Gravitational wave hints black hole remnants as dark matter

Abstract

The end state of Hawking evaporation of a black hole is uncertain. Some candidate quantum gravity theories, such as loop quantum gravity and asymptotic safe gravity, 
hint towards Planck sized remnants. If so, the Universe might be filled with remnants of tiny primordial black holes, which formed with mass $M<10^9\,{\rm g}$. A unique scenario is the case of $M\sim 5\times10^5\,{\rm g}$, where tiny primordial black holes reheat the universe by Hawking evaporation and their remnants dominate the dark matter. Here, we point out that this scenario leads to a cosmological gravitational wave signal at frequencies $\sim 100{\rm Hz}$. Finding such a particular gravitational wave signature with, e.g., the Einstein Telescope, would suggest black hole remnants as dark matter.

Gravitational wave constraints on the primordial black hole dominated early universe

“…Interestingly, we may also be able to find hints of black hole remnants as dark matter [60], if PBH evaporation leaves Planck mass remnants behind as suggested by some quantum gravity theories [309][310][311][312][313] (see also Refs. [314][315][316][317]).…”

“…[314][315][316][317]). If so, there is a unique initial PBH mass, that is M ∼ 5 × 10 5 g, that can reheat the universe and its remnants be the dark matter, with a sharp prediction for the frequency of induced GWs peaked at 100Hz [60].…”

“…PBH remnants after complete evaporation, if any, could also play the role of dark matter[58,59]. See also[60] 4. Solar-mass BHs could also be explained by transmuted BHs from the capture of small PBHs by neutron stars[111] 5.…”

Enhancement of gravitational waves at Q-ball decay including non-linear density perturbations

“…Although the non-resonant contribution is smaller than the resonant contribution around the non-linear scale, k cut [108,109], it will become non-negligible in lower frequency regions. In addition, we can also consider the density perturbation of Q-balls in analogy to the analysis in the PBH evaporation scenario [71,[110][111][112][113][114][115]. In the PBH scenario, such a contribution produces gravitational waves in a higher frequency region [112], which will also be true in our scenario.…”

“…The poltergeist effect has been studied in various contexts such as PBH evaporation [71,78,[109][110][111][112][113][114][115], Q-ball decay [116,117], oscillon decay [118], and axion dynamics [119]. Among them, we studied the poltergeist effect in the Q-ball decay linking to lepton asymmetry generation [120] motivated by the recent determination of the primordial helium abundance [121,122].…”