Abstract
The EDGES experiment has recently measured an anomalous global 21-cm spectrum due to hydrogen absorptions at redshifts of about $z\sim 17$. Model independently, the unusually low temperature of baryons probed by this observable sets strong constraints on any physical process that transfers energy into the baryonic environment at such redshifts. Here we make use of the 21-cm spectrum to derive bounds on the energy injection due to a possible population of ${\cal O}(1-100) M_\odot$ primordial black holes, which induce a wide spectrum of radiation during the accretion of the surrounding gas. After calculating the total radiative intensity of a primordial black hole population, we estimate the amount of heat and ionisations produced in the baryonic gas and compute the resulting thermal history of the Universe with a modified version of RECFAST code. Finally, by imposing that the temperature of the gas at $z\sim 17$ does not exceed the indications of EDGES, we constrain the possible abundance of primordial black holes. Depending on uncertainties related to the accretion model, we find that ${\cal O}(10) M_\odot$ primordial black holes can only contribute to a fraction $f_{\rm PBH}<(1-10^{-3})$ of the total dark matter abundance.
Highlights
The experiment to detect the Global EoR Signature (EDGES) has recently found an anomalously strong absorption in the 21-cm spectrum by the baryonic gas at redshifts in the range z ≈ 15–21 [1]
Depending on uncertainties related to the accretion model, we find that Oð10ÞM⊙ primordial black holes can only contribute to a fraction fPBH < ð1–10−3Þ of the total dark matter abundance
Because the intensity of the detected signal is proportional to I21 cm ∝ 1 − ðTRðzÞ=TSðzÞÞ, where TS is the spin temperature of the atomic hydrogen and TR is the temperature of background radiation, the very first studies of the EDGES anomaly ascribed the extra absorption to a new cooling mechanism for the hydrogen gas, based on baryon-dark matter (DM) interactions [2,3,4,5,6,7], on modified onset of star formation [11,12], or on the effects of dark energy [13,14]
Summary
The experiment to detect the Global EoR Signature (EDGES) has recently found an anomalously strong absorption in the 21-cm spectrum by the baryonic gas at redshifts in the range z ≈ 15–21 [1]. In spite of the mechanism responsible for its generation, the EDGES signal can be used to constrain all new physics scenarios that result in energy injection into the baryonic environment, thereby heating the hydrogen clouds responsible for the absorption signal. This observation was used for example in [16] to constrain DM annihilations. We adopt the same attitude to analyze the energy injection due to a population of Oð1–100ÞM⊙ primordial black holes (PBHs), with the aim of constraining the possible PBH DM abundance To this purpose, we disregard other sources of radiation, neglecting for instance all the astrophysical processes ongoing in the early Universe. In the Appendix, we calculate approximate CMB bounds for the considered parametrized accretion model, and compare them to the results obtained from the 21-cm spectrum
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