Ms. Alworth, let me offer you a few ideas on Primordial Black Holes (PBMs), which, hopefully, may offer you a few insights:
"Primordial Black Holes (PBHs) are, typically light, black holes which could have been formed in the early Universe. There are a number of formation mechanisms, including the collapse of large density perturbations, and possibly from cosmic string loops and bubble collisions (although I did not go into these more esoteric mathematical constructs here. Hartmann352). The number of PBHs formed is tightly constrained by the consequences of their evaporation and their lensing and dynamical effects. Therefore PBHs are a powerful probe of the physics of the early Universe, in particular models of inﬂation. They are also a potential cold dark matter candidate.
For instance PBHs formed at the QCD phase transition at t∼10−6 seconds would have a mass of the order a solar mass, MPBH ∼M⊙=2×10*30 kg."
-- Anne M. Green, School of Physics and Astronomy, University of Nottingham, University Park, Nottingham, UK.
During the period of radiation domination, see the schematic of the radiation era below, if a matter density ﬂuctuation is sufﬁciently large, as Anne Green states above, the mass must be greater than ∼M⊙=2×10*30 kg, then gravity will overcome the pressure forces of radiation and the ﬂuctuation will collapse to form a Primordial Black Hole (PBH) shortly after it enters the matter decoupling horizon.*
It has also been suggested that one of the most promising way to search for PBHs is to look for lensing effects, see images of gravitational lensing below using galactic structures as the lens in the examples, caused by these compact objects. Since the Schwarzschild radius of a PBH is comparable to the photon wavelength, the wave nature of electromagnetic radiation has to be taken into account. In such a case, lensing caused by PBHs introduces an interferometry pattern in the energy spectrum of the lensed object. The effect is called ’femtolensing’ due to very small angular distance between the lensed images.
The phenomenon has been a matter of extensive studies in the past, but the research was almost entirely theoretical since no case of femtolensing has been detected as yet. It was first suggested that the femtolensing of gamma-ray bursts (GRBs) at cosmological distances could be used to search for dark matter objects in the mass range 10*17 − 10*20 g not long ago.
Independent of the formation mechanism, Primordial Black Holes (PBH) are thereafter dynamically coupled to cosmic expansion. PBHs become gravitationally bound to each other, most likely, when their local density becomes equal to the density of the surrounding radiation, which generally happens after the matter-radiation reaches equality. However, due to large Poisson** fluctuations at small scales, some PBHs can decouple much earlier. When this happens, the closest PBH pairs start falling towards each other and their head-on collision may be prevented only by the torque caused by the gravitational field of the surrounding PBHs and other matter inhomogeneities. As a result a population of PBH binaries is or could be formed.
Primordial Black Holes could also form from the "baby universes" created during inflation, note the inflation era on the timeline below, that period of rapid expansion which is believed to be responsible for seeding the structures we observe today, such as galaxies and clusters of galaxies. During inflation, baby universes can branch off of our universe. A small baby (or "daughter") universe would eventually collapse, but the large amount of energy released in the small volume causes a black hole to form.
An even more peculiar fate awaits a bigger baby universe. If it is bigger than some critical size, Einstein's theory of gravity allows the baby universe to exist in a state that appears different to an observer on the inside and the outside. An internal observer sees it as an expanding universe, while an outside observer, such as we are, sees it as a black hole. In either case, the big and the small baby universes are seen by us as primordial black holes, which conceal the underlying structure of multiple universes behind their "event horizons." The event horizon is a boundary below which everything, even light, is trapped and cannot escape the black hole.
One of the consequences of the existence of PBHs with greater impact on different observables is the process of accretion. Infalling matter onto PBHs would release radiation, injecting energy into the surrounding medium, and strongly impacting its thermal state, leaving significant observable signatures. The physics of accretion is highly complex, but a simplified approach can be attempted when considering the spherical non-relativistic limit, following the groundbreaking work by Hermann Bondi
on accretion (1952). On Spherically Symmetrical Accretion. MNRAS
112, 195. doi:10.1093/mnras/112.2.195). In this framework, the Black Hole (BH) is treated as a point mass surrounded by matter, embedded in a medium which tends to be a constant density at a specific distance directly proportional to the gravity exerted by the BH.
The lower the Primordial Black Hole (PBH) mass, the earlier it evaporates. Those with masses of 10*15 g or below would have already evaporated by now, having lifetimes shorter than the age of the Universe (Don N. Page (1976). Particle Emission Rates from a Black Hole. II. Massless Particles from a Rotating Hole. Phys. Rev. D
14, 3260–3273. doi:10.1103/PhysRevD.14.3260), so they cannot contribute to the current Dark Matter (DM) abundance. These evaporation products or the effects they produce in different observables can be search for in a variety of experiments, probing different mass ranges.
The presence of Primordial Black Holes (PBHs) would dynamically heat star clusters, making them larger and with higher velocity dispersions, leading to an eventual dissolution into its host galaxy. Populations with high mass to luminosity ratios are more sensitive to this effect, as happens with ultra faint dwarf galaxies (UFDW), which would be disrupted by the presence of PBHs. As a result, these effects are evidence of PBHs.
Taking into account that PBHs could be immersed in Dark Matter (DM) halos with higher densities than the background, below find Primordial Black Holes (PBMs) as a fraction of Dark Matter (DM), their accretion rates would be increased, also leading to more stringent constraints. Cosmic Microwave Background (CMB) limits from accretion are currently the most stringent ones for masses ≳10M⊙. The main caveat is their dependence on some details of the accretion mechanisms, such as the effective velocity and the accretion rate, which may not be very well understood yet.
On the other hand, the energy injection from PBHs evaporation would produce anisotropies and spectral distortions in the CMB spectrum, which would also limit the maximum abundance, leading to similar constraints to those obtained from the extra-galactic Gamma-ray (γ
* See: Carr, B. J., Hawking S. W.: Black holes in the early Universe. Mon. Not. Roy. Astron. Soc. 168 399-415 (1974)
** Poisson fluctuations are a sequence of independently random events in which the occurrence of any event has no effect on the occurrence of any other.
Growing scientific evidence points to Primordial Black Holes (PBMs) forming from the collapse of density perturbations shortly after the Big Bang during the time of the radiation era if the coalescing mass is great enough to overcome the forces of radiation. After all is said and done, PBHs could still form a substantial part of the Dark Matter (DM), if not all. On the other hand, future experiments with better sensitivities may be able to reach yet unexplored regions farther back in the earlier times of the universe and tighten up current limits. Today, the 8.2-meter Subaru Telescope in Hawaii with its Hyper-Suprime Cam, can observe one hundred million stars in the Andromeda Galaxy simultaneously. And it can do it fast, taking new images every few minutes. Theorists at The Kavli Institute in Japan believe that if there are primordial black holes adrift out there, they should periodically pass in front of these stars and occult, attenuate or gravitationally lens their starlight. What an amazing time we live in when we can observe Andromeda, M-31, with such clarity while searching for such fleeting objects.