Primordial Black Holes in Inflationary Cosmology

Primordial black holes (PBHs) are hypothetical relics formed from the gravitational collapse of enhanced density fluctuations in the early Universe. In the context of inflationary cosmology, PBHs can arise when the inflaton field—the scalar degree of freedom driving accelerated expansion—experiences departures from the usual slow-roll regime. Brief phases of ultra-slow roll or rapid turns in field space amplify curvature perturbations on sub-horizon scales, seeding regions that collapse upon horizon re-entry. Alternative mechanisms invoke non-canonical kinetic terms or couplings between the inflaton and higher-curvature invariants, such as the Gauss–Bonnet term, further boosting small-scale power. The resulting PBH mass spectrum spans many orders of magnitude, from sublunar scales up to tens of solar masses, offering potential dark matter candidates or explaining gravitational wave events. Observational probes—ranging from microlensing surveys and cosmic microwave background anisotropies to stochastic gravitational-wave backgrounds—place stringent limits on their abundance. Modern models balance the need for sufficient amplification of the power spectrum against constraints on large-scale observables and non-Gaussian signatures. This interplay makes PBHs a valuable window onto high-energy physics, non-linear dynamics of inflation and the nature of dark matter.

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Primordial Black Holes in Inflationary Cosmology publication trend

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Technical terms

Inflation: A period of accelerated expansion in the early Universe driven by a scalar field.

Slow roll: A regime in which the inflaton’s potential energy dominates over its kinetic energy, ensuring gradual evolution.

Ultra-slow roll: A transient phase where the inflaton decelerates sharply, amplifying small-scale curvature perturbations.

Curvature perturbation: A measure of spatial fluctuations in the gravitational potential that seed structure formation.

Power spectrum: The variance of perturbations as a function of scale, indicating their amplitude across wavelengths.

Non-Gaussianity: Deviations from a purely Gaussian distribution of primordial fluctuations, sensitive to interaction dynamics.

Gauss–Bonnet term: A higher-order curvature invariant that can couple to the inflaton, altering perturbation growth.

Microlensing: The transient brightening of a background star due to the gravitational field of an intervening compact object.

References

1. Limits on Planetary-mass Primordial Black Holes from the OGLE High-cadence Survey of the Magellanic Clouds. The Astrophysical Journal Letters (2024).
2. Constraining Primordial Black Hole Formation from Single-Field Inflation. Physical Review Letters (2024).
3. Primordial Black Holes in Scalar Field Inflation Coupled to the Gauss–Bonnet Term with Fractional Power-law Potentials. The Astrophysical Journal (2024).
4. Observational Imprints of Enhanced Scalar Power on Small Scales in Ultra Slow Roll Inflation and Associated Non-Gaussianities. Galaxies (2023).
5. Non-canonical inflation and primordial black holes production. Physics Letters B (2019).
6. Seeding Primordial Black Holes in Multifield Inflation. Physical Review Letters (2020).