A Critical Review of Classical Bouncing Cosmologies

By Diana Battefeld, P. Peter. 2014

372 citations recorded by the source corpus.

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 these 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 also assess the fitness of different proposals with respect to the data. 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 dangerous instabilities at the classical and/or quantum level. If primordial gravitational waves are observed, certain bouncing cosmologies, such as the cyclic scenario, are in trouble, while others remain valid. 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 research.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/40008a2c2cddb18e6b49fc3d922b66866967b632

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In Astronomy and Astrophysics

Citations in the corpus

Horndeski theory and beyond: a review (2019, background): “cenario, inﬂationary spacetime is past incomplete [88, 89, 90] (see also [91]), which motivates nonsingular alternatives to Horndeski theory and beyond 18 inﬂation such as bouncing models (see, e.g., [92, 93, 94, 95, 96] for a review). In nonsingular cosmologies, there must be some interval during w”
Cosmological perturbations in modified teleparallel gravity models (2018, background): “Non-singular background cosmologies often need to resort to matching of the perturbations across the bounce, if not plagued by ghosts or already classical instabilities of linear perturbations [36].”
Generic instabilities of nonsingular cosmologies in Horndeski theory: A no-go theorem (2016, neutral): “Non-singular stages in the early universe, such as contracting and bouncing phases [5], cannot only be something that replaces inflation, but also “early-time” completion of inflation just to get rid of the initial singularity.”