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Testing Gravitation from Light-second to Cosmological Scales with Radio Pulsars

Emmanuel Fonseca

TL;DR

Radio pulsars provide a unique laboratory for gravitation by acting as ultra-stable clocks that reveal spacetime structure through timing variations. It organizes tests into two scales: light-second-scale binary pulsars probing strong-field GR via post-Keplerian effects, and cosmological-scale GW searches with pulsar timing arrays targeting the nanohertz ($n\mathrm{Hz}$) GW background. Prominent results include GR-consistent orbital decay to the $0.01\%$ level in the Hulse-Taylor system and in the double pulsar PSR J0737-3039A/B with seven PK effects, plus de Sitter and Lense–Thirring tests; PTAs have begun to constrain a stochastic GW background through the Hellings-Downs correlation. These findings, together with neutron-star mass measurements that constrain the equation of state, illustrate pulsars' central role in multi-messenger gravity and point toward future continuous-wave and memory-burst detections.

Abstract

Pulsars are spinning neutron stars typically observed as pulses emitted at radio wavelengths. These pulsations exhibit a rotational stability that rival the best atomic clocks, making pulsars one of the most important tools for resolving gravitational phenomena in extreme environments. I will present an overview of the ways in which radio pulsars can be used to test strong-field gravity and observe gravitational radiation, both in the context of historical and ongoing experiments. I will also describe how these measurements can be translated to sought-after quantities like the masses and moments of inertia of neutron stars.

Testing Gravitation from Light-second to Cosmological Scales with Radio Pulsars

TL;DR

Radio pulsars provide a unique laboratory for gravitation by acting as ultra-stable clocks that reveal spacetime structure through timing variations. It organizes tests into two scales: light-second-scale binary pulsars probing strong-field GR via post-Keplerian effects, and cosmological-scale GW searches with pulsar timing arrays targeting the nanohertz () GW background. Prominent results include GR-consistent orbital decay to the level in the Hulse-Taylor system and in the double pulsar PSR J0737-3039A/B with seven PK effects, plus de Sitter and Lense–Thirring tests; PTAs have begun to constrain a stochastic GW background through the Hellings-Downs correlation. These findings, together with neutron-star mass measurements that constrain the equation of state, illustrate pulsars' central role in multi-messenger gravity and point toward future continuous-wave and memory-burst detections.

Abstract

Pulsars are spinning neutron stars typically observed as pulses emitted at radio wavelengths. These pulsations exhibit a rotational stability that rival the best atomic clocks, making pulsars one of the most important tools for resolving gravitational phenomena in extreme environments. I will present an overview of the ways in which radio pulsars can be used to test strong-field gravity and observe gravitational radiation, both in the context of historical and ongoing experiments. I will also describe how these measurements can be translated to sought-after quantities like the masses and moments of inertia of neutron stars.

Paper Structure

This paper contains 5 sections, 2 figures.

Table of Contents

  1. Introduction
  2. The Key Method: Pulsar Timing
  3. The Light-second Scale: Pulsars in Orbital Systems
  4. The Cosmological Scale: GWs and Pulsar Timing Arrays
  5. Future Prospects

Figures (2)

  • Figure 1: PK effects observed in the double-pulsar binary system, each represented as a set of shaded curves. The extent of each shaded region corresponds to the 68.3% confidence region. All curves intersect at a common region as shown in the inset; their union illustrates the self-consistency of GR in explaining these phenomena. This figure is reproduced from ksm+21 under Creative Commons BY 4.0.
  • Figure 2: Spatial correlations between pairs of MSPs observed by various PTAs. In each panel, the solid- or dashed-line curves represent the expected HD correlation due to a stochastic GW background. Each panel is adapted from recent works produced by: the Chinese PTA (CPTA; xcg+23); the European and Indian PTAs (EPTA + InPTA; aaa+23a); the MeerKAT PTA (MPTA; msr+25); the North American Nanohertz Observatory for Gravitational Waves (NANOGrav; aaa+23a); and the Parkes PTA (PPTA; rzs+23). The CPTA panel is reproduced with permission granted by the publishing journal; all other panels are reproduced from the cited works under Creative Commons BY 4.0.