The great theoretical physicist Steven Weinberg passed away on July 23. This is our tribute.
- The recent passing of the great theoretical physicist Steven Weinberg brought back memories of how his book got me into the study of cosmology.
- Going back in time, toward the cosmic infancy, is a spectacular effort that combines experimental and theoretical ingenuity. Modern cosmology is an experimental science.
- The cosmic story is, ultimately, our own. Our roots reach down to the earliest moments after creation.
When I was a junior in college, my electromagnetism professor had an awesome idea. Apart from the usual homework and exams, we were to give a seminar to the class on a topic of our choosing. The idea was to gauge which area of physics we would be interested in following professionally.
Professor Gilson Carneiro knew I was interested in cosmology and suggested a book by Nobel Prize Laureate Steven Weinberg: The First Three Minutes: A Modern View of the Origin of the Universe. I still have my original copy in Portuguese, from 1979, that emanates a musty tropical smell, sitting on my bookshelf side-by-side with the American version, a Bantam edition from 1979.
Inspired by Steven Weinberg
Books can change lives. They can illuminate the path ahead. In my case, there is no question that Weinberg's book blew my teenage mind. I decided, then and there, that I would become a cosmologist working on the physics of the early universe. The first three minutes of cosmic existence — what could be more exciting for a young physicist than trying to uncover the mystery of creation itself and the origin of the universe, matter, and stars? Weinberg quickly became my modern physics hero, the one I wanted to emulate professionally. Sadly, he passed away July 23rd, leaving a huge void for a generation of physicists.
What excited my young imagination was that science could actually make sense of the very early universe, meaning that theories could be validated and ideas could be tested against real data. Cosmology, as a science, only really took off after Einstein published his paper on the shape of the universe in 1917, two years after his groundbreaking paper on the theory of general relativity, the one explaining how we can interpret gravity as the curvature of spacetime. Matter doesn't "bend" time, but it affects how quickly it flows. (See last week's essay on what happens when you fall into a black hole).
The Big Bang Theory
For most of the 20th century, cosmology lived in the realm of theoretical speculation. One model proposed that the universe started from a small, hot, dense plasma billions of years ago and has been expanding ever since — the Big Bang model; another suggested that the cosmos stands still and that the changes astronomers see are mostly local — the steady state model.
Competing models are essential to science but so is data to help us discriminate among them. In the mid 1960s, a decisive discovery changed the game forever. Arno Penzias and Robert Wilson accidentally discovered the cosmic microwave background radiation (CMB), a fossil from the early universe predicted to exist by George Gamow, Ralph Alpher, and Robert Herman in their Big Bang model. (Alpher and Herman published a lovely account of the history here.) The CMB is a bath of microwave photons that permeates the whole of space, a remnant from the epoch when the first hydrogen atoms were forged, some 400,000 years after the bang.
The existence of the CMB was the smoking gun confirming the Big Bang model. From that moment on, a series of spectacular observatories and detectors, both on land and in space, have extracted huge amounts of information from the properties of the CMB, a bit like paleontologists that excavate the remains of dinosaurs and dig for more bones to get details of a past long gone.
How far back can we go?
Confirming the general outline of the Big Bang model changed our cosmic view. The universe, like you and me, has a history, a past waiting to be explored. How far back in time could we dig? Was there some ultimate wall we cannot pass?
Because matter gets hot as it gets squeezed, going back in time meant looking at matter and radiation at higher and higher temperatures. There is a simple relation that connects the age of the universe and its temperature, measured in terms of the temperature of photons (the particles of visible light and other forms of invisible radiation). The fun thing is that matter breaks down as the temperature increases. So, going back in time means looking at matter at more and more primitive states of organization. After the CMB formed 400,000 years after the bang, there were hydrogen atoms. Before, there weren't. The universe was filled with a primordial soup of particles: protons, neutrons, electrons, photons, and neutrinos, the ghostly particles that cross planets and people unscathed. Also, there were very light atomic nuclei, such as deuterium and tritium (both heavier cousins of hydrogen), helium, and lithium.
So, to study the universe after 400,000 years, we need to use atomic physics, at least until large clumps of matter aggregate due to gravity and start to collapse to form the first stars, a few millions of years after. What about earlier on? The cosmic history is broken down into chunks of time, each the realm of different kinds of physics. Before atoms form, all the way to about a second after the Big Bang, it's nuclear physics time. That's why Weinberg brilliantly titled his book The First Three Minutes. It is during the interval between one-hundredth of a second and three minutes that the light atomic nuclei (made of protons and neutrons) formed, a process called, with poetic flair, primordial nucleosynthesis. Protons collided with neutrons and, sometimes, stuck together due to the attractive strong nuclear force. Why did only a few light nuclei form then? Because the expansion of the universe made it hard for the particles to find each other.
What about the nuclei of heavier elements, like carbon, oxygen, calcium, gold? The answer is beautiful: all the elements of the periodic table after lithium were made and continue to be made in stars, the true cosmic alchemists. Hydrogen eventually becomes people if you wait long enough. At least in this universe.
In this article, we got all the way up to nucleosynthesis, the forging of the first atomic nuclei when the universe was a minute old. What about earlier on? How close to the beginning, to t = 0, can science get? Stay tuned, and we will continue next week.
To Steven Weinberg, with gratitude, for all that you taught us about the universe.
Scientists do not know what is causing the overabundance of the gas.
- A new study looked to understand the source of methane on Saturn's moon Enceladus.
- The scientists used computer models with data from the Cassini spacecraft.
- The explanation could lie in alien organisms or non-biological processes.
Something is producing an overabundance of methane in the ocean hidden under the ice of Saturn's moon Enceladus. A new study analyzed if the source could be an alien life form or some other explanation.
The study, published in Nature Astronomy, was carried out by scientists at the University of Arizona and Paris Sciences & Lettres University, who looked at composition data from the water plumes erupting on Enceladus.
The particular chemistry, discovered by the Cassini spacecraft which flew through the plumes, suggested a high concentration of molecules that have been linked to hydrothermal vents on the bottom of Earth's oceans. Such vents are potential cradles of life on Earth, according to previous studies. The data from Cassini, which has been studying Saturn after entering its orbit in 2004, revealed the presence of molecular hydrogen (dihydrogen), methane, and carbon dioxide, with the amount of methane presenting a particular interest to the scientists."We wanted to know: Could Earthlike microbes that 'eat' the dihydrogen and produce methane explain the surprisingly large amount of methane detected by Cassini?" shared one of the study's lead authors Régis Ferrière, an associate professor in the department of Ecology and Evolutionary Biology at the University of Arizona.
Earth's hydrothermal vents feature microorganisms that use dihydrogen for energy, creating methane from carbon dioxide via the process of methanogenesis.
Searching for such microorganisms known as methanogens on the seafloor of Enceladus is not yet feasible. Likely, it would require very sophisticated deep diving operations that will be the objective of future missions.
So, Ferrière's team took a more available approach to pinpointing the origins of the methane, creating mathematical models that attempted to explain the Cassini data. They wanted to calculate the likelihood that particular processes were responsible for producing the amount of methane observed. For example, is the methane more likely the result of biological or non-biological processes?
They found that the data from Cassini was consistent with either microbial activity at hydrothermal vents or processes that have nothing to do with life but could be quite different from what happens on Earth. Intriguingly, models that didn't involve biological entities didn't seem to produce enough of the gas.
"Obviously, we are not concluding that life exists in Enceladus' ocean," Ferrière stated. "Rather, we wanted to understand how likely it would be that Enceladus' hydrothermal vents could be habitable to Earthlike microorganisms. Very likely, the Cassini data tell us, according to our models."
Still, the scientists think future missions are necessary to either prove or discard the "life hypothesis." One explanation for the methane that does not involve biological organisms is that the gas is the result of a chemical breakdown of primordial organic matter within Enceladus' core. This matter could have become a part of Saturn's moon from comets rich in organic materials.
Jupiter's mysterious auroral events are caused by vibrating waves of plasma.
- For 50 years, astronomers have known that Jupiter has frequent auroral displays, but not why.
- The bursts are a combination of visible and invisible light.
- The presence of NASA's Juno spacecraft around Jupiter allowed scientists to solve the mystery.
Here on Earth, an aurora borealis is a wondrous natural event that too few of us ever get a chance to see. Their occurrence remains unpredictable enough that a glimpse of one may remain elusive even for people who live in the United States' northern latitudes.
Imagine, though, that you could see one every few minutes. That's what happens at Jupiter's north and south poles every 27 minutes. Not only that, but each auroral event blasts out enough X-ray energy to power our entire civilization. A new study, from University College London and the Chinese Academy of Science and published in Science Advances, solves the mystery of how and why this occurs.
Surfing waves of plasma
Credit: ESA / NASA / Yao / Dunn
"We have seen Jupiter producing X-ray aurora for four decades," says the study's co-lead author William Dunn, "but we didn't know how this happened. We only knew they were produced when ions crashed into the planet's atmosphere."
To unravel the mystery behind what is happening, the researchers aligned observations made over a 26-hour period by NASA's Juno spacecraft (which orbits Jupiter) with X-ray measurements made by the European Space Agency's XMM-Newton Observatory (which orbits Earth). Having time-aligned the two sets of observations, computer modeling revealed the mechanics behind the auroral bursts.
Jupiter has a massive magnetic field — some 20,000 times stronger than Earth's — extending out around the planet. Plasma, or ionized gas whose atoms have been stripped of electrons as they collide with each other, races along these lines. Periodic vibrations in the magnetic field lines, the cause of which is still unknown but thought to involve interactions with the solar wind or magnetosphere, produce waves in the plasma.
Jupiter's moon Io releases ion particles from gigantic volcanoes, which get swept up and carried along by the plasma waves, eventually smashing into Jupiter's atmosphere. This results in the massive release of visible and invisible light, including X-rays.
Dunn says, "Now we know these ions are transported by plasma waves — an explanation that has not been proposed before, even though a similar process produces Earth's own aurora. It could, therefore, be a universal phenomenon, present across many different environments in space."
"Now we have identified this fundamental process, there is a wealth of possibilities for where it could be studied next," says co-lead author Zhonghua Yao. "Similar processes likely occur around Saturn, Uranus, Neptune, and probably exoplanets as well, with different kinds of charged particles 'surfing' the waves."
A black hole lab
Co-author Graziella Branduardi-Raymont says, "X-rays are typically produced by extremely powerful and violent phenomena such as black holes and neutron stars, so it seems strange that mere planets produce them too."
Jupiter thus represents a promising research opportunity.
"We can never visit black holes," says Branduardi-Raymont, "as they are beyond space travel, but Jupiter is on our doorstep. With the arrival of the satellite Juno into Jupiter's orbit, astronomers now have a fantastic opportunity to study an environment that produces X-rays up close."
A new artificial intelligence method removes the effect of gravity on cosmic images, showing the real shapes of distant galaxies.
A new AI-based tool developed by Japanese astronomers promises to remove unwanted noise in data to generate a cleaner view of the true shape of galaxies. The scientists successfully tried this approach on real data from Japan's Subaru Telescope and discovered that the distribution of mass produced by their technique corresponded to the established models.
The scientists from the National Astronomical Observatory of Japan (NAOJ) in Tokyo believe their method could be very useful in the analysis of big data from large astronomy surveys. These surveys help us study the structure of the universe by focusing on gravitational lensing patterns.
The trouble with gravitational lensing
Gravitational lensing refers to the phenomenon whereby massive space objects like a cluster of galaxies can distort or bend the light that comes from objects in their background. In other words, images of distant space bodies can be made to look strange by the gravitational pull of objects in the foreground.
One example of this is the "Eye of Horus" galaxy system, discovered by NAOJ astronomers in 2016. The striking images of the system, named in honor of the sacred eye of an ancient Egyptian sky god, are the byproduct of two distant galaxies being lensed by a closer galaxy.
The issue with gravitational lensing for astronomers is that it can make it hard to differentiate galaxy images that are distorted by gravity from galaxies that are actually distorted. This so-called "shape noise" undermines confidence in research into the universe's large structures.
Eye of Horus galaxy system. The yellow object at the center represents a galaxy about 7 billion light-years away that bends the light from two galaxies in the background that are even farther away.Credit: NAOJ
A new approach
The new study, published in the Monthly Notices of the Royal Astronomical Society, shows how the research team was able to counteract shape noise by utilizing ATERUI II, the most powerful astronomy supercomputer in the world. By feeding it pretend and real data from the Subaru Telescope, the scientists had the computer simulate 25,000 mock galaxy catalogs. They added realistic noise to these data sets while teaching their artificial intelligence network through deep learning to pick out the correct data from the noise.
"This research shows the benefits of combining different types of research: observations, simulations, and AI data analysis," shared team's leader Masato Shirasaki. He added, "In this era of big data, we need to step across traditional boundaries between specialties and use all available tools to understand the data. If we can do this, it will open new fields in astronomy and other sciences."
How the AI works
Employing a generative adversarial network (GAN), the Japanese astronomers' AI learned to find details that previously could not be seen, explained the observatory's press release. The GAN developed by the scientists actually uses two networks — one of them generates an image of a lens map without noise, while the other one compares it to the real noise-free lens map, tagging the created images as a fake. By running this system through a large number of noise and denoised map pairs, both of the networks are trained. The first one makes lens maps that are closer to the real ones, while the other network does a better job of identifying fakes.
The diagram of the AI (generative adversarial network) utilized in the study. Credit: NAOJ
To further test their method, the scientists turned their AI's attention to real data from 21 square degrees of the sky, showing that the distribution of foreground mass is in accordance with what is predicted by the standard cosmological model.
Tiny fluctuations in old Kepler data reveals four runaway planets that are reminiscent of Earth.
- Scientists discover four rogue planets that are unbound to any star.
- The discovery was made thanks to tiny microlensing light curves in data from the retired Kepler telescope.
- There may be billions of such rogue planets in our galaxy.
It's a familiar model: a solar system comprised of several planets orbiting a sun. And it's mostly true. However, it's not always true. A study just published in the Monthly Notices of the Royal Astronomical Society announces the discovery of four Earth-sized planets roaming freely through the universe on their own.
One last victory for Kepler
The find is a late-life triumph for the aging Kepler Telescope, which was retired in October 2018. Evidence for the planets appears in data from its K2 data collection mission that began when the craft's four gyroscope wheels, which allowed scientists to point Kepler at specific objects, failed in 2013.
"Our observations pointed an elderly, ailing telescope with blurred vision at one of the most densely crowded parts of the sky," says lead author of the study Iain McDonald of the University of Manchester, "where there are already thousands of bright stars that vary in brightness, and thousands of asteroids that skim across our field."
The K2 missions consisted of a series of campaigns in which the space telescope could observe star fields visible from the Earth's ecliptic plane as it orbited the planet. During these campaigns, mission controllers were able to keep Kepler in position by firing its thrusters and using its remaining pair of wheels. Each campaign lasted 80 days, after which the sun's angle changed and solar winds disrupted control of Kepler's attitude — that is, its angular orientation.
General relativity and microlensing
Kepler was originally designed to detect exoplanets using the "transit method" that looks for dips in the brightness of celestial objects (such as stars) as other objects (such as planets) pass in front of them. The craft detected thousands of exoplanets during its primary mission. However, this method only works for planets orbiting stars, and the Royal Academy astronomers were interested in looking for rogue planets.
Enter microlensing. Albert Einstein predicted 85 years ago that general relativity would allow objects to be seen due to microlensing, an effect that increases a distant object's brightness as nearby objects pass in front of it. This brief magnification effect could last anywhere from hours to a few days. These signals might be so subtle, says co-author Eamonn Kerins of the University of Manchester, that "Einstein himself thought that they were unlikely ever to be observed."
Poring through 2016 K2 data, the study's authors identified 27 short-duration microlensing candidates in the crowded star field, most of which were confirmed by observations from other systems. Four, however, were brand-new discoveries. Their super-brief durations suggest four lonely objects approximating the Earth in mass floating through space.
It is not yet known what would cause a planet to break free from its star, though it has been theorized that the gravitational pull of large nearby objects may yank them from the grasp of their solar system.
Still, rogue planets may not be uncommon. NASA says there may be twice as many free-floating Jupiter-mass planets as stars. Astronomers from the University of Leiden in the Netherlands say there are likely to be billions of rogue planets careening through the Milky Way. And astronomer Simon Portegies Zwart proposes that even our own solar system may have lost a Neptune-sized planet long ago.