then it's gonna stay at this rapid rate and keep expanding at this rapid rate, and it's gonna do it again and again and again and again. So you're gonna double in size and double in size and double in size, and then you're gonna double again in size and again in size and again in size when that same amount of time elapses. And it's gonna double and double and double, and it's gonna keep doubling relentlessly. So the idea of cosmic inflation was that instead of being filled with matter and radiation always, there was a phase that preceded our universe being filled with matter and radiation, where instead it was filled with a form of energy that was intrinsic to space itself. And while it was in that phase, space got stretched to be enormous in a minuscule tiny amount of time. And Guth, back in 1981, when his paper was published. He started thinking about it '79, paper was published early 1981. He realized all of those three big puzzles about the big bang. Why is it the same temperature in these different regions that haven't had a chance to talk to each other yet? Oh, because in inflation said that these regions were together and were in contact with each other in the past, and then inflation expanded or inflated them away. So they were in contact at some point in the past, and that's how they can have the same properties. And why did the expansion rate and the matter and energy density balance each other so much? Why did this make the universe spatially flat? Why is this balance so perfect? And Guth said, "Oh, well, look, the universe didn't have to be flat. It could be any shape you want." But just like if you take a balloon and you blow up the balloon to be bigger and bigger and bigger, if all you can see is a tiny little square on that balloon, you won't know whether it's flat or not because the part you can see is gonna be indistinguishable from flat. So he said, "Oh, it's going to stretch the universe to appear flat regardless of how it actually started. And that flatness is going to balance the expansion rate with the matter and energy density." And why are there no leftover high-energy relics? Why are there no magnetic monopoles or other exotic things we would expect to arise at these arbitrarily high temperatures? Because the universe didn't get arbitrarily hot. It only ever reached the energy scale of inflation, whatever that was. So therefore, if the universe only reached a certain temperature, maybe it didn't get hot enough to make those leftover relics. Sure, it was a hot big bang, but it wasn't the hottest imaginable state that we could have achieved. So when Alan Guth came out with this theory of inflation, its great power was that it could solve these three great problems about the universe. It could solve the horizon problem by putting what we see today as disconnected regions of the universe in contact in the past, when inflation was first occurring. It took this question of why is there this balance between the expansion rate and the energy density of the universe? Or why is the universe spatially flat? And answered it by saying it didn't matter how it began. When inflation happens, it fills the universe with this uniform energy density that determines the expansion rate, which stretches it to be spatially flat or indistinguishable from it. And why are there no leftover high-energy relics from these infinitely arbitrarily high temperatures? 'Cause the universe didn't get infinitely hot, it only came up to the energy scale of inflation, which means maybe it only got hot enough to be below the temperature needed to create those exotic high-energy relics. Inflation, thought up by Alan Guth and then since worked on by many, many others, was capable all at once of solving these three existential problems that you ran into if you assumed that the hot big bang could and must be extrapolated all the way to a singularity. So the idea is that inflation is now the first thing we can say something sensible about in the universe, not a singularity. The idea is not that, well, we had a singularity and all of this stuff emerged, and then we got this hot, dense, uniform, rapidly expanding state. The idea instead is instead of the singularity, we had this inflationary state; we had this state where space has energy to it and gets inflated, becomes large, becomes uniform, gets the same properties everywhere, becomes spatially flat, and then inflation comes to an end. I like to think about it the way I think about a ball atop a plateau that ends in a valley. As the ball stays on the plateau, you get inflation. But as the ball rolls off of the plateau into the valley, it loses energy. And all of that energy, whereas on a plateau, we would say that's gravitational potential energy, it gets turned into kinetic energy, and then it oscillates at the bottom of the hill. For the universe, it starts off as that energy inherent to space, and as it rolls into the valley, that energy gets converted into matter and radiation. And that is what sets up and creates the hot big bang that is expanding, that is uniform, that is hot and dense. That's how we go from inflation, which is relentless and rapidly expanding, into the hot big bang, where the energy is now in the form of matter and radiation.

- [Host] How can we test cosmic inflation?

- So we've got these two ideas now that are not compatible with each other: either we had a singular hot big bang that gave rise to the hot, dense, rapidly expanding state, or we had cosmic inflation that came to an end and gave rise to this hot, dense, rapidly expanding state. In science, we often think about, like, "Oh, well, we had this theory, and then this new one came along and we all switched over to that." But it's never that simple. There are always three things we have to look at together to say, "Can this new theory defeat the old theory?" And the first one is, can it reproduce all of the old successes that the old theory did? And if you can get to that hot, dense, uniform, rapidly expanding state with inflation, then yes, you can do that just as well as the singular big bang can. The second thing is, well, we had these puzzles with the old theory that we couldn't solve, which were the horizon problem, the flatness problem, and the monopole problem. And inflation does a great job of solving all three of those. So two for two. But then you've got the third critical test. That is, I have my new theory, I have my old theory, they both make predictions about something I haven't gone out and measured about the universe just yet. Can I make a prediction where the new theory and the old theory differ? And can I then go out and test those predictions to see which way is the universe? Does the universe support the new theory or the old theory? And that is the critical test for inflation. So how does inflation make predictions that differ from a big bang singularity? Well, we talked about one of them already, which is that if in inflation only gives you a certain amount of energy inherent to space, and then it decays into matter and radiation, that's a different prediction right there. 'Cause first off, this tells you for a non-inflationary universe, a universe that starts with a singularity, we're not gonna have a maximum temperature or a maximum energy that it gets up to. It can go arbitrarily high. We can go infinitely high if we want. Anything that we make at some super high energy should be made in a universe without inflation, whereas with inflation, no, no, no, we were at the top of a plateau, and we rolled down. And there's a limit to how hot the universe can get. So if that's the case, there should be a difference that we see imprinted in all things of the cosmic microwave background in that leftover primeval fireball from the big bang. If we have a different energy scale that we reach, we should see that when we look at what we call the imperfections in the temperature of this cosmic microwave background. There's a second thing that comes about from inflation that is remarkable, and that is due to the fact that right now, we've been thinking about inflation as being what we call a classical field. We've been picturing it like it's a ball on top of a hill, but it's more than that. We live in a quantum universe, and inflation should be a quantum field. And one thing that quantum fields do is fluctuate. They don't stay at a constant value. They have fluctuations inherent to them. So think about what happens if you have a little fluctuation in space and your universe is inflating. You make a little fluctuation. Think about it like a little wave, and I'm going to stretch it to larger and larger and larger scales. So this fluctuation gets stretched, and then the universe keeps inflating so it gets bigger and bigger and bigger, and that fluctuation gets stretched more, while on tiny quantum scales, new fluctuations get created. So I should have, if my universe is inflating, a set of quantum fluctuations that exist all throughout the universe, and they should be there on all scales. Now, the beautiful thing about this is when inflation comes to an end, when the ball rolls down into the valley to convert things to matter and radiation, that means those smallest scales, they get stretched just a little bit less than the larger scales. They should have slightly smaller quantum fluctuations than the largest scales. So inflation gives us this specific prediction that when we look at the fluctuations imprinted on the universe, they should be almost perfectly uniform on all scales. Except on smaller scales, the fluctuations should be slightly lower in magnitude than the larger-scale ones. So we call this an almost perfectly scale-invariant spectrum, and that's something we should be able to go out and look for. So one test, is do we see a maximum temperature imprinted in the cosmic microwave background? Second test is do we see an almost perfectly scale-invariant spectrum but with smaller fluctuations on small scale slightly than on large scale. A third prediction that should be made is, oh, well, inflation doesn't just stretch things to a maximum size that's the size of our observable universe. If we have inflation going on for a long enough duration of time, it shouldn't just be stretching quantum fluctuations to a maximum scale of the observable universe. It should be stretching them continuously to all scales, including scales that are larger than the universe we can see, what we would call super horizon fluctuations. Those should be imprinted in the leftover glow from the big bang as well. Whereas in a universe without inflation, you don't get those. And finally, there's a very esoteric thing about the nature of these fluctuations. What should these fluctuations be in nature? Should they be adiabatic, which means they have constant entropy within those fluctuations? Or should they be isocurvature fluctuations which have uniform curvature to each of the fluctuations? And this is a little technical, but inflation predicts 100% of them should be adiabatic and 0% of them should be isocurvature. So right there is four observable tests that we were able to tease out during the 1980s and the early part of the 1990s, and starting in the late 1990s and into the 2000s, the 2010s, and even today, we've now been able to go out and test those predictions of inflation against the singular big bang without inflation. And what do we find? Well, we find that there is a maximum temperature that the universe got up to, and it's a high, high temperature. It could be as high as about 10 to the 16 gigaelectron volts, which is a very high energy. The highest energy we've ever created at the Large Hadron Collider is only about 10,000 gigaelectron volts. So we're talking about something like 10 quadrillion gigaelectron volts. Wow, lot of energy. Guess what? That's not arbitrarily high. That's about a factor of 1,000 below the Planck energy. That is something that we can see. There's a limit to how hot the universe got, and it wasn't arbitrarily high. We can look at the fluctuations we see, and guess what? They are almost perfectly scale-invariant, but the fluctuations on the largest cosmic scales are about 3% larger than the ones on smaller cosmic scales. Consistent with what inflation tells you. We can look for do we see evidence for the existence of super-horizon fluctuations? And the answer is yes. Starting with the WMAP satellite in the 2000s, it became able to measure a polarization signal in the leftover glow from the big bang. And that said, "Look, right here. If this is the scale of the cosmic horizon, on the smaller side, we have all these fluctuations. But on the larger side, we get a bump, we see a peak, and those fluctuations exist, too." And also using the most recent data from the Planck satellite said, "Hey, we've been able to constrain adiabatic versus isocurvature fluctuations; at least 98.3% of those fluctuations are adiabatic or of constant entropy in nature. And at most, 1.7%, consistent with 0%, are isocurvature or of constant spatial curvature in nature." So this sets us up to look at how did inflation do versus the hot big bang singularity with no inflation? And the answer is, it's four for four. Inflation's four for four, big bang without inflation is zero for four. Inflation predicts we're gonna have a maximum temperature that isn't arbitrarily high, and we see that not in arbitrarily high temperature. Inflation predicts we're gonna have an almost perfectly scale-invariant spectrum of fluctuations that's just 3% larger on large cosmic scales than small cosmic scales. And we see that. Big bang gives no such prediction. Inflation tells you we should see on scales smaller than the cosmic horizon plenty of fluctuations, but also super-horizon fluctuations on scales larger than the cosmic horizon. And we see those. Hot big bang without inflation, we shouldn't see it. And inflation tells us that these fluctuations should be all adiabatic and not isocurvature in nature. And what we see is completely consistent with that. So inflation has done all three things that we need a theory to do. It's reproduced all the successes of the hot big bang. It's explained puzzles that the hot big bang without inflation can't explain. And it's made new predictions that differ from the hot big bang without inflation's predictions. And so far it's passed 100% of those tests. You might ask, what's left? What's left for inflation to do? And the big thing we'd like to know is I drew you a potential where I said, "Imagine a plateau, and we fall into a hill, and we oscillate in that valley at the bottom of the hill." We wanna know, is that the right model for inflation? There are alternatives. What is the shape of that potential look like specifically? We aren't exactly sure. We have a general idea, but we don't know the specifics. There are two big frontiers that we're trying to push to gain answers to those questions today. One is to look at the spatial curvature. I know we already talked about the flatness problem, and we said, "Oh, the universe is flat." But one of the places we get quantum fluctuations is in the curvature of space itself. They should be small. Inflation tells us that if we look to high enough precisions, somewhere between about one part in 10,000 and one part in one million, somewhere in that regime of precision, we should see departures from the universe being perfectly spatially flat. So far we've only measured it down to about the 1% level, or the one part in 100 level, and it is spatially flat down to that. But if we can get a few orders of magnitude more precise, we'll be able to see what is the precision to which our universe is flat. Is it consistent with what inflation predicts, or does it not match? And finally, there's another type of fluctuation that should exist that isn't these density or temperature fluctuations that show up in the early universe. There should be gravitational wave fluctuations imprinted by inflation on the universe. And there is an effort to look for these with very small angular scale measurements of the cosmic microwave background that are being conducted at the South Pole. We would like to make these experiments more precise because if we have even a weak signal left over from inflation, we wanna be able to find it. So the more precisely we look for this signal, the better chance we have of not only finding these leftover signals but of learning exactly what the properties of our cosmic origins of cosmic inflation were at those critical, very, very first moments that led up to the hot big bang. Since time immemorial, no creature came along on Earth that understood where we were in the universe, where we came from, and how we got to be here. All of this is new, like within many of our lifetimes new. We now know the universe began not from a big bang singularity but from a state that was rapidly and relentlessly inflating, where space was empty, filled only with the energy that was inherent to space itself. That inflationary phase stretched the universe flat, created quantum fluctuations that got stretched across the universe, and then that inflationary state came to an end. It gave rise to a hot, dense, almost perfectly uniform, but with these slight imperfections that are different on large versus small cosmic scales, that gave rise to the structure that came about in all the time since: stars, galaxies, planets, and human beings. Here we are today, 13.8 billion years later, and we know our cosmic origins up until that moment. But big questions about the specifics of what inflation was and how and whether inflation began, as well as what came before it, are still mysteries waiting to be solved. When I look at how far we've come, I am so proud of the human enterprise of civilization for reaching this moment where we can talk about our cosmic origins with some confidence. But as far as the question of our ultimate origin, where did inflation come from? Was it eternal to the past? Was it preceded or set up by some additional state? And did that state have a singular beginning or not? These are questions I don't know how to answer today, but as long as humans are capable of wondering about them, I hope that these are questions we continue to invest in and investigate with all of our might.

- [Host] Is there a multiverse?

- Welcome to the multiverse, or should I say our one universe within a multiverse of possibilities? We know that as we approach life, as we move through life, we have many different decisions that we make. Not just us, but all things around us, including inanimate things that have quantum outcomes, that could have gone many different ways but only wind up ever occurring in one specific way. Although there's a multiverse of possibilities, we think we are the only one universe out there. But is that actually true? Let's explore. Our idea of the multiverse generally comes about from the notion of quantum mechanics. We think every time we enter into a situation or a setup, there are a whole slew, maybe even an infinity of possibilities that could arise for our outcome. But whenever we make a key measurement or observation, we only ever get one specific outcome. Electrons in a hydrogen atom could be almost anywhere in space, even inside the atomic nucleus itself. But when we make that measurement and we ask the electron with a high-energy photon, "Where are you?" We only ever get one specific answer at that moment in time. We then say, "Well, hey, this is fine. This is a property of quantum mechanics." We have, instead of we're gonna get a definitive outcome that we can predict, we only know instead, there's a probability distribution of outcomes. Where we have a whole set of outcomes, quantum mechanics can tell you how probable each outcome is, but only one thing will ever actually ensue. You have to make that measurement or live through it in order to know which one it is. The multiverse appears in science fiction, in media, in film, in many different ways. Often with people experiencing life in one way then being transported to a different reality where a different set of outcomes occurred. They can pass through from one universe to another. They can travel back in time and alter things in their own universe. From "Back to the Future" to "Primer" to "Groundhog Day" to "Everything Everywhere All at Once," the multiverse is a staple of something that occurs in our imaginations. Do not sue us, Marvel Cinematic Universe. And what does this mean in physics terms? Well, for a long time, ideas about the multiverse were really just philosophical. You say, well, there are many ways to interpret the wave function in quantum mechanics and in what's called the many worlds interpretation of quantum mechanics, you can imagine that all of these universes simultaneously exist and that whenever an outcome gets made, it sort of tells us, oh, well, we live in this branch of the multiverse rather than any of the other branches. But what's fascinating about cosmic inflation is some people have had the idea that what if these different regions of the multiverse, these different parallel universes that we envision in our minds mathematically, what if the multiverse from cosmic inflation gave all of these different universes a place to live? This is a possibility that's exciting and that's worth exploring. One of the big concepts in physics and mathematics both is the notion of infinity. You know that if something goes on forever, it can approach infinity, and you can have infinity just by counting one, two, three, four, five, so on, so on, so on. If you counted for an infinitely long amount of time, you would approach infinity. It's not a number. It's something that goes on and on and on and never ends. It's truly bigger than any number or concept you can imagine, but there's more than one type of infinity. For example, if I said, "Well, you can go one, two, three, four, five, six, onto infinity," you might say, "Oh, well, can't I go two, four, six, eight, 10, 12, and get to infinity like that?" Sure, you can. It turns out, though, that those two types of infinities are actually the same type of infinity. One is just multiplied by a factor of two over the other. You're not getting a bigger infinity by counting by twos instead of counting by ones. However, there are several different types of infinities. So three different types of infinities that I want people to be aware of come from one, two, three, four, five, six. That's what I'll call a linear or first-order infinity. Then I want you to think about exponentials like two to the one, two to the two, two to the three, or 10 to the one, 10 to the two, 10 to the three. That's an exponential infinity. That's what I'll call a second type of infinity. But then I also want you to think of these combinatoric or these factorial types of infinities: 10 factorial, 100 factorial, 1,000 factorial, or different permutations that you can get by combining objects together in sequence, like having a room full of gas molecules colliding together and then picking the winning lottery particle one after the other in a row. That's a third type of infinity of what I'll call a combinatoric infinity, and a combinatoric infinity is bigger than even an exponential or second type of infinity. In fact, there's a concept called a combinatoric explosion that you get if you say, "Okay, I have a bag filled with maybe a million, maybe a billion marbles, maybe even an infinite number of marbles." If I go in and I pull out a marble, what are the chances that I'm gonna get a specific marble? Now, if I put the marble back in there and it's gonna collide with other marbles, what are the chances that I'm gonna pull out all the marbles it collided with? The numbers, the probabilities, they go to infinity. The numbers of itself, they go to infinity very, very rapidly. The probabilities of getting any particular outcome, they go infinitesimal very, very rapidly. There are different types of infinities, and which one you have depends on, in math, the type of mathematical infinity you're considering, and in physics, the type of physical system or process you're considering. When we talk about the multiverse, what we're asking is if we want this to be physically real, if we want this quantum mechanical multiverse of all of the possibilities we can imagine happening somewhere, if we want that to be physically real, then we need our actual physical universe to at least be big enough to hold all of those possibilities, and in order to know whether it is or not, we have to compare those types of infinity. The infinity you get from a quantum mechanical multiverse with the infinity that's predicted from an inflationary multiverse. If we wanna figure out which type of infinity do we have for our quantum mechanical physical universe versus our inflationary multiverse, we have to actually look at what's going on in each of these two systems. If you want a spoiler, spoiler is the inflationary multiverse is that second infinity type of an exponential infinity, whereas the quantum mechanical multiverse is an infinity of that third type of a combinatoric infinity. So one important concept in quantum mechanics is that things exist. Quantum systems exist in a superposition of all possible states until you make a measurement that does something that we call collapses the wave function. And all that means is when you make a measurement, you get one definitive outcome. So I can say there are two types of degrees of freedom. There's something that we call a discrete degree of freedom where if I have a quantum particle and I say, "I wanna measure this quantum particle's spin," I can measure it in the up-down direction, and I'm either gonna find that the quantum particle is spin-up or spin-down. If I make that measurement and I've got a spin-up or a spin-down particle, I can then say, "Well, I'm now gonna go and measure it in the left/right direction, in the X direction, and once again, I'll have a 50/50 chance of either it being spin to the left or spin to the right. Every time I make a measurement in a new dimension, X, Y, Z, I'm destroying the information I made in the previous direction. Quantum mechanical spin is something that is what we call a discrete degree of freedom, but it's discrete in each of the three dimensions that we have. There are also continuous degrees of freedom. So if I take a particle of matter like an electron, and an antiparticle of matter, a positron, an antimatter counterpart of the electron, and I bring them together, I know what they're going to do; they're going to annihilate into two photons, each of a very specific energy that's given by the rest mass of the electron and positron, E = mc^2. The energy of those photons is known exactly, but the direction that those photons go off in, yeah, they're gonna go off equal and opposite to each other, but will they go off in the X direction or the Y direction or the Z direction or some combination that makes an angle in any of the three dimensions? That's what we call a continuous degree of freedom. It can take on any of those possibilities. And in our universe, both the discrete degrees of freedom and the continuous degrees of freedom exist in almost all quantum mechanical systems. We have to look at how many particles are there in our observable universe. Our observable universe is pretty big. It goes off for 46 billion light-years in all directions and contains somewhere around 10 to the 90 particles if you include photons, neutrinos, protons, electrons, and every other quantum particle that makes us up. We have had 13.8 billion years since the start of the hot big bang for all of these particles to interact with one another. To have a huge variety of potential quantum outcomes. We normally think about the multiverse as something that arises when we make a conscious decision, but in reality, this effect of quantum splitting or having a huge set of possibilities arise for an outcome of a physical system, it happens irrespective of us as well. It doesn't need human intervention. Things that happen in interstellar space, things that happen in the interior of the Sun, things that happen when sunlight strikes the surface of the Earth, these all lead to a multiverse of possibilities. This is the largest type of infinity possible. When you ask about all of the different possibilities that exist for quantum mechanical outcomes in our universe, you truly get this combinatoric explosion. You get this largest type of infinity of all. So now we have to think about how much actual universe is there and is it big enough to hold all of these quantum mechanical possibilities? If it is, then the multiverse, as we conceive of it in fiction, could be physically real even if it's not here in our universe. But if the multiverse from inflation is not big enough to hold all of these possibilities, then this multiverse doesn't exist anywhere but in our minds and in, I guess, what we would call a mathematical rather than a physical space. So, let's think about it. I like to think about inflation as you have a plateau. The plateau has an edge. If you roll off the plateau, you roll down into a valley below, and inflation comes to an end. As long as you are on top of the hill, on top of the plateau, inflation is going to continue, but once it falls off the hill and rolls into the valley, inflation comes to an end. Now, we can think about this as a single ball on top of a hill, and maybe it starts off rolling, or maybe something gives it a nudge, and eventually, it'll reach the end and roll off, and it'll only happen for a finite amount of time. However, inflation, like all real physical theories, has to also be quantum mechanical in nature. We live in a quantum-mechanical universe, and we have no choice but to assume that inflation itself is quantum-mechanical. So what happens if I have a quantum mechanical ball on top of a plateau instead of a regular ball? Well, one property of quantum mechanical systems is that their position is inherently uncertain and that position will spread out like a quantum mechanical wave function over time. So if my ball were rolling very quickly towards one end of the plateau, the quantum spreading that happens would be slow compared to the rolling, and I would only inflate for a certain amount of time and then I'd roll into the valley below. But if my ball started off rolling slowly, then as it quantum mechanically spread out, there would be a chance that the ball would go closer to the hill. There would be a chance that ball would just keep rolling slowly as it did in the absence of quantum mechanical spreading, but there's also a chance that the ball would actually go back up towards the center of the plateau, that it would actually move away from the valley on either side. And it turns out when you work out the mathematics of inflation, the slowly rolling ball is the only option. You cannot have a fast-rolling ball, or you do not get enough inflation to reproduce the universe that we see. If the ball rolls slowly, then that means all these different parts of the wave function spread out, and sure, there will be some parts of the wave function that reach the end of the plateau, that roll down into the valley below, and that have in that once-inflating region of space, inflation comes to an end, and when it does, as the ball rolls down into the valley and oscillates the energy that was inherent to space itself gets converted into matter and radiation, and we get a hot big bang. That's great. This happened to us at least once here, 13.8 billion years ago. That's what triggered the end of inflation for us and the start of our hot big bang. We have every reason to believe that our observable universe, 46.1 billion light-years in radius from where we are, is only a fraction of however large our true unobservable universe is. What about the rest of it? What about the rest of the universe? The universe beyond our observable universe, the universe beyond our bubble of inflation that came to an end. Well, parts of it roll back towards the central part, where they continue to roll slowly to quantum mechanically spread out and where parts of it will roll off the plateau, bringing it into inflation and start a new hot big bang, while other parts continue to inflate for longer and longer and longer. This sets up a fascinating picture of how we think our inflationary multiverse looks where we have these large bubbles where inflation ends and hot big bangs begin and you start to get matter and radiation-filled universes that have their own quantum mechanical experiences with their own sets of outcomes and a huge variety of possible outcomes. They are separated by more inflating space that drives them all mutually apart from one another. And even though we are constantly getting new regions where inflation ends and the hot big bang begins, these two regions, where you have a hot big bang here and a hot big bang there, they never overlap. They never intersect; they never collide with each other because there's always more inflating space between them that's constantly pushing them apart. How fast do these new universes get created? How many new universes get created? This is something that precedes at an exponential rate. That's the second type of infinity. It's faster than the one, two, three, four, five, six, infinity, but it's slower than the factorial or combinatoric-driven infinity that represents our quantum mechanical universe. So when we say, "Hey, we've been around for 13.8 billion years, you know, with an observable universe full of 10 to the 90 particles that have all been interacting ever since, how many possible outcomes do we have?" You can calculate that, and you'll get a huge, not infinite, but huge number. It's not infinite because it hasn't been around for an infinite amount of time. We have had all of these particles interacting for all of this time, and it's a large number that's growing. It's tending towards infinity, but it hasn't reached it yet. Same thing, we think, with the inflationary multiverse. How many different possible universes are there in the inflationary multiverse? Well, again, it's a number that's growing. It's growing towards infinity, but it's a smaller infinity. It's going towards infinity more slowly than this larger type of infinity. An exponential infinity is smaller than a combinatoric type of infinity. Does this mean that the multiverse is truly fiction and the inflationary multiverse cannot hold all of the possibilities for our quantum mechanical multiverse that we envision in science fiction and perhaps our imaginations? That's true if inflation hasn't been going on for an infinite amount of time to the past. This picture we drew of a ball atop a plateau that spreads out over time and where the ball reaches the end of a plateau, inflation ends, but where the ball stays atop the plateau, inflation continues, this will give you a large number of universes that will go to infinity, but it won't go to infinity fast enough to hold all of the quantum mechanical possibilities. The only way that we can say the multiverse is physically real and all these possibilities do exist somewhere, you need one of two things to be true: either not only will inflation continue in places between different big bangs for all of eternity to the future, but inflation must have been going on for an infinite duration to the past. If that is true, if our universe has been undergoing inflation for an infinity of time, then perhaps there really are enough pockets of universe to hold this multiverse of possibilities. The other way that we could still have this be true is if the universe that is inflating was born infinite, that it was born infinite in spatial extent. If that's the case, then it was already infinite when it was born, and there were already enough possible volumes that it could occupy that no matter how many different quantum mechanical outcomes we have, we arrive at, we will have enough physical universe to hold it. We don't know whether either of those things is true or not. All that we have access to in our universe, in our observable universe to look for, is the part of the universe we can observe, and that only corresponds to that final tiny fraction of a second before inflation came to an end and resulted in our hot big bang. We don't know the answer to whether there is truly an infinity of worlds out there, but that's what it would take. The universe would have to be truly infinite, either in space or in time, in order for the science fiction, quantum mechanical multiverse to be physically real. If our universe is not infinite in size and it has not been inflating for a true eternity, then this quantum mechanical multiverse might be a compelling story, but it's one that falls into the realm of science fiction, not science fact. For as long as humans have been alive on Earth, we've always had regrets, either about things that we did where we got a bad outcome and we wished afterwards we hadn't done it or regrets for the things we didn't do and the chances we didn't take and the times we chickened out and we had an opportunity and we didn't seize it. How many of us have wondered "What if?" about some aspect of our life? "What if I had been brave and gone for the low likelihood but high-risk, high-reward opportunity? What if I had attempted to kiss the one who got away at the end of the night instead of letting them get away? What if I had been bold and took a chance on myself instead of taking the safe route that led me to a known but low-reward outcome?" Most of us will never know what that's like. The multiverse is fascinating because it gives us that extra hope that perhaps somewhere out there is the version of me who took the chance, who took the risk, who went for it, who got the reward. But unless our universe is bigger or older than we have any way of ever finding out about, it's not something that's physically real. Unless the universe is truly infinite, either in time to the past or truly infinite in spatial extent, there simply isn't enough universe to hold all of those quantum mechanical possibilities in a physical way. That doesn't mean the many-worlds interpretation of quantum mechanics is invalid, but it means that there is no physical universe out there where you lived your life without the regret you carry with you today. What this means is that in all of the multiverse, as vast as it is, there's only you. There's only this one version of you with the one life you've led up to this point, with the one opportunity to live your life to the fullest with the rest of it that remains, and I encourage you to do exactly that.

- [Host] How will the universe end?

- In our universe, we have two impetuses that work against one another. We have cosmic expansion, right? Our universe, we say, began with the hot big bang, and it was expanding. We also have the force of gravity, which works to pull everything back together. For a long time, we thought these would be the two main players in a great cosmic race: expansion versus gravitation, who would win? And that, we thought, would determine the fate of the universe. Fast forward to today, and the story is deeper and richer than that, and perhaps we're not so sure of the fate of our universe after all. So the big idea of how the expansion versus gravitation in our universe would determine our fate goes all the way back to 1922, when Alexander Friedman derived how a universe under the laws of general relativity evolves based on its contents, evolves based on what's in that universe. So you can imagine, because we have matter in our universe, like Friedman imagined, and say, "Oh, there's three possible fates for the universe." You start out with an expanding universe that's full of matter, and there are three possible fates. Either the universe is expanding, and it's full of stuff. The expansion works to drive everything apart. The gravitation of the stuff works to pull the universe back together. In modern terms, we might say the universe is a race and the big bang is the starting gun. So what happens? Well, one possibility is that there is enough matter and gravitation in the universe to pull it back together. The universe would expand, but gravitation would slow the expansion down and it would keep expanding and slow and slow and slow. And finally, we would reach a point where the universe reached its maximum size, and now, because it's still full of stuff that gravitates, gravity would pull it back together, and it would contract and shrink and shrink and shrink, and eventually it would collapse. Just as it began in a big bang; it would end in a big crunch. A reverse of that. Second is the opposite way. Well, it's expanding and gravitating, and gravity works to slow it down and slow it down, but there isn't enough stuff to overcome the expansion. So the universe keeps expanding forever, and it just continues to coast and gets bigger and bigger as time goes on with no end. Or you can play the Goldilocks story: instead of the porridge being too hot or too cold, it could be just right. And instead of the universe expanding too rapidly or of gravitation being too powerful for cosmic expansion, they could be perfectly balanced. We could say the universe expands and expands and expands, but it approaches a maximum size. The expansion rate approaches zero, but it never turns around. It never recollapses. It continues expanding, but at a slower and slower rate, eventually approaching zero as time goes on. It was these three possibilities that we were considering throughout most of the 20th century. And in the 1990s, we got a big surprise. We actually measured not only how the universe was expanding but how its expansion rate had changed over time, and what we found is of those three options, the universe was doing none of them. It was doing a fourth unexpected thing. It's like the universe was on that Goldilocks track for a while where it was expanding and slowing down and slowing down, and it looked like it would be right on the border between expanding forever and recollapsing. And then all of a sudden, about six billion years ago, the expansion rate stopped getting smaller. It went larger and larger and larger. A distant galaxy, as seen from our perspective, would recede from us, and its speed would slow and slow and slow. And then six billion years ago, its speed receding from us stopped slowing down, and it started to move away faster and faster and faster and faster. Today, in our universe, when we look out at the galaxies that we see, we can see up to 46 billion light years away, and every galaxy that's more than about 16 or 18 billion light years away is already receding from our view faster than the speed of light. If we sent a signal to it today, right now, even at the speed of light, it would never reach it. The universe is expanding faster than a light signal can reach it. 94% of what we can observe in the universe is already incommunicable and unreachable to us even at the speed of light. So what does this mean for the fate of the universe, right? We would think, "Oh, well, if it's expanding faster and faster, then things are going to continue to recede, and we'll just be left with a cold, empty universe where the things inside our local group, which is gravitationally bound together, things like Andromeda, the Magellanic Clouds, maybe 100 or more other little galaxies all within a few million light-years of each other, will stay bound together, and all the other galaxies and galaxy groups and clusters of galaxies will recede and accelerate away faster and faster, and they'll disappear." That's what we think the fate of the universe is going to be. But do we have it right? Maybe not. When we first thought about the possible fates of the universe, we assumed that our universe was only made out of matter and radiation. Will it expand forever, will it collapse, or will it be on that Goldilocks case? What we found in the 1990s by looking at distant supernova that were going off in the universe we saw that once you looked more than a certain distance away, it looked like the energy contents of the universe were evolving, that it wasn't just matter and radiation, but there was a new species of energy that was also present named dark energy. This dark energy was behaving like it was a form of energy that wasn't matter or radiation but rather was a form of energy inherent to space itself. As the universe expanded, dark energy looked like it was maintaining a constant or unchanging energy density. And so as the universe expands and matter and radiation get less and less and less important 'cause the volume increases and their density goes down, dark energy becomes the dominant form of energy in the universe and dominates and determines the future of cosmic expansion. But there's recent evidence that shows, or at least suggests, that perhaps dark energy strength is changing over time. Some of the recent evidence indicates that dark energy is actually weakening today, that the amount of energy inherent to space is dropping, and if that is the case, if dark energy is not a constant form of energy intrinsic to space itself, then our fate of being a cold, empty universe with only our local group to hang onto may not be true at all. We could have a universe where dark energy gets weaker and weaker and weaker and eventually decays away entirely. And if that happens, then not only will these distant galaxies stop receding from us at a faster and faster rate, they may yet stop moving away from us entirely, and the universe may yet collapse. We could still end in a big crunch if dark energy weakens so thoroughly that it reverses sign and starts pulling everything back in towards us. We could also imagine the other possibility that what if dark energy gets stronger and stronger over time? What if dark energy's strength were to increase, and it could maybe even do that after a period of decreasing where it turns around and gets stronger again? That would change our fate too in a very different way. It would mean that structures that are bound together today, like our local group, our galaxy, or even our solar system, would someday become unbound. If dark energy gets stronger and stronger, it will push these bound structures apart. In the very last moments of the universe, stars, planets, even atoms, and subatomic particles themselves would rip themselves apart as dark energy got stronger and stronger, ending the universe in a big rip. It was a huge quest of 20th-century physics to discover what the fate of our universe will be. And at the end of the century, we thought we had it. We thought we knew our universe will just expand forever into oblivion with different bound structures getting farther and farther apart with only whatever they're bound to to hang on. But now that fate is uncertain and whether we're going to have that fate where things drift apart forever and ever, or whether dark energy weakens and decays away and perhaps things end in a big crunch, or whether its strength increases and everything will rip itself apart, that remains one of today's greatest questions. We have a signal that gets imprinted into the universe from very, very early times. When you have this hot, dense, expanding big bang state, you have little clumps of matter made out of atoms and whatever dark matter is trying to pull themselves together under the force of gravity. But in the early universe, things are very hot, and radiation made mostly of photons is still very important. So if something tries to gravitationally collapse, it will heat up, and the photons inside of it will push back outwards. Photons are very good at pushing against normal matter, things made of protons, neutrons, and electrons, but not good at pushing against dark matter. So this is going to make a little bounce happen where the normal matter gets pushed outwards. This makes a feature that we see in the cosmic microwave background's fluctuations that says, "Hey, you have what we call an acoustic scale, which corresponds to this bounce of matter that exists at a certain distance scale, and as the universe expands, that distance scale expands." So when we measure the universe today, we should say, "Well, if I have a galaxy here, then I should be more likely to find a galaxy 500 million light-years away than either 400 million or 600 million light-years away. But as I look back in the universe's past at galaxies that I see farther and farther and farther away, the universe was smaller 'cause it had expanded less, and that acoustic scale was smaller." So by measuring large surveys of galaxies, large, huge numbers of galaxies over large volumes of space, we can see how has this acoustic scale evolved, and what does that tell us about how the universe has expanded? And in turn, what does it tell us about dark energy? If dark energy were a constant, we would expect to see one specific smooth scale change, and what we're seeing instead is that at the very end, at very recent times, it looks like that scale isn't growing as fast as dark energy predicted, which would mean perhaps dark energy is weakening or changing in how it affects our universe. That is something that could change our fate from the expected; everything gets driven apart to perhaps a different outcome.

- [Host] What was it like when the first stars began to shine?

- Here's something we haven't talked about yet that I think is one of the most fascinating aspects of our universe's history. One of the great questions we have about our universe is what were the first stars like and when did they form? If we came from a hot, dense, rapidly expanding state, we had an era in the universe's past where there was not enough time for matter to gravitate together into large enough clumps to form stars. How long did it take before the first ones formed? The stars that we see today are not made out of pristine material. They are not made of that same material that was left over from the big bang. Every star that we see today was made out of material that has already lived and died inside of previous generations of stars. In theory, we expect these first stars that were made to be much more massive than the stars that exist today and much, much shorter-lived. Whereas the largest, heaviest star we've ever found today is about 260 times the mass of our Sun, we expect these early stars will be up to thousands of times as massive as our Sun. The reason is very simple: we didn't have those heavy elements; they weren't made early on in the big bang. The big bang gave us a universe that was made of 99.999999% hydrogen and helium with only a tiny amount of heavy elements. Without those heavy elements, these big gas clouds can't cool efficiently. So in order to make stars, you need hugely massive clouds. Once you form those stars, they burn through their fuel really quickly, and they die. Even with the James Webb Space Telescope, we've only ever seen stars that look like other stars have lived and died before them. This quest for the very first stars in the universe is one that's still ongoing. We may have to look farther than we've ever seen, maybe to only 50 to 100 million years after the big bang, if we want to truly find them, but we know there was a time in the universe where there were no stars, and then there's a later time where there are plenty of stars. When did those first stars form and what did they look like? This is one of the biggest open questions and one of the biggest quests in astronomy that we're striving to finish today.

- [Host] What was it like when life first became possible?

- Here on Earth, we take for granted that the ingredients needed for life are everywhere, but this was not always the case in the universe. In order to have life, a few ingredients are really absolutely necessary, at least for life as we know it. You need a star that's going to provide a source of energy for life. You need a rocky planet and an interface between a surface of a world with an atmosphere and space. We think you need liquid water made out of hydrogen and oxygen on the surface of that planet with a thick enough atmosphere and the right temperatures to keep it that way. The universe when it was first born did not have those raw ingredients. Without heavy elements, elements like carbon, oxygen, nitrogen, phosphorus, and more, we were not able to make rocky planets around those first stars. It takes millions or maybe even hundreds of millions of years of cosmic evolution to bring about the first generation of stars, to have them live and die, and to have them make enough heavy elements so that you can have, when you form stars again, second or third generation stars; they will have the chance to have rocky planets with heavy elements around them. Then you need enough time to pass for that planet to cool, to have a stable surface so that any chemical bonds that form aren't immediately destroyed by having too much energy. It may not be until the universe is more than a billion years old that the first sustainable forms of life could possibly arise. We know that Earth is probably not the first example of life arising in the universe. Probably it happened many, many billions of years before, but could it have happened very early on in the universe? Chances are at least several hundred million years and maybe even a few billion years were required for all those key ingredients to finally be put in place.

- [Host] How are supermassive black holes formed?

- At the center of almost every massive galaxy that we know of today lies a black hole, not a puny, stellar-mass black hole that formed from the death of a star, a little bit more massive than the Sun, but a supermassive one. At the Milky Way, we have a black hole that's four million times the mass of our Sun. In many galaxies, there are black holes billions or tens of billions times as massive as our Sun. How did the universe get these black holes? Did black holes form and grow up with galaxies, or did the black holes form first, and did the galaxies form around them? We are getting hints from our leading observatories, including the James Webb Space Telescope, that these supermassive black holes may have been in place long before the galaxies that we associate them with were ever created. We have now observed supermassive black holes from very early on in the universe, even when the universe was just a few hundred million years old, that are already millions or even a billion solar masses in size. We see today that the most massive black holes are maybe about 0.1% as massive as the stars in their galaxy. But when we look to earlier and earlier times, we found black holes that are maybe 1%, 10%, or even 100% as massive as all the stars in their galaxies combined. We now think these black holes could not have formed from the stars and the corpses of stars that lived and died already, but instead were formed from a process known as direct collapse where huge clouds or streams of gas collided and collapsed to make seed black holes that may have been tens of thousands of times as massive as a star like the sun. They then grow initially faster than any of the stellar mass components of the galaxy grow. These overmassive black holes, a new discovery in the era of JWST, are changing what we thought of as the origin stories for the most massive single objects in the universe, supermassive black holes.

- [Host] When will the last star die?

- Right now, our universe is full of stars of all different masses and all different varieties. It's a little strange to think about it, but the stars that live the longest are actually the lowest in mass, the ones with the least amount of fuel in it. Why? Because the smaller in mass a star is, the slower it burns through its nuclear fuel. Our sun will live for about 10 to 12 billion years. A very massive star might only live for a few million years, but the lowest mass stars that we know of will live for tens or even over 100 trillion years. But our universe is gonna have stars for longer than that. Our universe continues to have reservoirs of hydrogen gas, and whenever they collapse, whenever these molecular clouds of gas collapse, they're going to form new stars, the heavy ones that will live only for a short while and the low-mass ones that will live for up to over 100 trillion years. We're going to keep forming new stars for not just billions or trillions but quadrillions of years. However, even after those stars that regularly form die out, there's still a chance to make new stars. We have all throughout our universe brown dwarfs, also known as failed stars. These are stars that are less than 8% the mass of our Sun. They often form in singlet systems, but they also often form in binary systems. If you have two brown dwarfs whose combined mass would be enough to ignite nuclear fusion in a star's core, over time, they will emit gravitational waves and spiral into each other. And when they touch, when they collide, they will merge together, and these will finally ignite nuclear fusion in the star's core and create the very last stars that will ever exist in our universe. It may take somewhere around a quintillion years for these stars to form, somewhere around 10 to the 18 or maybe even more years, much longer, more than a million times longer than the current age of the universe, but stick around long enough, watch these systems and for a few trillion years, they will for the very last time light up the night sky. When all the stars die, when the last sources of energy are exhausted, what will the state of the universe be? We call this the heat death of our universe. This is when the last bits of energy have been extracted. Where will those come from? There are two candidates for the types of system that will emit the last blips of energy in the universe. One candidate for what will make the last gasps of energy in the universe are these dead stellar systems, corpses of stars that are orbited by either stellar or planetary corpses. As they orbit each other, they will emit gravitational radiation. They will spiral into each other, and they will merge. And both these mergers and the in-spiral phase will lead to the emission of energy. These will be some of the last sources of energy that we could possibly use to extract energy from, do work, and perhaps make something happen that fights against the inevitable rising tide of thermodynamics. The other thing that happens is that black holes, the most massive black holes in the universe, will all eventually decay, and the most massive black holes will take the longest to decay. Black holes are not completely black but, in fact, emit very small amounts of radiation known as Hawking radiation from near their event horizons. And as they emit this radiation, that energy comes from the mass of the black hole. It may take more than a googol years, somewhere around 10 to the more than 100 years, in order for these black holes, the most massive ones, to fully decay away. But when they do, there will be one last flash of energy that propagates throughout the universe at the speed of light. And after all that happens, we will have achieved a state of thermal equilibrium where no further energy can be extracted, and that is the heat death of the universe.

- [Host] How does the James Webb Space Telescope change our understanding of space?

- NASA's newest flagship observatory is the James Webb Space Telescope. Most of us know about the Hubble Space Telescope as our great eye on the universe that has shown us what the universe looks like from the 1990s up until the present day. The James Webb Space Telescope is our newest flagship observatory. It is larger than Hubble. It is colder than Hubble. It is farther from Earth than Hubble, and it is optimized not for visible light observations like our eyes can see, but for infrared observations, which are longer wavelength types of light than Hubble is sensitive to. This has profoundly changed how we view our universe. The reason the James Webb Space Telescope is so much more powerful than Hubble is not only because it's larger but because it's optimized for these cold temperatures and infrared wavelengths. It has a state-of-the-art new technology known as a sunshield, which is a five-layer passive cooling system that always faces the Sun and keeps all of the observatory's instruments and optics shielded. The hot side of the sunshield is always extremely hot. You would get the worst sunburn on that side of the sunshield, but the cold side is cold enough to freeze liquid nitrogen. The cold side is only about 40 degrees above absolute zero. It is outfitted with a special state-of-the-art cryo cooler that keeps its mid-infrared instrument even colder than that. These long wavelengths of light enable scientists looking with the James Webb Space Telescope to see things that are colder at longer wavelengths and that radiate light that Hubble is not sensitive to. Where Hubble has all of this thermal noise, the James Webb Space Telescope is pristine and can see things that are not only colder than what Hubble can see, it can see things that are farther. Because as light travels through the universe and the expansion of the universe stretches that light to longer and longer wavelengths, Hubble runs into its limits and can't see the most distant objects of all. James Webb still keeps going and has shown us the most distant objects we've ever seen in the history of science. So we started with one ultra-distant galaxy that Hubble found named GN-z11. That was, from Hubble, the most distant galaxy in the universe. Today, GN-z11 is not even in the top 10 of most distant galaxies in the universe. All of the most distant galaxies have been discovered and measured by the James Webb Space Telescope, by JWST. It holds all 10 of the top 10 spots for the most distant galaxy ever. It has found the most distant cluster of proto-galaxies ever, and it has shown us the universe past the limits of what Hubble can detect. The most distant cluster of galaxies shouldn't form until a few billion years after the big bang. But JWST has discovered a proto-cluster, or a young, still-forming cluster of galaxies, just 650 million years after the big bang. It's one of the earliest discoveries, and it happened earlier than we even knew it was possible to happen. It also found not just black holes of the supermassive varieties at the center of galaxies, but it found what we call overmassive black holes, black holes that are far more massive than you would expect from the paltry masses of the galaxies that house them. We've even found one galaxy whose black hole may be more massive than all of the stars within it combined. This is changing our picture for how we think galaxies and black holes grow up and evolve together, suggesting that perhaps black holes may even form before the stars in a galaxy do. The James Webb Space Telescope is also optimized to show us how planets form around nearby stars. We did not know whether all solar systems were like ours with inner planets and an asteroid belt, intermediate planets and a Kuiper belt beyond it, or whether they were different. When looking at the star Fomalhaut, JWST found that we are not necessarily typical of planetary systems because, in addition to an inner disc with an asteroid belt and an outer Kuiper belt with gaps representing planets in between them, it also found a surprising intermediate belt, something that no one had expected. And now it raises the possibility that perhaps planetary systems are richer and more diverse, not even than we had imagined, but than we knew what to imagine. It's possible that there are many more possibilities for forming belts, planets, and gaps in discs than we ever knew. We had a big problem when we first started seeing galaxies with JWST. They looked brighter and more abundant than we expected. Was this a problem for our theories of the universe and how it formed and grew up and what it was made out of, or was this a learning opportunity to teach us something about how the universe grew up that's consistent with our best cosmic theories? It turned out that there were three reasons, all combined, that led JWST to find these galaxies. One is that JWST was cleaner than any observatory we had built before it and cleaner than we were expecting. So it was producing objects that appeared brighter because there was less dirt on the instruments interfering with the light. The second reason has to do with the simulations we ran. It turned out that you need high-resolution simulations in order to find the rarest, most over-dense regions, which is what leads to the first stars and galaxies altogether. And the third aspect of this was star formation is not something that happens continuously and steadily, but rather it happens in bursts. And when you see a burst of star formation happening, it makes a brighter light than when you have slow, steady star formation within galaxies. Those three factors combined: that JWST is cleaner than we thought, that JWST is revealing where the densest regions of matter are forming stars the most quickly, and that it is revealing the sites of ongoing bursts right now of star formation all combined to lead us to believe that, in fact, we understand the universe correctly. And the reason things look so bright and numerous is because of these three factors all put together. One of the things people were wondering is if what JWST sees, if this does not align with what we expected to see, does that mean that our understanding of the universe is incomplete or incorrect? Or does it just mean there are some small caveats and nuances that we need to better understand? When we first saw the huge abundance of bright early galaxies, many wondered, "Oh, my goodness, has JWST broken the universe?" And the answer now appears to be, no, our standard cosmological model has proven to be very robust and very hard to break. And it turns out that there are just a few small nuances and caveats that, if we include them, bring the full suite of JWST observations in line with what we expected. There's still much, much more to learn. Will JWST keep giving us surprises? We hope so. In fact, I bet on it. But will it break the universe? I doubt it. What we understand has proven more resilient and adaptable to what we see with just a few small caveats than almost anyone had expected.

- [Host] When will the next generation of telescopes be built?

- We have the Hubble Space Telescope still operational after all these years. We have the James Webb Space Telescope, launched in 2021, with an expected lifetime of more than two decades. Is that it? Will these be the last flagship observatories for astronomy and astrophysics? They shouldn't be. Each time we look at the universe, not only with unprecedented power but also in a wavelength of light that we have not looked at it in before or with a certain level of precision before, we have a whole new set of discovery potential. What can we find? If we look at the universe in X-ray light, in ultraviolet light, visible infrared, far infrared, microwave, and radio light, many of these wavelengths can only be observed from space. The Earth's atmosphere is opaque to many of them. So what would I recommend? What do you want to learn? Well, I would say the two biggest neglected windows... Yes, of course, we can always build bigger and better telescopes in visible and infrared. And I'm very excited about NASA's Habitable Worlds Observatories, which should be the first space mission that's capable of measuring Earth-like planets around Sun-like stars directly. If there is a twin or cousin of Earth out there, that will be the observatory that finds it. However, we have two fascinating wavelengths of light, the X-ray and ultraviolet, and on the longer end, the far infrared, that we have not built and flown a space telescope by NASA this century to investigate. With current technology, we could increase our knowledge of everything from supermassive black holes to how stars and planets form by orders of magnitude, by not just a factor of 10, but by factors of thousands or more. If we are truly serious about uncovering the mysteries of the universe, that's exactly what we should be investigating in, in order to bring our scientific dreams to fruition.