system
Given the following post, respond as if you're commenting on Reddit:
human
If space is ever so expanding, do we seen new/farther everyday we take photos of the outer edges of space? Do we add on to “observable universe” everyday too?
gpt
Kind of.

So, the universe as a whole does not have a boundary. The expansion of the universe is not the expansion of the *edge* of the universe, but rather everything getting further away from everything else - it's not expanding at the edges, it's expanding *everywhere*. In fact, the evidence is somewhat pointing towards the universe being *infinite* in size.

However, the *observable* universe *is* expanding. The edge of the observable universe represents the distance that light could have travelled since the beginning of the universe. The universe is about 13 billion years old, so the light from the edge of the observable universe is the light that has travelled about 13 billion light years (although the universe has continued to expand since the light was emitted and the images we see from this light are from objects that are now much more than 13 billion light years away).

This means that the images we see of the very edge of the observable universe are images of the very very young universe. This is the universe before there was any structure - it was a dense, uniform, opaque fluid of energy and particles. Because the universe was opaque, we can't actually see the light from the very beginning of the universe. Instead, we see the light that was emitted just as the universe got cool enough and sparse enough to become transparent. This light forms a background behind everything else in the universe. It has been redshifted over billions of years into microwave frequencies, which is why we call it the Cosmic Microwave Background, or the CMB.

So, as the observable universe expands, what happens is that the CMB radiation that we see is from slightly further away. What's happening is that the CMB radiation was emitted *everywhere*, but because it takes time for light to reach us, every second we're seeing CMB radiation that was emitted from gas about a light-second more distant than what we received the previous second.

This means we are technically seeing new parts of the observable universe, but it's not like we're seeing new galaxies turn up - we're just seeing slightly more distant "slices" of the CMB. The CMB is far too uniform over time for us to really see any change over decades though.