Primordial gravitational waves from NANOGrav: a broken power-law approach

TL;DR

This paper explores a broken power-law model for primordial gravitational waves to reconcile NANOGrav's detection with existing cosmological constraints, offering a flexible framework that aligns with multiple observational bounds.

Contribution

It introduces a broken power-law parametrization for the SGWB spectrum, moving beyond the simple power-law assumption to better fit observational data across a wide frequency range.

Findings

The broken power-law model can explain NANOGrav data without violating CMB and LIGO/Virgo constraints.

The model remains consistent with BBN and interferometer upper limits.

Next-generation GW detectors can test this phenomenological scenario.

Abstract

We revisit the possibility that the stochastic common-spectrum process recently detected by the NANOGrav pulsar timing array experiment could be due to primordial gravitational waves (GWs). A na\"{i}ve extrapolation down to interferometer scales of the blue GW spectrum required to explain NANOGrav consistently with Cosmic Microwave Background (CMB) observations would strongly violate upper limits on the stochastic GW background (SGWB) amplitude from LIGO/Virgo. In combination with the fact that there are over 19 decades in frequency between CMB and interferometer scales, this motivates us to move beyond the commonly adopted approximation of a pure power-law GW spectrum. We consider a broken power-law parametrization for the SGWB spectrum, which turns from blue to red above the break frequency: while phenomenological, this choice maps to various well-motivated early-Universe models, including scenarios featuring non-instantaneous reheating or a non-standard background expansion following reheating. After a detailed discussion of the contribution of the resulting SGWB to the early-Universe radiation energy density, we constrain the broken power-law model against a wide variety of multi-frequency cosmological and GW observations. We find that this phenomenological model is able to explain the NANOGrav signal while remaining in agreement with upper limits on the tensor-to-scalar ratio on CMB scales, Big Bang Nucleosynthesis constraints on the early-Universe radiation energy density, and upper limits on the SGWB amplitude on interferometer scales. We briefly discuss the very bright prospects for testing this model with next-generation probes across the GW frequency landscape, which motivate further exploring connections to specific well-motivated early-Universe models.