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Could regular primordial black holes be dark matter?

Mohsen Khodadi

TL;DR

The study critically evaluates Davies et al.'s claim that regular black holes (RBHs) can form zero-temperature remnants that act as dark matter. By enforcing adiabatic and quasi-static Hawking evaporation for RBHs with a finite horizon extremal limit, the authors show the mass-loss rate vanishes as $m\to m_*$ and the total evaporation time diverges, preventing true remnants from forming in any finite time. When one folds in cosmological constraints from the CMB and the extragalactic gamma-ray background, the near-extremal emission from a population of RBHs would exceed observed backgrounds, contradicting data (e.g., Fermi-LAT bounds). Consequently, low-mass RBHs are not viable dark matter candidates within this semi-classical framework; only potential non-perturbative quantum-gravity effects could modify this conclusion, which are outside the current RBH proposal. The work highlights the tension between regular black hole models and empirical constraints, and stresses the need for stable core dynamics beyond adiabatic approximations.

Abstract

The recent proposal proposed by Paul Davies and colleagues [Phys. Rev. D \textbf{111} (2025) no.10, 103512] that regular primordial black holes (RPBHs) form stable, zero-temperature remnants and could thereby constitute dark matter is critically examined. While the introduction of a fundamental length scale indeed regulates the Hawking temperature, preventing its divergence, we show that the evaporation timescale for such RPBHs is infinite. This result holds generically for analytic regular black hole spacetimes under standard adiabatic and quasi-static evolution. Consequently, RPBHs never actually reach a true remnant state within any finite time, but instead persist as slowly evaporating objects with a non-zero luminosity. When the combined emission from a cosmological population of these near-remnants is considered, the resulting radiation is found to violate stringent observational constraints from the cosmic microwave background and extragalactic gamma-ray backgrounds. Therefore, low-mass RPBHs are not viable dark matter candidates.

Could regular primordial black holes be dark matter?

TL;DR

The study critically evaluates Davies et al.'s claim that regular black holes (RBHs) can form zero-temperature remnants that act as dark matter. By enforcing adiabatic and quasi-static Hawking evaporation for RBHs with a finite horizon extremal limit, the authors show the mass-loss rate vanishes as and the total evaporation time diverges, preventing true remnants from forming in any finite time. When one folds in cosmological constraints from the CMB and the extragalactic gamma-ray background, the near-extremal emission from a population of RBHs would exceed observed backgrounds, contradicting data (e.g., Fermi-LAT bounds). Consequently, low-mass RBHs are not viable dark matter candidates within this semi-classical framework; only potential non-perturbative quantum-gravity effects could modify this conclusion, which are outside the current RBH proposal. The work highlights the tension between regular black hole models and empirical constraints, and stresses the need for stable core dynamics beyond adiabatic approximations.

Abstract

The recent proposal proposed by Paul Davies and colleagues [Phys. Rev. D \textbf{111} (2025) no.10, 103512] that regular primordial black holes (RPBHs) form stable, zero-temperature remnants and could thereby constitute dark matter is critically examined. While the introduction of a fundamental length scale indeed regulates the Hawking temperature, preventing its divergence, we show that the evaporation timescale for such RPBHs is infinite. This result holds generically for analytic regular black hole spacetimes under standard adiabatic and quasi-static evolution. Consequently, RPBHs never actually reach a true remnant state within any finite time, but instead persist as slowly evaporating objects with a non-zero luminosity. When the combined emission from a cosmological population of these near-remnants is considered, the resulting radiation is found to violate stringent observational constraints from the cosmic microwave background and extragalactic gamma-ray backgrounds. Therefore, low-mass RPBHs are not viable dark matter candidates.

Paper Structure

This paper contains 5 sections, 38 equations, 1 figure.

Table of Contents

  1. Introduction
  2. Reviewing the proposal for regular PBHs as dark matter
  3. Evaporation time of regular BHs
  4. Physical Viability and Observational Constraints
  5. conclusion

Figures (1)

  • Figure 1: Qualitative behavior of the Hawking temperature correspond to 4D Schwarzschild BH (black curve) and 4D regular BHs (blue curve) in terms of the mass.