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.
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