Towards anisotropy-free and nonsingular bounce cosmology with scale-invariant perturbations

Abstract

We investigate nonsingular bounce realizations in the framework of ghost-free generalized Galileon cosmology, which furthermore can be free of the anisotropy problem. Considering an ekpyroticlike potential we can obtain a total equation of state larger than 1 in the contracting phase, which is necessary for the evolution to be stable against small anisotropic fluctuations. Since such a large equation of state forbids the Galileon field to generate the desired form of perturbations, we additionally introduce the curvaton field which can in general produce the observed nearly scale-invariant spectrum. In particular, we provide approximate analytical and exact semianalytical expressions under which the bouncing scenario is consistent with observations. Finally, the combined Galileon-curvaton system is free of the big rip after the bounce.

A critical review of classical bouncing cosmologies

“…IV E 4. The latter models, dubbed non-minimal entropic, are based on replacing the tachyonic isocurvature field during the ekpyrotic phase, which we focus on in the following, by a non-minimally coupled entropic field as first proposed in [169,170]. These models still encompass non-Gaussianity from the conversion mechanism.…”

“…However, after the publication of PLANCK, the emphasis was put onto the possibility to counterbalance different contributions to non-Gaussianities to enable f local NL of order 1 [152]. To this end, the focus shifted to potentials approximately symmetric transverse to the adiabatic direction, as well as non-minimal entropic models [169][170][171][172]. All these models entail an unobservable primordial gravitational wave spectrum; they are therefore ruled out if the BICEP2 detection of r ∼ 0.2 is confirmed to be a signal of primordial origin by the future Keck Array observations at 100 GHZ and PLANCK observations at higher frequency [33,34], see Sec.…”

“…This work superseded that of [170] which set up the building blocks to obtain scale-invariant entropy perturbations within the ekpyrotic scenario via non-minimally coupled massless scalar fields. There, it was suggested that the entropy perturbation could be converted into curvature perturbations by means of a curvaton, as done in [169], or modulated (p)reheating [245]. Non-Gaussianities for the model in [169] where computed in [171], see the non-minimal entropic mechanism in Table II.…”

“…There, it was suggested that the entropy perturbation could be converted into curvature perturbations by means of a curvaton, as done in [169], or modulated (p)reheating [245]. Non-Gaussianities for the model in [169] where computed in [171], see the non-minimal entropic mechanism in Table II. This model does not entail intrinsic non-Gaussianities, but the ones arising from the conversion mechanism.…”

“…In [169], a nonsingular bounce in the framework of galileon cosmology with an ekpyrotic phase was investigated, with the addition of a curvaton instead of a matter phase to generate the scale-invariant spectrum of perturbations. This work superseded that of [170] which set up the building blocks to obtain scale-invariant entropy perturbations within the ekpyrotic scenario via non-minimally coupled massless scalar fields.…”