Ekpyrotic and cyclic cosmology

Abstract

Ekpyrotic and cyclic cosmologies provide theories of the very early and of the very late universe. In these models, the big bang is described as a collision of branes -and thus the big bang is not the beginning of time. Before the big bang, there is an ekpyrotic phase with equation of state w = P ρ ≫ 1 (where P is the average pressure and ρ the average energy density) during which the universe slowly contracts. This phase resolves the standard cosmological puzzles and generates a nearly scale-invariant spectrum of cosmological perturbations containing a significant nongaussian component. At the same time it produces small-amplitude gravitational waves with a blue spectrum. The dark energy dominating the present-day cosmological evolution is reinterpreted as a small attractive force between our brane and a parallel one. This force eventually induces a new ekpyrotic phase and a new brane collision, leading to the idea of a cyclic universe. This review discusses the detailed properties of these models, their embedding in M-theory and their viability, with an emphasis on open issues and observational signatures.

Cosmological super-bounce

“…On the one hand, it lends further credence to ekpyrotic/-cyclic models [15,64,65] as viable alternatives to inflationary models (with predictions in good agreement with data, plus a number of conceptual advantages [55]). On the other hand, the existence of cosmic bounces dramatically changes the predictions in a landscape context -see [66][67][68][69][70][71][72] for recent work in that direction.…”

A critical review of classical bouncing cosmologies

“…While attempts to construct alternative proposals to inflation in string gas cosmology, such as in [204], are still subject to unsolved problems [151]. [19,205,206], it was shown in a series of recent articles [66,[207][208][209]] that a string gas can be used successfully to describe the matter content in a Hagedorn phase, while providing the possibility of violating the NEC in a controlled manner in string theory.…”

“…which can be computed by evaluating (151), as well as the particular background solution providing the volume expansion rate N (ξ); the index i denotes the field that freezes in the late-time single field solution. Such a large and negative contribution imposes severe constraints on the relevant scenario.…”

An extended matter bounce scenario: current status and challenges

“…The Matter Bounce Scenario (MBS) (see [1,2,3,4,5] for recent reviews on a variety of bouncing cosmologies) suggests that the Big Bang singularity is replaced by a nonsingular bounce. It is essentially characterized by the Universe being nearly matter dominated at very early times in the contracting phase (to obtain an approximately scale invariant power spectrum) and evolving towards a bounce where all the parts of the Universe become in causal contact [6], solving the horizon problem, to enter afterwards into a regular expanding phase, in which it matches the behavior of the standard hot big bang cosmology.…”