NASA/Adboe Stock/ Ryan WillsOur universe is expanding, so it must have been smaller in the past. Indeed, if we rewind our cosmological movie, we see the universe shrinking back almost to a point the big bang some 13.8 billion years ago. Is this when time began? Alas, things arent so simple. Albert Einsteins general theory of relativity tells us that the backdrop of the universe is a fluid continuum, space-time, in which neither space nor time has an absolute meaning. Whats more, at the big bang, space-time distorts into a point of infinite density called a singularity. We cant say this is where time begins, only that it marks a rupture beyond which we cannot extrapolate.Even so, some cosmologists believe there was a before the big bang. Some suggest that another universe preceded ours, and that this one contracted and then bounced at the big bang, resulting in the expanding era we now observe. More radically, cosmologist Roger Penrose has proposed that new universes can emerge from ones that dont contract, through a dramatic rescaling of all space-time.In both these scenarios, time is eternal, but thats just one possibility. The late cosmologists Stephen Hawking and James Hartle suggested that time was once an ordinary dimension like space, which got derailed at the big bang into space-time. Another outlandish idea is that space-time is made of particle-like pieces. If so, these could be arranged in different phases, akin to steam and liquid water. Maybe the big bang was the point at which they condensed into the fluid, continuous space-time we observe today.