2012/12/31 by Keisuke Izumi, Yen Chin Ong · 2 citations
Physics and Astronomy · #astro-ph.CO #gr-qc #hep-th
paper · pdf · doi:10.1088/1475-7516/2013/06/029
published as JCAP06(2013)029 · Final version, with improvement on the writing, as well as correction to some typos
arxiv created 2013/06/21 · arxiv updated 2013/06/24
We perform detailed investigation of cosmological perturbations in f(T) theory of gravity coupled with scalar field. Our work emphasizes on the way to gauge fix the theory and we examine all possible modes of perturbations up to second order. The analysis includes pseudoscalar and pseudovector modes in addition to the usual scalar, vector, and tensor modes. We find no gravitational propagating degree of freedom in the scalar, pseudoscalar, vector, as well as pseudovector modes. In addition, we find that the scalar and tensor perturbations have exactly the same form as their counterparts in usual general relativity with scalar field, except that the factor of reduced Planck mass squared Mpl2 ≡ 1/(8πG) that occurs in the latter has now been replaced by an effective time-dependentgravitational coupling -2 (df/dT)|T=T0, with T0 being the background torsion scalar. The absence of extra degrees of freedom of f(T) gravity at second order linear perturbation indicates that f(T) gravity is highly nonlinear. Consequently one cannot conclusively analyze stability of the theory without performing nonlinear analysis that can reveal the propagation of the extra degrees of freedom.