The rise of the primordial tensor spectrum from an early scalar-tensor epoch

TL;DR

This paper investigates how early scalar-tensor epochs can enhance primordial gravitational wave spectra, introducing a new graphical tool to assess detectability with future GW observatories.

Contribution

It introduces the broken power-law sensitivity curve as a visual tool and analyzes GW spectra for various scalar-tensor models, highlighting their detectability prospects.

Findings

Disformal epochs produce peaked GW spectra with broken power-law profiles.

The sensitivity curve helps determine detectability of different GW spectra.

Certain models show promising detection prospects with upcoming GW detectors.

Abstract

Primordial gravitational waves (PGW) produced during inflation span a large range of frequencies, carrying information on the dynamics of the primordial universe. During an early scalar-tensor dominated epoch, the amplitude of the PGW spectrum can be enhanced over a wide range of frequencies. To study this phenomenon, we focus on a class of scalar-tensor theories, well motivated by high energy theories of dark energy and dark matter, where the scalar is conformally and disformally coupled to matter during the early cosmological evolution. For a conformally dominated epoch, the PGW spectrum has a flat step-like shape. More interestingly, a disformally dominated epoch is characterised by a peaked spectrum with a broken power-law profile, with slopes depending on the scalar-tensor theory considered. We introduce a graphical tool, called broken power-law sensitivity curve, as a convenient visual indicator for understanding whether a given broken power-law profile can be detected by GW experiments. We then analyse the GW spectra for a variety of representative conformal and disformal models, discussing their detectability prospects with the Einstein Telescope (ET), Laser Interferometer Space Antenna (LISA), DECi-hertz Interferometer Gravitational wave Observatory (DECIGO), and Big Bang Observer (BBO).