[Paper Review] Supermassive primordial black holes in multiverse: for nano-Hertz gravitational wave and high-redshift JWST galaxies

H. X. Huang, Yong Cai+3 more|arXiv (Cornell University)|Jun 30, 2023

Cosmology and Gravitation TheoriesPhysics and Astronomy34 citations

TL;DR

The paper proposes a multiverse PBH formation mechanism where supercritical bubbles during slow-roll inflation near neighboring vacua produce a multi-peaked mass spectrum, including supermassive PBHs up to ~10^11 solar masses, potentially explaining nano-Hertz GW hints and early JWST galaxies.

ABSTRACT

Recently, observational hints for supermassive black holes have been accumulating, which has inspired ones to wonder: Can primordial black holes (PBHs) be supermassive, in particular with the mass $M\gtrsim 10^{9}M_\odot$? A supercritical bubble (with an inflating baby universe inside it) that nucleated during inflation can develop into a PBH in our observable Universe. Here, we find that when the inflaton slowly passes by a neighboring vacuum, the nucleating rate of supercritical bubbles would inevitably attain a peak, so the mass distribution of multiverse PBHs, and the mass of peak can be up to $M\gtrsim 10^{11}M_\odot$. Thus our mechanism naturally provides a primordial origin of supermassive BHs.

Motivation & Objective

• Motivate the existence of supermassive primordial black holes (PBHs) and their potential connection to nano-Hertz GWs and high-redshift JWST galaxies.
• Introduce a slow-roll inflation model with neighboring vacua that enhances the nucleation rate of supercritical bubbles.
• Demonstrate that the resulting PBH mass spectrum can have peaks up to M ~ 10^11 M_sun and discuss observational implications.

Proposed method

• Model the inflaton dynamics with a two-field potential V(phi1, phi2) where phi1 slow-rolls along V_inf and a neighboring vacuum with V_b < V_inf exists at (phi2, phi1) ~ (phi2,F, phi1,*).
• Use the thin-wall approximation to estimate bubble nucleation rate per Hubble spacetime volume, lambda ~ e^{-B} with B ~ 2π^2 σ / H_i^3 and σ approximated by a path integral along a least-σ path.
• Relate the bubble size to the Hubble scale and expansion to derive PBH mass M ~ H_i r_i^2 M_P^2, leading to M ~ M_P^2/H_i e^{2N} and a mass spectrum f(M) ∝ M^{-1/2} in the simplest case.
• Derive B_* as a baseline and show how inflaton roll (Δφ1) modifies B through B ≈ B_* sqrt(1 + (Δφ1^2/φ2,F^2)).
• Explore how different slow-roll potentials V_inf, including power-law and KKLT-inspired forms, shape the peak structure f(M) and its dependence on N_* and model parameters.
• Provide qualitative discussion linking peak masses to observational hints and the potential multi-peak spectrum in a string landscape.

Experimental results

Research questions

• RQ1 Can supermassive primordial black holes with M ≥ 10^9 M_sun form from primordial processes during inflation?
• RQ2 Does a slow-roll inflation with neighboring vacua produce a peaked multiverse PBH mass spectrum with peaks reaching up to M ~ 10^11 M_sun?
• RQ3 Can the resulting PBH population account for nano-Hertz gravitational wave signals and the existence of early massive JWST galaxies?
• RQ4 How do different inflationary potentials (e.g., V ∝ φ^p, KKLT-type) affect the peak mass and height in the PBH mass spectrum?

Key findings

• A mechanism where inflaton passing near a neighboring vacuum enhances bubble nucleation, yielding a peaked multiverse PBH mass spectrum.
• The peak mass can reach M ≳ 10^{11} M_sun under reasonable slow-roll conditions, naturally providing supermassive PBHs.
• The PBH mass spectrum scales roughly as f(M) ∝ M^{-1/2} in simple scenarios, with the peak position set by N_* and the inflationary potential.
• Different slow-roll models (including chaotic-like φ^p and KKLT-inspired forms) produce distinct multi-peak spectra as shown in the plotted examples.
• The proposed multiverse PBHs could serve as seeds for early supermassive black holes and potentially explain nano-Hertz GW backgrounds and JWST-detected high-redshift massive galaxies.
• The multi-peaks spectrum potentially encodes information about both slow-roll inflation and string vacua.