Probing the gravitational wave background from cosmic strings with LISA

TL;DR

This paper evaluates LISA's potential to detect a stochastic gravitational wave background from cosmic strings, demonstrating it can significantly improve current constraints and explore various cosmic string models and early universe scenarios.

Contribution

It provides the first detailed analysis of LISA's sensitivity to cosmic string-induced gravitational waves across different theoretical models and cosmic histories.

Findings

LISA can probe cosmic string tensions down to Gμ ≈ 10^{-17}.

LISA's sensitivity surpasses current pulsar timing array constraints by about 6 orders of magnitude.

The study explores modifications in the gravitational wave spectrum due to early universe physics.

Abstract

Cosmic string networks offer one of the best prospects for detection of cosmological gravitational waves (GWs). The combined incoherent GW emission of a large number of string loops leads to a stochastic GW background (SGWB), which encodes the properties of the string network. In this paper we analyze the ability of the Laser Interferometer Space Antenna (LISA) to measure this background, considering leading models of the string networks. We find that LISA will be able to probe cosmic strings with tensions $G\mu \gtrsim \mathcal{O}(10^{-17})$, improving by about $6$ orders of magnitude current pulsar timing arrays (PTA) constraints, and potentially $3$ orders of magnitude with respect to expected constraints from next generation PTA observatories. We include in our analysis possible modifications of the SGWB spectrum due to different hypotheses regarding cosmic history and the underlying physics of the string network. These include possible modifications in the SGWB spectrum due to changes in the number of relativistic degrees of freedom in the early Universe, the presence of a non-standard equation of state before the onset of radiation domination, or changes to the network dynamics due to a string inter-commutation probability less than unity. In the event of a detection, LISA's frequency band is well-positioned to probe such cosmic events. Our results constitute a thorough exploration of the cosmic string science that will be accessible to LISA.