Direct determination of cosmic string loop density from simulations

TL;DR

This paper directly measures cosmic string loop distributions from large-scale simulations, confirming theoretical power-law behaviors and challenging models predicting small-scale divergence, with implications for gravitational wave detection.

Contribution

It introduces a direct simulation-based method for determining cosmic string loop densities, extending previous work with longer durations and larger volumes, and clarifies the transient nature of loop distribution exponents.

Findings

Loop distribution follows a power law with exponent -2.5 in radiation era.

Exponents decline over time, indicating transient regimes.

Results challenge models with rapid small-scale divergence.

Abstract

We determine the distribution of cosmic string loops directly from simulations, rather than determining the loop production function and inferring the loop distribution from that. For a wide range of loop lengths, the results agree well with a power law exponent -2.5 in the radiation era and -2 in the matter era, the universal result for any loop production function that does not diverge at small scales. Our results extend those of Ringeval, Sakellariadou, and Bouchet: we are able to run for 15 times longer in conformal time and simulate a volume 300-2400 times larger. At the times they reached, our simulation is in general agreement with the more negative exponents they found, -2.6 and -2.4. However, our simulations show that this was a transient regime; at later times the exponents decline to the values above. This provides further evidence against models with a rapid divergence of the loop density at small scales, such as ``model 3'' used to analyze LIGO data and predict LISA sensitivity.