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Did time go slower just after the Big Bang?

My impression from what I have read (interested lay person, not trained in astronomy) was that there is a close relationship between things like time, mass, speed and maybe even the overall size (admittedly another problematic concept in the context of the big bang) of the universe. So my question is, do these statements about specific quantities of time (aside from the fact that they are estimates) make sense? Or what I'm really wondering is, was time right after the big bang the same thing we think of now? Obviously there was nobody there to measure it (extra credit points to you for not mentioning obvious facts like this! :-)). But I'm wondering if perhaps from the perspective of a particle or sub-particle in that primordial soup, could time have been really slow, for example? How does time fit into our picture of what happened at the very beginning of the big bang?

The expression "clocks run slower in a gravitational field" should more precisely read "clocks run slower in a gravitational field than they do in empty space" or "clocks run slower in a deep gravitational field than they do in a shallower gravitational field". So it's true that if you compare a clock in the present epoch to a hypothetical clock in the early universe, just after the Big Bang, you'll find that time ran "slower" in the early universe because the mass density was so high. But does such a statement have any physical meaning?

Not much. At the time when it was so dense that time went appreciably "slower", the whole Universe had that high density. So some hypothetical observer wouldn't notice anything strange, as they'd have no way of comparing their clocks to someone in a low density universe. Physics would have worked just the same.

There would be an observational effect though, although it's unclear that we'll ever manage to observe it. Since we have telescopes that can look at the universe when it was younger, we would be able to observe the effect of time going slower in the early universe, if we had the ability to look back that far. The problem is that the universe was opaque during its first 300,000 years or so, and gravitational relativistic effects when the Universe was 300,000 years old would have been negligible. So, for now anyway, the prospects for observing the Universe when it was young enough for this effect to be noticeable are slim.

So does that mean that when we look out at different galaxies, we see no noticeable relativistic slow-down? It doesn't mean that. You actually don't have to look that far out before you start to see relativistic effects. But this is due to a completely different phenomenon from the gravitational effect I mentioned. It's due to the fact that the universe is expanding, so galaxies are moving away from us at speeds that are fast enough that we need to take relativity into account. This is the "cosmological redshift" effect.

October 2003, Christopher Springob (more by Christopher Springob) (Like this Answer)

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