For a long time now I've wondered about time within the universe.
Since the universe is expanding, when it was smaller, the mass was more concentrated. With mass more concentrated, gravity should be higher, and in higher gravity, clocks run slower (relative to other clocks in lesser gravity that is). Does this mean that time ran more slowly when the universe was older relative to today? First, I'm not sure the effect would be large enough to be measurable, let alone if you could even detect it at all since we're in the universe. Or could this effect be measurable or contribute to the red-shift seen by Hubble in receding galaxies? And furthermore, if this effect was large enough, could it compound the red-shift to make it look like the universe was accelerating?
One thing is clear to me, if you roll time back far enough, as you get closer to the big bang, matter (and at some point energy) was all close enough together that time must have been dilated with respect to today. At what point in time did this effect become negligible? How does this effect the age of the universe? If you run things back to the big bang, back to the singularity, time must have been moving infinitely slowly relatively to today. It's interesting that time is sort of pinched off at the beginning like that. Almost like a movie that starts slowly and eventually comes up to speed.
And that lead me to my next thought: if matter (which energy) creates gravity which dilates time, could it be a necessary component of time itself? After all, Einstein's famous equation e=mc² showing how matter is equivalent to energy contains the constant c². c is the speed of light, a speed is a distance something covers in a unit of time. It seems to me that if you could flip this equasion around a bit, you can show that time depends on energy and mass. I'm not sure if this equasion can be manipulated like this though.
It has been said that time is part of the universe. Many folks like to try to think what happened before the universe as if time existed before and will after. If you think of time as an integral part of the universe inextricable from the mass and energy in the universe, you can start to see why it's not possible to think of a "before the universe" within the universe's timeline. It's perhaps easier to visualize the universe as a movie film starting with the big bang with actors having no sense of time before the film and no sense that the film could be running faster or slower at any given point because you're within that film.
Without matter and energy, time does not exist. Time is, as they say, of the essence.
