Strong scale-dependence does not enhance the kinematic boosting of gravitational wave backgrounds

TL;DR

The paper demonstrates that Doppler boosting does not significantly amplify gravitational wave backgrounds with strong scale dependence, correcting previous approximations that suggested an enhancement.

Contribution

It provides an exact expression for the Doppler boost effect, showing that previous Taylor expansion approximations overestimated the amplification for scale-dependent spectra.

Findings

Doppler boosting causes only red- and blue-shifting, not amplification.

Previous Taylor expansion methods overestimate the boost effect.

Exact treatment shows no additional spectral features from motion.

Abstract

Existing expressions in the literature appear to indicate that Doppler boosting, due to our proper motion with respect to the isotropic frame of the universe, can amplify stochastic gravitational wave backgrounds whose energy spectra exhibit strong scale dependence, for example, those generated by large scalar perturbations in models of primordial black holes or by astrophysical populations with broken power-law behaviour. It has been suggested that this enhancement could increase the signal-to-noise ratio of such backgrounds in pulsar timing measurements, as well as in ground- and space-based observatories. We show that the reported enhancement is an artefact of a Taylor expansion of the boosted signal, typically performed in the literature under the assumption of a small boosting parameter. This approximation fails to reproduce the correct result for signals with strong scale dependence. When Doppler boosting is treated exactly, the apparent amplification disappears. Using representative spectra, we demonstrate that Doppler motion induces only blue- and red-shifting by the expected amount; it does not lead to additional amplification or introduce new spectral features. The exact expression for the kinematic boost can and should be easily applied in analysing such backgrounds.