A new study proposes that Hawking radiation could be used to find dark matter in places like primordial black holes.
- A new paper narrowed down what type of black holes may be the best candidates for containing dark matter.
- So far, dark matter has not been directly observed.
- The research team also developed new techniques to spot Hawking radiation that potentially comes from black holes.
Predicted to account for over 80 percent of all matter in the universe, so far, no one has directly seen dark matter. This is perhaps not surprising for a substance that doesn't reflect or emit any light. Now, a new study examines the possibility of finding dark matter in primordial black holes (PBHs), structures that hypothetically formed in the early life of the universe.
The paper, authored by scientists at the University of Amsterdam and the University of California-Santa Cruz and published in Physical Review Letters, looked to narrow down the parameters PBHs would need to contain dark matter. The authors also proposed a technique that could find dark matter by looking for so-called Hawking radiation.
What is Hawking radiation?
The late Stephen Hawking proposed the existence of thermal radiation that spontaneously emanates from black holes. He hypothesized the radiation was created by quantum effects near the black hole's event horizon, the boundary beyond which no light can escape. Furthermore, Hawking believed that over time, the radiation would result in enough energy and mass being taken away from a black hole to make it evaporate completely.
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In the new paper, the researchers calculated the likely mass constraints of PBHs that could be composed of dark matter. Specifically, they concluded that PBHs similar to an asteroid in size (around 1017 grams to 1022 grams) could "make up all the dark matter" in the universe. Furthermore, the study looked at new techniques for finding dark matter, examining the possibility of using MeV (megaelectron volt) gamma-ray telescopes to detect Hawking radiation coming from the primordial black holes.
In an interview with Phys.org, researcher Adam Coogan explained why their approach could work.
"The main idea behind our work was to think about a particular way of looking for asteroid-mass PBHs," Coogan shared. "Light PBHs are expected to emit Hawking radiation consisting of a mix of photons and other light particles, such as electrons and pions. Telescopes can then search for this radiation by observing our galaxy or other galaxies."
Paving the way for future telescopes
Coogan added that the goal of their paper was to evaluate if future telescopes would be able to spot this radiation and "how much of the asteroid-mass PBH parameter space they could probe."
What the researchers discovered is that previous studies have not yet analyzed data from NASA's COMPTEL gamma-ray telescope aboard the Compton Gamma Ray Observatory (CGRO). Utilizing the telescope's data could help narrow down the PBHs that need to be examined to those just below the asteroid-mass gap (that is, below 1017 grams). These would comprise the strongest constraints found so far and could lead to further discovery.
Two ways dark energy could destroy the universe | Katie Mack | Big Think www.youtube.com
The scientists also refined the calculations necessary to spot the spectrum of the hypothesized Hawking radiation supposedly emitted by a primordial black hole. Specifically, they improved upon the detection of radiation produced by electrons and pions within the spectrum.
The team's calculations could help determine how much PBHs of particular masses contribute to the overall amount of dark matter in the universe. Comparing their calculations of the radiation spectrums to observed data from areas believed to contain a lot of dark matter, like the center of the Milky Way, could help scientists rule out or zero in on certain black holes as dark matter candidates.
Looking ahead, the researchers believe that the next generation of MeV gamma-ray telescopes would be able to find dark matter in primordial black holes by directly detecting Hawking evaporation.
A new AI-generated map of dark matter shows previously undiscovered filamentary structures connecting galaxies.
- Scientists use artificial intelligence to produce a new map of dark matter in the local universe.
- The map's precision may lead to new insights into dark matter and the future of our universe.
- The map contains previously unknown "hidden bridges" that link galaxies.
A new map derived with the help of artificial intelligence reveals previously unknown "bridges" linking galaxies in the local universe. The bridges are in the form of filamentary structures. The scientists hope their map, published along with their paper in the Astrophysical Journal, can provide fresh insights into dark matter and the history of our universe.
While dark matter is an accepted notion, thought to make up 80 percent of all the matter in the universe, it has been hard to find. Scientists have, however, inferred much about the existence and behavior of dark matter by observing its gravitational influence on other space objects.
The universe has a dark matter skeleton
Cosmologists believe that dark matter serves as the filamentary skeleton of the cosmic web, which in turn, makes up the large-scale structure of the universe that partially controls the motion of galaxies and other cosmic systems.
While it's not proven possible yet to directly measure how dark matter is distributed in our local universe, the international team behind the research used AI to create a new map. The "local universe," which includes us, is an area about 1 billion light-years in radius where galaxies and related space objects are "essentially frozen in their present day configurations" and cosmic evolution effects are negligible, the astronomers explain.
"Ironically, it's easier to study the distribution of dark matter much further away because it reflects the very distant past, which is much less complex," said one of the study's authors, Donghui Jeong, associate professor of astronomy and astrophysics at Penn State. "Over time, as the large-scale structure of the universe has grown, the complexity of the universe has increased, so it is inherently harder to make measurements about dark matter locally."
A map of dark matter within the local universe. Smaller filamentary features (yellow) act as hidden bridges between galaxies. Dark matter's gravitational influence on galaxies is indicated by black dots. Prominent features of the universe are shown by red dots and X marks the Milky Way. CREDIT: Hong et. al., Astrophysical Journal.
Creating a better dark matter map
Cosmic web maps created previously relied on simulating the 13.8-billion-year evolution of the universe from early stages to present day. Such efforts required a tremendous amount of computation and did not yet produce accurate representations of the local universe, leading researchers to devise a novel approach. For the new map, they focused on utilizing machine learning to create a model based on the distribution and motion of galaxies. This allowed them to estimate how dark matter is distributed.
The AI was trained on simulated galaxies similar to the Milky Way using Illustris-TNG — an ongoing series of simulations that features galaxies, dark matter, gasses, and other matter.
Jeong explained that if you feed specific information into the model, it can fill out the gaps, relying on what it has already processed. The scientists further confirmed the mapping by applying it to real local galaxy data from the Cosmicflows-3 catalog of distance information about nearly 18 thousand galaxies.
The resulting map features major structures in our local universe like the "local sheet," which contains the Milky Way. Nearby galaxies and the "local void" — a nearby region of empty space — are also represented. What's more, the map allowed researchers to spot new structures. In particular, they hope to study in greater depth the small filamentary structures they discovered that appear to link galaxies. Jeong called them "hidden bridges."
Jeong believes these filaments can provide insight into the future of our galaxy. One particular question of note is whether the Milky Way would eventually collide with the Andromeda galaxy.
"Because dark matter dominates the dynamics of the universe, it basically determines our fate," shared Jeong. "So we can ask a computer to evolve the map for billions of years to see what will happen in the local universe. And we can evolve the model back in time to understand the history of our cosmic neighborhood."
Further studies that include galaxy data from new astronomical surveys will be needed to perfect the map's accuracy.
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.
We're cautiously optimistic about our new findings.
Dark matter, microscopic black holes and hidden dimensions were just some of the possibilities. But aside from the spectacular discovery of the Higgs boson, the project has failed to yield any clues as to what might lie beyond the standard model of particle physics, our current best theory of the micro-cosmos.
So our new paper from LHCb, one of the four giant LHC experiments, is likely to set physicists' hearts beating just a little faster. After analysing trillions of collisions produced over the last decade, we may be seeing evidence of something altogether new – potentially the carrier of a brand new force of nature.
But the excitement is tempered by extreme caution. The standard model has withstood every experimental test thrown at it since it was assembled in the 1970s, so to claim that we're finally seeing something it can't explain requires extraordinary evidence.
The standard model describes nature on the smallest of scales, comprising fundamental particles known as leptons (such as electrons) and quarks (which can come together to form heavier particles such as protons and neutrons) and the forces they interact with.
There are many different kinds of quarks, some of which are unstable and can decay into other particles. The new result relates to an experimental anomaly that was first hinted at in 2014, when LHCb physicists spotted "beauty" quarks decaying in unexpected ways.
Specifically, beauty quarks appeared to be decaying into leptons called "muons" less often than they decayed into electrons. This is strange because the muon is in essence a carbon-copy of the electron, identical in every way except that it's around 200 times heavier.
You would expect beauty quarks to decay into muons just as often as they do to electrons. The only way these decays could happen at different rates is if some never-before-seen particles were getting involved in the decay and tipping the scales against muons.
While the 2014 result was intriguing, it wasn't precise enough to draw a firm conclusion. Since then, a number of other anomalies have appeared in related processes. They have all individually been too subtle for researchers to be confident that they were genuine signs of new physics, but tantalisingly, they all seemed to be pointing in a similar direction.
The big question was whether these anomalies would get stronger as more data was analysed or melt away into nothing. In 2019, LHCb performed the same measurement of beauty quark decay again but with extra data taken in 2015 and 2016. But things weren't much clearer than they'd been five years earlier.
Today's result doubles the existing dataset by adding the sample recorded in 2017 and 2018. To avoid accidentally introducing biases, the data was analysed "blind" – the scientists couldn't see the result until all the procedures used in the measurement had been tested and reviewed.
Mitesh Patel, a particle physicist at Imperial College London and one of the leaders of the experiment, described the excitement he felt when the moment came to look at the result. "I was actually shaking", he said, "I realised this was probably the most exciting thing I've done in my 20 years in particle physics."
When the result came up on the screen, the anomaly was still there – around 85 muon decays for every 100 electron decays, but with a smaller uncertainty than before.
What will excite many physicists is that the uncertainty of the result is now over "three sigma" – scientists' way of saying that there is only around a one in a thousand chance that the result is a random fluke of the data. Conventionally, particle physicists call anything over three sigma "evidence". However, we are still a long way from a confirmed "discovery" or "observation" – that would require five sigma.
Theorists have shown it is possible to explain this anomaly (and others) by recognising the existence of brand new particles that are influencing the ways in which the quarks decays. One possibility is a fundamental particle called a "Z prime" – in essence a carrier of a brand new force of nature. This force would be extremely weak, which is why we haven't seen any signs of it until now, and would interact with electrons and muons differently.
Another option is the hypothetical "leptoquark" – a particle that has the unique ability to decay to quarks and leptons simultaneously and could be part of a larger puzzle that explains why we see the particles that we do in nature.
Interpreting the findings
So have we finally seen evidence of new physics? Well, maybe, maybe not. We do a lot of measurements at the LHC, so you might expect at least some of them to fall this far from the standard model. And we can never totally discount the possibility that there's some bias in our experiment that we haven't properly accounted for, even though this result has been checked extraordinarily thoroughly. Ultimately, the picture will only become clearer with more data. LHCb is currently undergoing a major upgrade to dramatically increase the rate it can record collisions.
Even if the anomaly persists, it will probably only be fully accepted once an independent experiment confirms the results. One exciting possibility is that we might be able to detect the new particles responsible for the effect being created directly in the collisions at the LHC. Meanwhile, the Belle II experiment in Japan should be able to make similar measurements.
What then, could this mean for the future of fundamental physics? If what we are seeing is really the harbinger of some new fundamental particles then it will finally be the breakthrough that physicists have been yearning for for decades.
We will have finally seen a part of the larger picture that lies beyond the standard model, which ultimately could allow us to unravel any number of established mysteries. These include the nature of the invisible dark matter that fills the universe, or the nature of the Higgs boson. It could even help theorists unify the fundamental particles and forces. Or, perhaps best of all, it could be pointing at something we have never even considered.
So, should we be excited? Yes, results like this don't come around very often, the hunt is definitely on. But we should be cautious and humble too; extraordinary claims require extraordinary evidence. Only time and hard work will tell if we have finally seen the first glimmer of what lies beyond our current understanding of particle physics.
Harry Cliff, Particle physicist, University of Cambridge; Konstantinos Alexandros Petridis, Senior lecturer in Particle Physics, University of Bristol, and Paula Alvarez Cartelle, Lecturer of Particle Physics, University of Cambridge
Researchers propose a new method that could definitively prove the existence of dark matter.
- Scientists identified a data signature for dark matter that can potentially be detected by experiments.
- The effect they found is a daily "diurnal modulation" in the scattering of particles.
- Dark matter has not yet been detected experimentally.
Dark matter, a type of matter that is predicted to make up around 27 percent of the known universe, has never been detected experimentally. Now a team of astrophysicists and cosmologists think they found a clue that may lead them to finally detect the elusive material, so hard to find because it does not absorb, reflect, or emit light.
The existence of dark matter has so far been predicted by inference from its gravitational effects on the motion of the stars and galaxies rather than direct observation. No existing technologies can pick it out. This has led researchers at the Shanghai Jiao Tong University and the Purple Mountain Observatory of the Chinese Academy of Sciences to identify characteristic dark matter signatures that would be easier to detect.
Their new paper proposes a new type of effect that relates to the so-called "sub-GeV dark matter" which is boosted by cosmic rays. Looking for this effect can potentially allow direct detection of dark matter using nuclear recoil techniques.
The diurnal effect of accelerated dark matter rays. Credit: Ge et al.
The research team included Shao-Feng Ge and Qiang Yuan, who explained that their approach is to look for a prominent signature of accelerated dark matter particles that come from the galaxy's center, where dark matter and cosmic rays are at high density. They found that these particles have a "diurnal modulation" – a scattering pattern that is linked to the time of day. At periods when the Galaxy Center faces the side of the planet that's opposite the location of the detector, the Earth shadows a large amount of these particles. At other times, they come in as a signal with "higher recoil energy."
"The conventional diurnal effect is only for slow moving (nonrelativistic) DM particles in our galaxy (so-called standard DM halo)," Ge and Yuan said to Phys.org. "The effect is negligibly small either from direct experimental constraints, or due to the detection threshold. For light DM particles, on the other hand, the DM-nucleus interaction is much less constrained, which leaves room for strong diurnal modulation."
Researchers Ning Zhou and Jianglai Liu, who were also involved in the study, said in an interview that the signature they are proposing could be "a smoking gun of cosmic ray boosted dark matter detection".
The researchers plan next to look for the signature in previously gathered data, as well as in underground dark matter experiments.
They are also encouraging scientists around the world to look for this signature in their data.
Check out the new paper "Diurnal Effect of Sub-GeV Dark Matter Boosted by Cosmic Rays" published in Physical Review Letters.