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Given that the universe is expanding in the three spatial dimensions, is it possible that the time dimension is also changing over time?

If the rate of the passage of time is changing over time, would this effect match or partially match the observed and theorised effects of dark energy, inflation or both?

Qmechanic
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ShadowCrypt
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  • What do you mean by the "rate of the passage of time"? For example, how do you imagine an hour passing in two hours? It sounds like measuring time from a different frame of reference. What frame of reference are you referring to? What would be one use case of a measurement in this frame? – safesphere Jul 12 '18 at 14:51
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    @safesphere How are those objections different to exactly-analogous ones you could pose about expansion in space? They probably are different, but the difference isn't obvious and it's well worth a question. +1. – Emilio Pisanty Jul 12 '18 at 14:53
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    @EmilioPisanty Conceptually, if S is space and T is time and the rate of expansion of space in time is, say, 2, then I could write S=2T (oversimplified for clarity). This is an equation that makes logical sense. Now, with the "rate of the passage of time" being also 2, what similar equation would you write? T=2T makes no sense. – safesphere Jul 12 '18 at 14:58
  • @safesphere: Still oversimplifying, but I think what you want to say is that $\Delta S=2\Delta T$ makes sense, whereas $\Delta T=2\Delta T$ does not. – WillO Jul 12 '18 at 15:02
  • Vote to close as a duplicate. – safesphere Jul 12 '18 at 15:07
  • @safesphere in a small spatial reference I agree this would apply, im more meaning at cosmological scales of multiple billions of light years, would the effect of time passing differently in the past be observable and would those effects match effects of dark energy etc? – ShadowCrypt Jul 12 '18 at 15:09
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    Like I said, you didn't define what "time passing differently" actually means. Thus your question is meaningless. – safesphere Jul 12 '18 at 15:12
  • @safesphere Sorry, I think I lack experience with the technical terminology to correctly answer your question but I'll try again. An observer 12 billion years ago perceives time passing at a rate of T. I in our current time frame perceive time passing at a rate of t. However I can also observe the other observer at a distance of 12 billion light years. Must T=t? Just as in observations between a stationary and a relativistic observer, any variation in perceived time would be constant between the two observers, but it should be possible for there to be an observed difference, right? – ShadowCrypt Jul 12 '18 at 15:35
  • @safesphere Alternatively. If an observer is in a room with clock A, no matter how fast or slow time passes, the observer would perceive time to be passing as the same speed as A is in the same temporal reference frame as them. If the observer observed clock B outside of their temporal frame say at a distance of 10 billion light years, and compared the passage of time on B and A is it technically possible for there to be a difference (after accounting for the redshift of B retreating as the universe expands)? – ShadowCrypt Jul 12 '18 at 16:05
  • What on earth does "time passing at a rate of $T$" mean? – WillO Jul 12 '18 at 18:38
  • Your last question boils down to this: Is there a time dilation component in the cosmological redshift? In the current cosmology the answer is no. All cosmological redshift is due only to the expansion of space: $1+z=\dfrac{a_{now}}{a_{then}}$ where $a$ is the space expansion scale factor and z is the cosmological redshift. The formula is derived here: https://en.wikipedia.org/wiki/Redshift#Expansion_of_space – safesphere Jul 12 '18 at 19:26
  • @safesphere Isn't it be equivalent? If two photons were fired 1 sec apart from A, but the time separation was larger than 1 sec at B (due to time passing faster now), then can't it be perceived as an isotropic expansion in space? – aiwl Oct 03 '21 at 09:03
  • @aiwl Consider a hypothetical mirror that you hold on a very very long rigid rod. You emit two photons 1 sec apart. They fly through the uniformly expanding space to the mirror, reflect, and again fly through the expanding space back to you. What redshift and return time difference would you measure? They will arrive 1 sec apart and with no redshift. Also, as mentioned above time and again, “time passing faster now” has no meaning. If you think it does, write a formula to explain what you mean. – safesphere Oct 03 '21 at 09:36
  • @safesphere Thanks for the example, I think it reveals a misunderstanding I have. I don't understand why they would still arrive 1 sec apart. If they were sub-luminal particles, wouldn't the distance between them increase? From the reference frame of one of the particles, shouldn't the other particle move away according to the Hubble law? Regarding "time passing faster", I mean that a time interval of t1 measured in the past, will be t2 > t1 if measured now. Whether it'll affect things like radioactive half lives, I don't know. But it'll have the effect on the particles in my first example. – aiwl Oct 03 '21 at 10:05
  • @safesphere Nevermind, if that were the case, then the speed of light won't be constant. – aiwl Oct 03 '21 at 10:18
  • Your confusion is that the space expansion carries stuff away. It doesn't. Space expanding without acceleration applies no forces on matter. If two particles in an expanding space are initially at a certain distance static relative to each other, they will remain at the same distance forever despite space around them expanding all the time. Think of space as an empty container. If your living room expands, you might see the walls flying away (if there are "walls"), but it would have no effect on you or things around you. Your just get more room, but no forces would pull your furniture away. – safesphere Oct 04 '21 at 08:56
  • @aiwl Galaxies flying apart per the Hubble law don't automatically prove the expansion of space. They can fly apart, simply because of the initial push. Currently there is no experimental proof that space actually expands. It is an unconfirmed theory. However, this theory says, if things move randomly, they will eventually end up in the regions of space expanding at the same speed as theirs. So they become static relative to the expansion of these regions. This is why galaxies follow the Hubble law, but not because the space expansion carries them away. They simply catch up with it over time. – safesphere Oct 04 '21 at 09:10

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