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

TL;DR

Massive primordial black holes are proposed as the dominant dark matter, formed from broad peaks in the primordial curvature spectrum and yielding a broad, clustered mass distribution from ~0.01 to $10^5$ M_sun. The paper develops a multi-faceted phenomenology: MPBH growth via accretion and mergers, seeding of early structure, and a wide array of observational signatures spanning CMB distortions, reionization, microlensing, X-ray/gamma sources, and gravitational waves. It highlights a multi-wavelength, multi-frequency approach to test the MPBH-DM hypothesis with current and future observatories, including GAIA and LISA/PTA networks. A sub-solar-mass BH detection by LIGO would strongly indicate a primordial origin, reinforcing the MPBH scenario as a powerful framework to probe the early Universe and structure formation.

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.

Figures (11)

• Figure 1: The Black Holes of Known Mass detected by LIGO. It is clear that they correspond to a new population of black holes unheard off before. While IMBH and SMBH were known to populate the centers of globular clusters and galaxies, respectively, this new class of black holes in binaries had not been detected before. Figure from [LIGO webpage].
• Figure 2: The strain sensitivity and amplitude of the three GW events detected by advanced LIGO as they inspiral towards merging. The inspiralling binary black hole (BBH) coalescence time-span after the 30 Hz mark of the three events. More massive binaries would not be seen by LIGO, since $f_{\rm ISCO} = 44\ {\rm Hz}~(100~M_\odot/M_{\rm tot}) > 30$ Hz implies $M_{\rm tot} < 147~M_\odot$, but better seismic attenuation by Virgo and KAGRA may help explore the higher mass range. Figures from Ref. TheLIGOScientific:2016pea.
• Figure 3: Limits on the abundance of PBH today, from extragalactic photon background (orange), femto-lensing (red), micro-lensing by MACHO (green) and EROS (blue), from wide binaries (light brown), and CMB distortions by FIRAS (cyan) and WMAP3 (purple). The constraints from star formation and capture by neutron stars in globular clusters are displayed for $\rho_{\rm DM}^{\rm Glob. Cl.} = 2 \times 10^3 \ {\rm GeV/cm}^{3}$ (dark brown). The black dashed line corresponds to a particular realization of our scenario of PBH formation. Figure adapted from Ref. Clesse:2015wea.
• Figure 4: Generic inflationary potential giving rise to peaks in the power spectrum. In two-field models like hybrid inflation the inflection point is substituted by the symmetry breaking point. In both cases there are a few-folds of inflation after the critical point $\phi_c$.
• Figure 5: The primordial power spectrum of curvature fluctuations induced during inflation. Those fluctuations that enter during the radiation era collapse to form black holes within a range of masses, and spatially clustered around the more massive ones. The dotted line corresponds to the Gaussian primordial spectrum predicted by inflation and at the core of the $\Lambda$CDM paradigm, and consistent with the observed CMB anisotropies; the continuous line corresponds to the original model of GBLW (1996) GarciaBellido:1996qt, with a sharp peak in the spectrum, that gives rise to a monochromatic mass spectrum, and the dashed line corresponds to the recent proposal of CGB (2015) Clesse:2015wea for a broad peak in the spectrum, giving rise to a broad mass spectrum of PBH, which are strongly clustered.