More power on large scales

TL;DR

The paper investigates whether a macroscopic dark matter component in the form of mass-losing primordial black holes (PBHs) can generate more power on large scales than standard LCDM by seeding early structure. It introduces a toy N-body model in which PBHs evaporate via Hawking radiation, leading to early high-density seeds and to a time-varying matter density parameter $\Omega_m'$, which effectively boosts the radiation density and can delay matter–radiation equality. The simulations show that early PBH seeding and mass loss produce bulk flows roughly twice as large as in constant-mass scenarios, potentially reconciling observations of large-scale flows (≈400 km s$^{-1}$ on >100 Mpc scales) with theory, and offer a mechanism to mitigate the Hubble tension by modifying the pre-recombination expansion history. The work highlights the need to test these ideas with standard cosmology codes, explore PBH initial mass functions, and consider related macroscopic DM candidates (e.g., axion miniclusters) in future, more realistic simulations.

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.

Figures (9)

• Figure 1: The temperature evolution of low mass PBHs versus cosmic temperature. In the radiation dominated era PBHs of mass 10$^{-23}$ M$_\odot$ (denoted m23) and higher, if they form, do so on the green diagonal line, and evolve with little mass loss until $\dot{{\rm M}}$ is of the same order as M, at which point they rapidly heat and evaporate. Kelvin units are on the top and right borders. PBHs change their evolutionary rate as T $\sim$ M$^{-1}$, from radiation (solid line), to matter (dashed line), to dark energy dominated (dotted line). The t8 line marks the start of galaxy formation. The three dotted vertical lines are from left to right the QCD transition at 220 MeV, the surface of last scattering of the CMB and the temperature today, 2.7K. The first of these is discussed as a possible PBH trigger by Alonso-Monsalve & Kaiser (2023). As the universe cools, the lowest mass PBHs take a right angle turn to high temperatures, rapidly becoming Planck mass relics. Higher masses are shown by Mould (2025).
• Figure 1: a. Enlargement of Figure 1 for z $<$ 10$^5$. Redshift log(1+z) is denoted by the numbers towards the top. The CMB is shown by the vertical dotted line, and 10$^8$ years by the green vertical dashed line.
• Figure 2: Evolution of the velocity dispersion of models 102 and 103 (green) with redshift. The dashed line is the fall of density due to dark matter mass loss. If the density is 0.3 at z = 1300, at the current epoch it is approximately 0.27, which is within observational uncertainties.
• Figure 3: The central part of run 104, timestep 99 in 3D.
• Figure 4: The resultant bulk flow velocity increased with increasing initial redshift, z$_0$. Solid symbols are from Table 3, representing assemblies of dark matter with more than 100 particles. Open symbols are from the Appendix and are unrestricted as to the number of particles. Simulations with mass loss are in red.