For example. A star is 8B light years away, we’re told that that the light has taken 8B years to get to us.
BUT the universe is constantly expanding therefor was the star not much much closer to us therefor making it’s light younger?
In: 10
There are a few different things going on here. There is the “lookback time” which is how long the light has been traveling to reach us. There is the comoving distance, which is the distance that the object we are seeing is from us *right now* due to expansion. These numbers don’t match, because like you said, for light to take 8 billion years to get here it would have to be emitted from closer than 8 billion lightyears, because of the expansion of the universe. But by the time the light gets here the object is now more than 8 billion lightyears away from us. So the light from a star with a comoving distance of 8 billion lightyears would have a lookback time less than 8 billion light. And light with a lookback time of 8 billion years would come from an object with a comoving distance greater than 8 billion lightyears.
The actual calculations of the lookback time and comoving distance (which is almost always the number reported as the “distance” in cosmology) require a complex model of how the entire universe evolves over time and knowledge of how redshifted the light is when it gets to us.
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