rearls t1_ivovu5u wrote on November 9, 2022 at 3:17 PM

Observable universe question.

My mental model is that as we look at things further and further away we are seeing then as they were when the light left them. The further out we look in space the further back we are looking in time and newer better telescopes like JWST allow us see further back in time.

However

There is also a limit as to how much we can see as there are parts of the universe so far away from us that no light from them has reached us yet.

The conclusion is no matter how powerful the telescope there is only so far out (or back) we can see? How long after the Big bang is that threshold?

JensAypa t1_ivp2deu wrote on November 9, 2022 at 4:01 PM

After the Big Bang, the matter was so dense that any light that was emitted was instantly reabsorbed by the surrounding matter. So we cannot see light from that time. It's only when Universe had expanded enough that some photons were able to travel without being disturbed. These photons were emitted approximately 380'000 years after the Big Bang, and they constitute what is called the "cosmic microwave background". You cannot see further than that (with light).

But still, that's very far, and the first galaxies are no more than a few pixels in our better telescopes. So building better telescopes does help us finding further and further objects (the JWST for example, has already found a galaxy further than all the ones previously known, dating from around 300 million years after the Big Bang).

rearls t1_ivpm9uy wrote on November 9, 2022 at 6:10 PM

Ok so it's not like the Big Bang threw things apart so fast and so far that light hasn't got to us yet?

JensAypa t1_ivpr13w wrote on November 9, 2022 at 6:41 PM

Yes it is, too ! What you said in your message was perfectly true. The further we look, the further back we are looking in time, and we cannot see further and older than around 380'000 years after the Big Bang because the Universe was opaque before that. So an object that formed and emitted its first light some millions of years after the Big Bang, but whose light has to travel more than 13.8 billion light years (the Universe is 13.8 billion years old), is not visible by any telescope.