Looking out for the Galileon in the nanohertz gravitational wave sky

TL;DR

This paper investigates how the Galileon scalar field could produce detectable gravitational wave polarizations in pulsar timing arrays, using NANOGrav data to constrain its properties and potential as a source of observed correlations.

Contribution

It introduces a novel analysis of Galileon-induced gravitational wave polarizations in pulsar timing data and constrains the effective mass of the Galileon field based on observations.

Findings

Longitudinal spatial correlation consistent with a $10^{-22}$ eV Galileon

Galileon polarizations could explain observed pulsar correlations

Constraints on the Galileon effective mass from NANOGrav data

Abstract

We study the polarizations induced by the Galileon as a stochastic gravitational wave background in the cross correlated power in a pulsar timing array. Working within Galileon gravity, we first show that the scalar gravitational wave signature of the Galileon is encoded solely in its effective mass, which is controlled by the bare mass, conformal coupling, and a tadpole. Then, we study the phenomenology of the Galileon induced scalar polarizations and place observational constraints on these using the present NANOGrav data set. Our results feature longitudinal spatial correlation, indicative of a $10^{-22}$ eV {subluminal} Galileon, and show the Galileon polarizations as a statistically compelling source of the observed spatial correlation across millisecond pulsars, if there is any.