An expandable image is available at https://en.wikipedia.org/wiki/Observable_universe
|By Pablo Carlos Budassi - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=74584660|
Light takes time to get to us, so the farther away an object us, the farther back in time we can see.
The very earliest stage of the universe is invisible - photons could not get through the dense fog of subatomic particles. After c. 370,000 years, atoms began to form, so creating empty spaces that let photons start their long journey. We are still able to observe the radiation emitted at that time, because it has taken billions of years to reach us.
More on the early universe and cosmic background microwave radiation in these two short clips by Professor David Butler:
Is the Universe gradually disappearing?
Yes - and no.
The fabric of the Universe is expanding, so that the farther away an object is, the faster it will seem to be receding. (This is "on the whole" - some objects, such as the Andromeda galaxy, happen to be moving in our direction. But an otherwise "stationary" object will still be carried away by space-time expansion.)
The logic of this seems to be that with enough of this stretching, the farthest parts of the Universe will be going faster than the speed of light and so information from them can never reach us. In a sense, they will have torn free of our observable Universe and will cease to exist as far as we are concerned. An August 2018 article in Forbes magazine appears to be thinking on these lines:
But if Einstein is right, then no matter is rushing away from us at or above light-speed.
This is because of the way you add two speeds together.
For everyday purposes, two cars approaching each other, each travelling at 30 mph, are closing the gap at 60 mph...
... very, very nearly, but not quite! For in reality, there is a microscopic reduction in the total, which becomes much more significant as velocities get closer to light-speed. The formula is this:
u is the combined speed, as seen from our point of view
v is the speed of the first object
u' is the speed of the second object, as seen from v
c is the speed of light (and c2 is the speed of light times itself)
So if we see a galaxy moving away from us at 60% of the speed of light (i.e. 0.6 C), and there is a quasar moving away from the galaxy in the same direction, also at 0.6 C (as seen from the galaxy), then (if you can do the math) Einstein's formula says the quasar is receding from ourselves not at a total of 1.2 C (20% faster than light) but at 15/17ths of C - i.e. lower than light-speed.
Therefore, a graph of celestial objects plotting distance against velocity would appear to be very nearly straight-line to start with (as per Hubble) but curving as the velocities approached C.
So information from the most distant reaches of the Universe can never be completely lost, but the frequencies will ultimately be lengthened to the point where we would have no means to detect them.
Like old soldiers, the remotest bits of the Universe don't die (leave us altogether); they just fade away.