I don't think it's correct to say that we 'know' that. It's currently one of our best guesses. The inflation model is a way that we can explain the homogeneity of the expansion that we see now. The idea is that at one point during the expansion right after the Big Bang, time and space expanded, but the energy density of time and space did not drop, in fact I think it goes the other way. Anyway, it's still a theory. It has its successes and problems. Some are for, some are against. Alan Guth's The Inflationary Universe is a good read. I remember that I wrote him when I was an undergrad with a little knowledge and a lot of ideas. -No reply. (BTW, Douglas Hostadter did reply to me. That guy is awesome.)In fact, we know that spacetime has already been ‘warped’ faster than light once before – during the resulting ‘cosmic inflation’ that took place just moments after the big bang.