Gravitational waves from a universe filled with primordial black holes

• Published in: Journal of cosmology and astroparticle physics Vol. 2021; no. 3; p. 53
• Main Authors: Papanikolaou, Theodoros, Vennin, Vincent, Langlois, David
• Format: Journal Article
• Language: English
• Published: Bristol IOP Publishing 01.03.2021; Institute of Physics (IOP)
• Subjects: Astrophysics; Big bang cosmology; Black holes; Constraints; Evaporation; General Relativity and Quantum Cosmology; Gravitational effects; Gravitational waves; Heating; High Energy Physics - Theory; Nuclear fusion; Nuclei (nuclear physics); Physics; Relative abundance; Universe; Waves; gravitational radiation: emission; power spectrum; gauge; reheating; energy: density; black hole: mass; gravitational radiation: induced; tensor: energy-momentum; black hole: formation; black hole: evaporation; space-time: fluctuation; gravitational waves / theory; inflation; perturbation: scalar; nucleosynthesis: big bang; formation: time; primordial black holes; back reaction; matter: power spectrum; physics of the early universe; evaporation; power spectrum: tensor; black hole: primordial
• ISSN: 1475-7516; 1475-7508; 1475-7516
• DOI: 10.1088/1475-7516/2021/03/053

Ultra-light primordial black holes, with masses m PBH < 10 9 g, evaporate before big-bang nucleosynthesis and can therefore not be directly constrained. They can however be so abundant that they dominate the universe content for a transient period (before reheating the universe via Hawking evaporation). If this happens, they support large cosmological fluctuations at small scales, which in turn induce the production of gravitational waves through second-order effects. Contrary to the primordial black holes, those gravitational waves survive after evaporation, and can therefore be used to constrain such scenarios. In this work, we show that for induced gravitational waves not to lead to a backreaction problem, the relative abundance of black holes at formation, denoted Ω PBH,f , should be such that Ω PBH,f < 10 -4 ( m PBH /10 9 g) -1/4 . In particular, scenarios where primordial black holes dominate right upon their formation time are all excluded (given that m PBH > 10 g for inflation to proceed at ρ 1/4 < 10 16 GeV). This sets the first constraints on ultra-light primordial black holes.