Primordial Black Holes as Generators of Cosmic Structures (1801.00672v3)

Abstract: Primordial black holes (PBHs) could provide the dark matter in various mass windows below $10^2 M_{\odot}$ and those of $30 M_{\odot}$ might explain the LIGO events. PBHs much larger than this might have important consequences even if they provide only a small fraction of the dark matter. In particular, they could generate cosmological structure either individually through through the seed' effect or collectively through thePoisson' effect, thereby alleviating some problems associated with the standard CDM scenario. If the PBHs all have a similar mass and make a small contribution to the dark matter, then the seed effect dominates on small scales, in which case PBHs could seed the supermassive black holes in galactic nuclei or even galaxies themselves. If they have a similar mass and provide the dark matter, the Poisson effect dominates on all scales and the first bound clouds would form earlier than in the usual scenario, with interesting observational consequences. If the PBHs have an extended mass spectrum, which is more likely, they could fulfill all three roles - providing the dark matter, binding the first bound clouds and generating galaxies. In this case, the galactic mass function naturally has the observed form, with the galaxy mass being simply related to the black hole mass. The stochastic gravitational wave background from the PBHs in this scenario would extend continuously from the LIGO frequency to the LISA frequency, offering a potential goal for future surveys.

• The paper demonstrates that an extended PBH mass function can simultaneously explain dark matter and the early seeding of cosmic structures such as SMBHs.
• The paper shows that both the seed and Poisson effects from PBHs can accelerate galaxy formation, offering resolutions to challenges in standard CDM models.
• The paper predicts a continuous gravitational wave background from PBH populations, providing a testable target for future gravitational wave observatories.

Primordial Black Holes as Generators of Cosmic Structures

The paper "Primordial Black Holes as Generators of Cosmic Structures" by Bernard Carr and Joseph Silk explores the potential for primordial black holes (PBHs) to play a significant role in the formation of cosmic structures and as candidates for dark matter. This research addresses several open questions in cosmology, particularly the nature of dark matter and the early formation of supermassive black holes (SMBHs) in galactic nuclei.

Overview of the Research

The paper considers a spectrum of PBH masses and their implications for different cosmic scales. PBHs could plausibly constitute the dark matter in various mass windows below $10^2 M_{\odot}$ and the $30 M_{\odot}$ black holes observed by LIGO might be explained by PBHs. Significantly larger PBHs, despite providing only a small fraction of the dark matter, might still contribute to cosmological structure formation through either the "seed" effect, where individual PBHs act as seeds for larger structures, or the "Poisson" effect, where collective fluctuations influence structure formation.

The seed effect could potentially explain the presence of SMBHs and even galaxies at high redshifts, offering solutions to the classic conundrum of how such large structures could form so early in cosmic history. The Poisson effect, on the other hand, could dominate if PBHs account for all the dark matter, leading to an acceleration of the formation of the first bound clouds compared to the standard cold dark matter (CDM) model.

Key Claims and Results

1. Mass Function of PBHs: The paper explores both monochromatic and extended mass functions of PBHs, suggesting that an extended mass spectrum is more practical for reconciling the role of PBHs in both dark matter and structure formation.
2. Cosmic Structure Contribution: Provided that PBHs are not significantly inhibited by current observational constraints (such as CMB distortions), they might have seeded the first SMBHs and contributed to the formation of early galaxies via hierarchical growth mechanisms.
3. Gravitational Wave Background: The model predicts a continuous background of gravitational waves stemming from PBH populations, detectable by observatories spanning the frequency range from LIGO to LISA. This signature offers a compelling target for future gravitational wave surveys.

Implications and Speculations

The implications of PBHs as generators of cosmic structures are profound. Practically, they could resolve several persistent issues within the CDM framework, such as the formation of large structures at early times and the nature of dark matter itself. Theoretically, this model prompts a re-evaluation of early universe conditions and the potential mechanisms of PBH formation, including inflationary scenarios and non-standard collapse conditions.

The research also speculates on future developments in AI, particularly in refining data analysis techniques for gravitational wave detection and cosmological simulations, which will be vital for testing the predictions put forth in this paper.

Conclusion

Carr and Silk's work posits a unified framework in which PBHs could simultaneously account for the dark matter and provide the seeds necessary for observed cosmic structures. While observational constraints remain a key challenge, ongoing and future advancements in gravitational wave astronomy and cosmological observations will be crucial in validating or refuting this model. The paper ultimately underscores the importance of exploring non-standard cosmological models to solve long-standing astronomical puzzles.