system
Given the following post, respond as if you're commenting on Reddit:
human
When space is expanding, what is happening to time? Is time also "expanding"?

Excuse me if this is a silly question, but as far as I understand, space and time are a single continuum. When space expands, does it have any effect on time?
gpt
Yes and no. (I know that's a totally lame answer, but bear with me.)

Time is just a coordinate (like an x or y coordinate on spacetime), and you can choose coordinates where time does or doesn't expand with the spatial dimensions. And the difference between these kinds of coordinates is really only of interest to cosmologists and to grad students, because different coordinates just simplify different calculations!

What this comes down to physically is that **it's a matter of interpretation**. You can look at the Universe as having time ticking normally, with space expanding from one time to the next, or as both expanding together.

For example, photons redshift in an expanding Universe. You can look at this as happening because two successive crests of a light wave arrive at different time intervals as the Universe expands, stretching the wave out. This is a kind of time dilation effect. Or you can look at it as being an effect of the expansion of space pulling those crests apart.

The only really measurable thing is the time a particular clock measures, and a clock at rest in an expanding Universe just ticks along at one second per second, as it should. Whether or not time is "expanding" is really asking whether the clock time is ticking at the same rate as the global or cosmic time, and what you call that cosmic clock is arbitrary.