# Space is expanding so what is time doing? [duplicate]

Space is expanding and as we know space and time are intrinsically linked to be now known as spacetime. What is happening to time during expansion? Is there more time, longer time or is the time part not affected at all by expansion?

• Time Dilation is also a consequence of Cosmological Expansion.. Commented Mar 14, 2015 at 20:17

It's really an either/or proposition, i.e. either space is expanding or the time experienced by distant objects is dilated, depending on how you view the situation. We choose the former description because it is better.

To expand (excuse the pun!) on what I mean, the measurable result of time dilation is red-shift and indeed distant cosmological objects appear red-shifted and thus "slowed down". It may be tempting to think of faraway objects as being time dilated, however time dilation is not the only cause of red-shift. For example, in special relativity, time dilation is not defined directly from red-shift, but from the relativistic correction to non-relativistic Doppler shift.

In cosmological models we choose coordinates where the red-shift is implicitly and entirely caused by the expansion of space as opposed to time dilation. We could take a different view and try to make the red-shift as entirely due to gravitational time dilation, except the concept of gravitational time dilation doesn't fit that well with cosmological spacetimes and the expansion of space fits much better with the idea of a homogeneous, isotropic Universe.

In summary there is no cosmological time dilation as an analogy to cosmological expansion, at least in the way that we choose to view the situation.

• My problem is that if ther is no space and no time and only a thing called spacetime ten expansion must push on it as a whole. It has to be 50 50 if you try dividing something that in reality can't be divided. Commented Mar 14, 2015 at 21:46
• One for the math guys, does 0/2 + 0/2 = 0? Commented Mar 14, 2015 at 21:54
• Any 'split' would be completely arbitrary and unnecessary. There are good reasons for viewing cosmic red-shift as the result of cosmic expansion and not many good reasons for viewing it as the result of cosmic time dilation, even though we could in principle take the latter view. Commented Mar 14, 2015 at 21:55
• If x = 5 and y = 10, does x = y/2 or does y = x*2? Reality really doesn't care which way you look at it, but if your models are easier to follow if you choose to claim y = x*2, it makes sense to use those models. Remember, even Einstein's idea of relativity was little more than a clever way to fit the real life data that had been collected. Commented Mar 15, 2015 at 1:29
• @Jitter Yes 0/2 + 0/2 = 0. 0/practically-any-value = 0. Though x/0 is undefined. Commented Mar 15, 2015 at 5:16

You ask Is there more time, longer time or is the time part not affected at all by expansion?

First off, Is there more time?

I don't know because I dont know exactly what time is? Do you? Did Einstein? No, I think he said he didn't in one of his books? Does anybody? Probably not.

Is there longer time is an easier question because I don't think that depends on cosmic expansion so much as on ordinary GR effects of say, time slowing down (not to you but to another observer) as you travel faster. I don't think any Cosmic expansion is going to change that.

is the time part not affected at all by expansion? This is anwered, hopefully correctly, in the paragraph above, its the same question as is there longer time?

Hope this helps (and that i'm correct!!)

• Thankyou, I ask because I am wondering about dark energy and how it pushes on galaxies if it is vacuum energy and how that would push on time? Commented Mar 14, 2015 at 21:17
• @jitter if you google something like "is vacuum energy the cause of cosmic expansion" or "cosmological constant problem" you will find that the cause of cosmic expansion is not vacuum energy and you can read the reasons why. (there is an extremely large difference in their values). As I said in my answer, I don't know what time "is" (apart from the simple wording "a measure of change", which does not apply here) so I don't have an answer for how time would be pushed. I'm not avoiding your question, but google "what is time" and you will get 1,000 different explanations. regards
– user74893
Commented Mar 14, 2015 at 22:41

The idea that space is expanding is a relation between it's size and time. Time expanding would be a relation between time and time, so it can't???

• This seems to be an unpopular answer, but I besides brevity I'm not sure there's anything wrong with it. It highlights the fact that having time itself change over time... doesn't even make sense.
– user10851
Commented Mar 15, 2015 at 8:34