Alexander C. Jenkins
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 have recently demonstrated a novel PBH formation mechanism: the collapse of a small segment of cosmic string in the neighbourhood of a cusp (sharp, highly compact features which generically form on cosmic string loops). 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.
Slide 2: What are cosmic strings? Cosmic strings are macroscopic, stable, line-like configurations of quantum fields that are predicted by many extensions to the Standard Model of particle physics. Detecting them would give us unprecedented insights into the laws of nature at extreme energies. What are primordial black holes? By “primordial”, we mean any black hole which is not formed from stellar evolution. There are several exotic processes through which PBHs may have been formed in the early Universe, including the collapse of circular cosmic string loops (shown below). Our cusp-collapse mechanism occurs under much more generic conditions, resulting in many more PBHs than previously thought. The figure on the left shows a numerical simulation of a cosmic string network; "long" strings larger than the Hubble volume are shown in black, while smaller loops are shown in red. The figure on the right shows a cartoon illustration of circular loop collapse, with a perfectly circular cosmic string loop (shown in blue) contracting at an accelerating rate until it is compact enough to form a PBH (shown in grey).
Slide 3: A unique black hole population. We find that cusp-collapse PBHs have
large spins χ = 2/3, with χ = 1 the maximum allowed by general relativity. This is in contrast with other PBH formation mechanisms, which lead to χ ≈ 0 (or in some scenarios, χ ≈ 1). Astrophysical black holes can have spins in this range, but their masses are much larger (M_BH > 5M_sun). This means cusp-collapse PBHs are a "smoking gun" signal of cosmic strings. Cusp-collapse PBHs are also born with velocities near the speed of light, which could lead to interesting observational consequences. The figure shows the locations of various black hole populations in mass-spin parameter space; stellar-mass black holes observed by LIGO/Virgo are shown in blue, supermassive black holes are shown in green, and "generic" PBHs (i.e. those formed through previously-studied mechanisms) are shown in grey and pink. Cusp-collapse PBHs are shown in red, clearly occupying a unique region of the parameter space.
Slide 4: Effects on the gravitational-wave signal. Cusps on cosmic string loops are a promising source of gravitational waves (GWs) for observatories like LIGO/Virgo. Cusp collapse will leave clear imprints on the emitted GW signal, with important implications for LIGO/Virgo searches. Shown on the right is a heuristic model of the GW strain signal. The collapse happens just before the peak of the standard cusp signal. We also expect a high-frequency component from the quasi-normal ringing of the newly-formed PBH horizon. The figure shows two GW signals: the standard cusp signal (shown in blue) is sharply peaked, and symmetric around the peak; the cusp-collapse signal (shown in red) is initially the same as the standard signal, but is truncated just before the peak, eventually culminating in a damped sinusoid corresponding to the PBH ringdown.