Effect of Primordial Black Holes on the Cosmic Microwave Background and Cosmological Parameter Estimates

Abstract: We investigate the effect of non-evaporating primordial black holes (PBHs) on the ionization and thermal history of the universe. X-rays emitted by gas accretion onto PBHs modify the cosmic recombination history, producing measurable effects on the spectrum and anisotropies of the Cosmic Microwave Background (CMB). Using the third-year WMAP data and FIRAS data we improve existing upper limits on the abundance of PBHs with masses >0.1 Msun by several orders of magnitude. Fitting WMAP3 data with cosmological models that do not allow for non-standard recombination histories, as produced by PBHs or other early energy sources, may lead to an underestimate of the best-fit values of the amplitude of linear density fluctuations (sigma_8) and the scalar spectral index (n_s). Cosmological parameter estimates are affected because models with PBHs allow for larger values of the Thomson scattering optical depth, whose correlation with other parameters may not be correctly taken into account when PBHs are ignored. Values of tau_e=0.2, n_s=1 and sigma_8=0.9 are allowed at 95% CF. This result that may relieve recent tension between WMAP3 data and clusters data on the value of sigma_8. PBHs may increase the primordial molecular hydrogen abundance by up to two orders of magnitude, this promoting cooling and star formation. The suppression of galaxy formation due to X-ray heating is negligible for models consistent with the CMB data. Thus, the formation rate of the first galaxies and stars would be enhanced by a population of PBHs.

• The paper demonstrates that PBHs, through gas accretion and X-ray output, modify the universe’s ionization and thermal history, impacting CMB signals.
• The paper refines estimates of key cosmological parameters, including σ8 and nₛ, by incorporating non-standard energy inputs from PBHs.
• The paper employs semi-analytical methods and updated Monte Carlo codes to tighten constraints on PBH abundance, enhancing our understanding of early star formation.

Overview of “Effect of Primordial Black Holes on the Cosmic Microwave Background and Cosmological Parameter Estimates”

This paper, authored by Ricotti, Ostriker, and Mack, presents an in-depth investigation of the effects of non-evaporating primordial black holes (PBHs) on the ionization and thermal history of the universe and their implications on the Cosmic Microwave Background (CMB). The study leverages data from the Wilkinson Microwave Anisotropy Probe (WMAP) and the Far-Infrared Absolute Spectrophotometer (FIRAS) to refine existing constraints on the abundance of PBHs with masses exceeding 0.1 M$_\odot$.

Cosmological Impacts of PBHs

The paper highlights that X-rays emitted from gas accreting onto PBHs modify the cosmic recombination history, potentially creating observable spectral and anisotropic effects on the CMB. Such effects could significantly affect cosmological parameter estimates such as the amplitude of linear density fluctuations ($\sigma_8$) and the scalar spectral index ($n_s$). A notable result is the proposition that models ignoring early energy sources like PBHs might underestimate these parameters. This consideration is especially relevant given the possible alleviation of tensions in $\sigma_8$ values derived from WMAP3 data versus cluster observations.

Impact on Cosmic Structures

PBHs are shown to influence molecular hydrogen abundance by increasing it by up to two orders of magnitude, thus promoting early cooling and star formation. Contrary to potential concerns, the paper finds that galaxy formation suppression through X-ray heating is negligible under models consistent with CMB data. Consequently, the presence of PBHs can enhance the formation rate of the first stars and galaxies.

Numerical Analysis and Implications

The paper employs semi-analytical calculations to model the ionization and thermal history of the primordial plasma, incorporating effects such as Compton drag and feedback processes. By modifying the Monte-Carlo code COSMOMC, the authors fit models accounting for PBH effects to third-year WMAP data. Their analysis revises the upper limits for PBH abundance, providing more stringent constraints.

Future Developments and Considerations

The implications of PBHs span both practical stakeholder interests, such as those concerning dark matter constituents, and theoretical cosmology, fostering a nuanced understanding of early universe energy interactions. As future CMB experiments enhance sensitivity and range, these results underscore the potential necessity of incorporating non-standard recombination histories into cosmological models to achieve more accurate parameter estimates and align multiple observational datasets.

In summary, this paper contributes significantly to our understanding of the potential roles and constraints of PBHs within cosmology, presenting methodological advances and results that challenge and refine current cosmological parameter estimates. This work serves as a pivotal piece in the broader puzzle of understanding the universe's early epochs and the constituents of its dark matter.