Limits on scalar-induced gravitational waves from the stochastic background by pulsar timing array observations

Area of Science:

• Cosmology and Astrophysics
• Gravitational Wave Astronomy
• Particle Physics and Early Universe Models

Background:

• Multiple collaborations (NANOGrav, PPTA, EPTA, CPTA) have reported evidence for a Stochastic Gravitational Waves Background (SGWB).
• The observed SGWB characteristics align with predictions from supermassive black-hole binaries (SMBHBs), but new physics explanations remain possible.
• The early universe is a potential source for gravitational waves beyond standard astrophysical phenomena.

Purpose of the Study:

• To investigate scalar-induced gravitational waves (IGWs) from the very early universe as an alternative explanation for the observed SGWB.
• To constrain parameters of IGW energy spectra and associated early universe models.
• To explore implications for inflation models and primordial black hole (PBH) production.

Main Methods:

• Utilized a parameterized broken power-law function to model the SGWB energy spectrum.
• Fitted this model to recent SGWB data from NANOGrav, PPTA, and EPTA.
• Analyzed the constraints derived from the spectral fitting on IGW parameters and inflationary models.

Main Results:

• The parameterized model successfully fits the observed SGWB data.
• The analysis places constraints on the parameters characterizing the IGW energy spectrum.
• Restrictions are imposed on specific inflation models that predict PBH formation.

Conclusions:

• Scalar-induced gravitational waves from the early universe offer a plausible explanation for the observed SGWB.
• This research provides valuable constraints on fundamental physics, including inflation and PBH cosmology.
• The findings complement ongoing and future gravitational wave experiments, such as space-based missions.