With the notion of causality, firmly fixed by GR, we derived the concept of a singular point from where space-time begun. Causality alone gives us the possibility to talk about a known past (i.e. every effect has a cause) and by this we can trace back in time every event. To reach the moment of the Big Bang we only have to extrapolate backwards the Hubble Law. (I see this like a reversal of effect with cause in every point of space-time.)
With all this said, my question is about the moment of Big Bang; if even time started there, we have an effect without a cause and causality breaks. In this perspective, when did causality started to play a role in the evolution of the universe? Does this path allow for a pre-Big Bang theory? Like a universe, where all its mass has been turned into energy (and in this case, with no mass present, the ideas of small and large are equivalent. And an infinite universe would be equivalent with a point sized one. Hence, the possibility of point like starting place for the Big Bang, but with preexisting time).
From the point of view of causality we reach a logical nonsense at the moment of the BB. How can we modify the BBT to avoid this problem? (Or is has been done already?)
Best Answer
You are confusing causality with determinism. Causality implies, nothing can travel faster than light and future can not affect the past.
According to the singularity theorems of GR there was a singularity and a singularity means certain physical quantities were infinite. In other words GR is not capable to handle such badly behaved point. The notion of time does not have well defined meaning at this point and even if there were events before the big bang those events can not have any observational consequence in the post bang universe. Hence any such prebang events will be causally disconnected from the post bang universe. One can therefore say that time indeed began at the big bang. There was always this belief that in a proper quantum theory of gravity, somehow this singularity can be avoided. But it is far from clear now and any prebang physics is pure speculation at this stage.
Note that even in the classical GR, two events which are spacelike separated are causally disconnected. They are outside of each other's light cone and therefore they can't have any causal link between them.