Cosmic microwave background limits on accreting primordial black holes

Abstract

Interest in the idea that primordial black holes (PBHs) might comprise some or all of the dark matter has recently been rekindled following LIGO's first direct detection of a binary-black-hole merger. Here we revisit the effect of accreting PBHs on the cosmic microwave background (CMB) frequency spectrum and angular temperature/polarization power spectra. We compute the accretion rate and luminosity of PBHs, accounting for their suppression by Compton drag and Compton cooling by CMB photons. We estimate the gas temperature near the Schwarzschild radius, and hence the free-free luminosity, accounting for the cooling resulting from collisional ionization when the background gas is mostly neutral. We account approximately for the velocities of PBHs with respect to the background gas. We provide a simple analytic estimate of the efficiency of energy deposition in the plasma. We find that the spectral distortions generated by accreting PBHs are too small to be detected by FIRAS, as well as by future experiments now being considered. We analyze Planck CMB temperature and polarization data and find, under our most conservative hypotheses, and at the order-of-magnitude level, that they rule out PBHs with masses 10 2 M as the dominant component of dark matter.

Constraining primordial black holes with the EDGES 21-cm absorption signal

“…5 corresponds to the collisional ionization case in Ref. [42], which is subdominant with respect to the constraint given, for instrance, in Ref. [43], that assumes a different accretion model.…”

“…The yellow, purple and light blue regions are excluded by the microlensing results from EROS [58] and MACHO (M) [59], and by lack of lensing signatures in type Ia supernovae (SNe) [60], respectively. The dark blue, orange, red and green regions are ruled out by Planck data [42], survival of stars in Segue I (Seg I) [61] and Eridanus II (Eri II) [62], and the distribution of wide binaries (WB) [63], respectively. The gray region (LIGO) is excluded by the nonobservation of the stochastic GW background [56].…”

Cosmic microwave background bounds on primordial black holes

“…Pioneering and extremely stringent bounds obtained a decade ago [5] have been shown to be incorrect in Ref. [6]. In a sufficiently simple, spherical accretion flow model, Ref.…”

“…In [7], we have recently argued that the situation considered in [6] is too idealized, and that in a physically plausible framework, stellar mass PBH form accretion disks. Despite lower accretion rates, these configurations are characterized by a significantly larger luminosity than what appropriate for the spherical models of [6]. This results into more stringent bounds, reported in Fig.…”

“…In a sufficiently simple, spherical accretion flow model, Ref. [6] derived the very conservative bound f PBH < 1 for M > 10 − 100 M , the exact value still depending if the ionizing energy of the accreting material is provided by the photons escaping from the hot interior or via collisional effects in the gas. In [7], we have recently argued that the situation considered in [6] is too idealized, and that in a physically plausible framework, stellar mass PBH form accretion disks.…”