The Influence Of Primordial Black Holes On The Radio Background And 21-cm Overall Spectrum During The Dawn Of The Univers

The 21-cm global spectrum is one of the most important tools for detecting the cosmic Dark Ages and Cosmic Dawn.It contains important information such as the history of the evolution of the universe,the properties of the first luminous objects,and new physics.The strong absorption trough（～-500 mK）corresponding to the Cosmic Dawn（z～17）was found in the 21-cm global spectrum measured by the EDGES experiment in 2018.It has more than two or three times the maximum depth given by the standard model.Moreover,the shape of it is similar to a trough with a flat bottom.Therefore,these have aroused great interest and extensive discussion.In order to explain such an enhanced signal,one consideration is to have an extra cosmic radio background based on the cosmic microwave background（CMB）.It must be supplied by the radiation sources which can emit strong radio signals but a little bit of X-ray for having an effective effect on the early 21-cm signal.So that the intergalactic medium is not heated and ionized up too early by X-ray photon.In my study,I consider the possibility of the primordial black hole（PBH）as such an additional radio source.PBHs are considered to have special effects on the evolution of the universe and the 21-cm spectrum of neutral hydrogen due to their special properties,such as very early birth,large mass and number.I assume that the PBHs have a single mass for Facilitate discussion and not affect research conclusions.I first calculate the X-ray luminosity of a PBH from its accretion rate and calculate the radio luminosity by using the relationship between the radio luminosity and the X-ray luminosity.This relationship is derived from observations of astrophysical black holes.While the generated radio radiation is not sufficient to achieve a greater intensity of the 21-cm absorption signal.I find that the X-ray radiation generated by PBHs is too strong,and the heating and ionization effects are stronger the contribution to the cosmic radio background.However,even so,there is still a reasonable parameter range(the PBHs’massM_.and abundancef_PBH)that can generate sufficient Ly-alpha radiation to bring the 21-cm signal closer to the maximum value（～-200 mK）given by the standard cosmological model.If the PBHs are“naked”（meaning that there is only a uniform IGM around the PBHs）,this range is roughly around log(f_PBH)=-2.5 log（M_./M_⊙）-2.5.If the PBHs are surrounded by a massive dark matter halo which is formed under the influence of the gravitational force of the central PBHs,this range is roughly around log(f_PBH)=-2.4 log（M_./M_⊙）-5.0.Greater mass and abundance mean stronger X-ray heating and ionization,which on the one hand weakens the 21-cm absorption signal,and on the other hand,contradicts the observed thermal history of the universe;the smaller mass and abundance means that it is not sufficient to generate enough Ly-alpha radiation to make the spin temperature of neutral hydrogen_S and gas temperature_k coupling.Next,I use the jet radiation model of a black hole to calculate the radiation of the PBHs.I find that if the PBH’abundancef_PBH（ratio of total PBH mass density to total matter density）and mass satisfy the relation log(f_PBH)～-1.8 log（M_./M_⊙）-3.5 for1M_⊙（?）M_.（?）300M_⊙,they can generate a cosmic radio background required for reproducing the 21-cm absorption signal seen by the EDGES and keep the thermal history of the universe unchanged.The accretion rate can be boosted if the PBHs are surrounded by massive dark matter halos,which permits lowerM_.,f_PBH values to satisfy the EDGES observation.Dark matter halos have lower mass and weaker impact at higher redshift;At lower redshift,the mass increases to a far greater mass than the central PBH,which can have sufficient impact.Therefore,the accretion rate of the PBH decreases with the decrease of the redshift value at a high redshift,while changes to increase with the decrease of the redshift value after a certain redshift.The 21-cm absorption signal can evolve rapidly during the Cosmic Dawn and has an absorption trough shape.the frequency（redshift）and depth of the absorption trough can determine the mass and abundance of the PBHs simultaneously.For absorption trough redshift～17 and depth～-500 mK,it corresponds toM_.～1.05M_⊙andf_PBH～1.5×10^-4.