A Brief Review on Primordial Black Holes as Dark Matter

Primordial black holes (PBHs) represent a natural candidate for one of the components of the dark matter (DM) in the Universe. In this review, we shall discuss the basics of their formation, abundance and signatures. Some of their characteristic signals are examined, such as the emission of particles due to Hawking evaporation and the accretion of the surrounding matter, effects which could leave an impact in the evolution of the Universe and the formation of structures. The most relevant probes capable of constraining their masses and population are discussed.

Hawking evaporation of black holes in massive gravity

We study the Hawking evaporation of a class of black hole solutions in dRGT massive gravity, in which the graviton mass gives rise to an effective negative cosmological constant. We found that the effective emission surface can be either proportional to the square of the effective AdS length scale, or corresponds to the square of the impact parameter of the null geodesic that falls onto the photon orbit of the black hole. Furthermore, depending on the black hole parameters, the emission surface could switch from one to another as the black hole loses mass during the evaporation process. Furthermore, the black holes can either evaporate completely or become a remnant at late time. Our result is more generally applicable to any asymptotically anti-de Sitter-like black hole solution in any theory whose metric function has a term linear in the coordinate radius, with massive gravity being only a concrete example.

Black Holes and Wormholes in Extended Gravity

We discuss black hole type solutions and wormhole type ones in the effective gravity models. Such models appear during the attempts to construct the quantum theory of gravity. The mentioned solutions, being, mostly, the perturbative generalisations of well-known ones in general relativity, carry out additional set of parameters and, therefore could help, for example, in the studying of the last stages of Hawking evaporation, in extracting the possibilities for the experimental or observational search and in helping to constrain by astrophysical data.

Hawking radiation may violate the Penrose cosmic censorship conjecture

We analyze the Hawking evaporation process of Reissner–Nordström black holes. It is shown that the characteristic radiation quanta emitted by the charged black holes may turn near-extremal black-hole spacetimes into horizonless naked singularities. This analysis therefore reveals the intriguing possibility that the semi-classical Hawking evaporation process of black holes may violate the fundamental Penrose cosmic censorship conjecture.

Lower-dimensional corpuscular gravity and the end of black hole evaporation

Black holes in [Formula: see text] spatial dimensions are studied from the perspective of the corpuscular model of gravitation, in which black holes are described as Bose–Einstein condensates (BEC) of (virtual soft) gravitons. In particular, since the energy of these gravitons should increase as the black hole evaporates, eventually approaching the Planck scale, the lower-dimensional cases could provide important insight into the late stages and end of Hawking evaporation. We show that the occupation number of gravitons in the condensate scales holographically in all dimensions as [Formula: see text], where [Formula: see text] is the relevant length for the system in the [Formula: see text]-dimensional spacetime. In particular, this analysis shows that black holes cannot contain more than a few gravitons in [Formula: see text]. Since dimensional reduction is a common feature of many models of quantum gravity, this result can shed light on the end of the Hawking evaporation. We also consider [Formula: see text]-dimensional cosmology in the context of corpuscular gravity and show that the Friedmann equation reproduces the expected holographic scaling as in higher dimensions.

A Status Report on the Phenomenology of Black Holes in Loop Quantum Gravity: Evaporation, Tunneling to White Holes, Dark Matter and Gravitational Waves

The understanding of black holes in loop quantum gravity is becoming increasingly accurate. This review focuses on the possible experimental or observational consequences of the underlying spinfoam structure of space-time. It addresses both the aspects associated with the Hawking evaporation and the ones due to the possible existence of a bounce. Finally, consequences for dark matter and gravitational waves are considered.