Cosmic Conundra Explained by Thermal History and Primordial Black Holes (1906.08217v4)

Abstract: A universal mechanism may be responsible for several unresolved cosmic conundra. The sudden drop in the pressure of relativistic matter at $W^{\pm}/Z^{0}$ decoupling, the quark--hadron transition and $e^{+}e^{-}$ annihilation enhances the probability of primordial black hole (PBH) formation in the early Universe. Assuming the amplitude of the primordial curvature fluctuations is approximately scale-invariant, this implies a multi-modal PBH mass spectrum with peaks at $10^{-6}$, 1, 30, and $10^{6}\,M_{\odot}$. This suggests a unified PBH scenario which naturally explains the dark matter and recent microlensing observations, the LIGO/Virgo black hole mergers, the correlations in the cosmic infrared and X-ray backgrounds, and the origin of the supermassive black holes in galactic nuclei at high redshift. A distinctive prediction of our model is that LIGO/Virgo should observe black hole mergers in the mass gaps between 2 and $5\,M_{\odot}$ (where no stellar remnants are expected) and above $65\,M_{\odot}$ (where pair-instability supernovae occur) and low-mass-ratios in between. Therefore the recent detection of events GW190425, GW190814 and GW190521 with these features is striking confirmation of our prediction and may indicate a primordial origin for the black holes. In this case, the exponential sensitivity of the PBH abundance to the equation of state would offer a unique probe of the QCD phase transition. The detection of PBHs would also offer a novel way to probe the existence of new particles or phase transitions with energy between $1\,{\rm MeV}$ and $10^{10}\,$GeV.

Overview of "Cosmic Conundra Explained by Thermal History and Primordial Black Holes"

The paper "Cosmic Conundra Explained by Thermal History and Primordial Black Holes," authored by Bernard Carr and his collaborators, explores the hypothesis that primordial black holes (PBHs) could be key to explaining several unresolved issues in cosmology and astrophysics. These PBHs might have been formed during the early Universe due to pressure drops in relativistic matter following specific phase transitions, such as the $W^{\pm}/Z^{0}$ decoupling, quark-hadron transition, and $e^{+}e^{-}$ annihilation.

PBH Mass Spectrum

The research posits that the PBH formation is enhanced during particular epochs when the thermal state of the Universe changes without requiring a large enhancement of primordial curvature perturbations. The authors propose a multi-modal PBH mass spectrum with peaks at $10^{-6}$, 1, 30, and $10^{6} M_\odot$, based on an assumed scale-invariant power spectrum of primordial curvature fluctuations. These specific mass scales align with transitions in the relativistic degrees of freedom that typically occur at critical points in the Universe’s thermal history. This spectrum is claimed to justify the PBHs' role in explaining several cosmic phenomena.

Implications for Cosmic Conundra

The proposed PBH mass spectrum offers unified explanations for various observational cosmic conundra. These include:

1. Dark Matter Composition: PBHs could comprise a significant part or even the entirety of dark matter.
2. LIGO/Virgo Observations: The detected black hole mergers by LIGO/Virgo, particularly events like GW190425, GW190814, and GW190521, are posited to support the presence of PBHs with masses and mass ratios that correspond to gaps expected from stellar evolution scenarios.
3. Supermassive Black Holes and Galactic Nuclei: The observed SMBHs in galactic centers at high redshifts can be accounted for by PBHs formed during the early Universe, specifically those at the higher end of the mass spectrum.
4. Microlensing and Cosmic Backgrounds: The paper suggests that certain microlensing effects and correlations in the cosmic infrared and X-ray backgrounds can be attributed to PBHs in respective mass windows.

Theoretical and Practical Implications

The paper presents the view that PBHs serve as probes for the Universe's detailed thermal history, especially given that their abundance is exponentially sensitive to the equation of state during formation epochs such as the QCD phase transition. Furthermore, the detection of PBHs could potentially reveal new physics, such as unknown particles or transitional states with energy levels between $1~{\rm MeV}$ and $10^{10}~{\rm GeV}$, which lie beyond current observational capabilities.

Future Directions

The paper anticipates further validation from upcoming observations, like enhanced gravitational wave detections and refined cosmic background analyses. By mapping PBH abundances and properties more precisely, researchers might pinpoint phases in the Universe's history otherwise inaccessible through conventional astrophysical means.

In summarizing, the paper provides a coherent argument that reconciles diverse astronomical observations within the framework of PBH formation and existence, contingent on complex phase transitions within the early Universe. However, as with many theoretical constructs, empirical evidence will be essential to substantiate such bold claims, pushing the boundaries of standard models in particle physics and cosmology.