– Why can we still detect photons from the CBR?


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

Imagine you have a time machine that can take you back to the very first moment of a grand fireworks show. As the fireworks explode and light up the sky, they create beautiful bursts of colourful light. Now, imagine that instead of staying in the past, you hop back into your time machine and fast forward to the present day.
Even though you’ve returned to the present, you can still see some faint traces of those fireworks in the sky. The light they produced, even though it happened a long time ago, is still traveling through space and reaching you in the present. That lingering glow is like the Cosmic Background Radiation (CBR).
The CBR is like the afterglow of the Big Bang, the spectacular event that started our universe. Just like the fireworks’ light travels through the sky, the photons from the Big Bang have been traveling through space for billions of years, reaching us now. Scientists can detect these ancient photons using special tools, giving us valuable information about the early universe and how it all began, much like studying the afterglow of the fireworks show helps us understand the initial explosion.

The radiation emitted from that early stage of the universe spread out and is just now after billions of years reaching us. That’s why the term light-year is a thing. It describes how far light travels in a year. The star Sirius is like 8 light-years away from us so the light needs 8 years to reach us. The CMB or CBR is Billion’s of light-years away from us. The further we look into the Universe, the further we also look back in time.

What we see so far out is loong gone, but the light isn’t.

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.

Thanks everybody for your explanations. I understand the distance – time relationship, expressed in light years.
What I am having difficulty with is the timing, I guess. If I think of the CBR being emitted as a fireworks being lit up ( and then fading away), the fireworks lasted 100 k years or so. So it strikes me as a coincidence, that we are seeing photons from an event that only lasted 100 k years.
Or phrased differently: why are there CBR photons all over the place apparently?

The big bang was an explosion where you – yes you! – is sitting in its middle, as all of the universe. When we look into the sky you look into far distances and at the same moment backwarts in time. And if you look far enough away then you also look back into a time when the explosion occures. The light is streched to the expanding universe and therefore its frequency, too. And it is now so much streched, that we “see” it as a microwave radiation.