Wikipedia:Reference desk/Archives/Miscellaneous/2023 July 5

= July 5 =

Time appears to run five times slower in the early universe
"This expansion means that light from ancient cosmic events must travel increasingly longer distances to reach Earth, and therefore takes more time to arrive. As a result, cosmic events that are extremely distant or far back in time appear to unfold more slowly compared to the same event happening nearby, right now. That isn’t to say the early universe was in slow motion, however – anyone present billions of years ago would have seen time evolving normally." - NewScientist.

I was under impression that the light only gets red shifted due to expanding space. Article mentions early universe was not in slow motion, and later (earliest quasar) appeared to run five times more slowly than quasars from today.

What I am missing? manya (talk) 08:23, 5 July 2023 (UTC)
 * Well, you always carry your own time with you (it's called "proper time"). No matter where you are, your wrist watch, your heart beat, the physiological processes in your brain (these are all "clocks") always run at the same rate. It's only when you observe a clock at a distance, in a different environment, that you see that it runs at a different rate compared to your own clock. In the quasar paper, the clock that we're looking at is a variability in the quasars' light output: quasars further appear to vary more slowly than nearby ones, even if the physical conditions in the quasars are the same. The redshift of light is the exact same thing, it is primarily also an effect of time dilation (usually people talk about expansion of space but these things are inextricably linked and basically the same). The "clocks" are the atoms that emit the light, and their "rate" is given by the energy difference between two electronic levels in the atoms. These are the same here and there, but we observe clocks in the early universe as running slower, which manifests itself by the light (the time signal) being redshifted, i.e. by a measured frequency that is lower (or wavelength higher) than what you'd see if the emitting atom was sitting on your desk. --Wrongfilter (talk) 08:43, 5 July 2023 (UTC)
 * I think I understand 'not in slow motion' and 'appears to run slow' part. What I am wondering is how is it derived from the red shift.
 * Say, someone from earliest quasar flashes a light beam towards us after every five seconds for one second each time. When we observe it, it appear to be flashing after every twenty five seconds for five seconds, right?
 * For these five seconds, we will see the red shifted light, but I thought the red shift only tells us about object's velocity away from us. Looks like it also quantifies the object's time dilation. manya (talk) 09:02, 5 July 2023 (UTC)
 * Your description of the phenomenon is perfect. However, none of this has anything to do with velocity, cosmological redshift and time dilation (as I said, same thing) only depend on the size of the Universe then and now. The Universe now is five times as large as it was when the light was emitted by those quasars. Our particular universe has expanded monotonously, first decelerated, then accelerated. Imagine a universe that was static (not expanding) when the light was emitted, then some time later went through a brief period of expansion to five times its initial size, then became static again until today. Redshift and time dilation would be exactly the same even though nothing was happening either at emission or observation. The universe could have done wild things in between, like oscillate between phases of expansion and contraction, without any change to this observation (we would notice by looking at objects at other distances, of course). In all of this, there is no velocity that could be related to redshift or time dilation! The Doppler effect of course is caused by velocity and also leads to redshift and time dilation, but the cosmological effects are not Doppler. --Wrongfilter (talk) 14:34, 5 July 2023 (UTC)