From Cosmic Bounces to Primordial Black Holes: The Role of Primordial Magnetic Fields
Abstract
We investigate primordial black hole (PBH) formation from primordial magnetic fields generated during a non-singular ekpyrotic bounce. A time-dependent coupling between the electromagnetic field and a scalar background breaks conformal invariance and amplifies gauge field fluctuations during contraction, generating magnetic fields whose anisotropic stress dominates during radiation domination. By retaining finite-wavenumber cutoffs, we compute the ultraviolet-dominated curvature spectrum, evaluate density-perturbation variance, and determine PBH abundances using both monochromatic and critical-collapse prescriptions. For benchmark parameters, the critical collapse mass spectrum peaks at M≃ 2.0×10−3M⊙ , with a total PBH dark matter fraction of fPBH≈ 10−5, forming a subdominant subsolar PBH population. Enforcing the cosmological overclosure bound (fPBH ≤ 1) constrains the primary background parameters, namely the end-of-contraction Hubble scale (Hend ≤ 1.6 × 103 GeV) and the contraction exponent (q ≤ 0.247), establishing a physical boundary consistent with the electromagnetic backreaction limit. Shifting to asteroid-mass PBHs or a larger dark matter fraction requires modifying the ultraviolet cutoff and contraction duration. This analysis highlights the essential role of finite-wavenumber cutoffs and assumed-collapse prescriptions in linking bounce magnetogenesis to the predicted PBH abundance.
Related
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BibTeX
@misc{singhal2026from,
title = {{From Cosmic Bounces to Primordial Black Holes: The Role of Primordial Magnetic Fields}},
author = {Ajeet Singhal and Shreya Banerjee},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.21658},
eprint = {2610.21658},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.21658},
note = {CC0},
}
RIS
TY - UNPB
TI - From Cosmic Bounces to Primordial Black Holes: The Role of Primordial Magnetic Fields
AU - Ajeet Singhal
AU - Shreya Banerjee
PY - 2026
DA - 2026/10/15
PB - arXiv
DO - 10.48550/arxiv.2610.21658
UR - https://arxiv.org/abs/2610.21658
AB - We investigate primordial black hole (PBH) formation from primordial magnetic fields generated during a non-singular ekpyrotic bounce. A time-dependent coupling between the electromagnetic field and a scalar background breaks conformal invariance and amplifies gauge field fluctuations during contraction, generating magnetic fields whose anisotropic stress dominates during radiation domination. By retaining finite-wavenumber cutoffs, we compute the ultraviolet-dominated curvature spectrum, evaluate density-perturbation variance, and determine PBH abundances using both monochromatic and critical-collapse prescriptions. For benchmark parameters, the critical collapse mass spectrum peaks at M≃ 2.0×10^−3M_⊙ , with a total PBH dark matter fraction of f_PBH≈ 10^−5, forming a subdominant subsolar PBH population. Enforcing the cosmological overclosure bound (f_PBH ≤ 1) constrains the primary background parameters, namely the end-of-contraction Hubble scale (H_end ≤ 1.6 × 10^3 GeV) and the contraction exponent (q ≤ 0.247), establishing a physical boundary consistent with the electromagnetic backreaction limit. Shifting to asteroid-mass PBHs or a larger dark matter fraction requires modifying the ultraviolet cutoff and contraction duration. This analysis highlights the essential role of finite-wavenumber cutoffs and assumed-collapse prescriptions in linking bounce magnetogenesis to the predicted PBH abundance.
ER -