# Expansion of what? [duplicate]

When people say "the expansion of the Universe" they seem to mean the expansion of the space between mass-dense regions. However, my understanding is that space and time are intrinsically linked, so ① shouldn't time be "shrinking" (ticking slower) as space expands? ② Wouldn't that account for for redshift variances?

If not, please explain how/why time is unaffected by expansion.

Note: I have read spacetime expansion and universe expansion?, but it hasn't cleared up these questions for me.

• You're trying to reason about general relativity using words rather than equations, and it isn't working very well. Your #1 doesn't follow logically from the earlier material, so the "gee whiz" stuff in #2 and #3 is unfounded. – user4552 Mar 4 '18 at 19:40
• I wasn't aware that words weren't allowed. Are you attempting to say "① No. (No follow up.) ② No. See 1. ③ No. (No follow up.) ④ I refuse to answer." Because I'm not understanding your comment other than to say "You're doing it wrong," which is discouraging. I'm not sure why this is so common among the online physics communities. – Rubellite Fae Mar 4 '18 at 19:50
• @TomB. : I think we can all agree that if the OP were to emulate Einstein's ratio of equations to words, the question could be much improved. – WillO Mar 4 '18 at 21:13
• To say that the universe expands is to say that spatial distances are greater at one time than they are at another. But it makes no sense to ask whether spacetime expands (or shrinks) because any expansion or shrinkage is already built into what spacetime is. In other words, spacetime cannot be different today than it was yesterday, because today and yesterday were both part of spacetime all along. – WillO Mar 4 '18 at 21:17
• @WillO, but I think we can also agree that if we all emulated Einstein's ratio of answers to condescensions, this site could be much improved. – Tom B. Mar 4 '18 at 22:01

Yes, time and space are, to some extents, "linked". I'm not sure exactly what you mean by "shrink" in this question. It is true that the expansion not only makes distant galaxies recede at velocities proportional to their distances, but also makes their time run at a slower pace, as observed by us.

This general relativistic time dilation is observed all the time, e.g. in supernova lightcurves which show how their brightness of supernovae decline with time after the explosion. The farther away a galaxy is, the slower it is obseved to decline.

This does not cause their redshift; rather, both effects are caused by the same phenomenon, namely expansion. But both effects add to diminish the flux that we receive from distant sources. The wavelength of light that we receive from a galaxy is "stretched" by a factor $(1+z)$, where $z$ is called "it's redshift" and is given by the size of the Universe when the light was emitted, relative to the size it has now. General relativity predicts, and observations confirm, that time is dilated by the same factor, $(1+z)$.

That is, the flux we receive is diminished by a factor $(1+z)^2$ more than the usual inverse square law predicts — one factor for the less energy per photon (due to the redshift), and one factor for the smaller number of photons emitted per time interval (due to the galaxy's time interval taking longer than our time interval).

All of this is true irrespective of the rate of the expansion; in particular whether the rate is accelerating.

• This was well put together and the explanation is great for a broader audience. Thanks. – Rubellite Fae Mar 4 '18 at 22:02
• @RubelliteFae Let me know if you want a more mathematically rigorous explanation. – pela Mar 5 '18 at 8:38
• No, your explanation is great for my level, but it seems to conflict with other answers I've come across. Otherwise I would choose it as the answer. – Rubellite Fae Mar 5 '18 at 13:48
• @RubelliteFae Well… I don't mean to say that I'm right and they're wrong, but… if they're in conflict with this answer, they are wrong. The time dilation for objects at cosmological distances is very real, and frequently observed. Trust me, I'm an astronomer :) – pela Mar 5 '18 at 17:12