Bounce behaviour in Kantowski-Sachs and Bianchi Cosmologies

TL;DR

This paper investigates the possibility of cosmological bounces in Kantowski-Sachs and Bianchi models, showing that such bounces are generally forbidden under certain energy conditions, with exceptions in specific models, and explores braneworld scenarios as a potential resolution.

Contribution

It demonstrates that bounce behavior cannot occur in most homogeneous cosmologies without violating fundamental physical conditions, except in certain models like closed FRW and Bianchi IX, and considers braneworld scenarios as alternatives.

Findings

Bounces are forbidden in Kantowski-Sachs models without violating the reality condition.

Bounce behavior violates the weak energy condition in most Bianchi models.

Braneworld scenarios may offer a way to realize bounces without violating energy conditions.

Abstract

Many cosmological scenarios envisage either a bounce of the universe at early times, or collapse of matter locally to form a black hole which re-expands into a new expanding universe region. Energy conditions preclude this happening for ordinary matter in general relativistic universes, but scalar or dilatonic fields can violate some of these conditions, and so could possibly provide bounce behaviour. In this paper we show that such bounces cannot occur in Kantowski-Sachs models without violating the {\it reality condition} $\dot{\phi}^2\geq 0$. This also holds true for other isotropic spatially homogenous Bianchi models, with the exception of closed Friedmann-Robertson-Walker and Bianchi IX models; bounce behaviour violates the {\em weak energy condition} $\rho\geq 0$ and $\rho+p\geq 0$. We turn to the Randall-Sundrum type braneworld scenario for a possible resolution of this problem.