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Surely this is an observer dependent effect due to the expansion of the universe? For example, if a light ray was emitted each second billions of years ago, by

Surely this is an observer dependent effect due to the expansion of the universe? For example, if a light ray was emitted each second billions of years ago, by now we observe that a redder light is emitted every 5 seconds. Because while it was travelling, the space through which it travelled expanded. When the article says things like "time was slower in the past" it makes my head hurt. Don't they mean "events in the past appear to run 5 times slower when observed today?"

MrBodangles 3y ago
I've actually had this question for a while regarding the JWST and newly discovered distant galaxies being stated as 13+ billion years old — isn't our estimation of their age based on the distance the light has traveled (red shifted) from emission? If so, wouldn't that mean that the 13+ billion year old galaxy's would actually be much younger. e.g. the light could've been emitted 10 billion years ago, but 3 billion light years of additional space has grown between us since then. My understanding is that this is dependent on hubble's law, and we do not know the exact rate of expansion now or in the past, but stating an object is 13+ billion years old without that knowledge seems... Unscientific.
- MattPalmer1086 3y ago
  I believe cosmologists account for all of this. It takes about 13 billion years for space to expand enough to give the observed red shift. The distance between us is now greater than 13 billion light years. Yes, there is uncertainty about the rate of expansion, with different techniques disagreeing. So none of this is fully settled. Which is very scientific!
orlp 3y ago
Time is relative. Events near heavy objects appear slower to observers not near heavy objects. An observer in a similar gravitational field as the event (which is implied if the observer and event are in the same place) will see the events at "normal" speed. The separation of event and observer can be mostly in space when observing a relatively close object falling into a black hole, but can also be in both space and time when looking far into the universe (and thus back in time).
- MattPalmer1086 3y ago
  Isn't cosmological time dilation a different thing though, entirely due to the expansion of space? It is not due to differences in gravitational fields or relative speeds approaching the speed of light.

MrBodangles 3y ago

I've actually had this question for a while regarding the JWST and newly discovered distant galaxies being stated as 13+ billion years old — isn't our estimation of their age based on the distance the light has traveled (red shifted) from emission? If so, wouldn't that mean that the 13+ billion year old galaxy's would actually be much younger. e.g. the light could've been emitted 10 billion years ago, but 3 billion light years of additional space has grown between us since then. My understanding is that this is dependent on hubble's law, and we do not know the exact rate of expansion now or in the past, but stating an object is 13+ billion years old without that knowledge seems... Unscientific.

MattPalmer1086 3y ago
I believe cosmologists account for all of this. It takes about 13 billion years for space to expand enough to give the observed red shift. The distance between us is now greater than 13 billion light years. Yes, there is uncertainty about the rate of expansion, with different techniques disagreeing. So none of this is fully settled. Which is very scientific!

orlp 3y ago

Time is relative. Events near heavy objects appear slower to observers not near heavy objects. An observer in a similar gravitational field as the event (which is implied if the observer and event are in the same place) will see the events at "normal" speed. The separation of event and observer can be mostly in space when observing a relatively close object falling into a black hole, but can also be in both space and time when looking far into the universe (and thus back in time).

MattPalmer1086 3y ago
Isn't cosmological time dilation a different thing though, entirely due to the expansion of space? It is not due to differences in gravitational fields or relative speeds approaching the speed of light.