Cosmic microwave background limits on accreting primordial black holes

Abstract: Interest in the idea that primordial black holes (PBHs) might comprise some or all of the dark matter has recently been rekindled following LIGO's first direct detection of a binary-black-hole merger. Here we revisit the effect of accreting PBHs on the cosmic microwave background (CMB) frequency spectrum and angular temperature/polarization power spectra. We compute the accretion rate and luminosity of PBHs, accounting for their suppression by Compton drag and Compton cooling by CMB photons. We estimate the gas temperature near the Schwarzschild radius, and hence the free-free luminosity, accounting for the cooling resulting from collisional ionization when the background gas is mostly neutral. We account approximately for the velocities of PBHs with respect to the background gas. We provide a simple analytic estimate of the efficiency of energy deposition in the plasma. We find that the spectral distortions generated by accreting PBHs are too small to be detected by FIRAS, as well as by future experiments now being considered. We analyze Planck CMB temperature and polarization data and find, under our most conservative hypotheses, and at the order-of-magnitude level, that they rule out PBHs with masses >~ 10^2 M_sun as the dominant component of dark matter.

• The paper recalculates the radiative efficiency and accretion rate, showing that energy deposition from PBHs into the CMB is significantly lower than earlier estimates.
• The analysis of Planck data indicates that PBHs with masses above roughly 100 solar masses cannot account for the majority of dark matter.
• Detailed modeling of thermal and Compton cooling processes underscores the complexity of PBH accretion dynamics and the need for refined future investigations.

Cosmic Microwave Background Limits on Accreting Primordial Black Holes

The research article authored by Yacine Ali-Ha\"imoud and Marc Kamionkowski focuses on exploring the constraints that primordial black holes (PBHs), specifically those that accrete matter, impose on the cosmic microwave background (CMB). This study revisits and challenges prior constraints established by Ricotti, Ostriker, and Mack (ROM), offering a revised and more conservative assessment of the role PBHs might play as a component of dark matter.

Primordial black holes are a theoretical class of black holes thought to have formed in the early universe due to density fluctuations. Their relevance has increased following the detection of gravitational-wave events by LIGO, which detected merging black holes of stellar mass scales, renewing speculation over whether PBHs could constitute all or a fraction of dark matter. This prospect is particularly appealing in light of null results from dark matter particle searches, thus warranting a re-examination of viable PBH constraints through new observational data from instruments like Planck.

The paper provides a comprehensive analysis of how accreting PBHs might affect the CMB through various mechanisms, primarily focusing on the spectral and angular temperature/polarization power spectra. This involves evaluating the accretion rate, luminosity, and the subsequent energy deposition into the CMB. Notably, the authors incorporate Compton drag and cooling effects from CMB photons and consider the accretion dynamics in distinct regimes based on ionization and thermal equilibrium assumptions.

A central finding of the study is that the luminosity and resultant energy deposition rate from accreting PBHs are significantly lower than previously estimated by ROM. This divergence arises from a more nuanced calculation of the radiative efficiency and accretion rate, showcasing that thermal and Compton cooling significantly impact the accretion process near the Schwarzschild radius of the PBH. Importantly, the study finds that the radiation-induced spectral distortions of the CMB are too minimal to be detected by the currently available instruments, such as FIRAS or even those proposed for the future.

Analyzing the Planck CMB temperature and polarization data, the authors assert that PBHs with masses greater than approximately 100 solar masses cannot be the principal constituent of dark matter, in stark contrast to ROM who indicated stricter mass limits. This paper suggests that earlier estimates might have overestimated the effect of PBHs on the CMB due, in part, to simplified models that overlooked the detailed mechanics of radiative processes.

The implications of this research are profound, as they potentially relax some of the tighter constraints on PBHs and suggest avenues for ongoing and future observational strategies to further probe their role within the cosmological context. While this work provides a robust and revised lower bound on PBH dark matter contributions, it highlights substantial theoretical uncertainties, such as non-spherical accretion flows and dynamic interactions with the surrounding medium, leaving open compelling lines of investigation in astrophysics and cosmology.

In summary, while the article substantially re-evaluates the landscape of contraints on primordial black holes as dark matter candidates, it also underscores the complexity and need for future work to incorporate detailed modeling of PBH environments. This research paves the way for renewed investigations into the compatibility of PBHs within the broader cosmological model and realigns our understanding of their potential observability through the lens of CMB studies.