Primordial black hole formation from transient $f(T)$ cosmology

TL;DR

This paper explores how a transient phase in $f(T)$ teleparallel gravity can significantly enhance primordial black hole formation, potentially explaining dark matter without altering the radiation sector.

Contribution

It introduces a novel $f(T)$ cosmological model where transient effects lower collapse thresholds, leading to increased PBH production and a distinctive mass function feature.

Findings

Transient $f(T)$ effects lower collapse thresholds.

Enhanced PBH formation during transient phase.

PBHs can account for dark matter with realistic parameters.

Abstract

We study primordial black hole (PBH) formation in a minimally coupled $f(T)$ teleparallel cosmology that generates a transient departure from standard radiation domination. The model is constructed so that modified-gravity effects are negligible at early and late times, but become dynamically relevant over a finite epoch, during which an effective torsion component reduces the total equation-of-state parameter below 1/3.We show that this transient softening lowers the collapse threshold for overdensities at horizon re-entry, leading to an exponential enhancement of PBH formation. In addition, the modified background alters the relation between temperature and horizon mass, producing a localized feature in the PBH mass function. For representative parameters, PBHs with asteroid-scale masses can account for a significant fraction, or even the entirety, of dark matter for perturbation amplitudes $\sigma^2 \sim \mathcal{O}(10^{-3})$, while remaining consistent with current constraints. Our results demonstrate that modified gravity alone can efficiently generate PBHs, without requiring ad hoc modifications of the radiation sector.