http://arxiv.org/abs/1710.06945
If primordial black holes (PBHs) form directly from inhomogeneities in the early universe, then the number in the mass range $10^6 -10^9M_{\odot}$ is severely constrained by upper limits to the $\mu$-distortion in the cosmic microwave background (CMB). This is because inhomogeneities on these scales will be dissipated by Silk damping in the redshift interval $5\times 10^4\lesssim z\lesssim2\times 10^6$. If the primordial fluctuations on a given mass-scale have a Gaussian distribution and PBHs form on the high-$\sigma$ tail, as in the simplest scenarios, then the $\mu$ constraints exclude PBHs in this mass range from playing any interesting cosmological role. Only if the fluctuations are highly non-Gaussian, or form through some mechanism unrelated to the primordial fluctuations, can this conclusion be obviated.
T. Nakama, B. Carr and J. Silk
Fri, 20 Oct 17
12/42
Comments: 10 pages, 5 figures
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