Matter sources for a Null Big Bang

TL;DR

This paper explores the properties of matter and stress-energy tensors compatible with a Null Big Bang, revealing conditions under which regular cosmological evolution can start from a horizon and the role of different types of matter.

Contribution

It generalizes previous findings by analyzing matter behavior near horizons in Kantowski-Sachs cosmologies, including the effects of phantom matter and emergent universe scenarios.

Findings

Regular cosmological evolution starts from a Killing horizon under NEC.

Matter is absent at the horizon and appears due to vacuum interaction.

Certain phantom matter types are compatible with Null Big Bang without vacuum interaction.

Abstract

We consider the properties of stress-energy tensors compatible with a Null Big Bang, i.e., cosmological evolution starting from a Killing horizon rather than a singularity. For Kantowski-Sachs cosmologies, it is shown that if matter satisfies the Null Energy Condition (NEC), then (i) regular cosmological evolution can only start from a Killing horizon, (ii) matter is absent at the horizon, and (iii) matter can only appear in the cosmological region due to interaction with vacuum. The latter is understood phenomenologically as a fluid whose stress tensor is insensitive to boosts in a particular direction. We also argue that matter is absent in a static region beyond the horizon. All this generalizes the observations recently obtained for a mixture of dust and a vacuum fluid. If, however, we admit the existence of phantom matter, its certain special kinds (with the parameter $w \leq -3$) are consistent with a Null Big Bang without interaction with vacuum (or without vacuum fluid at all). Then in the static region there is matter with $w\geq -1/3$. Alternatively, the evolution can begin from a horizon in an infinitely remote past, leading to a scenario combining the features of a Null Big Bang and an emergent universe.