“Despite its name, the big bang theory is not really a theory of a bang at all. It is really only a theory of the aftermath of a bang.” -Alan Guth
When we look to the deepest reaches of space, we can look for the “first” of any type of object. The first galaxy, the first star, the first light from the CMB, or even the first signals from the Big Bang, like gravitational waves. Yet these signals all have very different redshifts from one another, and perhaps more puzzlingly, lie at extraordinarily different cosmic distances from us.
The reason for this is the intricate relationship between the expansion of the Universe and the contents of what’s in it, and how that changes over time. When the Universe was hotter and denser in the past, it was also expanding much faster. It expanded so much faster in the past that looking back to the edge of the observable Universe (when the big bang occurred) versus looking to the cosmic microwave background (380,000 years later) corresponds to a 900 million light year difference today!