• Primordial black holes
• dark matter
• 21 cm
• radio background

The discovery of gravitational waves from black hole mergers detected by the LIGO/VIRGO interferometers have been brought renew interest to the field of primordial black holes (PBHs). These objects have many fields of application.
They can be used to probe primordial inhomogeneities, the properties of the early Universe and they are considered possible non-baryonic Dark Matter candidates.
PBHs are formed by gravitational collapse of overdense regions in the early Universe, during the radiation dominated era.
The presence of a population of Black Holes accreting at high redshift could be used to investigate unclear results obtained by observational experiments such as EDGES or ARCADE2. These results revealed, respectively, an anomalous amplitude in the 21cmabsorption signal at z = 17 and an extragalactic radio excess over the CMB at redshift z = 0. In this thesis we model the accretion of PBHs in dark matter halos to compute their radio emission, with the final aim to understand whether the EDGES and ARCADE2 results can be explained by a radio background of PBHs.
To compute the radio emission from PBHs we proceed as follows. Given a dark matter halo of mass M_H at a given redshift, we first compute the expected number of PBHs for given value of f_DM (namely the fraction of dark matter that can be explained through PBHs) and PBHs mass (we assume M = 30 M_sun). For each PBH, we compute its accretion rate considering the Bondi-Hoyle Littleton prescription according to which the accretion rate depends on the gas density distribution and the sound speed in the gas surrounding the black hole. Given the accretion rate, the bolometric luminosity (Lbol epsilon ˙M c2, where epsilon is the radiative efficiency) can be converted into an X-Ray luminosity through bolometric corrections (L_X = 0.3 Lbol ). Finally, we adopt the Fundamental Plane relation to compute the radio emission starting from the X-Ray luminosity.
The main results of our work are the following:
• for fDM = 0.05 ( fDM = 1) we find that the expected radio luminosity of
DM halos with 10^8 < M_H/M_sun < 10^10 at redshift z = 10 is 10^32 < L_rad/erg/s < 10^36
(10^32 < L_rad/ergs < 10^36);
• At z = 17 our model predicts a radio background intensity from accreting
PBHs (4x10^(-21) < I21,PBH < 9x10^(-18)[erg/scm^2srHz]) depending on the
choice of parameters ( f_DM, epsilon);
• By taking into account the radio background expected from accreting
PBHs, we find that these sources can only explain <=30% of the EDGES
signal.
We discuss possible improvements of the model that can be further constrained through ARCADE2 data.