Testing Gravity with Binary Pulsars in the SKA Era

TL;DR

This work details how binary pulsars function as precise laboratories for strong-field gravity and how the SKA will revolutionize this field. It outlines GR predictions across orbital dynamics, radiation, and propagation, and surveys extensions such as scalar-tensor, quadratic, massive gravity, and PFEs. The text discusses current constraints from pulsars, the scientific payoff of discovering shorter-period binaries, and the transformative role of SKA capabilities—instantaneous sensitivity, observing strategies, VLBI, multiwavelength data, and multimessenger synergy with LISA and GW detectors. Together, these elements promise stringent tests of cosmic censorship, no-hair theorems, SEP, and the radiative properties of gravity, with unprecedented precision and breadth.

Abstract

Binary (and trinary) radio pulsars are natural laboratories in space for understanding gravity in the strong field regime, with many unique and precise tests carried out so far, including the most precise tests of the strong equivalence principle and of the radiative properties of gravity. The Square Kilometre Array (SKA) telescope, with its high sensitivity in the Southern Hemisphere, will vastly improve the timing precision of recycled pulsars, allowing for a deeper search of potential deviations from general relativity (GR) in currently known systems. A Galactic census of pulsars will, in addition, will yield the discovery of dozens of relativistic pulsar systems, including potentially pulsar -- black hole binaries, which can be used to test the cosmic censorship hypothesis and the ``no-hair'' theorem. Aspects of gravitation to be explored include tests of strong equivalence principles, gravitational dipole radiation, extra field components of gravitation, gravitomagnetism, and spacetime symmetries. In this chapter, we describe the kinds of gravity tests possible with binary pulsar and outline the features and abilities that SKA must possess to best contribute to this science.

Figures (12)

• Figure 1: The mass-mass diagram of the Hulse-Taylor pulsar, PSR B1913+16 based on the PK parameters measured by WeisbergHuang2016. In the figure the underlying gravitational theory is assumed to be GR. Under this theory, we can calculate bands denoting the $1-\sigma$ uncertainties in the component masses inferred from various relativistic effects. The fact that all bands meet at the same region in the diagram implies that GR passes these tests and provides a self-consistent description of the masses. Figure from FreireWex2024, reused without changes under a https://creativecommons.org/licenses/by/4.0/.
• Figure 2: Comparison of different gravity experiments in terms of spacetime curvature probed and maximum spacetime curvature possible. The y-axis gives the maximum spacetime curvature in the system. Since the Y axis is the maximum value of the X- axis, the lower diagonal is greyed out as impossible. The curvature is calculated as the square-root of the Kretschmann scalar $R_{\alpha\beta\gamma\delta} R^{\alpha\beta\gamma\delta}$ (full contraction of the Riemann tensor). 'Earth' stands for near-Earth orbit experiments, like Gravity Probe B. Figure adapted from Wex2020Univ.
• Figure 3: The mass-mass diagram of PSR J0737$-$3039A/B, also known as the double pulsar Kramer2021. In the figure the underlying gravitational theory is assumed to be GR. The inset is an expanded view of the region of principal interest, where the intersection of all curves within a small region within measurement uncertainties means that GR has passed all these tests. For more details, see Kramer2021, figure from FreireWex2024, reused without changes under a https://creativecommons.org/licenses/by/4.0/.
• Figure 4: Fractional error of PK parameters for J0737$-$3039A with simulated future data. The solid lines represent results using AA* and the dash-dotted lines represent results using AA4.
• Figure 5: Required number of circular orbit templates as a function of minimum spin period of the pulsar for a SKA-mid survey for DNS and PSRBH systems denoted as red and gray lines respectively. The dotted and solid lines assume that there is 33.3% and 50% of the orbit within the observation respectively.