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.
The discoveries of a number of binary black hole mergers by LIGO and VIRGO has reinvigorated the interest that primordial black holes (PBHs) of tens of solar masses could contribute non-negligibly to the dark matter energy density. Should even a small population of PBHs with masses O(M ) exist, they could profoundly impact the properties of the intergalactic medium and provide insight into novel processes at work in the early Universe. We demonstrate here that observations of the 21cm transition in neutral hydrogen during the epochs of reionization and cosmic dawn will likely provide one of the most stringent tests of solar mass PBHs. In the context of 21cm cosmology, PBHs give rise to three distinct observable effects: (i) the modification to the primordial power spectrum (and thus also the halo mass function) induced by Poisson noise, (ii) a uniform heating and ionization of the intergalactic medium via X-rays produced during accretion, and (iii) a local modification to the temperature and density of the ambient medium surrounding isolated PBHs. Using a four-parameter astrophysical model, we show that experiments like SKA and HERA could potentially improve upon existing constraints derived using observations of the cosmic microwave background by more than one order of magnitude. I. INTRODUCTIONThe canonical cosmological model assumes that dark matter is comprised of a cold gas of weakly interacting particles. Despite its simplicity, this minimal scenario provides an excellent fit to both cosmic microwave background (CMB) and large-scale structure measurements [1][2][3][4]. However, a precise understanding of the fundamental nature of dark matter is missing and remains at the forefront in the current list of unsolved problems in modern physics.Although dark matter is usually interpreted in terms of a new elementary particle, other alternatives exist. Black holes (BHs) produced prior to big bang nucleosynthesis (BBN), i.e., primordial black holes (PBHs), represent such an alternative -remarkably, this solution is as old as particle dark matter [5]. In particular, this scenario has recently attracted much attention [6][7][8][9][10][11][12] in the context of the LIGO and VIRGO discoveries of several binary BH mergers [13][14][15][16][17].The idea that PBHs could be formed by strong accretion during the radiation-dominated epoch was first introduced five decades ago [18]. It was later suggested that initial fluctuations in the early Universe could give rise to a large number of gravitationally collapsed objects with masses above the Planck mass [19], that however, would not grow substantially by accretion [20]. Very light PBHs would not survive until the present epoch, though; the prediction that any BH should emit particles with a black body spectrum [21,22] implies that PBHs with masses below 10 −18 M (with M the Sun's mass) would have evaporated on cosmological times scales [21][22][23]. While the evaporation of such light PBHs would have interesting observational consequences [24][25][26], only heavier PBHs coul...
The elastic scattering between dark matter particles and radiation represents an attractive possibility to solve a number of discrepancies between observations and standard cold dark matter predictions, as the induced collisional damping would imply a suppression of small-scale structures. We consider this scenario and confront it with measurements of the ionization history of the Universe at several redshifts and with recent estimates of the counts of Milky Way satellite galaxies. We derive a conservative upper bound on the dark matterphoton elastic scattering cross section of σ γDM < 8×10 −10 σ T (m DM /GeV) at 95% CL, about one order of magnitude tighter than previous constraints from satellite number counts. Due to the strong degeneracies with astrophysical parameters, the bound on the dark matter-photon scattering cross section derived here is driven by the estimate of the number of Milky Way satellite galaxies. Finally, we also argue that future 21 cm probes could help in disentangling among possible non-cold dark matter candidates, such as interacting and warm dark matter scenarios. Let us emphasize that bounds of similar magnitude to the ones obtained here could be also derived for models with dark matter-neutrino interactions and would be as constraining as the tightest limits on such scenarios.
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