rckrusekontrol t1_jad91sz wrote on February 28, 2023 at 4:36 PM

You might not mean time dilation- since space is expanding it will not change the speed of objects. Like stretching a balloon, two dots will grow apart, but that doesn’t mean they are traveling relative to each other.

Light doesn’t experience time. But what does the expansion mean for observing distant galaxies? I do know that there are some galaxies that we will never be able to see. The expansion of space will happen faster than their light can traverse that distance.

Other galaxies will red shift until they aren’t detectable any more- light may still be reaching us but that frequency of the light as it travels towards us will slow. This is sort of like being in a wave pool and walking backwards. The time between waves grows greater the further away you get from the source.

When we calculate the age of galaxies, we have to consider expansion, and the degree of red shift helps us figure it out. Light that has been traveling since right after the Big Bang may have only been traveling for 13 billion years… But it will have travelled many many more light years in that time.

HunkyMump OP t1_jadba6p wrote on February 28, 2023 at 4:50 PM

Great answer, thanks for your time. So even though due to expansion of space-time some galaxies will moving away from us at effectively the speed of light, they won’t be “frozen in time” from our perspective because that increase in distance doesn’t equate to an increase in velocity?

rckrusekontrol t1_jadfwek wrote on February 28, 2023 at 5:19 PM

Well, in a manner of speaking, they might appear frozen in time- we can only see light at that distance, but let’s say we could see our friend Harry.

Since the light from Harry has to cross an increasingly large distance to reach us, he will appear to move slower and slower. It’s taking longer for light to reach us, hence longer for us to see change. Instead he just gets redder and redder, and kind of fades out.

This applies for space time dilation in other areas, ie, Black holes. If you watched Harry head into a black hole, you’d never see him suck in. Since gravitational dilation is rapidly increasing, Harry will slow to a freeze and red shift out. He is already gone but his image/light is taking longer and longer to reach you, until it can’t reach you at all.

Edit; I’ll add that due to the sheer magnitude of time on this scale, everything we observe seems pretty much “frozen in time”. It just takes too long anything big enough to observe to happen , we only get a snap shot. We get lucky and find this or that happening to a distant Galaxy, but we probably won’t be around long enough to see any process play out.