Primordial gravitational waves in the nano-Hertz regime and PTA data — towards solving the GW inverse problem

Abstract

In recent years, several pulsar timing array collaborations have reported first hints for a stochastic gravitational wave background at nano-Hertz frequencies. Here we elaborate on the possibility that this signal comes from new physics that leads to the generation of a primordial stochastic gravitational wave background. We propose a set of simple but concrete models that can serve as benchmarks for gravitational waves sourced by cosmological phase transitions, domain wall networks, cosmic strings, axion dynamics, or large scalar fluctuations. These models are then confronted with pulsar timing data and with cosmological constraints. With only a limited number of free parameters per model, we are able to identify viable regions of parameter space and also make predictions for future astrophysical and laboratory tests that can help with model identification and discrimination.

Probing the equation of state of the early Universe with pulsar timing arrays

“…Recently, four major pulsar timing array (PTA) collaborations, namely the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) [1,2], the Parkes PTA (PPTA) [3,4], the European PTA (EPTA) along with the Indian PTA (InPTA) [5,6], and the Chinese PTA (CPTA) [7], have jointly announced compelling evidence of a commonspectrum signal consistent with the Hellings-Downs spatial correlations [8] in their latest data sets, thus suggesting a gravitational wave (GW) origin for this signal. While various physical phenomena [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] can produce GW signals within the PTA frequency band, the precise origin of this signal, whether attributed to supermassive black hole binaries [24][25][26][27] or other cosmological sources , remains an ongoing subject of investigation.…”

Metastable cosmic strings

“…Beyond the JCAP11(2023)020 astrophysical interpretation in terms of inspiraling supermassive black-hole binaries [27][28][29], possible cosmological interpretations include stable and metastable cosmic strings [30][31][32][33][34][35] (see, e.g., refs. [29,36,37] for an overview of possible cosmological signals). However, the originally favoured GUT-scale strings with a tension in the range Gµ ≃ 10 −(8•• •6) are firmly excluded by these results, as they would lead to too large an SGWB signal in the PTA band.…”

Probing stochastic gravitational wave background from SU(5) × U(1)χ strings in light of NANOGrav 15-year data

“…Likewise, there are other potential explanations that account for the Nanograv 2015 data see refs. [26,27] The process of breaking the SU(5) symmetry can lead to an undesired consequence: the generation of magnetic monopoles. The presence of monopoles is incompatible with observations and poses a challenge to the model.…”