I know that this may sound as a very basic question, but how come that we can detect CMB radiation, light or gravitational waves from the big-bang era? Shouldn't this radiation has overtaken us a long time ago?
EDIT: if i got it right , the answer to my question has to do with space-expansion , but to be honest i can't understand why.
In an hypothetical situation that universe stops expanding and starts shrinking, we won't be able to detect such radiations anymore?
The universe is expanding - and it is expanding, as far as we can tell, in all directions, at the same rate, everywhere all at once. So over a given period of time, a distance of 1000 units will become 1100 units, and in another passing of the same time, that distance will become 1210 units long, then 1321 units, and so on. Any radiation emitted beyond a certain distance will find it difficult to catch up, given that it must travel at a fixed speed.
As a result, and due to a point in time at the beginning of the Big Bang where everything was intensely small, hot and energetic, we're unable to see beyond a particular point in time. It's this hot and energetic state we see evidence of in the background radiation. It's both the edge, and the beginning of our universe.
EDIT: if i got it right , the answer to my question has to do with space-expansion , but to be honest i can't understand why.
In an hypothetical situation that universe stops expanding and starts shrinking, we won't be able to detect such radiations anymore?
One has to think of our three dimensions we live in and the effect of expansion or shrinking, in a novel way. Intuition comes with the model of the surface of a balloon and the raisin bread analogies.
Let us take the surface of a balloon, and imagine that before we blow it out it was very small ( the beginning of the expansion) . As we blow it out the surface expands. In the two dimensional world of the surface of the balloon, an ant, for example will see all points on a ( for the ant hypothetical/mathematical) sphere surface receding from each other. In the ants world at the beginning of time all surface points were one and the same point.
The raisin bread analogy is a three dimensional analogue how space defined by the dough expands, and all points move away from each other.
In the current model of the universe, each of us are at the center of the original expansion, and the light that decoupled at 380.000 years after the big bang is coming at us from all other points in the universe, and the light from our point streams out to the rest of the universe. The red shift of the spectra of known atoms allows us to measure the expansion of galaxies from each other. The CMB keeps on coming because time keeps on counting the beginning of the universe in the four dimensional space. It is not a "wave front" that can pass , because it is everywhere due to the four dimensional space.
If there were shrinking, i.e. galaxies would be nearing each other and us, there would be a universal blue shift and we would be measuring shrinking and not expansion. We would still be detecting the radiation, and it would tell us of the shrinking of the universe instead of the expansion it is telling us now.
