Whispers from the dark side: Confronting light new physics with NANOGrav data (2009.11875v1)

Abstract: The NANOGrav collaboration has recently observed first evidence of a gravitational wave background (GWB) in pulsar timing data. Here we explore the possibility that this GWB is due to new physics, and show that the signal can be well fit also with peaked spectra like the ones expected from phase transitions (PTs) or from the dynamics of axion like particles (ALPs) in the early universe. We find that a good fit to the data is obtained for a very strong PT at temperatures around 1 MeV to 10 MeV. For the ALP explanation the best fit is obtained for a decay constant of $F \approx 5\times 10^{17}$ GeV and an axion mass of $2\times 10^{-13}$ eV. We also illustrate the ability of PTAs to constrain the parameter space of these models, and obtain limits which are already comparable to other cosmological bounds.

• The paper demonstrates that NANOGrav data can be explained by models of cosmic phase transitions and axion-like particle dynamics, suggesting new physics.
• It employs detailed spectral fits using parameters such as vacuum energy ratio and transition timescale to constrain dark sector physics.
• The study links astrophysical observations with particle models by evaluating axion mass and decay constant, providing actionable insights for future experiments.

An Expert Analysis of "Whispers from the Dark Side: Confronting Light New Physics with NANOGrav Data"

The exploration of gravitational wave backgrounds (GWBs) has been significantly advanced by the recent data from the North American Nanohertz Observatory for Gravitational Waves (NANOGrav). Ratzinger and Schwaller investigate the potential of attributing this gravitational wave signal to new physics scenarios beyond the Standard Model, namely through processes such as cosmic phase transitions (PTs) and the dynamics of axion-like particles (ALPs).

The paper revisits the NANOGrav data, which suggest the presence of a stochastic GWB consistent with the power-law spectrum. The analysis departs from traditional sources, proposing that the observed GWB could be an indication of phenomena like PTs or radiated energy from ALP dynamics.

Phase Transitions as a Source of Gravitational Waves

Phase transitions in the early universe, particularly strong first-order transitions, are well-known to produce detectable gravitational waves. The authors propose that the NANOGrav data could be consistent with such a PT. They perform a fit to the NANOGrav frequency spectrum, focusing on the scenarios of phase transitions with varying degrees of vacuum energy contribution and duration. Particularly, they identify conditions where the phase transition occurs at temperatures between 1-10 MeV, which is notably low, necessitating involvement from a dark sector with strongly suppressed interactions with the Standard Model.

The theoretical framework utilizes parameters such as the energy density ratio $\alpha$ and the transition timescale $\beta/H$ to describe the potential of detectable gravitational wave emissions. The PTs providing a strong astronomical signal fit well within known constraints, suggesting new avenues for probing dark sector dynamics.

Audible Axions and Gravitational Waves

The consideration of axion-like particles introduces another captivating dimension. ALPs interacting with dark photons can give rise to gravitational waves through tachyonic instabilities during the axion decay process. This paper identifies a potential parameter space that matches the NANOGrav findings, identifying that an axion mass around $2 \times 10^{-13}$ eV with a decay constant of approximately $5\times 10^{17}$ GeV offers the best-fit scenario.

The authors assert that such axion models fall under scrutiny from cosmological constraints such as $N_{\rm eff}$, which bounds the relativistic degrees of freedom during the Big Bang nucleosynthesis epoch. The implications of these models extend beyond providing a match to the NANOGrav data; they may also intersect with ongoing experimental efforts, such as the CASPEr-wind experiment and potential signals in black hole superradiance.

Constraints and Future Directions

By confronting these models with the data, the authors do not just propose alternative explanations but also derive constraints on possible dark-sector physics. Specifically, the requirements for the phase transition to not disrupt established cosmological processes like BBN and the relic density limits on axion theories are crucial constraints.

In summary, the paper by Ratzinger and Schwaller takes significant steps toward integrating potential new physics explanations for observed astrophysical phenomena, offering profound insights into gravitational wave science's future. The exploration of these phenomena provides not only constraints but also an essential foundation for future experimental and theoretical advancements in understanding the early universe's hidden dynamics. This work sets the stage for future studies to further refine and test these alternative models in light of growing Gravitational Wave (GW) datasets, possibly differentiating them from known astrophysical sources such as supermassive black hole binaries.