http://arxiv.org/abs/1406.2790
Given the proliferation of bouncing models in recent years, we gather and critically assess these proposals in a comprehensive review. The PLANCK data shows an unmistakably red, quasi scale-invariant, purely adiabatic primordial power spectrum and no primary non-Gaussianities. While observations are consistent with inflationary predictions, bouncing cosmologies aspire to provide an alternative framework to explain them. Such models face many problems, both, of the purely theoretical kind, such as the necessity of violating the NEC and instabilities, and at the cosmological application level, as exemplified by the possible presence of shear. We provide a pedagogical introduction to these problems and assess the fitness of different proposals. For example, many models predict a slightly blue spectrum and must be fine-tuned to generate a red spectral index; as a side effect, large non-Gaussianities often result. We highlight several promising attempts to violate the NEC without introducing fatal instabilities at the classical and/or quantum level. However, gravitational waves at the level indicated by BICEP2 cannot be achieved in almost all bouncing models in which they have been calculated, with the exception of models in string gas cosmology, which are problematic for other reasons. We conclude that, while most bouncing cosmologies are far from providing an alternative to the inflationary paradigm, a handful of interesting proposals have surfaced, which warrant further research. The constraints and lessons learned as laid out in this review might guide future model builders.
D. Battefeld and P. Peter
Thu, 12 Jun 14
46/50
Comments: 53 pages, 19 figures, review article
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