A spiral galaxy with an associated dwarf galaxy (the faint blue spot at the upper left).
Dark matter has had incredible explanatory power. After the concept was introduced in the early 1980s, dark matter quickly became a central feature of our cosmological view of the Universe. The current leading dark matter model is called Lambda Cold Dark Matter, or ΛCDM, and its predictions have consistently come true. A large number of observations confirm that our galaxy, like any other, resides in a halo (basically a spherical blob) of invisible, faintly or non-interacting matter, with a mass far greater than any of the stars in it.
Of course there was some skepticism. Some have argued that there may be no dark matter and instead our understanding of gravity does not apply to galactic scales, which could eliminate the discrepancies we explain with dark matter. But as time goes on, the evidence continues to support dark matter. Those alternative models are still possible, but no convincing evidence has come to light.
But that does not mean that the ΛCDM model is perfect. Two problems have arisen in the form of predictions not matching certain observations. It may be that these problems can be solved by modifying the model or by taking into account other processes that may occur with normal, “baryonic” matter. A group of researchers has written a piece for the journal PNAS a summary of the various possible solutions to the problems.
Cusps, cores and satellites (oh, my)
The first problem became known as the ‘cusp-core problem’. It is predicted that dark matter would accumulate close to the core of its galaxy and that the large amount of dark matter towards the core should detectably increase the velocities of visible objects. such as stars near the centers of galaxies. However, this is not what is observed. ΛCDM predicts much more dark matter in galactic nuclei than we’ve seen so far. The problem is observable in typical galaxies like our own Milky Way, but it becomes much more blatant in smaller galaxies.
At first, scientists wondered if this discrepancy could indicate that the problem was essentially an optical illusion — an artifact of objects in the smaller galaxies that don’t track circular motion, leading us to misinterpret their velocities. But as better and better measurements came in, it became clear that such a simple solution was not in sight; the problem really exists.
And then there’s the “missing satellite problem”. Just as the Earth has a satellite in the Moon, the Milky Way has a number of satellite galaxies, smaller celestial bodies that orbit the Milky Way. These satellite galaxies rest within their own dark matter halos, themselves within the confines of the Milky Way’s main halo. For this reason, they are considered “sub-halos” of the Milky Way’s halo.
Before the year 2000, only nine satellite galaxies were known, including the Large and Small Magellanic Clouds. But in addition to these satellite galaxies, the Milky Way also has a number of “empty” subhalos of dark matter. These blobs of dark matter could have played host to satellite galaxies, but they’re not. Therein lies the problem: According to predictions, there should be up to 20 more satellite galaxies than we observed, with only the mysteriously empty subhalos making the numbers seem reasonable.
Diagram of the Milky Way and some of its satellite galaxies.
There were some proposed solutions for this. It is possible that early in the subhalo history, when galaxies began to form, background UV light pushed the gas outward, preventing galaxies from forming. Another possibility is that stellar winds, charged particles from the first generation of massive stars or from supernovae, expelled the accreting gas.
After the Sloan Digital Sky Survey (SDSS), things got more complicated. About 15 new satellite galaxies were detected, so faint they were previously missed. That survey covered only about 20 percent of the sky; if similar dwarfs were further away in the Milky Way’s halo, they would be too faint to be detected, and there could be several hundred dwarf galaxies that orbit the Milky Way.
This makes for a confusing picture in terms of the formation of dwarf galaxies. But in light of the newer findings, the missing satellite problem is no longer a problem for ΛCDM. “Despite the gaps in understanding,” the authors write in their paper, “it seems reasonable for now to view the relationship between low-mass subhalos and ultrafaint dwarfs as a puzzle of the physics of galaxy formation rather than a contradiction to CDM.”
But just when we thought it was safe to go back into the dark matter subhalo, a new form of the missing satellite problem emerged. Because when comparing the empty or dark subhalos to the bright ones – the ones that harbor dwarf galaxies – a difference became apparent. The mass of dark matter in the centers of dark subhalos is about five times greater than the mass in the centers of bright ones.
Some researchers suggested that this could be a coincidence – the satellite galaxies of the Milky Way just happen to be in the least massive subhalos of dark matter. But this was a somewhat unsatisfactory solution, and the discrepancy was called “too big to fail” by one researcher. Even with the lowest estimate of the mass of the Milky Way halo, which would maximize the chance of the dwarf galaxies forming by chance in the least massive subhalos, the discrepancy is still too large to be a statistical fluke. Not only that, but the Andromeda galaxy has the same problem, making it even less likely to be a coincidence.
In its new form, the missing satellite problem is starting to look a lot like the core cusp problem. Both problems involve galactic nuclei that have much less mass than predicted. Therefore, the authors argue that the two problems have effectively merged into one.
The question then is how to solve the problem. Is there some unknown or unknown process by which baryonic matter causes the discrepancies? And if not, what does this mean for our models of dark matter: should the properties of the mysterious substance be changed?
Solutions in baryonic physics?
After the cusp-core problem first became apparent, it was thought that baryonic physics could not provide a solution. After all, one of the properties of dark matter is that it only interacts with baryonic matter through gravity (and perhaps through the weak force, but such interactions would be rare and make little difference in this context).
But if you were to place a lot of baryonic matter, such as a galaxy, in a halo of dark matter, the added gravity should pull more dark matter toward the center, making the problem worse, not better. If, instead of dark matter, the galaxy were floating in a cloud of gas particles with a similar mass to the dark matter halo, the galaxy could help keep the center relatively free of gas by heating it. When gas heats up, it expands, meaning it doesn’t clump in the center. But with dark matter, there’s no way to transfer heat from the baryonic matter, so that can’t happen.
But this common wisdom hasn’t stopped scientists from looking for ways around the problem: complex scenarios involving baryonic matter that could affect dark matter. One such solution, considered extreme at the time, involved supernova explosions. When a massive star explodes, it blows off its outer layers, which in turn can push away nearby matter. This wave of outgoing baryonic matter may have an effect on dark matter.
These supernovae are part of a feedback mechanism: the faster star formation occurs, the more gas is ejected from the galaxy by supernovae. This, in turn, prevents more stars from forming and expels baryonic matter over time.
This feedback affects the dark matter. During the stage of the system’s evolution when baryonic matter is still abundant, the added gravity pulls dark matter deeper into the center of the galaxy, just as common wisdom predicts. But then, when the feedback mechanism clears most of the baryonic matter, the dark matter is suddenly released. This speeds up the dark matter particles quite a bit, broadening their individual orbits and reducing clumping in the galaxy’s core.