featureless), but that there is a sudden change in the plasma pressure at a particular cosmological epoch, allowing PBHs to form more easily. The required amplitude of the inhomogeneities must be much larger than that observed on cosmological scales but not too large, so this requires fine tuning of both the scale and amplitude.Īn alternative approach is to assume the power spectrum is smooth (i.e. One approach is to choose an inflationary scenario which produces a peak in the power spectrum of curvature fluctuations at the required scale. Given the revival of interest in PBHs, one must explain why they have the mass and density required for explaining the LIGO/Virgo events, and why these values are comparable to the mass and density of stars. An extended mass function with a peak in the range 1–10 M ⊙ could explain LIGO/Virgo observations and would only be constrained by lensing probes (microlensing and supernovae lensing) which are strongly debated and subject to large uncertainties. Moreover, the LIGO/Virgo observations seem to favour mergers with low effective spins, as expected for PBHs, but are hard to explain with models of stellar evolution. The observed merger rate is compatible with what would be expected if PBHs constitute an appreciable fraction, and possibly all of the cold dark matter (CDM). Primordial black holes (PBHs) in the solar-mass range have attracted a lot of attention since the LIGO/Virgo detection of gravitational waves from coalescing black holes. This article is part of a discussion meeting issue ‘Topological avatars of new physics’. Next generation gravitational wave and microlensing experiments will be able to test this scenario thoroughly. We predict the future detection of binary black hole (BBH) mergers in LIGO with masses between 1 and 5 M ⊙, as well as above 80 M ⊙, with very large mass ratios. Moreover, the sudden drop in radiation pressure of relativistic matter at H 0/ W ±/ Z 0 decoupling, the QCD transition and e + e − annihilation enhances the probability of PBH formation, inducing a multi-modal broad mass distribution with characteristic peaks at 10 −6, 1, 30 and 10 6 M ⊙, rapidly falling at smaller and larger masses, which may explain the LIGO–Virgo black hole mergers as well as the OGLE-GAIA microlensing events, while constituting all of the cold dark matter today. Therefore, in this scenario there is a common origin of both the dark matter to baryon ratio and the photon to baryon ratio. Baryons are efficiently produced in relativistic collisions around the black holes and soon redistribute to the rest of the universe, generating the observed matter–antimatter asymmetry well before primordial nucleosynthesis. We review here a new scenario of hot spot electroweak baryogenesis where the local energy released in the gravitational collapse to form primordial black holes (PBHs) at the quark-hadron (QCD) epoch drives over-the-barrier sphaleron transitions in a far from equilibrium environment with just the standard model CP violation.
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