Gravitational Light Deflection with SKA-VLBI and Its Application to Precision Tests of General Relativity

Abstract

Experimental test of general relativity remains an ongoing endeavour. Radio astrometry provides a vital tool for precisely measuring the light deflection caused by the Sun, testing general relativity, and discriminating between gravitational theories. The best accuracy for the post-Newtonian relativistic parameter, ${\hat{I}}^3$, achieved with very long baseline interferometry is $9\times 10^{-5}$. With 300-sec integration, SKA-VLBI can achieve a sensitivity of $\sim$15 $\hat{I}¼$Jy at 15 GHz over a bandwidth of 0.256 GHz. This enables detection of $\sim$36 extragalactic radio sources per square degree with flux densities of $\sim$1.5 mJy, and potentially detecting in-beam radio sources. Single-epoch SKA-VLBI observations may achieve an astrometric precision of $\sim$2 $\hat{I}¼$as. Utilising the Sun as a gravitational lens, 10-epoch positional tracking of extragalactic sources could improve ${\hat{I}}^3$ accuracy to $\sim$10${}^{-7}$. Even with Jupiter as a lens, SKA-VLBI can measure ${\hat{I}}^3$ to $\sim$10${}^{-4}$. Critically, it may conduct the first measurement of quadrupolar deflection of light caused by Jupiter, determining the physical oblateness of Jupiter, $J_{2,\mathrm{J}}$, to within $\sim${}$10^{-3}$. These advances are expected to rigorously test and improve gravitational theories or high-order parameterized post-Newtonian formalisms, while laying the foundations for (sub)$\hat{I}¼$as astrometry.

Published in Advancing Astrophysics with the SKA II (AASKAII), 2026 (arXiv:2606.20366). Report-no:AASKAII/Li01