Massive Primordial Black Holes as Dark Matter and their detection with Gravitational Waves

Abstract: Massive Primordial Black Holes (MPBH) can be formed after inflation due to broad peaks in the primordial curvature power spectrum that collapse gravitationally during the radiation era, to form clusters of black holes that merge and increase in mass after recombination, generating today a broad mass-spectrum of black holes with masses ranging from 0.01 to $10^5~M_\odot$. These MPBH could act as seeds for galaxies and quick-start structure formation, initiating reionization, forming galaxies at redshift $z>10$ and clusters at $z>1$. They may also be the seeds on which SMBH and IMBH form, by accreting gas onto them and forming the centers of galaxies and quasars at high redshift. They form at rest with zero spin and have negligible cross-section with ordinary matter. If there are enough of these MPBH, they could constitute the bulk of the Dark Matter today. Such PBH could be responsible for the observed fluctuations in the CIB and X-ray backgrounds. MPBH could be directly detected by the gravitational waves emitted when they merge to form more massive black holes, as recently reported by LIGO. Their continuous merging since recombination could have generated a stochastic background of gravitational waves that could eventually be detected by LISA and PTA. MPBH may actually be responsible for the unidentified point sources seen by Fermi, Magic and Chandra. Furthermore, the ejection of stars from shallow potential wells like those of Dwarf Spheroidals (DSph), via the gravitational slingshot effect, could be due to MPBH, thus alleviating the substructure and too-big-to-fail problems of standard collisionless CDM. Their mass distribution peaks at a few tens of $M_\odot$ today, and could be detected also with long-duration microlensing events, as well as by the anomalous motion of stars in GAIA. Their presence as CDM in the Universe could be seen in the time-dilation of lensed images of quasars.

• The paper introduces a framework linking the formation, growth, and gravitational wave detection of massive primordial black holes.
• It models an MPBH mass spectrum from 0.01 to 10^5 solar masses, highlighting their role in dark matter and early galaxy formation.
• It proposes gravitational wave observations, cosmic background anomalies, and microlensing events as practical avenues for detection.

Overview of "Massive Primordial Black Holes as Dark Matter and their detection with Gravitational Waves"

This paper explores the concept that massive primordial black holes (MPBH) could constitute a substantial portion of dark matter, presenting a framework that links MPBH formation, growth, and detection through gravitational waves. The study explores the theoretical underpinnings, observational implications, and potential avenues for empirical verification through upcoming gravitational wave observations and other astrophysical phenomena.

Formation of MPBH

The hypothesis revolves around the notion that MPBH can form after cosmic inflation, arising from broad peaks in the primordial curvature power spectrum. During the radiation era, these peaks result in the gravitational collapse of high-density regions leading to the formation of black holes. Post-recombination, these black holes merge and accrete mass, generating a broad spectrum of black hole masses observed today, which range from 0.01 to $10^5 \, M_\odot$.

The theory posits that these MPBH serve as initial seeds for galaxies and influence the rapid onset of structure formation, potentially initiating reionization and contributing to the formation of early galaxies and clusters. Furthermore, they may act as the progenitors for supermassive black holes (SMBH) and intermediate-mass black holes (IMBH), by gathering accreting gas, forming gravitational centers around which galaxies and quasars can emerge.

Detection through Gravitational Waves

A significant portion of the paper is devoted to discussing the detectability of MPBH via their gravitational wave emissions. The hypothesis suggests that MPBH have played a significant role in the observations made by LIGO concerning massive black hole mergers. If MPBH exist in sufficient quantities, they would not only be integral to the current dark matter content but might also be responsible for creating a stochastic gravitational wave background. This suggestion aligns with potential detection capabilities of future missions like LISA and existing ones like pulsar timing arrays (PTA).

Moreover, MPBH interactions could explain anomalies observed in cosmic infrared background (CIB) fluctuations and X-ray background levels, possibly correlating with unresolved point sources seen by observatories such as Fermi, Magic, and Chandra.

Implications for Astrophysical Phenomena

The framework introduces potential solutions to the so-called "too-big-to-fail" and substructure problems in standard cold dark matter (CDM) models. MPBH within dwarf spheroidal galaxies could eject stars via gravitational slingshot effects, alleviating some discrepancies regarding the expected versus observed number of satellite galaxies. The paper also considers the role of MPBH in creating microlensing events and influencing stellar motion anomalies, such as those that could be detected by the GAIA mission through induced time dilation in strong lensing systems.

Future Prospects

The paper implicitly speculates on future directions in cosmology and astrophysics if MPBH are revealed to be a significant component of dark matter. This includes a deeper understanding of dark matter's nature, potentially shifting paradigms from particle-based explanations to macroscopic objects like black holes. Furthermore, with the continued advancement of gravitational wave astronomy, MPBH may provide insights into the fundamental physics of the early universe by allowing us to probe the cosmic inflationary period's dynamics.

In summary, this paper provides a comprehensive theoretical exploration of MPBH as potential dark matter candidates and their detection prospects through gravitational wave astronomy, alongside their profound implications for our understanding of cosmic evolution and the nature of dark matter. Such studies pave the way for corroborative efforts across observational and theoretical domains, potentially reshaping our grasp of the universe's composition.