New limits on cosmic strings from gravitational wave observation

TL;DR

This paper sets new upper bounds on cosmic string energy scales using gravitational wave data, discusses detection prospects, and highlights challenges in distinguishing signals from black hole binaries.

Contribution

It provides the most stringent current limits on cosmic string parameters from gravitational wave observations and analyzes future detection prospects across different observatories.

Findings

Upper bound on Gμ: < 1.5×10^{-11} at 95% confidence

PTA detection of gravitational waves may be confused with black hole signals

Future observatories like LISA and BBO are crucial for probing lower Gμ values

Abstract

We combine new analysis of the stochastic gravitational wave background to be expected from cosmic strings with the latest pulsar timing array (PTA) limits to give an upper bound on the energy scale of the possible cosmic string network, $G\mu < 1.5\times 10^{-11}$ at the 95% confidence level. We also show bounds from LIGO and to be expected from LISA and BBO. Current estimates for the gravitational wave background from supermassive black hole binaries are at the level where a PTA detection is expected. But if PTAs do observe a background soon, it will be difficult in the short term to distinguish black holes from cosmic strings as the source, because the spectral indices from the two sources happen to be quite similar. If PTAs do not observe a background, then the limits on $G\mu$ will improve somewhat, but a string network with $G\mu$ substantially below $10^{-11}$ will produce gravitational waves primarily at frequencies too high for PTA observation, so significant further progress will depend on intermediate-frequency observatories such as LISA, DECIGO and BBO.