Primordial black holes may make up only a small part of dark matter

This artist's illustration shows what primordial black holes might look like. In reality, black holes would compete to form accretion disks, as shown in the figure.

What is dark matter? This question figures prominently in discussions about the nature of the universe. There are many proposed explanations for dark matter, both within the Standard Model and beyond.

One of the proposed components of dark matter is primordial black holes, which arose in the early Universe without a collapsing star as a progenitor.

The problem of dark matter is the problem of missing mass. Galaxies should not be able to hold together and not fly apart, judging only by their apparent mass. Their observed mass is stars, gas, dust and a scattering of planets.

For galaxies to survive, there must be some other form of mass in space. Dark matter is a conventional name for the mass that is missing. Astronomer Fritz Zwicky first used the term in 1933 when he observed the Coma cluster and discovered evidence of missing mass. About 90% of the Coma cluster is missing mass, which Zwicky called “dark matter.”

In this mosaic of a space telescope "Hubble" depicts part of the enormous Coma Galaxy Cluster, containing more than 1,000 galaxies and located 300 million light years away. The rapid movement of its galaxies was the first clue to the existence of dark matter. — This mosaic from the Hubble Space Telescope shows part of the vast Coma galaxy cluster, containing more than 1,000 galaxies and located 300 million light-years away. The rapid movement of its galaxies was the first clue to the existence of dark matter.

Primary black holes (PBHs) are one of the leading candidates for dark matter. At the dawn of the universe, pockets of dense subatomic matter may have formed naturally. Once they became dense enough, they could collapse, turning directly into black holes. Unlike their astrophysical counterparts, they had no stellar predecessors.

Recent Webb observations and LIGO/Virgo results support the idea that primordial black holes are dark matter. Some researchers go further and say that these data support the idea that dark matter consists solely of primordial black holes and has no other components.

A new study suggests that some of the early primordial black holes merged, and that LIGO/Virgo may detect gravitational waves from the mergers. The study is called “Constraints on primordial black holes based on LIGO-Virgo-KAGRA O3 events“The lead author is M. Andres-Carcasona, a graduate student at the Institute of High Energy Physics at the Barcelona Institute of Science and Technology.

An image based on a supercomputer simulation of a cosmological environment in which primordial gas undergoes direct collapse into a black hole.

In 2015, LIGO (Laser Interferometer Gravitational-Wave Observatory) discovered the first black hole merger. At the time, researchers welcomed this new window into the universe. Previously, astronomical observations were based on electromagnetic radiation, but LIGO/Virgo changed this situation.

Now Japan has joined the LIGO/Virgo collaboration with its Karga gravitational-wave observatory, and the international effort is called LIGO/Virgo/Karga (LVK). Together, these three observatories collect data on gravitational waves.

“Previous work has explored the use of gravitational wave data to search for direct or indirect evidence for the existence of primordial black holes,” the authors write. “Specific targeted searches for compact subsolar mass objects that could serve as a signal for the existence of primordial black holes have so far been unsuccessful.”

The authors note that our growing body of gravitational wave data may contain signatures of primordial black holes that have been missed by other researchers. They write that some component masses “… fall into regions where astrophysical models do not predict them, potentially indicating the presence of a population of primordial black holes,” they write.

This ESA graphic shows how Webb and LISA, the spaceborne laser interferometer antenna, can detect primordial black holes and unravel the mystery of dark matter. Unfortunately, LISA is still at least ten years away from launch.

The primordial black hole mass function plays a large role in the formation of primordial black holes. Their goal is to update constraints on the mass of primordial black holes in gravitational wave data. “One of our goals is to derive constraints that do not depend significantly on the underlying formation scenario. Thus, we consider many different functions of the primordial black hole mass,” they explain.

The two main formation scenarios they mention are astrophysical and primordial. Within the framework of the primary, there are various ways of forming primary black holes, and all of them are associated with the mass function. The authors explain that primary black holes can explain the entire population of dark matter, but only if they are in the range of 10^-16 to 10^-12 solar masses.

“Lighter primordial black holes are already evaporating today and may constitute only a small part of dark matter,” they write.

Astrophysical black holes form binaries and can merge, sending gravitational waves outward. If primordial black holes merge, they will also send out gravitational waves. It is possible that some of these mergers are behind some of the gravitational wave data detected by LIGO/Virgo/Karga during the third observation cycle. The researchers present their results in the form of pessimistic and optimistic options. According to the pessimistic version, all observations of gravitational waves are due to mergers of astrophysical black holes, while the optimistic version suggests that some of them are the result of primordial black hole mergers.

Their research and its results involve an awful lot of complex physics terms and relationships. But the main question is whether primordial black holes could be partially or entirely dark matter. In this context, what do the obtained results amount to?

This artist's illustration shows small black holes in the accretion disk of a supermassive black hole. In early 2024, a team of researchers discovered evidence of a small black hole inside the accretion disk of a supermassive black hole. A small black hole, if it exists, has a mass between 100 and 10,000 solar masses. At the lower end of this range, it has the same mass as a supermassive black hole. It's not supposed to be primordial, but it shows how much we still have to learn about black holes. — This artist's illustration shows small black holes in the accretion disk of a supermassive black hole. In early 2024, a team of researchers found evidence of a small black hole inside the accretion disk of a supermassive black hole. A small black hole, if it exists, has a mass between 100 and 10,000 solar masses. At the lower end of this range, it has the same mass as a supermassive black hole. It's not supposed to be primordial, but it shows how much we still have to learn about black holes.

The researchers say that, according to their analysis of populations of astrophysical and primordial binary black holes, primordial black holes cannot be entirely composed of dark matter. In extreme cases, they may form a small part of it.

“…in a population of binaries consisting of primordial and astrophysical black holes, we found that under any scenario, primordial black holes can account for no more than 10^-3 of the total dark matter mass in the mass range from 1 to 200 solar masses.”

You don't need to be a physicist to understand what they are saying. Based on their analysis, PBHs may only make up a tiny fraction of dark matter.

This study may not be sensational. This is a look under the hood of astrophysics and cosmology, where teams of researchers work hard to gradually constrain and define various phenomena. But this does not detract from its significance.

One day the headline may appear: “Physicists have identified dark matter! Answers to the big questions of the Universe!”

If this ever happens, there will be hundreds and thousands of studies like this behind it.