Primordial black holes from cusp collapse on cosmic strings

Abstract

Primordial black holes (PBHs) are of fundamental interest in cosmology and astrophysics, and have received much attention as a dark matter candidate and as a potential source of gravitational waves. One possible PBH formation mechanism is the gravitational collapse of cosmic strings. Thus far, the entirety of the literature on PBH production from cosmic strings has focused on the collapse of (quasi)circular cosmic string loops, which make up only a tiny fraction of the cosmic loop population. We demonstrate here a novel PBH formation mechanism: the collapse of a small segment of cosmic string in the neighbourhood of a cusp. Using the hoop conjecture, we show that collapse is inevitable whenever a cusp appears on a macroscopically-large loop, forming a PBH whose mass is smaller than that of the loop by a factor of the dimensionless string tension Gµ. Since cusps are generic features of cosmic string loops, and do not rely on finely-tuned loop configurations like circular collapse, this implies that cosmic strings produce PBHs in far greater numbers than has previously been recognised. The resulting PBHs are highly spinning and boosted to ultrarelativistic velocities; they populate a unique region of the BH mass-spin parameter space, and are therefore a "smoking gun" observational signature of cosmic strings. We derive stringent new constraints on Gµ from the evaporation of cusp-collapse PBHs, and update existing constraints on Gµ from gravitational-wave searches.

Quantum gravity phenomenology at the dawn of the multi-messenger era -- A review

“…Several mechanisms could have led to the formation of primordial black holes in the early universe: the gravitational collapse of large inhomogeneities [780][781][782][783][784] produced during inflation [785,786] or phase transitions [787], scalar field fragmentation [788][789][790], vacuum bubbles [791,792] or the collapse of topological defects like cosmic strings [793,794]. They can constitute from a negligible fraction to the totality of dark matter.…”

Can we distinguish astrophysical from primordial black holes via the stochastic gravitational wave background?

“…4, which indicates that accurate measurement of the parameter α, and constraints on the time-delay parameter t ef f d , characteristic mass scale of PBHs Mc, and the fraction of PBH/ABH f P BH/ABH should be feasible at O5 (Abbott et al 2016a) and A+ (Abbott et al 2018a;Barsotti et al 2020) sensitivities. By using the posteriors on the PBH population parameters such as α, Mc and f P BH/ABH , one can explore different models of PBH formation scenarios and can constrain the parameter space of those models (Zel'dovich & Novikov 1967;Hawking 1971;Carr 1975;Khlopov & Polnarev 1980;Khlopov et al 1985;Carr 2005;Clesse & García-Bellido 2017;Ali-Haïmoud et al 2017;Raidal et al 2017Raidal et al , 2019Sasaki et al 2018;Jenkins & Sakellariadou 2020;Atal et al 2020).…”

Extreme-Value Statistics of the Spin of Primordial Black Holes

“…For instance, accretion and merger can increase the fraction of black holes at the tail of the spin distribution [35,36], and thus generally driving up the skewness and kurtosis. Fattening of the tail of the spin pdf can also be achieved by more exotic PBH formation mechanisms such as scalar-field fragmentation [37] and formation of PBHs through the collapse of cosmic-string cusps [38]. We leave a more qualitative investigation of such effects for future work.…”