Illuminating the dark ages: Cosmic backgrounds from accretion onto primordial black hole dark matter

Abstract

The recent interpretation of cold dark matter as the sum of contributions of different mass Primordial Black Hole (PBH) families [1] could explain a number of so far unsolved mysteries, like e.g. the massive seed black holes required to create the supermassive black holes in the earliest QSOs, the ubiquitous massive LIGO/VIRGO massive binary black holes, or even the putative "Planet X" PBH in our Solar System. By far the most abundant family of PBH should be around the Chandrasekhar mass (1.4 M ). This prediction may already have been vindicated by the recent OGLE/GAIA discovery of a sizeable population of putative black holes in the mass range 1-10 M .Here I assume that all dark matter exists in the form of 1.4 M PBH and estimate the contribution of baryon accretion onto this population to various cosmic background radiations, concentrating first on the cross-correlation signal between the Cosmic X-ray and the Cosmic infrared background fluctuations discovered in deep Chandra and Spitzer surveys. I assume Bondi accretion with reasonable parameters like baryon density and the effective relative velocity between baryons and PBH as well as appropriate accretion and radiation efficiencies and integrate these over cosmic time. I indeed predict a contribution of PBH accretion to the X-ray background consistent with the residual X-ray fluctuation signal. This signal peaks at redshifts z≈17-30, consistent with other constraints requiring the signal to come from such high redshifts. The PBH contribution to the 2-5 µm Cosmic Infrared background fluctuations is only about 1%, so that these likely come from star formation processes in regions associated with the PBH.I discuss a number of other phenomena, which could be significantly affected by the PBH accretion. Magnetic fields are an essential ingredient in the Bondi accretion process, and argue that the PBH can play an important role in amplifying magnetic seed fields in the early universe and maintaining them until the galactic dynamo processes set in. Next I study the contribution of the assumed PBH population to the re-ionization history of the universe and find that they do not conflict with the stringent ionization limits set by the most recent Planck measurements. X-ray heating from the PBH population can provide a contribution to the entropy floor observed in groups of galaxies. The tantalizing redshifted 21-cm absorption line feature observed by em EDGES could be connected to the radio emission contributed by PBH to the cosmic background radiation. Finally, the diffuse X-ray emission in the Galactic Center region is not violated by the assumed PBH dark matter, on the contrary, some of the discrete sources resolved in the deepest Chandra observations of the Galactic Ridge could indeed be accreting PBH. Contents 1 Introduction 1 2 Basic parameters of the toy model 3 3 Accretion spectrum and radiative efficiency 6 4 The contribution of PBH to the extragalactic background light 8 5 Discussion 11 5.1 Linear versus post-linear growth 11 5.2 Magnetic fields in the early...

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