When we look in the night sky, we can see hundreds of stars. In remote and dark areas we can see a few thousand stars with the naked eye. But imagine a night so bare you could only see a couple dozen stars. Most of them distant and dim. This would be our sky if our solar system existed in an ultra-diffuse galaxy. 

Our galaxy spans 100,000 light years, and contains nearly 400 billion stars. Ultra-diffuse galaxies can have a similar size, but only contain a few billion stars.  They are so fluffy that they raise several questions. Since these galaxies contain so few stars despite their size, they either failed to produce stars during their evolution, or they had most of their stars stripped away by some cosmic event. Then there is the question of how long they can last. With fewer stars, the galaxies aren’t tightly bound by gravity compared to galaxies like the Milky Way.  Recently in an attempt to answer some of these questions, a team of astronomers found an interesting clue.

These diffuse galaxies are also known as low surface brightness galaxies. Because they have fewer stars, they aren’t as bright as other galaxies of similar size. The team looked at 89 of these galaxies in a dense supercluster of galaxies known as the Perseus cluster. Since the Perseus cluster has many large and dense galaxies, we would expect that diffuse galaxies would tend to be torn apart by the gravity of more massive galaxies. But the team found that many of the diffuse galaxies were largely intact. In order to survive the tidal forces of nearby galaxies, they must have much more mass than their stars alone, and that means they likely contain large amounts of dark matter.

If that’s the case, it could also explain why they contain so few stars. These diffuse galaxies could have formed with a mass similar to our Milky Way, but with much less gas and dust, producing much fewer stars. To know for sure we’ll need a better understanding of dark matter, but that’s another story.

Paper: C. Wittmann, et al. A population of faint low surface brightness galaxies in the Perseus cluster core. MNRAS, 470, 1512 (2017)

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In the battle to describe the motion of galaxies, two models fight head to head. In one corner is dark matter, a strange form of matter that surrounds galaxies, and can be seen only through its gravitational tug on light. In the other corner is Modified Newtonian Dynamics (MoND), which proposes a new approach to gravitational interactions, just as Einstein’s model superseded Newton’s. 

Dark matter has long been the favorite contender, since it explains a range of observed phenomena such as the large scale clustering of galaxies as well as the motion of gas and dust within spiral galaxies.  But MoND also has a few things going for it. Overall, it’s predictions agree with dwarf galaxies better than dark matter, and MoND also matches the rotation of spiral galaxies. As long as direct detection of dark matter continues to elude us, modified gravity models will be eager challengers.

Recently MoND regained some interested after a study showed that the rotation of galaxies seemed to correlate with the distribution of visible matter in those galaxies. This is exactly what one would expect from MoND models, and seemed to go against the predictions of dark matter. But new computer simulations show that dark matter can account for these correlations after all.

The team developed a computer model that accounted not just for stars and dark matter, but also the interstellar gas. In modeling the formation and evolution of galaxies, they found that a correlation forms between the distribution of matter and their rotation. They also found the relation between mass, size and luminosity of galaxies in their model matched that of observed galaxies.

So dark matter can account for the rotational dynamics of a galaxy just as MoND models can. That doesn’t rule out MoND, but it does mean that MoND isn’t necessary in this case.

Paper: Aaron D. Ludlow, et al. Mass-Discrepancy Acceleration Relation: A Natural Outcome of Galaxy Formation
in Cold Dark Matter Halos. PRL 118, 161103 DOI: 10.1103/PhysRevLett.118.161103 (2017)

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Dark matter is difficult to study. Since it doesn’t interact with light, it is basically invisible. But it does have mass, and that means it deflects light ever so slightly, an effect known as weak gravitational lensing. By observing the way light from distant galaxies is distorted, we can map the distribution of mass between us and the galaxies. Comparing this to the visible matter of galaxies allows us to map the presence of dark matter. This technique works well when measuring large regions of dark matter, such as the halos around galaxies, but gravitational lensing is such a weak effect it’s difficult to study the detailed structure of dark matter. That’s unfortunate, because the details are what we need to understand the nature of dark matter. 

A computer simulation showing filaments of dark matter between clusters of galaxies. Credit: Michael L. Umbricht

The dominant model for dark matter makes several predictions we can test. For example, it predicts that dark matter will clump together gravitationally, and that means galaxies will also cluster together at a particular scale. This is exactly the clumping scale we observe across the cosmos. But there are also predictions we can’t easily test, such as dark matter filaments. As dark matter clumps together, some of the dark matter should be left behind, forming filaments of dark matter that connect galaxies and clusters of galaxies. These filaments have long been thought to exist, but detecting them is extremely difficult. Their gravitational influence is so small any weak lensing they produce is almost indistinguishable from random noise.

There has been some evidence of dark matter filaments. Comparisons of faint lensing between galaxies agrees with models of dark matter filaments, but with weak data you have to be careful not to presume too much about your model. A new paper in MNRAS tries to overcome this issue by taking a different approach. Rather than trying to observe filaments within a single cluster of galaxies, the team looked at data from thousands of filaments.

Taking data from the Baryon Oscillation Spectroscopic Survey, they focused on about 23,000 pairs of Luminous Red Galaxies (LRGs). These galaxies are particularly bright, and are easy to distinguish from other galaxies. They also have very similar structures, which makes them useful to study statistically. The team then measured the weak lensing between these pairs of galaxies. Individually the lensing between them would be hard to distinguish from random distortions, but they then combined the data from the pairs to create an overall average. In this way any random distortions would tend to wash out, leaving only the effects of dark matter. The result is a statistical image of the dark matter filaments between galaxy pairs.

While the result is statistical, it doesn’t rely upon a dark matter model to infer its presence. It also agrees with the statistical predictions of dark matter filaments. It’s yet another success for the dark matter model.

Paper: Seth D. Epps et al. The weak-lensing masses of filaments between luminous red galaxies. Monthly Notices of the Royal Astronomical Society. DOI: 10.1093/mnras/stx517 (2017)

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Over the years, dark matter has remained an enigma. Observations of things like large scale galaxy distribution and the motion of stars and gas within galaxies points to the existence of some sort of weakly interacting matter, we still haven’t figured out what this dark matter could be. We know a lot of things it can’t be, such as neutrinos, but the solution still eludes us. So one idea that keeps coming back is that dark matter might be due to black holes. Not stellar mass black holes formed from dying stars, or supermassive black holes found in the centers of galaxies, but smaller black holes that may have formed in the early universe. 

The idea that small black holes could be dark matter is a pretty good one. Small black holes could be widely distributed, giving the halo effect we see around galaxies. Their small size would mean they wouldn’t absorb much light (making them “dark” in astronomical terms). Plus, the idea doesn’t need to call upon any kind of exotic yet to be seen type of matter. Small, primordial black holes could have formed from regular matter during the early moments of the big bang.

Observational constraints on dark matter black holes.

It’s also a theory we can test, and that’s where the idea starts to fall apart. Dark matter comprises the majority of mass within galaxies, so if it was comprised of small black holes, there would have to be a lot of them. If these black holes were fairly large (say, on the order of a solar mass or more) then we should observe them distort starlight that they pass in front of though an effect known as gravitational microlensing. We’ve watched a lot of stars over time, and there has been no microlensing. If they were small black holes (about the mass of a moon) then there would be so many of them that they would distort the light from gamma ray bursts, and again we haven’t seen any evidence of that. If the black holes were really tiny, then Hawking radiation would have caused them to evaporate away long before now.

So most sizes of black holes have been ruled out as a possibility, but not all. If these primordial black holes were around the mass of a few Jupiters, then they would be too small to be observed through microlensing, but too large to have an effect on things like gamma ray bursts. As it stands, we don’t have an observational test that can rule out dark matter black holes with a narrow range of masses. Of course that would raise the question about why primordial black holes would only be in this mass range and not others. The good news is that with the dawn of gravitational astronomy we should be able to test the idea further in the future. If dark matter is comprised of Jupiter-mass black holes, then some of them will merge and create gravitational waves. Either we will see these waves eventually, or we will basically rule out black holes as an option for dark matter.

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Gamma rays are the most energetic forms of light in the Universe. They’re generated by a variety of sources, from the heated material surrounding supermassive black holes, to the supernova explosions of dying stars. But some have theorized they might also be produced by dark matter. 

Dark matter doesn’t interact strongly with regular matter, nor does it interact strongly with light. But since efforts to detect dark matter directly have failed so far, we aren’t entirely sure what makes up dark matter. This has led theorists to develop lots of models about how dark matter might interact with itself. Some dark matter models propose that while dark matter doesn’t interact with regular matter, dark matter particles can collide and annihilate to produce gamma rays, similar to the way matter and antimatter can produce gamma rays through annihilation. Since dark matter is fairly spread out throughout our galaxy, this would produce a diffuse background of gamma rays.

Interestingly, the gamma ray background we observe is diffuse. About 80% of the gamma rays we observe don’t come from a specific source such as supermassive black holes. It’s possible that they come from distant sources we can’t pinpoint, but it could also come from diffuse dark matter interactions. At least that’s been one idea.

But a recent survey of the gamma ray background doesn’t support the dark matter models. Using 81 months of data from the Fermi telescope, the team was able to distinguish the energy levels of different gamma rays, and found that they tend to occur at two energies. The highest energy gamma rays seem to come from known sources such as black holes and supernovae, while lower energy gamma rays don’t have a clear source. However, the distribution and energy range of the lower energy gamma rays is inconsistent with dark matter models, so most of them can be ruled out as the source.

To be clear, this does not mean that dark matter doesn’t exist (as some popular articles have claimed). It does, however, mean that dark matter doesn’t emit gamma rays. So the dark matter enigma continues to evade a solution, and this new study simply adds to the mystery.

Paper: Mattia Fornasa, et al. Angular power spectrum of the diffuse gamma-ray emission as measured by the Fermi Large Area Telescope and constraints on its dark matter interpretation. Phys. Rev. D 94, 123005 (2016)  DOI: 10.1103/PhysRevD.94.123005

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As dark matter continues to vex astronomers, new solutions to the dark matter question are proposed. Most focus on pinning down the form of dark matter, while others propose modifying gravity to account for the effect. But a third proposal is simply to remove gravity from the equation. What if the effects of gravity aren’t due to some fundamental force, but are rather an emergent effect due to other fundamental interactions? A new paper proposes just that, and if correct it could also explain the effects of dark matter.

The idea of emergent gravity isn’t entirely new. The most popular variation was proposed in 2010, where Erik Verlinde argued that gravity is not a fundamental force, but rather an effect that arises from the entropy of the Universe. Entropy is a property of thermodynamics. It’s often described as the unusable part of a system (or the waste heat if you will) and while that’s sometimes a useful description, a better description involves the amount of information contained within a system. An ordered system (say, marbles evenly spaced in a grid) is easy to describe because the objects have simple relations to each other. On the other hand, a disordered system (marbles randomly scattered) take more information to describe, because there isn’t a simple pattern to them. Basically, the more information it takes to describe a system, the more entropy it has.

Verlinde’s model uses this connection between thermodynamics (heat, energy, and forces) and information through a mathematical method known as the holographic principle. Since the information contained within a region of space depends upon the arrangement of objects within that region, moving the objects can change the entropy within the region. Verlinde demonstrated that this produces an entropic force that acts like gravity. From the basic idea of information entropy, one can derive Einstein’s equations of general relativity exactly.

Entropic gravity is an interesting idea, and it would explain why gravity is so difficult to bring into the fold of quantum physics, but it’s not without its problems. For one, since entropic gravity predicts exactly the same gravitational behavior as general relativity, there’s no experimental way to distinguish it as a better theory. There are also theoretical problems with the model. For example, if you try to describe a gravitationally closed system of masses within the model it only matches experiment if you place weird constraints on the entropy of the system.

But despite its problems the idea is at least worth exploring, and this latest work adds a new twist by describing the effects of dark matter. In the original formulation, the model focused on standard gravity. Specifically, it excluded dark energy. This new paper notes that since the dark energy of a region of space requires additional information to describe, including it in the model changes the entropy of a region of space. The paper then goes on to show how this additional information creates an additional entropic force. One that might account for the effects of dark matter similar to other modified gravity models such as Modified Newtonian Dynamics (MoND). Thus gravity, dark matter, and dark energy might all be connected through entropy.

While this seems like an elegant solution to several cosmological problems, there are plenty of reasons to be skeptical. For one, this new variation of emergent gravity still has the same theoretical difficulties of the original. Then there’s the fact that modified gravity models fail to explain large scale effects such as the clustering of galaxies, which regular gravity and dark matter explains very well. This new work is still more of an idea and less of a robust theory.

But even if the model doesn’t work out in the end, it demonstrates how thermodynamics and gravity are deeply connected in ways that aren’t obvious at first glance.

Paper: E. P. Verlinde. Emergent Gravity and the Dark Universe. arXiv:1611.02269 (2016)

Paper: E. P. Verlinde. On the Origin of Gravity and the Laws of Newton.  arXiv:1001.0785 (2010)

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One point of evidence in support of dark matter is the way in which the speed of stars, gas and dust in a galaxy varies with their distance from the center of the galaxy, known as the galactic rotation curve. Most of the visible matter of a galaxy is concentrated near the center of a galaxy, so we would expect that more central stars should move much faster than stars on the outer rim. Thus the rotation curve should decrease with distance. However most galaxies have a fairly flat rotation curve, meaning outer stars move about as fast as inner stars. This and other evidence as led us to develop the theory of dark matter. But new research on galactic rotation curves has found an odd correlation, and it could mean that dark matter is wrong after all. 

The rotation curve for M33, also known as the Triangulum galaxy. Credit: Wikipedia

The orbital speed of stars in a galaxy depends upon the gravitational pull of a galaxies mass. The more strongly a star is pulled toward galactic center, the greater its radial acceleration, and the faster it needs to move to overcome that pull. This is similar to the planets in our solar system. Mercury is close to the Sun, and therefore has a large radial acceleration due to the Sun’s gravity. Distant Pluto has only a small radial acceleration. So Mercury zips around the Sun at 48 km/s, while Pluto chugs along at less than 5 km/s. According to the dark matter model, a galaxy’s mass isn’t concentrated in its center. Most of the visible matter is, but a galaxy is surrounded by a halo of dark matter. Most of a galaxy’s mass is dark matter, and most of it is in the halo.

There’s a lot of other evidence to support dark matter, but there have also been alternative models such as Modified Newtonian Dynamics (MoND). In this model, the radial acceleration of a star deviates very slightly from that predicted by Newtonian physics and general relativity. The difference is too small to notice on the scale of our solar system, but on a galactic scale that difference adds up, producing a galactic rotation curve just as we observe. MoND and related theories can accurately describe rotation curves, but they fail to describe other effects such as large scale galactic clustering and the mass distributions of colliding galaxies, so dark matter is the dominant model in astrophysics. But new research on the radial acceleration of stars could bring MoND back into favor.

The correlation between radial and gravitational accelerations is pretty strong. Credit: McGaugh, et al.

The researchers looked at the observed rotation curves for 153 galaxies, and calculated the radial acceleration at various distances in each galaxy. They then compared these results to the gravitational acceleration as predicted by the distribution of visible matter within a galaxy (technically the distribution of baryonic mass). They found a strong correlation between the two. When the gravitational acceleration was stronger, so was the radial acceleration, and when one was weaker, so was the other. What’s interesting is that this relation holds up in a range of galaxies. It didn’t matter whether most of the visible matter was clustered in the center or not, the relation still held. It’s also a purely empirical correlation, so there is no strong theoretical component to make it work.

So what gives? The researchers propose three possibilities. The first is that the correlation could be due to the dynamics of galaxy formation. It’s not clear how this would occur, but there are aspects of galactic evolution we don’t fully understand. The second is that the distribution of dark matter and baryonic matter within a galaxy are correlated. This would require some new kind of dark matter physics that makes dark matter clump in the same way that regular matter does. The third and perhaps most intriguing idea is that it really is due to some kind of modified dynamics.

The correlation between radial acceleration and mass distribution is strong enough that it points to baryonic matter being the source of the acceleration. A relation such as this is exactly the kind of thing MoND models would predict, and it contradicts current dark matter models. If this result is replicated, dark matter will have some explaining to do if it wants to remain the dominant theory. But the challenge for MoND is also just beginning. To succeed in the end it will have to account for things like large scale clustering, where it currently fails miserably.

The game between dark matter and MoND just got a bit more interesting.

Paper: Stacy McGaugh, et al. The Radial Acceleration Relation in Rotationally Supported Galaxies. arXiv:1609.05917 [astro-ph.GA] (2016)

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Dark matter is one of the great unsolved mysteries of modern astronomy. We’ve reached the point where we know most matter in the cosmos is made of matter that interacts weakly with light if at all, but drives much of the gravitational interactions between galaxies. While it’s often portrayed as a modern idea added simply to shoehorn observations into the standard model, it actually has a history spanning more than a century, and the theory of dark matter has been refined and improved as we’ve learned more about our Universe. 

The origins of dark matter can be traced to the 1600s. Soon after Isaac Newton presented his theory of universal gravity, some astronomers began to speculate about the existence of objects that might emit little or no light, but could still be known by their gravitational tug on bright objects like stars and planets. This idea was strengthened in the 1700s when Pierre Laplace argued that some objects might be massive enough to trap any light they emit (a simplistic idea of a black hole), and by the 1800s Urbain Le Verrier and John Couch Adams used gravitational anomalies in the motion of Uranus to predict the presence of Neptune. By this point astronomers had demonstrated the presence of dark nebulae, seen only by the light they absorb from bright objects behind them. It was clear that there was more in the Universe than could be seen by visible light.

Our modern take on dark matter as a major contributor to galactic mass can be traced to Fritz Zwicky. In 1933 he studied the motion of galaxies within the Coma Cluster. The Coma Cluster is a galactic supercluster containing more than 1,000 galaxies. Since these galaxies are gravitationally bound, the speed of these galaxies can provide a measure of the cluster’s mass. Basically, the more mass the cluster has, the wider the distribution of galactic speeds following a relation known as the virial theorem. A few years earlier Edwin Hubble had estimated that the Coma Cluster contained about 800 galaxies, each containing about a billion stars. Using the virial theorem Zwicky calculated a cluster mass more than 500 times larger than that of Hubble. Zwicky noted that if his measurements held true “dark matter is present in much greater amount than luminous matter.” Over the next couple decades the virial theorem was applied to other galaxy clusters with similar results. Not everyone accepted these results, largely because the virial theorem is a statistical calculation that depends upon certain assumptions. For example, it assumes the clusters are gravitationally bound. Perhaps the galaxies in these clusters are actually flying away from each other, so that the virial theorem simply doesn’t apply. But there was another line of evidence to support dark matter. One that couldn’t be so easily dismissed.

Dark matter shows its presence in large galactic clusters such as Abell 1689. Credit: HST, ACS, WFC, H. Ford (JHU)

In the early 1900s astronomers began to look at the spectra of galaxies. From this they could determine the speeds of stars as a function of their distance from galactic center, known as a galactic rotation curve. Seen in visible light, most galaxies have a bright center, dimming as you move away from the center. This would imply most of the stars (and thus most of the mass) is located near the center of a galaxy. If that’s the case, one would expect stars far from the center to move much more slowly than stars near the center, just as in our solar system Earth orbits the Sun much more quickly than distant Pluto. When Max Wolf and Vesto Slipher measured the rotation curve of the Andromeda galaxy, they found it was basically flat, meaning that stars moved at the same speed regardless of their distance from galactic center. One solution to this mystery was that Andromeda is surrounded by a halo of dark matter so that its mass is not concentrated in the center. While other galaxies showed similar rotation curves, seeming to support the presence of dark matter, even Fritz Zwicky was skeptical. Gas and dust within a galaxy might exert some kind of drag on fast moving stars, he argued, thus flattening the rotation curves. But by the 1950s radio astronomy had progressed to the point where it could detect monatomic hydrogen through the famous 21 centimeter line. Radio observations of both the Andromeda galaxy and our own Milky Way galaxy showed similarly flat rotation curves. Since hydrogen is by far the most abundant element in the Universe, the results proved that not only stars, but the gas of any dark nebulae were orbiting the galaxies at similar speeds. Either galaxies contained significant dark matter, or our understanding of gravity was very wrong.

As the evidence for dark matter grew, it soon became clear that there was a serious problem. Assuming our gravitational theories were correct, dark matter must be far more plentiful than luminous matter both in galaxies and among galactic clusters. If this dark matter consisted of things like dark nebulae, their presence should be detectable by the light they absorb. If so much dark matter exists, it must not only be non-luminous, it must not absorb light either. It couldn’t simply be regular matter that is cold and dark, but must be something very different. This was such a radial idea that many astronomers questioned the validity of Newtonian gravity. By the 1980s several alternative gravitational models, the most famous of which was Modified Newtonian Dynamics (MoND), proposed by Mordehai Milgrom. While these models did work well for things like dwarf galaxies, they worked horribly with things like galactic clusters. Dark matter models were not without their problems, but they agreed more readily with observations.

Dark matter in colliding galaxies like the Bullet Cluster show us how dark matter behaves. Credit: X-ray: NASA/CXC/CfA/M.Markevitch et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/ D.Clowe et al.; Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.

In the past couple decades data gathered from gravitational lensing and deep sky surveys have allowed us to further refine our dark matter models. From the large scale distribution of galaxies we know that dark matter must be cold and slow moving, so the countless neutrinos that zip through the cosmos at nearly the speed of light cannot account for dark matter. From gravitational lensing we know the distribution of dark matter within galaxies. By observing the distribution of dark matter within colliding galaxies we know that not only does dark matter not interact with light, it also doesn’t interact strongly with regular matter or itself. While this further verifies the existence of dark matter, it also makes it more difficult to determine just what dark matter is.

The most recent challenge for dark matter has been to determine its composition. The most popular idea is that they are Weakly Interacting Massive Particles (WIMPs), but these particles should be detectible by the same experiments used to observe astrophysical neutrinos. So far, no evidence for these particles has been forthcoming. Direct efforts to detect dark matter have only served to eliminate our options for dark matter. After studying dark matter for more than a century, it continues to elude us.

And so the dark history of dark matter continues.

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The Universe is driven by four fundamental forces: gravity, electromagnetism, and the strong and weak nuclear forces. The behavior of matter within the Universes governed by their interactions via these forces. If these four forces are truly the only forces there are, then by understanding these forces we should have a full understanding of how objects interact. So when matter behaves in a way that is odd or unexplainable, one idea that often gets considered is that there might be another force at work. Perhaps there is a fifth fundamental force we haven’t yet discovered. Take, for example, the mystery of dark matter. 

Dark matter was first proposed to account for the fact that the motion of of hydrogen in the Milky Way didn’t seem to follow the rules of gravity. Either our understanding of gravity didn’t apply on galactic scales, or our galaxy has far more mass than is visible through stars and dust. Over the last few decades other observational evidence supports the existence of dark matter, but pinning down the details has proved quite vexing. Efforts to detect dark matter directly have been unsuccessful, and creating a theoretical model that unifies dark matter with regular matter is equally elusive.

Based upon observation, we know a few facts about dark matter. It makes up about 27% of the matter in our Universe, it interacts gravitationally like regular matter, and it interacts weakly (if at all) with light. In other words, dark matter interacts via the gravitational force, but not the electromagnetic one. Perhaps the most popular model for dark matter proposes that they also interact with the weak force, making them Weakly Interacting Massive Particles (WIMPs). But if dark matter is comprised of WIMPs they should be detectible through weak interactions on Earth, and so far such interactions haven’t been observed.

So what else could dark matter be? If dark matter only interacts with other matter via the gravitational force, then one would assume it also only interacts with itself gravitationally. But there have been some interesting hints that perhaps dark matter can interact with itself in a non-gravitational way. It could be through the strong force, or it could be via some new, dark-matter force.

Every fundamental force has a corresponding force-carrying boson through which it interacts with matter. The strong force has gluons, electromagnetism and photons, etc. If there is a fifth dark-matter force, there should be some corresponding interaction boson. Several years ago Sean Carroll et al proposed an analogous force to electromagnetism known as dark electromagnetism.  Just as regular matter interacts with electromagnetism through photons, dark matter would interact through “dark photons.” Since dark photons wouldn’t interact with regular matter, the “light” from dark matter wouldn’t be seen, thus explaining its invisible nature.

According to the dark electromagnetism model, dark photons and regular photons would interact slightly through a process known as mixing, and this would have a subtle effect on particle interactions. When the model was proposed it was thought dark photons could explain a mystery in particle physics known as the g-2 anomaly, where the experimental value of a muon’s magnetic moment differs by three standard deviations from the theoretical prediction of the standard model. However subsequent experiments seem to eliminate dark photons as a viable solution.

The data seems to match the existence of a new boson. Credit: A.J. Krasznahorkay et al.

Recently studies of the decay rates of Beryllium-8 have led researchers to propose a new variation of the dark-matter force. Beryllium-8 nuclei are very unstable, so they decay into helium atoms very quickly. But if it has a lot of energy Beryllium-8 will emit a high energy photon before decaying. This photon can likewise decay into an electron-positron pair, which is much easier to detect. According to the standard model, the electron and positron are much more likely to have a similar direction, so the greater the difference in their directions, the less pairs you should see. But a team found a bump in the number when they were spaced about 140 degrees apart, which could be explained by an interaction with a new kind of boson. It’s energy is too high to be a dark photon, so the team has called it an x-boson, x being “unknown.”

It’s important to note that while this could be a big discovery, it needs to be replicated. Even if it does turn out to be legitimate there is no evidence connecting it to dark matter. The team was specifically looking for a dark-matter force boson, hence the connection to dark matter. As it stands it’s an interesting result, but there’s a lot more work to be done.

Paper: Jonathan L. Feng, et al. Protophobic Fifth-Force Interpretation of the Observed Anomaly in ⁸Be Nuclear Transitions. Phys. Rev. Lett. 117, 071803 (2016)  arXiv:1604.07411 [hep-ph]

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Most galaxies are dominated by dark matter. Our own galaxy is about 85% dark matter by mass. But recently a galaxy was discovered that is almost entirely dark matter. Known as Dragonfly 44, less than 0.01% of its mass is regular matter. 

Dark matter is highly mysterious due to the fact that we haven’t detected it directly. Since dark matter doesn’t interact with light, we can only observe its gravitational effect on light and the motions of stars. The elusive nature of dark matter has led some to propose other ideas such as modified gravity, but these don’t agree well with the Universe we observe. Given that dark matter is so mysterious, how do we know Dragonfly 44 is a dark matter galaxy?

To begin with the galaxy is very diffuse. It is only slightly smaller than the Milky Way in size, but has about 1% the number of stars. By itself that might not mean much, but Dragonfly 44 is part of a cluster of galaxies known as the Coma cluster. Over time such a diffuse galaxy would be torn apart by other nearby galaxies unless it had a large mass to hold it together. This is supported by the speeds of the stars within the galaxy. Stars within a galaxy generally orbit the center of a galaxy similar to the way planets orbit a star. The more mass a galaxy has, the faster the stars tend to orbit. By looking at the distribution of stellar speeds within a galaxy, we can get a measure of how much mass a galaxy has. Finally, Dragonfly 44 has about 100 globular clusters orbiting it. These are small but dense clusters of stars that generally form a kind of halo around larger galaxies. Typically, the larger the galaxy the more globular clusters it will have. All of this supports the idea that Dragonfly 44 has a mass about equal to that of our Milky Way. Given the low number of visible stars, that means 99.99% of its mass must be dark matter.

The spectra of Dragonfly 44. Credit: P. van Dokkum, A. Romanowsky, J. Brodie.

These kinds of Ultra Diffuse Galaxies (UDGs) could help us better understand dark matter. While galaxies such as the Milky Way are mostly composed of dark matter, their central regions where stars are most dense are dominated by regular matter. As a result it’s difficult to study the effects of dark matter in detail. With diffuse dark matter galaxies we can see the effects of dark matter more clearly. If dark matter does interact with itself to produce small amounts of light, as some have proposed, it might be visible within UDGs.

Paper: Pieter van Dokkum, et al. A High Stellar Velocity Dispersion and ~100 Globular Clusters for the Ultra Diffuse Galaxy Dragonfly 44. The Astrophysical Journal Letters, Volume 828, Number 1 (2016) arXiv:1606.06291 [astro-ph.GA]

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Dark matter is the most common matter in the Universe, accounting for nearly 85% of the total mass of the cosmos. But while we know dark matter is out there, we have no real idea what it is. Lots of ideas have been proposed, such as WIMPs or exotic particles known as axions, but another idea is that it might be caused by black holes. Not ones that formed from dying stars or the giants in the hearts of galaxies, but primordial black holes that formed soon after the big bang. 

Most models of primordial black holes propose that they formed during the first few milliseconds of the Universe. Depending on the model their hypothetical sizes range from tiny ones the mass of a small mountain, to large ones dozens of times more massive than the Sun. There has been a great deal of effort to observe primordial black holes, but so far there has been no signs of them. For this reason, primordial black holes have not been a popular solution to the dark matter problem. There would have to be lots of primordial black holes out there to account for dark matter, and their presence would be seen by things like the gravitational lensing of starlight.

The recent detection of gravitational waves, however, has rekindled interest in dark matter black holes. The LIGO observation saw the merger of two black holes roughly 30 solar masses in size. This is curious, because we’d expect stellar collapse black holes to have less mass than that, and intermediate mass black holes found in star clusters typically have more mass than that. It’s hard to imagine how the LIGO black holes could have formed from either stars or mergers, but they do fall within the upper range of primordial black hole models. They also fall within the mass range that’s hardest to rule out as a dark matter candidate.

A recent paper in the Astrophysical Journal Letters argues that black holes like the ones seen by LIGO could account for dark matter. To support this idea, Alexander Kashlinsky looked at fluctuations in the cosmic infrared background. Unlike the cosmic microwave background, which is the thermal remnant of the big bang, the cosmic infrared background is caused by a wide range of processes that produce infrared light, such as heated gas within galaxies. Fluctuations in this background can tell us about the structure of its sources. Kashlinsky compared infrared fluctuations with the distribution of known sources such as galaxies, and found some of the fluctuations couldn’t be accounted for by known sources. The scale of these fluctuations agrees with a dark matter distribution of LIGO-mass black holes. So it’s possible that black holes could explain dark matter after all.

There are reasons to be cautious however. In addition to black holes, there are other models that could explain the fluctuations of the infrared background. There’s also the fact that while LIGO has found black holes on the order of 30 solar masses, we don’t know if such black holes are common or rare. However further observations of the infrared background and the gravitational waves from black hole mergers can answer these questions. If LIGO-mass black holes turn out to be quite common, then the great mystery of dark matter could be solved by primordial black holes after all.

Paper: A. Kashlinsky. LIGO gravitational wave detection, primordial black holes and the near-IR cosmic infrared background anisotropies. The Astrophysical Journal Letters, Volume 823, Number 2 (2016) arXiv:1605.04023 [astro-ph.CO]

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Dark matter is one of the big mysteries of modern cosmology. We have lots of evidence to support its existence, but the enigmatic, invisible stuff has been notoriously resistant to direct detection. Because of this, there have been lots of ideas about just what dark matter might be made of, and just how such particles might be detected. One of these ideas focuses on particles known as axions, but recent work has eliminated at least some versions of axions as a dark matter candidate. 

Axions are a theoretical particle devised in the 1970s to address certain issues in quantum chromodynamics (a theory that describes the behavior of the strong force). According to the model, axions would be low mass, chargeless particles that don’t interact strongly with light, which sounds like a perfect dark matter candidate. While there have been attempts to observe the effect of axions, they haven’t been successful. New work in Physical Review Letters not only didn’t find evidence of axions, it places strong constraints on their existence.

Although axions don’t interact strongly with light, they can interact in subtle ways. By coupling with magnetic fields axions could decay into light, and they could also absorb gamma rays. So a team of astronomers combed through six years of gamma ray data from a galaxy known as Perseus A. The central region of this galaxy not only emits strong gamma rays, the region is known to have magnetic fields. If dark-matter axions exist, they should interact with the gamma rays, changing the spectrum from Perseus A. But the team found no evidence of any change in the spectrum. The results aren’t strong enough to rule out the existence of axions, but it is strong enough to rule them out as a source of dark matter.

So dark matter is still a mystery. We now have a better idea of what dark matter isn’t, but that still doesn’t tell us what it is.

Paper: M. Ajello et al. Search for Spectral Irregularities due to Photon–Axionlike-Particle Oscillations with the Fermi Large Area Telescope. Phys. Rev. Lett. 116, 161101 (2016)  arXiv:1603.06978 [astro-ph.HE]

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