利用RadioAstron（RA）开展从近地到月球距离的爱因斯坦等效原理引力红移检验
Gravitational redshift test of EEP with RA from near Earth to the distance of the Moon
AI总结：
本研究利用配备氢脉泽的RadioAstron空间VLBI卫星与地面站，通过2015-2017年多普勒跟踪测量检验爱因斯坦等效原理的引力红移，得到破缺参数ε=(2.1±3.3)×10^-4，并展望未来任务可达10^-7精度。
摘要：
爱因斯坦等效原理（EEP）是广义相对论的基石，预言了引力红移的存在。我们报告了利用RadioAstron（RA，即射电天文卫星）测量这一红移的新结果，该卫星是一台空间甚长基线干涉测量（VLBI）航天器，被发射到环绕地球的演化高偏心轨道，地心距离可达353000千米。该航天器以及位于俄罗斯普希诺、美国格林班克的地面跟踪站均配备了氢脉泽频率标准，使得我们能够测量预言的引力红移可能存在的破缺，其形式为破缺参数$\boldsymbol{\u03b5}$。通过在2015年至2017年的专门观测时段内切换RadioAstron的频率参考模式，记录的下行链路频率基本上可以校正非相对论性多普勒频移。我们对这些时段内获得的多普勒跟踪频率测量结果进行了分析，发现$\boldsymbol{\u03b5 = (2.1 \u00b1 3.3)\times10^{-4}}$。我们还讨论了改用航天器信号的时域记录来测量$\u03b5$、以显著降低不确定度的前景，并展望了未来的空间VLBI任务如何有望将测量精度提升至$10^{-7}$量级。
英文摘要：
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.
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