The expansion of time, probably ad nauseum...

Also consider that asking if time passes at the same rate requires a comparison with something else. If the whole universe experienced increased/decreased amounts of time dilation early in its history, how would we know? What would that even mean if everything experienced it? There would be nothing to compare against and say "Oh look, their clock is passing way faster than ours!".

Both "time" and "space" are imaginary mathematical coordinate systems, nothing more. They are not real and have utterly no empirical referent. You can not experiment with imaginary things and you can not manipulate imaginary things or use them to any real effect.

That's a very reasonable thing to worry about and be puzzled by! In popular accounts they don't tell you the whole story.

In bare general relativity (as distinct from cosmology, where you have matter e.g. a primordial hot gas, and the ancient light from it which we still see) there is no criterion of rest and no distinguished time.

However in cosmology we do have a criterion of rest. We live in a bath of ancient light.
this is redshifted by expansion so it is no longer in the visible range. It used to be the slightly orange-ish color light given off by 3000 kelvin (hot) gas. Like the surface of an orange-ish star. But now the wavelengths are stretched out by a factor of about 1000 and it is in the microwave range. Socalled microwave background. A kind of invisible soup of light that you need a horn antenna to detect.

IF YOU MOVE fast enough in some direction you will see a DOPPLER HOTSPOT in the ancient light in that direction.

If you are at rest the ancient light will be the same low temperature, approximately same mix of wavelengths, in all directions. No Doppler hotspot in the microwave sky, for an observer at rest.

So there is a criterion of rest. It has a definite meaning. And in accordance, then, with general relativity, there is a preferred time.

That is the basic time we use in cosmology and it is the time that the AGE of the expanding phase of the universe is stated in terms of.

Observers anywhere in the universe who are at rest relative to Background---who measure the same temperature we do in all directions---will estimate approximately the same age.

In effect, our clocks are synchronized with those of all those other observers---or would be if they existed (which we don't know for sure).

To reverse time you would have to reverse all the motion in the universe that we use to tell time.

Any source for this claim??

A better question here is why the forward direction of time is also the direction where the universe is expanding. There is nothing in principle impossible about observing a collapsing universe instead of an expanding one (either is a perfectly-valid solution to the Friedman equations). So why do we think of the forward direction as the one where things are further apart in the future?

I would think that in the beginning of a collapsing universe you would never get signals between points the signals would be getting smaller and moving away from all other points. I think that it all goes back to the block universe and the simultaneous nature of energy in the present so if you think of time as real and mass as a measure of the past then dark matter could be a measure of the future.

Hi Marcus,

Is the above equivalent to an obsever co-moving with the Hubble flow? I've heard that phrased used a few times to talk about a standard of rest.