Dark matter is probably not made up of black holes

One of the major outstanding mysteries of modern physics is the nature of dark matter.

According to observations, it should exist. Not only that, to explain gravitational anomalies across the cosmos, dark matter should be about 5 times more common in the universe than ordinary visible matter.

In our own galaxy, there is 15 times more dark matter than ordinary matter.

The issue, however, is that dark matter should also be by its very nature virtually impossible to detect, given it only very weakly interacts with normal matter except through gravity.

“The nature of dark matter remains a mystery. Most scientists think it is composed of unknown elementary particles,” says Przemek Mróz from the Astronomical Observatory at Poland’s University of Warsaw. “Unfortunately, despite decades of efforts, no experiment has found new particles that could be responsible for dark matter.”

Black holes are pretty dark

Mróz is lead author on 2 new papers published in Nature and the Astrophysical Journal Supplement which tests whether dark matter can be explained through another enigmatic class of objects in the universe: black holes.

Nearly 100 black hole mergers have been detected since the first in 2015. These black holes are usually 20–100 times heavier than our Sun. In contrast, those previously discovered in our Milky Way are typically only 5–10 solar masses.

“Explaining why these two populations of black holes are so different is one of the biggest mysteries of modern astronomy,” Mróz says.

It could be that the larger black holes detected by the LIGO and Virgo experiments are primordial black holes formed in the very early universe.

Since gravitational waves detectors have been used to find more black holes, physicists have been speculating that such primordial black holes could make up a significant fraction, if not all, of dark matter.

While black holes do not emit light, this theory can be tested.

Einstein comes to the rescue again

A consequence of Einstein’s general theory of relativity is that massive objects can bend light around them. This effect is called gravitational microlensing. When the massive object, like a black hole, comes between Earth and other objects like galaxies, then those galaxies become magnified and their brightness increases.

The higher the mass of the object causing the lensing, the longer the brightness of the bodies behind it is increased.

Lensing by objects about the size of the Sun lasts a few weeks. But gravitational lensing from black holes of more than 100 solar masses would last a few years.

The idea that gravitational lensing could help study dark matter was first put forward in the 1980s by Polish astrophysicist Bohdan Paczyński. Experiments showed that black holes smaller than the Sun could make up less than 10% of dark matter. But these first experiments weren’t sensitive to longer timescale microlensing.

Pricking a hole in black hole theory

Astronomers from the Optical Gravitational Lensing Experiment (OGLE) present new results from a 20-year-long monitoring of 80 million stars in the nearby Large Magellanic Cloud.

If the dark matter in the Milky Way was composed only of black holes, the researchers would have expected to see 258 microlensing events. Instead, their results offered up only 13.

Diagram showing galaxies with yellow dots
Expected vs. observed microlensing events by massive objects toward the Large Magellanic Cloud as seen through the Milky Way halo. Credit: J. Skowron / OGLE. Background image of the Large Magellanic Cloud: generated with bsrender written by Kevin Loch, using the ESA/Gaia database.

“That indicates that massive black holes can compose at most a few percent of dark matter,” says Mróz.

To be precise, black holes of 10 solar masses may comprise at most 1.2% of dark matter. 100-solar-mass black holes account for 3.0% of dark matter, and 1000 solar mass black holes – 11% of dark matter.

“Our observations indicate that primordial black holes cannot comprise a significant fraction of the dark matter and, simultaneously, explain the observed black hole merger rates measured by LIGO and Virgo,” says Andrzej Udalski, principal investigator of the OGLE survey.

The mystery around what makes up the majority of dark matter remains.

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