Thursday, July 18, 2013

1307.4598 (Flávio S. Coelho et al.)

PPN expansion and FRW scalar perturbations in n-DBI gravity    [PDF]

Flávio S. Coelho, Carlos Herdeiro, Shinji Hirano, Yuki Sato
n-DBI gravity explicitly breaks Lorentz invariance by the introduction of a unit time-like vector field, thereby giving rise to an extra (scalar) degree of freedom. We look for observational consequences of this mode in two setups. Firstly, we compute the parametrized post-Newtonian (PPN) expansion of the metric to first post-Newtonian order. Surprisingly, we find that the PPN parameters are exactly the same as in General Relativity (GR), and no preferred-frame effects are produced. In particular this means that n-DBI gravity is consistent with all GR solar system experimental tests. We discuss the origin of such degeneracy between n-DBI gravity and GR, and suggest it may also hold in higher post-Newtonian order. Secondly, we study gravitational scalar perturbations of a generic Friedmann-Robertson-Walker space-time. We show the dynamics of these perturbations is determined by an effective field theory for a single scalar. This scalar has an oscillatory solution which decays as the Universe expands; the solution, moreover, and in contrast with a canonical scalar field coupled to GR, does not freeze on superhorizon scales. Such behaviour leads to the following novel feature: the power spectrum depends on the complete time evolution of the scalar perturbations, including their superhorizon evolution and not just their amplitude at horizon exit during the inflationary era.
View original: http://arxiv.org/abs/1307.4598

No comments:

Post a Comment