Matter sources for a Null Big Bang

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 $w\le -3$) 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 $w\ge -1/3$. 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.

5 two-column pages, revtex4, no figures. One reference corrected. Final version accepted for publication in Class. Quantum Grav

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• Horizons in matter: black hole hair vs. Null Big Bang
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