The light of the oldest galaxies also shine the longest and in the beginning after the big bang they were also very close to us. Only the expansion of space cannot be the cause that this light is only now reaching us, can it? 13.7 billion years ago this was very close to us, so wasn’t the light from it faster to us?
Answer
As you may know, the expansion of the universe causes the redshift of galaxies, according to a law whereby the redshift (the amount of light shifted towards the red) increases with distance. The redshift of a galaxy tells us the rate at which that galaxy is moving away from us, actually the rate at which the universe is expanding at the distance of the galaxy.
The redshift of the big bang is infinite; so we can’t see the bang, all its light has disappeared from our spectrum. Redshift infinity corresponds to a speed equal to the speed of light. That’s right: the big bang happened everywhere and here (here and everywhere were the same back then), and its signal runs away as fast as it comes to us.
The redshift axis between 0 and infinity is therefore a timeline. What has redshift 0 is happening today, 13.8 billion years after the Big Bang. What has redshift infinity happened at the time of the big bang, 13.8 billion years ago for us. So everything in between happened between the big bang and today. A galaxy with a large redshift is in a place that is already far away, but not as far as the signal that could still reach us from the big bang. So we see that galaxy after the big bang, but a long time ago anyway, because its light had to travel a long distance to get to us.
Answered by
prof. Christopher Waelkens
Astronomy
Old Market 13 3000 Leuven
https://www.kuleuven.be/
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