Induced gravitational waves from flipped SU(5) superstring theory at $\mathrm{nHz}$

TL;DR

This paper predicts a unique gravitational wave signal in the nHz range from flipped SU(5) superstring cosmology, arising during a transition from an early matter era to radiation domination, potentially detectable by pulsar timing arrays.

Contribution

It is the first to connect flipped SU(5) superstring theory with a specific induced gravitational wave signal in the nHz range, linked to early matter era dynamics.

Findings

GW peak frequency depends on string slope and is in the nHz range.

The predicted GW signal is within detection capabilities of SKA, NANOGrav, and PTA.

The model links string theory parameters to observable cosmological signals.

Abstract

The no-scale flipped SU(5) superstring framework constitutes a very promising paradigm for physics below the Planck scale providing us with a very rich cosmological phenomenology in accordance with observations. In particular, it can accommodate Starobinsky-like inflation, followed by a reheating phase, which is driven by a light "flaton" field, and during which the GUT phase transition occurs. In this Letter, we extract for the first time a gravitational-wave (GW) signal which naturally arises in the context of the flipped SU(5) cosmological phenomenology and is related to the existence of an early matter era (eMD) driven by the flaton field. Specifically, we study GWs non-linearly induced by inflationary perturbations and which are abundantly produced during a sudden transition from the flaton-driven eMD era to the late-time radiation-dominated era. Remarkably, we find a GW signal with a characteristic peak frequency $f_\mathrm{GW,peak}$ depending only on the string slope $\alpha'$ and reading as $f_\mathrm{GW,peak} \propto 10^{-9} \left(\frac{\alpha'}{\alpha'_*}\right)^4 \mathrm{Hz}$, where $\alpha'_*$ is the fiducial string slope being related directly to the reduced Planck scale $M_\mathrm{Pl}$ as $\alpha'_* = 8/M^2_\mathrm{Pl}$. Interestingly enough, $f_\mathrm{GW,peak}$ lies within the $\mathrm{nHz}$ frequency range; hence rendering this primordial GW signal potentially detectable by SKA, NANOGrav and PTA probes at their very low frequency region of their detection bands.