Black Holes Alone Can’t Explain Dark Matter

Artist’s illustration of the distribution of dark matter surrounding the Milky Way. [ESO/L. Calçada]

Among the many unanswered astronomical questions, the problem of dark matter is one of the most difficult we are facing. If our physics is correct, 27 percent of the mass and energy in the universe is made of dark matter, and although there are many hypotheses, we still don’t what it is.

But according to Timothy Brandt, a postdoctoral fellow at the Institute for Advanced Study in Princeton, we know what dark matter isn’t. His research on dwarf galaxies, published in the Astrophysical Journal Letters, shows that dark matter cannot be made of black holes. Or at least of black holes alone.

The main characteristic of dark matter is that it interacts only through gravity, so explaining it with black holes is not too crazy. The theory becomes a lot more complex when all the characteristics are taken into account: Dark matter must have formed during the first instants of the Big Bang, it doesn’t interact with itself, etc.

One possibility for dark matter is massive compact halo objects (MACHOs), black holes that formed directly from the energy fluctuations of the Big Bang. MACHOs are actually attractive when it comes to testing hypotheses, as we can look for the effect of the individual objects on their surroundings.

By looking at how light is bent by gravity, the presence of MACHOs should be noted by astronomers. However, gravitational lensing surveys couldn’t find any evidence of them, indicating that if they exist they would have at least 30 times the mass of the Sun. At the same time, studies on stellar binaries in the outskirts of the Milky Way (the area known as the halo) put the maximum mass of the MACHOs at 100 solar masses.

So how about intermediate masses? The study of MACHOs got reinvigorated by the discovery of gravitational waves. The black holes responsible for the signal were in the allowed mass range, an interesting suggestion that dark matter was indeed MACHOs.

To test the remaining range, Brandt looked at how stars move around the center of Eridanus II, a small galaxy that orbits the Milky Way. The satellite is very dense and has a lot of dark matter, and Brandt has calculated that to explain the observation, the MACHOs have to be less than 10 solar masses.

Brandt's study, then, shows that there’s no range of MACHOs that can explain dark matter. This doesn’t mean that there are no primordial black holes out there, just that dark matter must be made of something else.

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