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

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, a space VLBI spacecraft launched into an evolving high eccentricity orbit around Earth with geocentric distances reaching 353,000km. 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 ε, 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 ε = (2.1 ± 3.3) × 10−4. We also discuss prospects for measuring ε with an uncertainty on the order of 10−5 using instead the time-domain recordings of the spacecraft signals and envision how 10−7 might be possible for a future space VLBI mission.

Preliminary sensitivity study for a gravitational redshift measurement with China’s Lunar exploration project

“…In addition, deep-space missions also are proposed to perform the precise measurement of the gravitational redshift, such as the VERITAS mission with Deep Space Atomic Clock (DSAC) [40]. The space VLBI RadioAstron(RA) mission recently reported a test of gravitational redshift with the uncertainty 3.3 × 10 −4 [41]. China's Lunar Exploration Project (CLEP) is actively underway for the purpose of lunar explorations and scientific experiments, which also includes proposal of using an H-maser clock with stability of 5 × 10 −13 / √ τ (where τ is the integration time in seconds), and using the microwave links for the time and frequency transfer between the CLEP satellites and the ground stations, such as the Chinese VLBI network [42,43].…”