Gravitational redshift test of EEP with RA from near Earth to the distance of the Moon

TL;DR

This paper reports a new test of the Einstein Equivalence Principle's gravitational redshift prediction using RadioAstron spacecraft data, achieving a measurement of the violation parameter with an uncertainty of a few times 10^{-4}.

Contribution

It introduces a novel method of analyzing Doppler-tracking frequency data from space VLBI to test gravitational redshift, with potential for improved future measurements.

Findings

Measured violation parameter ε = (2.1 ± 3.3)×10^{-4}

Achieved current measurement uncertainty of a few times 10^{-4}

Discussed future prospects for reaching 10^{-7} accuracy

Abstract

The Einstein Equivalence Principle (EEP) is a cornerstone of general relativity and predicts the existence of gravitational redshift. We report on new results of measuring this shift with RadioAstron (RA), a space VLBI spacecraft launched into an evolving high eccentricity orbit around Earth with geocentric distances reaching 353,000 km. The spacecraft and ground tracking stations at Pushchino, Russia, and Green Bank, USA, were each equipped with a hydrogen maser frequency standard allowing a possible violation of the predicted gravitational redshift, in the form of a violation parameter $\varepsilon$, to be measured. By alternating between RadioAstron's frequency referencing modes during dedicated sessions between 2015 and 2017, the recorded downlink frequencies can essentially be corrected for the non-relativistic Doppler shift. We report on an analysis using the Doppler-tracking frequency measurements made during these sessions and find $\varepsilon = (2.1 \pm 3.3)\times10^{-4}$. We also discuss prospects for measuring $\varepsilon$ with a significantly smaller uncertainty using instead the time-domain recordings of the spacecraft signals and envision how $10^{-7}$ might be possible for a future space VLBI mission.