Here’s an eye-opening snippet from Katie Mack’s excellent book The End of Everything (Astrophysically Speaking):
The expansion of the universe as it is occurring today does more than just stretch out the light of distant galaxies. It also stretches out and dilutes the afterglow of the Big Bang itself. One of the strongest pieces of evidence for the Big Bang [...] is the fact that we can actually see it, simply by looking far enough away. What we see, specifically, is a dim glow, coming from all directions, of light produced in the universe’s infancy. That dim glow is actually a direct view of parts of the universe that are so far away that, from our perspective, they are still on fire—they’re still experiencing the hot early stage of the universe’s existence, when every part of the cosmos was hot and dense and opaque with roiling plasma, like the inside of a star. The light from that long-burned-out fire has been traveling to us all this time, and, from sufficiently distant points, has just now arrived.
!
