ИСТИНА

The gravitational time dilation, or the gravitational redshift, is one of the cornerstones of general relativity and a direct consequence of the Einstein Equivalence Principle (EEP). Precise measurements of the gravitational redshift are of primary importance to measure violations that characterize possible unified theories. We report on new results of measuring the gravitational redshift with the spacecraft of the RadioAstron mission, which is in an eccentric orbit around Earth with geocentric distances up to 350,000 km. The spacecraft and ground stations at Pushchino, Russia, and Green Bank, USA, were each equipped with hydrogen maser frequency standards. Combining uplink and downlink modes in interleaved observations both at 8.4 and 15 GHz during dedicated sessions between 2015 and 2017, the recorded redshifted downlink frequencies from the spacecraft could be largely corrected for the non-relativistic Doppler shift. Our preliminary result for the relevant EEP violation parameter, using only the Doppler fre- quency measurements at the ground stations has an uncertainty of 3 × 10−4 and is consistent with zero. Never before has the gravitational redshift been probed with such high accuracy over such large distances near Earth. We also discuss the prospect for measuring the EEP violation parameter with an uncertainty approaching 10−5 using instead the higher-accuracy VLBI recordings made at numerous ground stations.