Luca Amendola

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Prehistoric Black Holes

Black hole masses (Courtesy Caltech/MIT/LIGO Laboratory)

Black holes are normally thought to be the end state of heavy stars. When no  force can resist the pull of gravity, matter collapses in a singularity and a horizon develops, from which no signal can escape, including light (save for a slow quantum mechanical evaporation). Supermassive black holes at the center of most galaxies can also be produced by accretion of stars and gas onto an initial seed.

Stellar sized black holes are responsible for the gravitational waves first discovered on September 14, 2015 by the LIGO team, leading to one of the fastest Nobel Prize recognition ever.

The LIGO black holes however seem to be unusually massive, in excess of 10 and up to 60 solar masses. A theory advanced many years ago, that black holes are after all not necessarily the final state of astrophysical structures but actually the initial one, could explain these massive black holes. And provide on the way a clue to another cosmic mystery, dark matter.

According to this idea, first proposed by George Chapline, and by Bernard Carr and Stephen Hawking in the 70s, black holes could in fact be produced in the very early Universe, when matter and light were tightly coupled into a hot plasma. This plasma is opaque and light cannot travel through, so we can have only indirect information, a bit like doing archaeology of prehistoric eras when no writing had yet been developed. These so-called primordial black holes might have any mass, depending  on the initial amount and size of matter fluctuations. Since they contain mostly radiation, they escape the strong observational constraints on baryonic dark matter (i.e. made out of ordinary massive particles like protons and neutrons), and could conceivably form all or most of today’s elusive dark matter.

Standard primordial black holes, however, have various problems. One is that one needs a contrived distribution of initial fluctuations for them to be produced in abundance. The other is that they are very compact objects and should distort via gravitational lensing the propagation of starlight  in our own Galaxy, an effect that is not observed in a sufficient amount.

So in a recent paper we proposed a model that can solve both problems. We proposed that primordial black holes arise not because of special initial conditions, but because an extra force, much stronger than gravity, attracted non-baryonic particles (but not radiation) in the very early Universe. The abundance and mass of these primordial black holes depend entirely on a single parameter, the strength of the new force, and one can obtain masses in the LIGO range and with the right abundance to produce all the dark matter required by observations.

In fact, we do not know if all objects formed in this way are actually black holes. Some or all of them could actually be just lumps of particles held together by gravity, like  dark matter mini-halos. In this case, the constraints from gravitational lensing would be much less severe, thereby solving the second problem.

What kind of non-baryonic particles could have ended up into the black holes? Particle physics models that go beyond the standard schemes predict many neutral massive particles that could serve to this scope. If the objects are genuine black holes, these particles do not even need to be stable, since they could have been locked up into the black hole horizon before having time to decay.

Black holes could then be a time capsule of prehistoric cosmology!