MICROSCOPE’s view at gravitation

Abstract

The weak equivalence principle (WEP) is the cornerstone of General Relativity (GR). Testing it is thus a natural way to confront GR to experiments, which has been pursued for four centuries with increasing precision. MICROSCOPE is a space mission designed to test the WEP with a precision of 1 in 10^15 parts, two orders of magnitude better than previous experimental constraints. After completing its two-year mission, from 2016 to 2018, MICROSCOPE delivered unprecedented precise constraints $\eta({\rm Ti, Pt})~=~ [-1.5 \pm 2.3~{\rm (stat)} \pm 1.5~{\rm (syst)}]~\times 10^{-15}$ (at 1σ in statistical errors) on the Eötvös parameter between one proof mass made of titanium and another made of platinum. This bound allowed for improved constraints on alternative theories of gravitation. This review discusses the science beyond MICROSCOPE – GR and its alternatives, with an emphasis on scalar-tensor theories – before presenting the experimental concept and apparatus. The mission’s science returns are then discussed before future tests of the WEP are introduced.

Probing ultralight isospin-violating mediators at GW170817

“…The parameter space of the isospin-violating mediators with mass ≲ 10 −11 eV is constrained by a number of experiments, including WEP, motions of asteroids [55,56] and planets [18,57], and black hole superradiance (BHSR) [58]. The dominant WEP experiments include the Eöt-Wash (EW) torsion balance experiment [9,10], the lunar laser ranging (LLR) experiments [19][20][21][22], and the MICROSCOPE experiment [11][12][13][14][15]. Note that the signals in the WEP experiments depend on the charges of both the test mass and the attractor (gravity source).…”

“…While substituting different test masses can be relatively straightforward, changing attractors often presents more of a challenge due to the limited number of nearby celestial bodies. The most commonly used attractors include the Earth [9][10][11][12][13][14][15] and the Sun [10,[19][20][21][22]. Another critical factor in the fifth force experiments is the distance.…”

“…For example, experiments using the Sun as the attractor probe mediators with mass ≲ 10 −18 eV, as the distance to the Sun is ∼ 1.5 × 10 8 km. In contrast, experiments that use the Earth as the attractor, such as MICROSCOPE [11][12][13][14][15], can probe mediators in a larger mass range, m ≲ 3 × 10 −14 eV, which is determined by the radius of the Earth. Consequently, the scenario where the charge of the Earth is zero and the mediator mass is in the range of ∼ (10 −18 , 3 × 10 −14 ) eV cannot be probed by WEP experiments using the Sun as the attractor.…”

“…Extensive experimental efforts have been devoted to searching for the effects of the fifth force across a wide range of distances and couplings [5]. One notable aspect of the fifth force searches is the exploration of the composition-dependent interactions [6], which could lead to violations of the weak equivalence principle (WEP) [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22]. The isospin-violating mediators, which have different couplings to neutrons and protons, are an intriguing type of compositiondependent interaction in fifth-force experiments and also of great interest in dark matter direct detection [23][24][25][26].…”

News from the Swampland — constraining string theory with astrophysics and cosmology

“…Still, the sharp MICROSCOPE bound can play a significant role in some specific varying-α models, further tightening the possible values for the coupling of the field with the rest of the standard model. For a review see[134].…”