Black Holes as Primordial Dark Matter

source: Physicists Argue That Black Holes From the Big Bang Could Be the Dark Matter

Primordial Black Holes #

A “Primordial Black Hole” (PBH), is a black hole which has been around since nearly the time of the Big Bang.

PBH as Dark Matter #

The hypothesis that PBHs could be the dark matter cosomolgists tangentially observe was put forth in many papers (like this one from Johns Hopkins U.) around 2016. This idea was dismantled by the calculation which showed that if black holes had been around as long as PBHs are/were thought to have been around for, and in high enough numbers, we would see more pairs forming. These pairs would, through gravity, begin to orbit each other and be detectable by LIGO, observationally. We don’t observe this, so the idea (then) was abandoned.

A new paper from Montpellier U. has shown that a large population of PBHs would result in orbital collisions that perfectly match what LIGO has observed.

If true this would add weight to the hypothesis that dark matter is just primordial black holes.

Black Holes in the early Universe #

How could a black hole form during/at the Big Bang?

Hawking/Carr proposed in the ’70s that within the first fractions of the Universe’s life density fluctuations could have resulted in some regions being more dense than others. At a high enough regional density a black hole could be formed. They reasoned that anything larger than a common asteroid would, probably, still be in the Universe today.

Then in the ’90s, with the Cosmic Inflation theory, the initial density fluctuations proposed in the ’70s could be explained. Alongside the density fluctuations, state transitions in the early plasma are thought to have aided the gravitational collapse.

This theory was (again) largely put by the wayside in the ’90s when Astronomers/Cosmologists began the search for dark matter particles (WIMPs). These were assumed to be found when the LHC was turned on, but none were and no physical evidence for WIMPs has been procured to date.

New Approaches to early Plasma #

Over the last few decades more knowledge has been produced/theorized on how the early universe could have produced the PBHs. We can now calculate the pressure and density that would have arisen in a quark/gluon plasma. Further, the size variation on the PBHs has grown with the knowledge at hand.

With quarks and gluons forming neutrons and protons a necessary pressure drop occurs, spawning a black hole as described above.

Numeric argument for PBH existence #

Simulations have shown that the argument of “seeing more, observationally, than we actually do with LIGO” actually falls apart beyond a binary system. That is, if a third black hole were to come into orbit with another two the rotational dynamics of the system would be such that the occurence of merges would be less frequent than initially postulated. The simulations gave the observational range that would fit in with what LIGO is seeing (see the Montpellier paper linked above). It also predicts how/where these PBHs might exist: in dark clusters, with cluster diameters up to 1AU, each harboring up to a thousand black holes each with the larger ones in the cluster’s center and the smaller ones on the outside edges.

Detractors #

Aside from the majority opinion that dark matter is composed of a hard-to-observe fundamental particle, others observe that LIGOs observational data is also explained by standard black holes formed during the lifetime of a star.

In the ’90s, astronomers began “microlensing searches”. These experiments focus on a bright patch in the sky and wait for a dark object to pass in front of it. If the PBH theory were true, we would have seen something in support of that, but nothing has been observed.