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 .