Dark Radiation and Superheavy Dark Matter from Black Hole Domination (1905.01301v1)

Abstract: If even a relatively small number of black holes were created in the early universe, they will constitute an increasingly large fraction of the total energy density as space expands. It is thus well-motivated to consider scenarios in which the early universe included an era in which primordial black holes dominated the total energy density. Within this context, we consider Hawking radiation as a mechanism to produce both dark radiation and dark matter. If the early universe included a black hole dominated era, we find that Hawking radiation will produce dark radiation at a level $\Delta N_{\rm eff} \sim 0.03-0.2$ for each light and decoupled species of spin 0, 1/2, or 1. This range is well suited to relax the tension between late and early-time Hubble determinations, and is within the reach of upcoming CMB experiments. The dark matter could also originate as Hawking radiation in a black hole dominated early universe, although such dark matter candidates must be very heavy ($m_{\rm DM} >10^{11}$ GeV) if they are to avoid exceeding the measured abundance.

• The paper demonstrates that primordial black holes can dominate early cosmic energy and generate dark radiation via Hawking radiation.
• It quantifies dark radiation with ΔNₑff contributions of 0.03–0.2 and predicts superheavy dark matter masses around 10¹¹ GeV to meet cosmic abundance.
• The study employs robust mathematical models to explore early universe dynamics and offers insights into resolving Hubble constant tensions and hidden sector implications.

Dark Radiation and Superheavy Dark Matter from Black Hole Domination

The paper undertaken by Hooper, Krnjaic, and McDermott explores a compelling hypothesis within the field of cosmology: the prospect of primordial black holes dominating the energy density of the early universe. This investigation is set against the backdrop of assessing Hawking radiation as a potential source of both dark radiation and superheavy dark matter. The paper presents a robust mathematical framework, detailing the conditions under which black holes could have substantially influenced the cosmic energy budget before Big Bang Nucleosynthesis (BBN).

In this scenario, if primordial black holes existed in sufficient numbers, they would contribute increasingly to the energy density of the universe as it expands. The evaporation of these black holes via Hawking radiation could produce dark matter and dark radiation particles. One of the critical findings is the estimated contribution to $\Delta N_{\rm eff}$, the effective number of neutrino species, due to light and decoupled particles emanating from Hawking radiation. These contributions are calculated to be in the range of 0.03-0.2, contingent upon the particle spin and initial black hole mass. This is particularly significant as it could help to address the tension in the Hubble constant measurements derived from local observations versus those inferred from the cosmic microwave background (CMB).

The paper also evaluates the potential of Hawking radiation to generate a significant component of dark matter. For the dark matter candidates to align with observed cosmological abundances, these particles would need to be sufficiently massive, in the order of $10^{11} \, \rm{GeV}$. This introduces the notion of superheavy dark matter, which would not have been in thermal equilibrium in the early universe—a paradigm shift from traditional thermal relic perspectives.

From a methodological standpoint, the authors provide a detailed evaluation of the factors influencing black hole formation, including initial density fractions and mass distributions. The analysis considers both radiation-dominated and black-hole-dominated early universe scenarios. The paper also explores implications of black hole mergers and accretion processes, suggesting these mechanisms only hold significance for mass growth at very early times ($T_{\rm eff} \gg 10^8 \, \rm{GeV}$).

Significantly, the notion of a large hidden sector (LHS) is addressed, wherein numerous otherwise undetectable particles could be produced by black hole evaporation. This presents a potential paradigm for string theory's proposed "axiverse" scenario, although constraints on $\Delta N_{\rm eff}$ imply that only a limited number of such species could exist without conflicting with current observations.

The implications of this research are far-reaching both practically and theoretically. The possibility of resolving existing discrepancies in Hubble constant measurements points towards practical impacts on our understanding of the universe's expansion history. Meanwhile, from a theoretical perspective, this framework opens new questions regarding the initial conditions of the universe and the potential for unknown particle species beyond the Standard Model.

Future developments in observational technologies, particularly stage IV CMB experiments, could provide empirical tests for these theoretical predictions. Accurate measurements of $\Delta N_{\rm eff}$ could validate or falsify these hypotheses, shedding light on the nature of dark matter and the fundamental constituents of the universe. As our observational capabilities advance, the models and predictions from this paper will undoubtedly contribute to the broader discourse on cosmology and particle physics. The interplay between primordial black holes and early universe dynamics represents a critical nexus point in our search to understand the true nature of cosmic evolution.