E8 Modified Gravity from the Earth’s outskirts to the cosmos
Fundamental physics has been challenged for decades by (1) the unification of General Relativity (GR) and of Quantum Physics (QP), and more recently by (2) the discovery of the acceleration of the expansion of the Universe. Known theories trying the reconcile GR and QP, e.g. string theory often introduce new scalar fields that could be detected through an associated fifth force. Similarly, the accelerated expansion of the Universe can be explained either by introducing a cosmological constant in Einstein’s equation, or alternatively by introducing extra fields that affect the Universe’s dynamics on cosmological scales or modify GR. However, the fifth force created by an extra field, if it affects the Universe on its larger scales, should also affect its dynamics on small scales, which contradicts past and current tests of GR. To avoid violating GR on smaller scales, screening mechanisms have been designed, that cancel the effect of the fifth force so that new theories are consistent with GR at the level of the current experimental constraints. One of the most stringent of these tests tackles putative violations of the equivalence principle which could occur in theories with certain types of screening, e.g. chameleons.
MICROSCOPE, launched on April 26, 2016, will perform a test of the Weak Equivalence Principle by measuring the Eötvös parameter down to 10-15, two orders of magnitude lower than the best current constraints. As theories with an extra scalar field may violate the WEP, this mission will allow for new constraints on the presence of such scalar fields. In particular, the chameleon screening mechanism could be tested.
The Lunar Laser Ranging Experiment and planetary ephemerids yield tight constraints on deviations from GR in the Solar System. In particular, it is possible to constrain other screening mechanisms such as the Vainshtein and K-mouflage ones as they affect the orbits of planets and interplanetary probes.
Cosmological surveys bring complementary constraints, as Modified Gravity (MG) affects structure formation. For instance, massive galaxy cluster counts and measurements of the matter power spectrum can discriminate against LCDM and MG models; therefore, making it possible to constrain different screening mechanisms.
The project aims to constrain (or investigate how to constrain) modifications to General Relativity, with an emphasis on screening mechanisms, on three complementary scales. We then aim to focus on the following three axes of research:
- Earth orbit scale: constrain the chameleon mechanism with MICROSCOPE.
- Solar System scale: development of an instrumental concept to experimentally constrain MG and screening mechanisms.
- Cosmological scales: constrain MG and screening mechanisms thanks to their effects on structure formation.
- Joël Bergé (ONERA),
- Sandrine Pires (CEA/AIM)
- Quentin Baghi (ONERA)
- Philippe Brax (CEA/IPhT)
- Bruno Christophe (ONERA)
- Emilie Hardy (ONERA)
- Martin Kilbinger (CEA/AIM)
- Françoise Liorzou (ONERA)
- Marguerite Pierre (CEA/AIM)
- Manuel Rodrigues (ONERA)
- Jean–Philippe Uzan (IAP)
- Patrick Valageas (CEA/IPhT)
MICROSCOPE Mission: First Results of a Space Test of the Equivalence Principle
Phys. Rev. Lett. 119, 231101 – Published 4 December 2017