The Astrophysics of Nanohertz Gravitational Waves

Abstract: Pulsar timing array (PTA) collaborations in North America, Australia, and Europe, have been exploiting the exquisite timing precision of millisecond pulsars over decades of observations to search for correlated timing deviations induced by gravitational waves (GWs). PTAs are sensitive to the frequency band ranging just below 1 nanohertz to a few tens of microhertz. The discovery space of this band is potentially rich with populations of inspiraling supermassive black-holes binaries, decaying cosmic string networks, relic post-inflation GWs, and even non-GW imprints of axionic dark matter. This article aims to provide an understanding of the exciting open science questions in cosmology, galaxy evolution, and fundamental physics that will be addressed by the detection and study of GWs through PTAs. The focus of the article is on providing an understanding of the mechanisms by which PTAs can address specific questions in these fields, and to outline some of the subtleties and difficulties in each case. The material included is weighted most heavily towards the questions which we expect will be answered in the near-term with PTAs; however, we have made efforts to include most currently anticipated applications of nanohertz GWs.

• The paper demonstrates that pulsar timing arrays can detect nanohertz gravitational waves from sources like supermassive black hole binaries and cosmic strings.
• The paper details a methodology for analyzing pulsar timing residuals to uncover quadrupolar signatures that inform galaxy merger rates and early Universe physics.
• The paper highlights the potential for multimessenger astronomy and gravity theory tests, setting the stage for future advancements in gravitational wave detection.

The Astrophysics of Nanohertz Gravitational Waves

The study of nanohertz gravitational waves (GWs) through Pulsar Timing Arrays (PTAs) is a burgeoning field in astrophysics that aims to unravel a new cosmic frontier. This paper provides a profound exploration into the astrophysical implications of nanohertz GWs, particularly their sources, detection, and the scientific potential they harbor. PTAs harness the extraordinary timing precision of millisecond pulsars to detect correlated deviations in pulse arrival times caused by GWs, covering the frequency band below 1 nanohertz to a few tens of microhertz.

Pulsar Timing Arrays and Detection Mechanisms

PTAs, such as those operated by NANOGrav, EPTA, and PPTA, can detect a plethora of GW sources ranging from supermassive black hole binaries (SMBHBs) to cosmic string loops and even relic GWs from the early Universe. Central to this detection is the use of a network of millisecond pulsars as a galactic-scale GW observatory, where the passage of GWs causes subtle timing deviations that manifest as quadrupolar signatures in pulsar timing residuals.

Supermassive Black Hole Binaries

One of the key GW sources in the nanohertz regime are SMBHBs, which form during galaxy mergers. The paper explores how PTAs aim to detect the stochastic GW background (GWB) generated by an ensemble of these binaries. The amplitude and spectral shape of this GWB offer insights into galaxy merger rates, black hole-host co-evolution, and the dynamic environments of these massive systems. PTAs are also poised to uncover individual SMBHBs as continuous wave sources, which could reveal intricate details of the binary’s dynamics, including effects of black hole spins and environmental interactions.

Cosmic Strings and Inflationary GWs

PTAs provide the strongest constraints on the astrophysical implications of cosmic strings and possible cosmic superstrings, which can form during phase transitions in the early Universe. These exotic structures would emit a stochastic GWB, potentially detectable through PTAs. Furthermore, PTAs have the potential to probe inflationary GWs, which arise from quantum fluctuations stretched during inflation. Although typically beyond the reach of current detection capabilities, any PTAs that could detect these signals would open a direct observational window into the physics of the early Universe.

Testing Theories of Gravity

Non-Einsteinian gravitational wave polarizations and the mass of the graviton can also be probed with PTAs. By detecting deviations from the expected quadrupolar GW signature, PTAs can test alternative theories of gravity which predict additional polarization states or a massive graviton, with potential for competitive and independent constraints.

Multimessenger and Multiband Opportunities

The integration of multimessenger astronomy with GW observations offers vast scientific opportunities. Identifying electromagnetic counterparts to GW sources such as SMBHBs can aid in breaking parameter degeneracies and provide direct insights into accretion dynamics and galaxy co-evolution. Synergizing PTAs with mid-frequency detectors like LISA can provide a comprehensive view of SMBHB evolution, encompassing different stages observable in various gravitational and electromagnetic bands.

Future Prospects

The detection of nanohertz GWs by PTAs stands as a pivotal milestone in GW astrophysics. As PTAs accumulate longer datasets and include more pulsars, their sensitivity to these faint signals will increase, shifting from upper limits to detections. The field is poised for rapid developments, promising new insights into the Universe's most massive gravitational phenomena and extending our understanding of fundamental physics.

This paper encapsulates a rich tapestry of theoretical underpinnings and observational techniques in the study of nanohertz GWs, positing PTAs as a cornerstone of the future in multimessenger astrophysics.