More power on large scales

TL;DR

This paper explores how primordial black holes as dark matter candidates could influence early universe structure formation, galaxy velocities, and the Hubble tension, suggesting earlier simulations and mass loss effects are significant.

Contribution

It introduces the idea that primordial black holes can seed early structure formation and affect cosmological parameters, offering new insights into dark matter and the Hubble tension.

Findings

Early PBHs lead to larger galaxy bulk flow velocities.

Mass loss from PBHs can reduce the Hubble tension.

Starting simulations before matter-radiation equality impacts results.

Abstract

The high value of the cosmic microwave dipole may be telling us that dark matter is macroscopic rather than a fundamental particle. The possible presence of a significant dark matter component in the form of primordial black holes suggests that dark halo formation simulations should be commenced well before redshift z = 100. Unlike standard CDM candidates, PBHs behave as dense, non-relativistic matter from their inception in the radiation-dominated era. This allows them to seed gravitational potential wells and begin clustering earlier. We find that starting N-body simulations at redshifts even before matter-radiation equality yield galaxy bulk flow velocities that are systematically larger than those predicted by standard LCDM models. The early, high-mass concentrations established by PBHs lead to a more rapid and efficient gravitational acceleration of surrounding baryonic and dark matter, generating larger peculiar velocities that remain coherent over scales of hundreds of Mpc. Furthermore, a sub-population of PBHs in the 10^-20 to 10^-17 solar mass range would lose a non-negligible fraction of their mass via Hawking radiation over cosmological timescales. This evaporation process converts matter into radiation, so a time-varying matter density parameter, Omega_m', is introduced, which behaves like a boosted radiation term in the Friedmann equation. This dynamic term acts to reduce the Hubble tension. A higher effective Omega_r in the early universe reduces the sound horizon at the epoch of recombination. PBH mass loss also influences fits to the equation of state parameter, w, at low redshift. The naive N-body modelling presented here suggests investigation with tried and tested cosmology codes should be carried out, by introducing mass losing PBHs and starting the evolution as early as practicable.