Science is an ongoing flirtation with the unknown.
- The history of modern cosmology is one of the great triumphs of the human imagination.
- Still, mysteries abound, particularly the nature of dark matter and dark energy.
- Science moves forward by embracing the unknown as a challenge; taking the wrong turn is part of the way forward.
"Where did everything come from?" is perhaps the most fascinating question we can ask — so much so, that it's much older than science itself, given that most religions have also wondered about our origins. That science joined in during the 20th century as a powerful new voice in this conversation is nothing short of extraordinary. How amazing is it that a mammalian species on a small planet could develop the intellectual and technological tools to say something concrete about the history of the universe itself? And how far can we go telling this story?
Dark matter and dark energy are vivid reminders that science is an ongoing flirtation with the unknown.
We know that the universe has a history that started some 13.8 billion years ago — hence, the name of our column, 13.8 — and that it has been expanding and cooling ever since. How do we know this? There are several ways: (1) Galaxies are receding from one another with speeds proportional to their distance, carried by the expansion of space itself; (2) A bath of microwave photons (i.e., the particles that make up light and all other forms of electromagnetic radiation) permeates the whole universe, serving as fossils from the time when the first hydrogen atoms formed, some 400,000 years after the Big Bang — as predicted by theory; and (3) Between a second and three minutes after the Big Bang, the first light atomic nuclei were formed by a process called "primordial nucleosynthesis" in quantities also predicted by theory and verified by observations.
The missing ingredients in the universe
All the above is solid science. But it's not enough. We want to go further back in time to explain some of the finer details of the cosmic expansion, before and beyond the formation of light nuclei and the microwave background. So we add two more components to the cosmic recipe, both suggested by observational evidence but still shrouded in mystery: dark matter and dark energy.
If we think of the material composition of the universe as a cake recipe, we find ourselves currently in the odd situation of knowing that we have three main ingredients — regular matter, dark matter, and dark energy — and how much of each we need, but we don't really know what the two most abundant are. We do know a lot about them, but certainly not enough. And that's the agony and the (potential) ecstasy of scientific research, the power of speculation to open new ways of thinking about nature or sinking us into further confusion.
A dark mystery
Credit: NASA, ESA, M. J. Jee and H. Ford et al. (Johns Hopkins Univ.)
Dark matter was first speculated to exist in the 1930s by the Swiss-American astronomer Fritz Zwicky as he noticed that galaxies in clusters moved faster than they should if the matter in the cluster was only the matter that shined (and, hence, was visible to our telescopes). Things evolved faster after American astronomer Vera Rubin and her collaborators noticed in the late 1970s that stars in galaxies rotated faster than they should if the matter within them was, again, only the matter that shined. An intense search for dark matter — so-named because we can't see it — has been ongoing for the past four decades or so, still with negative results. The puzzling thing is that we see its effects quite clearly as we look to objects in space. Having mass (and thus gravitational pull), it affects the stuff we can see. But efforts to collect particles of dark matter have been unsuccessful so far, a somewhat stressful tension between astronomical observations and fundamental theory.
Dark energy was discovered in 1998 and is even more mysterious and elusive. We know it's not made of particles or smaller chunks of material stuff as dark matter probably is; it seems to be an ethereal substance that permeates the whole cosmos with the bizarre property of making space stretch out faster than expected. We can't think of it as a localized thing but rather as a spread-out thing, like air in the atmosphere (sort of).
Efforts to collect particles of dark matter have been unsuccessful so far, a somewhat stressful tension between astronomical observations and fundamental theory.
Dark energy candidates are all quite weird. One candidate consists of quantum fluctuations of energy in empty space that materialize as particles that pop in and out of existence, the energy of the vacuum itself. Or it could be a mysterious property of space itself, something Einstein invented to save his 1917 failed model of a static universe, today called the "cosmological constant." Most probably, if this is dark energy, it is only an approximation for something much more complex and subtle that only looks constant to us now. Or perhaps dark energy is some unknown kind of substance modeled as a diaphanous field that pervades all of space, affectionally called "quintessence" by cosmologists, echoing the substance Aristotle proposed to make up celestial objects and fill up the heavens.
Like footprints in the snow
Whatever they are, dark matter and dark energy have the potential to revolutionize our understanding of the universe. Like subtle tracks of a fox on a vast snowfield, we know they are out there in some form due to the way they impress their presence on what we can see in the world. If we didn't know a fox existed, we would infer an animal made those tracks. We would then try to imagine what kind of animal it was that left tracks such as these using the evidence at hand.
Likewise, we see the tracks of dark matter and dark energy imprinted in the universe, and we are trying to determine what mysterious things they could be. Dark matter and dark energy are vivid reminders that science is an ongoing flirtation with the unknown. Even if our current speculations turn out to lead us in the wrong direction, we need to take risks to advance our understanding of the world.
A new study found the possible reason why some dwarf galaxies appear to not have dark matter.
- A new paper presents a possible reason for why some dwarf galaxies appear to be missing dark matter.
- The researchers at the University of California, Riverside ran cosmological simulations to find the answers.
- They discovered some galaxies were stripped of dark matter through extreme tidal loss.
Astronomers discovered that extreme tidal loss may be a possible explanation for why some galaxies seem to have no dark matter, a mystery type of matter that's supposed to take up to 27 percent of the universe, according to NASA. Dark energy takes up another 68 percent, creating a repulsive force that speeds up the universe's expansion. Neither has been directly seen so far but rather inferred through their effects on space.
The team from the University of California, Riverside, found anomalies in some smaller galaxies, known as "dwarf galaxies" (containing up to a billion stars, compared to the Milky Way's 200-400 billion). Some appear to have no dark matter at all. This is despite the fact that they were formed in galaxies that were teeming with dark matter previously. What is the explanation for this phenomenon, which muddies our understanding of dark matter?
The scientists used a cosmological simulation called Illustris on dark-matter-free galaxies DF2 and DF4. They wanted to understand how similar space objects would evolve and what might have happened that led them to lose dark matter. The simulation could create galaxies, with evolving stars, supernovas, and growing and merging black holes. Within the simulation, the researchers found "dwarf galaxies" similar to DF2 and DF4 which lost over 90 percent of their dark matter through the process of tidal stripping, in which material is stripped from the galaxy by galactic tidal forces.
The study's first author was the physics and astronomy graduate student Jessica Doppel, while the co-author Laura Sales, an associate professor of physics and astronomy, was Doppel's graduate advisor.
"Interestingly, the same mechanism of tidal stripping is able to explain other properties of dwarfs like DF2 and DF4 — for example, the fact that they are 'ultradiffuse' galaxies," said Sales. "Our simulations suggest a combined solution to both the structure of these dwarfs and their low dark matter content. Possibly, extreme tidal mass loss in otherwise normal dwarf galaxies is how ultradiffuse objects are formed."
Besides Sales and Doppel, the study involved Julio F. Navarro from the University of Victoria in Canada, Mario G. Abadi and Felipe Ramos-Almendares of the National University of Córdoba in Argentina, Eric W. Peng of Peking University in China, and Elisa Toloba of the University of the Pacific in California.
Laura Sales (seated, left) and her research group of students, including Jessica Doppel (seated, right).
Credit: UCR/Stan Lim
Sales's team is currently collaborating with the Max Planck Institute for Astrophysics in Germany to improve the simulations with more advanced physics and a resolution that's 16 times better than the Illustris they used on this study.
Check out the new paper, published in the Monthly Notices of the Royal Astronomical Society.
Dr. Katie Mack explains what dark energy is and two ways it could one day destroy the universe.
- The universe is expanding faster and faster. Whether this acceleration will end in a Big Rip or will reverse and contract into a Big Crunch is not yet understood, and neither is the invisible force causing that expansion: dark energy.
- Physicist Dr. Katie Mack explains the difference between dark matter, dark energy, and phantom dark energy, and shares what scientists think the mysterious force is, its effect on space, and how, billions of years from now, it could cause peak cosmic destruction.
- The Big Rip seems more probable than a Big Crunch at this point in time, but scientists still have much to learn before they can determine the ultimate fate of the universe. "If we figure out what [dark energy is] doing, if we figure out what it's made of, how it's going to change in the future, then we will have a much better idea for how the universe will end," says Mack.
Astronomers propose a new location for the mysterious force that accelerates the universe.
- Astronomers predict that dark energy is located in the voids between galaxies.
- Dark energy is thought responsible for the acceleration of our universe.
- The intergalactic voids are known as GEODEs.
Dark energy has been estimated to take up to 68 percent of the known Universe, accelerating its expansion. One problem? No one has definitively found dark energy. Now, a new study from astronomers at the University of Hawaii at Manoa predicts that the location of the mysterious force is in the compact objects found in voids between galaxies called Generic Objects of Dark Energy (GEODEs).
The possible existence of GEODEs was first suggested in the mid-1960s. They would be formed upon the collapse of stellar objects, which would not create black holes but these unusual structures, proposed scientists. While looking almost like black holes to an outside observer, GEODEs would be different in conforming to Einstein's equations on singularities. They would consist of a spinning layer around a core of dark energy.
The new research looked at how such GEODEs would move through space. The researchers concluded that the movement is affected by the spinning layer around a GEODE. If the layer spins slowly, the GEODE would group faster than black holes, explains the University's press release. This is due to the unusual fact that GEODEs increase in mass from the universe's growth. If the GEODE's outer layer spins close to the speed of light, another effect comes into play and GEODEs would repel each other.
The science team included Kevin Croker, Jack Runburg, and Duncan Farrah from the University of Hawaii.
"The dependence on spin was really quite unexpected," said Farrah. "If confirmed by observation, it would be an entirely new class of phenomenon."
What is Dark Energy made of?
Many of the ancient stars, from the time when the universe was less than 2 percent of its age today, would have formed GEODEs upon their demise. When these GEODEs consumed other stars and interstellar gas, they started to spin rapidly, creating mutual repulsion that pushed them apart from each other into what gradually became empty voids between galaxies.
The scientists think their study can explain where the elusive dark energy resides while staying consistent with what we were able to observe about our universe. The dark energy conundrum would be solved as the number of ancient stars corresponds to the number of ancient GEODEs necessary to make the math work.
Still, research is ongoing and the astronomers look to improve upon their results. Croker stated that "now that we have a clearer understanding of how Einstein's equations link big and small, we've been able to make contact with data from many communities, and a coherent picture is beginning to form."
Check out their new study published in The Astrophysical Journal.
An Oxford scientist claims a Nobel-Prize-winning conclusion is wrong.
- Paper by Oxford University physicist Subir Sarkar and his colleagues challenges how conclusions about cosmic acceleration and dark energy were reached.
- Physicists who proved cosmic acceleration shared a Nobel Prize.
- Sarkar used statistical analysis to question key data, but his methodology also has detractors.
Is our Universe's expansion speeding up? The 2011 Nobel Prize went to three scientists for proving just that. But what if the evidence they used to come up with this conclusion was wrongly interpreted and the supposed cosmic acceleration is simply an artifact of our movement through a local part of the Universe? In the big picture, there's no speeding up. What's also not there is the mysterious dark energy, thought to be creating that acceleration, says a new paper from a group of physicists who take issue with the supernovae-related evidence that was used to come up with the original Nobel-worthy conclusion.
The Nobel Prize for the cosmic acceleration idea, if you're wondering, was won by Saul Perlmutter, Brian Schmidt, and Adam Riess for "for the discovery of the accelerating expansion of the Universe through observations of distant supernovae". They used evidence from exploded stars called "la supernovae" or "standard candles" to show that the Universe's expansion was getting faster. These kinds of supernovas are so bright that we actually know their absolute brightness. This fact allows scientists to calculate the distance of these explosions from Earth, while studying the red shift in the light they emit points to the Universe's rate of expansion. In 1998, groups led by Perlmutter and Schmidt found light from 50 supernova to be dimmer than it was supposed to be, leading them to conclude that cosmic expansion was actually accelerating (thanks to dark energy – a yet-to-be-directly-observed enigmatic force that supposedly takes up 68% of all mass-energy in the Universe while causing it to expand).
But while the expansion has become accepted as science fact, there have been some who see things differently. Following up on his 2015 paper on this subject, Oxford University physicist Subir Sarkar and his colleagues at the Niels Bohr Institute and the Paris Institute of Astrophysics now published a second study taking issue with the idea of a Universe growing with acceleration.
As explained in Physics World, by statistically analyzing a sample of 740 la supernovae in their 2015 paper, Sarkar's team found "only marginal" support for cosmic acceleration with low statistical significance. The difference in their approach was in how they looked at the procedures used to calculate the absolute brightness of supernovae and how their light is absorbed by dust that gets in the way.
2011 Nobel Laureates in Physics, Saul Perlmutter, Brian P. Schmidt and Adam G. Riess2011 Nobel Laureates in Physics, Perlmutter, Schmidt and Riess, describe how an assumed error turned into the surprise discovery that the universe is expandi...
Critics of that paper abounded, taking issues with their methodology and pointing to other data that showed acceleration. Now, in the second paper, to be published in Astronomy and Astrophysics, the scientists continue to assail the supernovae evidence and the idea of cosmic acceleration by pointing to anomalies in the red-shift data and how calculations with respect to the Cosmic Microwave Background (CMB) are carried out.
"If you look at supernovae in only a small part of the sky, it would look like you had cosmic acceleration," Sarkar says. "But we are saying that it is just a local effect, that we are non-Copernican observers. It has nothing to do with the overall dynamics of the universe and therefore nothing to do with dark energy."
Riess disagrees with Sarkar's conclusions and data, calling it outdated. His team used data from 1,300 supernovae in their latest study and came up with clear-cut support for the acceleration's existence. Furthermore, he stated, "The evidence for cosmic acceleration and dark energy are much broader than only the supernovae Ia sample."
Who would argue with a Nobel Prize-winner? Subir Sarkar, who believes that "The CMB does not directly measure dark energy," adding "That is a widely propagated myth."
You can check out his new paper for yourself at arXiv.
Lisa Randall: Dark Energy Will Take Over
Physicist Lisa Randall on why dark energy doesn't dilute as the universe expands.