I am trying to grasp the concept of cosmic background radiation, specifically why we can still see it today. As the name implies, it is very faint radiation from the very early universe.
What I don’t get: if a very distant star stops emitting radiation then it will disappear from our view.
As far as I understand it, CBR originated with the recombination. That phase took a few hundred thousand years, then it stopped. So why can we still see the photons emitted then?
Thanks for illuminating me.
In: 6
Because photons take time to reach us, looking at something really far away is like looking back in time.
Turns out if we look out into the void we see a faint glow from the explosive beginning of everything long ago. The event that created those waves is long gone, but the waves keep travelling nonetheless.
If the sun were to extinguish, we wouldn’t know about it for several minutes. If a star goes supernova RIGHT NOW, 500 light-years away, it’ll take us 500 years to see it. If something is 5 light-years away, that means it’ll take you 5 years to see that it just changed colors.
The important thing to realize is: space grows much faster than light can travel across it. It is gravity that binds us close together in groups of stars. That is why it isn’t surprising that we can see evidence of objects that existed a long time ago.
That should hopefully motivate why we measure in light-years and why concepts like general relativity can be unintuitive sometimes.
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