Exploring the high-redshift PBH-$Λ$CDM Universe: early black hole seeding, the first stars and cosmic radiation backgrounds (2109.08701v2)

Abstract: We explore the observational implications of a model in which primordial black holes (PBHs) with a broad birth mass function ranging in mass from a fraction of a solar mass to $\sim$10$^6$ M${\odot}$, consistent with current observational limits, constitute the dark matter component in the Universe. The formation and evolution of dark matter and baryonic matter in this PBH-\LambdaCDM~Universe are presented. In this picture, PBH DM mini-halos collapse earlier than in standard \LambdaCDM, baryons cool to form stars at $z\sim15-20$, and growing PBHs at these early epochs start to accrete through Bondi capture. The volume emissivity of these sources peaks at $z\sim20$ and rapidly fades at lower redshifts. As a consequence, PBH DM could also provide a channel to make early black hole seeds and naturally account for the origin of an underlying dark matter halo - host galaxy and central black hole connection that manifests as the $M{\rm bh}-\sigma$ correlation. To estimate the luminosity function and contribution to integrated emission power spectrum from these high-redshift PBH DM halos, we develop a Halo Occupation Distribution (HOD) model. In addition to tracing the star formation and reionizaton history, it permits us to evaluate the Cosmic Infrared and X-ray Backgrounds (CIB and CXB). We find that accretion onto PBHs/AGN successfully accounts for the detected backgrounds and their cross-correlation, with the inclusion of an additional IR stellar emission component. Detection of the deep IR source count distribution by the JWST could reveal the existence of this population of high-redshift star-forming and accreting PBH DM.

• The paper introduces a PBH-ΛCDM model where primordial black holes act as dark matter and seed early structure formation during the universe's QCD phase transitions.
• The paper applies HOD and MCMC methods to demonstrate that early mini-halo collapse triggers star formation at redshifts around 15–20 and contributes to the cosmic X-ray and infrared backgrounds.
• The paper predicts observable CXB and CIB fluctuations and steep IR source count variations, offering testable signatures for missions like JWST, Athena, and LISA.

Overview of a Primordial Black Hole Dark Matter Cosmology

This paper investigates the potential implications of a universe where dark matter is largely composed of primordial black holes (PBHs) with a broad mass distribution. The paper examines how such a PBH-dominated (PBH-$\Lambda$CDM) cosmology could impact early structure formation, star formation rates, and the evolution of cosmic backgrounds at high redshift. The authors establish a framework wherein PBHs contribute significantly to the Cosmic Infrared Background (CIB), Cosmic X-ray Background (CXB), and serve as seeds for supermassive black holes (SMBHs), while also exploring how this model fits within the constraints provided by observed astrophysical phenomena.

Key Findings and Methodology

1. Primordial Black Holes as Dark Matter: The authors propose that a population of PBHs with masses ranging from a fraction of a solar mass to approximately $10^6 M_{\odot}$ could account for the observed dark matter. The model presupposes that these PBHs formed during the early universe's quantum chromodynamics (QCD) phase transitions, a concept consistent with various observational constraints.
2. Early Halo and Star Formation: In this PBH-$\Lambda$CDM model, the formation of dark matter mini-halos and their subsequent collapse occurs earlier compared to the standard cosmology. This early collapse facilitates the onset of star formation at redshifts $z\sim15-20$.
3. Accretion and Black Hole Growth: The model predicts that growing PBHs begin accreting matter via Bondi capture, which leads to significant contributions to the CXB and CIB. The authors use a Halo Occupation Distribution (HOD) model to estimate contributions to these backgrounds.
4. Fitting the Observational Data: The paper rigorously tests their model against existing observational constraints, including the Thompson optical depth from CMB observations, star formation rate densities (SFRD) from high-z galaxy surveys, and unresolved CXB measurements. MCMC methods are employed to explore the model parameter space, particularly investigating variations in IMF and enrichment histories of population III stars.
5. Predictions for Future Observations: The model anticipates significant contributions to faint IR source counts and makes predictions about fluctuations in the CXB and CIB. These source counts and fluctuations could be validated or constrained by upcoming observations from JWST, LISA, Euclid, and Athena, among other missions.

Implications

• Theoretical Implications: The insight that SMBHs correlate naturally with their host halo masses within a PBH-$\Lambda$CDM cosmology offers a coherent explanation for observed $M_{\text{bh}} - \sigma$ scaling relations and suggests an intrinsic connection between galaxies and dark matter halo properties that could originate from these early conditions.
• Observational Predictions: Upcoming observatory missions have the potential to critically test the model's predictions, particularly through high-redshift surveys targeting the structure and evolution of the early universe. The expected steepening in faint IR source counts at magnitudes visible to JWST provides a strong observational benchmark for this model.
• Future Developments: This research opens pathways for further exploration of the gravitational wave signatures from PBH mergers, possibly facilitating indirect detection of PBH dark matter through gravitational wave observatories like LIGO and LISA.

Critique and Conclusion

While the PBH-$\Lambda$CDM cosmology offers a compelling and coherent model with explanatory power for several cosmic phenomena, it also introduces new complexities. For instance, the broad mass spectrum of PBHs and their subsequent astrophysical histories add layers of variables that must be tightly constrained by observations.

The model's predictions related to CXB and CIB power spectra, star formation rates, and early black hole growth remain contingent upon the refinement of instrument sensitivities and the fidelity of upcoming observational data. Nonetheless, the paper provides a robust framework for considering PBHs as a significant constituent of dark matter, reinvigorating interest in primordial mechanisms of cosmic structure formation. As observational evidence accumulates, models like this will play a crucial role in refining our understanding of the universe’s composition and the nature of dark matter.