Are Friedman Equations Valid at the Moment of the Big Bang?

[haael]

Are known solutions to Friedman equation exact at the moment of Big Bang or do they fail?

I mean: do solutions of Friedman equations contain Big Bang as a point of time, or do they start "just after"?

Can Friedman equations be extended to the time before Big Bang? If so, what do they say?

 

[kimbyd]

Are known solutions to Friedman equation exact at the moment of Big Bang or do they fail?

I mean: do solutions of Friedman equations contain Big Bang as a point of time, or do they start "just after"?

Can Friedman equations be extended to the time before Big Bang? If so, what do they say?

Typically what is referred to as the "moment of Big Bang" is $t=0$ in the Friedmann equations. At this time, the Friedmann equations are invalid, as there is a singularity in the equations. These equations must be incorrect at times earlier than some unknown time t > 0. Current observations seem to indicate that the Friedmann equations are probably pretty solid after a few minutes, due primarily to Big Bang Nucleosynthesis observations, but it's hard to determine their accuracy before then.