Constraints on the cosmic string loop collapse fraction from Primordial Black Holes

Abstract

A small fraction, $f$, of cosmic string loops can collapse to form Primordial Black Holes (PBHs). Constraints on the abundance of PBHs can therefore be used to constrain $f$. We update these calculations, taking into account the PBH extended mass function, and find $f<10^{-31}(G\mu /c^2{)}^{-3/2}$. This is roughly two orders of magnitude tighter than previous constraints. The improvement from the tighter constraints on the abundance of PBHs is partly off-set by refinements to the theoretical calculation of the cosmic string loop formation rate.

Comments: 5 pages, v2: takes into account only a fraction of string network energy goes into long loops, constraints consequently weakened by one order of magnitude (thank you to referee for pointing this out) + other minor changes