As we’re talking of the subject of seeing the edge, or very close to the edge, of the universe. Maybe someone can answer or point me in the right direction to something I’ve never managed to wrap my head around.
Which is how can we see the beginning of the universe? 10 years ago we could see back to 1 second after the big bang, now we can see 0.0000001 second after it. The figures aren’t correct but they make the point, the light from the big bang can only “pass” us once so how can we see earlier light?
It is hard to get your head around, but it's mainly to do with inflation of the early universe which created the vast (infinite?) size of the universe already in a blink of an eye, before matter even came into existence and the subsequent expanansion that has gone on ever since.
Think of say the Sun and Alpha Centauria, our nearest star. When you see the light from the Sun it's from 8 minutes ago but when you see the light from A.C. its from 4.3 years ago since it's much further away and as such is dimmer than our star. So in a way we are looking back in time, as those stars were 8 mins and 4.3 years ago. It doesn't mean we could use a powerful telescope and look at the Sun as it was yesterday or A.C as it was 20 years ago, you are right in saying we are just seeing the current stream of photons as they reach the earth, right now. The ones from yesterday or 20 years ago have passed us by.
It's just with the size of the universe after inflation, and the expansion of space/time since, there is light from the early galaxies that were so far away already that their light really has just taken that long to reach us. The problem is, it is so faint (a few photons) and so stretched out in it's wavelength (due to expansion) that we haven't had a telescope sensitive enough to pick it up before, now hopefully we can.
So this is what's called the observable universe, the sphere around us of stuff whose light so far has taken the 13.8 billion years worth of time the universe has been around to get here. There will be objects even further away where the light still hasn't reach us yet. So that 'edge' of the observable universe does move outwards - by respect of the objects being further away and thus further back in time when the light was first emitted. So when we say looking back in time that's just a function of seeing objects that are further away.
Or put another way, when the first stars were born the light from them passed us, for examples sake, 20 years ago. If we were looking 20 years ago we’d see the star light up but today we can only see that star as it was when it was 20 years old. Yet with the ‘edge’ of the universe, as we’ve built better telescopes, we’ve been able to see further back in time.
In this example, no, if we were looking at the point of space where the star formed 20 years ago there would be nothing there (well, apart from the gas cloud of course!) we wouldn't see it light up until 20 years later, so we are looking back in time to how it was 20 years ago, even though it only lit up for us today.
This was supposed to be a quick post to try and help, it makes you realise there's no easy way to explain it
An article here probably does a better job than me!
http://curious.astro.cornell.edu/ab...3-billion-years-ago-not-passed-us-by-beginner
the end summary
So the key ideas:
The Big Bang occurs at all points in space, so relics associated with it like the Cosmic Microwave Background are emitted from all points in space and can be seen in all directions.
The Universe is thought to be infinite, so as time goes on we can see farther and farther away, and thanks to its infinite size and inflation, there are already objects at all distances in the universe for us to see there. These objects long ago emitted light that reflects their earliest state and since they are great distances away (and the journey is lengthened by expansion), that light reaches us only today.
Things that start emitting light from too far away won't ever be seen as expansion prevents photons from ever reaching us.
