Stringy bounces and gradient instabilities

Piao

2017

J. High Energ. Phys.

Abstract:The nonsingular bounce models usually suffer from the ghost or gradient instabilities, as has been proved recently. In this paper, we propose a covariant effective theory for stable nonsingular bounce, which has the quadratic order of the second order derivative of the field φ but the background set only by P (φ, X). With it, we explicitly construct a fully stable nonsingular bounce model for the ekpyrotic scenario.

Section: Introductionmentioning

confidence: 99%

A covariant Lagrangian for stable nonsingular bounce

Piao

2017

J. High Energ. Phys.

“…However, other studies based on modified gravity will also be interesting [62][63][64][65][66][67][68], especially their stabilities.…”

Section: Discussionmentioning

confidence: 99%

Higher order derivative coupling to gravity and its cosmological implications

Piao

2017

Phys. Rev. D

We show that the R (3) δK operator in effective field theory is significant for avoiding the instability of nonsingular bounce, where R (3) and K µν are the three-dimensional Ricci scalar and the extrinsic curvature on the spacelike hypersurface, respectively. We point out that the covariant Lagrangian of R (3) δK, i.e., L R (3) δK , has the second order derivative couplings of scalar field to gravity which do not appear in Horndeski theory or its extensions, but does not bring the Ostrogradski ghost.We also discuss the possible effect of L R (3) δK on the primordial scalar perturbation in inflation scenario.

“…We conclude that, based on EFT, not only a stable nonsingular cosmological scenario may be built without getting involved in unknown physics, but also the phenomenological possibilities of its implementation are far richer than expected (see also [52,53] for the higher spatial derivative operators). …”

Section: Resultsmentioning

confidence: 99%

The effective field theory of nonsingular cosmology: II

Qiu

et al. 2017

Eur. Phys. J. C

Based on the effective field theory (EFT) of cosmological perturbations, we explicitly clarify the pathology in nonsingular cubic Galileon models and show how to cure it in EFT with new insights into this issue. With the least set of EFT operators that are capable to avoid instabilities in nonsingular cosmologies, we construct a nonsingular model dubbed the Genesis-inflation model, in which a slowly expanding phase (namely, Genesis) with increasing energy density is followed by slow-roll inflation. The spectrum of the primordial perturbation may be simulated numerically, which shows itself a large-scale cutoff, as the large-scale anomalies in CMB might be a hint for.