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Characterizing The Preferred Mass Range For Primordial Black Holes & Dark Matter

The 2017 Nobel prize in physics has been awarded to Kip Thorne, Rainer Weiss, and Barry Barish, “for decisive contributions to the LIGO detector and the observation of gravitational waves”.  The LIGO discovery has ushered in a new era of gravitational wave astronomy.

The events generating the gravitation waves are consistent with the inspiral of two black holes, that began to coalesce, and upon merging resulted in the LIGO detected gravitational wave signals. Thus far all LIGO observations (several events) have suggested that the masses of these colliding black holes are around 30 times the mass of the Sun.  This can be contrasted with the mass of Sgr A* (the black hole in the center of our galaxy), which is nearly 4 million times the mass of the Sun.  Black holes are believed to live at the center of most spiral galaxies, like our Milky Way, but the origin of such lighter black holes – like those detected by LIGO – could present a challenge for astrophysicists and cosmologists to explain.

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The LIGO detector site in Livingston, Louisiana (Credit: Caltec/MIT/LIGO Lab via Wikimedia Commons). Image licensed under CC0

Another outstanding puzzle is the primordial origin and nature of dark matter. In particular, whether it is made up of fundamental particles, or perhaps it is of a completely different and unexpected nature.  The observational evidence for dark matter is indirectly inferred. It is not only required in order to account for the orbits of distant stars within our galaxy but also as a dominant ingredient of the primordial fluid in the early universe.  That is, without a majority of dark matter in the early universe, galaxies and galaxy clusters would not evolve to form the way we see them through our telescopes today.  So, what is the cosmological dark matter?

The recent LIGO observations are not only interesting because they give us observational evidence for the existence of gravitational waves, but also because they suggest another intriguing possibility – that black holes could be all or part of the cosmological dark matter.  Black holes were once dismissed as being the only component of dark matter, because of a number of observational constraints when a wide range of black hole masses are considered.

For example, strong constraints on some black hole masses result from the way they would distort the blackbody spectrum of the cosmic microwave background (the remnant light of the big bang).  However, the special mass of 30 solar masses (as detected by LIGO) lies within a special range where observation constraints are lacking.  Could these black holes be the dark matter, and if so, why is this mass favored over all others?

Finally, another puzzle of the early universe is – How did the universe evolve prior to the creation of the lightest elements (so-called Big Bang Nucleosynthesis)?  When did the universe thermalize into the hot soup needed to form these light elements?  It is believed that during the earliest fractions of a second the universe underwent a period of incredibly rapid acceleration – a period known as cosmic inflation. Although there is strong observational evidence for inflation, what followed inflation and how it led to the universe today is not established.

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Fundamental theory (notably string theory) suggests that during this transition from the end of inflation to the hot, thermalized universe, the universe may have been predominantly filled with a fluid similar to the recently discovered Higgs boson.  If this picture is accurate, the oscillations of these fields would have led to conditions in the early universe where it was much more favorable for black holes to form.  However, later these fields decay producing large amounts of radiation in the form of Standard Model particles, and then the lightest atoms proceed to form.  The production of this radiation dilutes the early formed, so-called ‘primordial’ black holes.  However, the key finding of this recent research is that the last black holes to form will suffer the least amount of dilution.

What is interesting is that when one asks what the masses of these surviving black holes are, one finds that their mass is around the 30 solar masses – exactly the mass required for the dark matter interpretation of the LIGO result.  This suggests the idea that these black holes could be all or part of the dark matter, and that the LIGO detection might not only be the first detection of gravitational waves, but also the first detection of dark matter.  Whether these black holes could be the main or only component of dark matter, remains an open question.  But these initial results suggest the intriguing possibility that with this particular mass of black holes, dark matter is closer to being understood.

This study, A preferred mass range for primordial black hole formation and black holes as dark matter revisited, was recently published in the Journal of High Energy Physics.

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