[Paper Review] The NANOGrav Program for Gravitational Waves and Fundamental Physics

Adam Brazier, Shami Chatterjee+12 more|arXiv (Cornell University)|Aug 14, 2019

Pulsars and Gravitational Waves ResearchPhysics and Astronomy65 references42 citations

TL;DR

NANOGrav uses precision timing of Galactic millisecond pulsars to search for nanohertz gravitational waves, aiming to detect a stochastic background, resolve individual supermassive binary black holes, and probe fundamental physics.

ABSTRACT

We describe the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) and its efforts to directly detect and study gravitational waves and other synergistic physics and astrophysics using radio timing observations of millisecond pulsars.

Motivation & Objective

• Detect a stochastic background of nanohertz gravitational waves (GWs).
• Characterize the GW background to infer demographics of the SMBHB population.
• Detect individual SMBHBs and enable multi-messenger follow-up.
• Constrain fundamental physics including neutron-star equation of state, cosmic strings, primordial GWs, and beyond-GR theories.

Proposed method

• Model pulsar timing residuals with white and red noise and a GW signal to capture correlated timing deviations across the array.
• Use the Hellings-Downs correlation to identify the quadrupolar GW signature across pulsar pairs.
• Combine Bayesian and frequentist detection methods to search for stochastic and deterministic GW signals.
• Incorporate Earth-term and pulsar-term GW effects; improve with precise pulsar distances to enable full GW parameter estimation.
• Develop and employ sophisticated noise models accounting for pulsar-intrinsic and propagation-induced effects.

Experimental results

Research questions

• RQ1 Can NANOGrav achieve a robust detection of a nanohertz GW stochastic background within the near term?
• RQ2 What are the spectral properties and turnover of the stochastic background, and what do they imply about SMBHB environments and galaxy cores?
• RQ3 Can individual SMBHBs be detected and localized, and how will multi-messenger follow-up proceed with upcoming surveys?
• RQ4 What constraints can PTA observations place on fundamental physics, such as cosmic strings, primordial GWs, and beyond-GR gravity?
• RQ5 How can improvements in pulsar samples, cadence, bandwidth, and noise modeling enhance GW sensitivity?

Key findings

• Robust detection of the nanohertz GW background is anticipated within the next 3–7 years under fiducial assumptions.
• Characterizing the GW background will yield insights into SMBHB populations, galaxy growth, and environmental coupling.
• Individual SMBHB detections as quasi-monochromatic GW sources are expected by 2030, with prospects for multi-messenger follow-up.
• PTAs offer sensitivity to early-Universe GW sources and to tests of gravity via polarization states beyond the standard + and × modes.
• Advanced noise modeling and array expansion (more MSPs, broader sky coverage) are essential for improving GW detection prospects and enabling precise parameter estimation.