Early modified gravity: implications for cosmology

Philippe Brax (1) , Carsten van de Bruck (2) , Sebastien Clesse (3, 4) , Anne-Christine Davis (5) , Gregory Sculthorpe (2)

1 IPHT - Institut de Physique Théorique - UMR CNRS 3681
2 SoMaS - School of Mathematics and Statistics [Sheffield]
3 NaXys - Namur Center for Complex Systems [Namur]
4 TUM - Technische Universität Munchen - Technical University Munich - Université Technique de Munich
5 DAMTP - Department of Applied Mathematics and Theoretical Physics [Cambridge]

We study the effects of modifications of gravity after Big Bang Nucleosynthesis (BBN) which would manifest themselves mainly before recombination. We consider their effects on the Cosmic Microwave Background (CMB) radiation and on the formation of large scale structure. The models that we introduce here represent all screened modifications of General Relativity (GR) which evade the local tests of gravity such as the violation of the strong equivalence principle as constrained by the Lunar Ranging experiment. We use the tomographic description of modified gravity which defines models with screening mechanisms of the chameleon or Damour-Polyakov types and allows one to relate the temporal evolution of the mass and the coupling to matter of a scalar field to its Lagrangian and also to cosmological perturbations. The models with early modifications of gravity all involve a coupling to matter which is stronger in the past leading to effects on perturbations before recombination while minimising deviations from Lambda-CDM structure formation at late times. We find that a new family of early transition models lead to discrepancies in the CMB spectrum which could reach a few percent and appear as both enhancements and reductions of power for different scales.

Domaines: Relativité Générale et Cosmologie Quantique [gr-qc]; Physique des Hautes Energies - Théorie [hep-th]

Philippe Brax, Carsten van de Bruck, Sebastien Clesse, Anne-Christine Davis, Gregory Sculthorpe. Early modified gravity: implications for cosmology. Physical Review D, 2014, 89, pp.123507. ⟨10.1103/PhysRevD.89.123507⟩. ⟨cea-00983437⟩