Testing the Equivalence Principle in space after MICROSCOPE

Tests of the Weak Equivalence Principle (WEP) can reveal a new composition dependent force of nature or disprove many models of new physics. For the first time this test is successfully carried out in space by the MICROSCOPE satellite. Early results show no violation sourced by the Earth for Pt and Ti test masses with random errors (after 8.26 d of integration time) of about 1e-14 and similar systematic errors. It improves by 10 times over the best ground tests with rotating torsion balances despite 70 times less sensitivity to differential accelerations, thanks to the much stronger driving signal in orbit. The test is limited by thermal noise higher than expected due to the poor quality factor of the gold wires used for electric grounding. This noise was shown to decrease when the spacecraft was set to rotate faster than planned. The result will improve by the end of the mission, as thermal noise decreases with more data. Not so systematic errors. We investigate major non-gravitational effects and find that the Pt-Pt zero-check sensor does not allow their separation from the signal. The early test itself reports systematics in the Pt-Ti sensor which are not detected in the Pt-Pt one, hence would not be distinguished from a violation. The improved test will need more measurements to check systematics, but there is not enough time left. MICROSCOPE demonstrates the huge potential of space for WEP tests of very high precision and indicates how to reach it. To realize the potential, a new experiment needs the spacecraft to be in rapid, stable rotation around the symmetry axis, needs high quality mechanical suspensions as in the most precise gravitational experiments on ground, and must allow systematic checks. The design of the `Galileo Galilei' (GG) experiment, aiming to test the WEP to 1e-17 and currently a candidate for a medium-sized mission of the ESA, unites all the needed features.