Primordial black holes

Black holes are intriguing objects. They are both simple and complex. As seen from the outside, they are simple because they only have gravitational interaction and can be characterized by just three quantities: mass, spin, and charge. However, they are complex because we don't know the physics inside the black hole. The inconsistency between the infalling observer and the external observer is the origin of the famous black hole's information paradox.

At the time of writing, we know that there are two types of black holes that exist in nature: stellar-mass black holes and supermassive black holes (SMBHs). Stellar-mass black holes with mass of order a few-100 solar masses could have been formed due to the collapse of massive stars at the end of their lifetime. SMBHs have mass above 100 000 solar masses and are typically located at the center of galaxies. The origin of SMBHs remains unknown, but the conventional wisdom is the "small-to-big" scenario in which many stellar-mass black holes accrete and/or merge to form a SMBH. This process could take billions of years to complete. This standard picture is challenged by the unusual rareness of intermediate-mass black holes and the existence of too early SMBHs observed by the infrared James Webb Space telescope.

There is also a hypothetical kind of objects called "Primordial Black Holes" (PBHs). PBHs were not formed from collapse of dead massive stars, but were instead formed in the very early Universe due to, for example, collapse of sufficiently large cosmological perturbation or collapse of Fermi balls in a strong first-order phase transition. Their mass could span a huge range from 1g to 100 000 solar masses. PBHs have very rich phenomenologies, to name just a few: they could be dark matter (DM) in a certain mass range, their evaporation product could be DM, they can emit gravitons which could form a gravitational waves (GWs) background at very high-frequency, their formation mechanisms could produce the GWs background at lower-frequency, their mergers could also produce a GWs background, they can also produce GWs due to population fluctuation if they temporarily dominates the early Universe, they could be invoked to explain the observed early SMBHs.

I'm interested in exploring the aspects of PBHs such as their formation, accretion, evaporation, mergers, associated GWs signals, and potential role in explaining the early SMBHs.

• Ngo Phuc Duc Loc, "Gravitational waves from burdened primordial black holes dark matter", Phys. Rev. D 111, 023509 (2025)

• Rouzbeh Allahverdi, James B. Dent, Ngo Phuc Duc Loc, and Tao Xu, "Gravitational wave signatures of primordial black hole accretion during early matter domination",  JCAP 10 (2025) 026