More power on large scales

¹ Swinburne University PO Box 218 Hawthorn 3121, Australia
² ARC Centre of Excellence for Dark Matter Particle Physics Victoria 3010, Australia

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 28 August 2025
Accepted: 11 January 2026

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 (PBHs) suggests that simulation of dark halo formation should be commenced well before redshift z = 100. Unlike standard cold dark matter candidates, which are initially relativistic or possess thermal velocities, 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 much earlier, which significantly alters the initial power spectrum on small scales. We find that when we start N-body simulations at redshifts even before matter-radiation equality (z ∼ 3400), the galaxy bulk flow velocities are systematically higher than those predicted by standard ΛCDM models. The early high-mass concentrations established by PBHs lead to a more rapid and efficient gravitational acceleration of the surrounding baryonic and dark matter and generate higher peculiar velocities that remain coherent over scales of hundreds of megaparsec. Furthermore, a sub-population of PBHs in the 10⁻²⁰ to 10⁻¹⁷ M_⊙ 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 that a time-varying matter density parameter, Ω_m′, is introduced that behaves like a boosted radiation term, in the Friedmann equation. This dynamic term, which is most active between recombination and the late Universe, acts to reduce the Hubble tension. A higher effective Ω_r in the early Universe (pre-evaporation) reduces the sound horizon at the epoch of recombination. This smaller standard ruler as imprinted on the cosmic microwave background (CMB) would result in a higher value of the Hubble constant (H₀) inferred from the CMB at the 1% level, which brings it into slightly closer agreement with local late-time measurements. PBH mass loss also affects fits to the equation of state parameter w at low redshift. The naive N-body modelling presented here suggests that an investigation with tried and tested cosmology codes should be carried out by introducing mass-losing PBHs and starting the evolution as early as practicable.