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Threshold of primordial black hole formation

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abstract

Based on a physical argument, we derive a new analytic formula for the amplitude of density perturbation at the threshold of primordial black hole formation in the universe dominated by a perfect fluid with the equation of state $p=w\rho c^{2}$ for $w\ge 0$. The formula gives $\delta^{\rm UH}_{H c}=\sin^{2}[\pi \sqrt{w}/(1+3w)]$ and $\tilde{\delta}_{c}=[3(1+w)/(5+3w)]\sin^{2}[\pi\sqrt{w}/(1+3w)]$, where $\delta^{\rm UH}_{H c}$ and $\tilde{\delta}_{c}$ are the amplitude of the density perturbation at the horizon crossing time in the uniform Hubble slice and the amplitude measure used in numerical simulations, respectively, while the conventional one gives $\delta^{\rm UH}_{H c}=w$ and $\tilde{\delta}_{c}=3w(1+w)/(5+3w)$. Our formula shows a much better agreement with the result of recent numerical simulations both qualitatively and quantitatively than the conventional formula. For a radiation fluid, our formula gives $\delta^{\rm UH}_{H c}=\sin^{2}(\sqrt{3}\pi/6)\simeq 0.6203$ and $\tilde{\delta}_{c}=(2/3)\sin^{2}(\sqrt{3}\pi/6)\simeq 0.4135$. We also discuss the maximum amplitude and the cosmological implications of the present result.

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representative citing papers

Angular momentum of vacuum bubbles in a first-order phase transition

hep-ph · 2025-05-14 · unverdicted · novelty 7.0

Computes the dimensionless spin parameter s = J/(G_N M^2) of false vacuum bubbles from density and velocity perturbations in FOPTs, yielding values from O(10^{-5}) to O(10) and a scaling relation with FOPT timescale, wall velocity, and temperature ratio.

Primordial Neutron Stars

astro-ph.CO · 2026-04-09 · unverdicted · novelty 6.0

Large initial baryon asymmetry allows Hubble patches to collapse into primordial neutron stars arrested by nuclear pressure, requiring later entropy dilution to match observed Y_B and BBN.

Constraints on Primordial Black Holes

astro-ph.CO · 2020-02-27 · accept · novelty 4.0

Updated compilation shows PBHs are tightly constrained across 55 orders of magnitude in mass, ruling out dominant dark matter contributions except in narrow windows, with many limits carrying observational uncertainties.

Scalar induced gravitational waves review

gr-qc · 2021-09-03 · unverdicted · novelty 2.0

A review that unifies analytical expressions for scalar-induced gravitational waves and emphasizes calculations for non-radiation-dominated cosmologies.

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