Pegajace t1_jcnwgbu wrote on March 18, 2023 at 5:13 AM

>would these photons traveling ad infinitum define the edge of the universe

They would if the Big Bang had been an explosion at a specific point in space, but it wasn’t. There isn’t an expanding sphere of photons defining the outer edge of the universe because the universe did not start at a central point. The Big Bang was a rapid growth of spacetime that happened to space, not in space, and it happened everywhere simultaneously.

FullOfStarships t1_jcoc9ps wrote on March 18, 2023 at 8:48 AM

This answer is simultaneously completely correct, and completely wrong.

TL;DR the question you are looking for is "edge of the Visible Universe", and the answer is "yes, that is the dictionary definition of Visible Universe".

The Cosmic Microwave Background was created 370,000 years after the big bang. The photons that reach us have travelled for the lifetime of the universe, minus 370,000 years. These have a redshift of 1,100.

The Cosmic Neutrino Background was created about 10 minutes after the big bang. The neutrinos that reach us have travelled for the lifetime of the universe, minus 10 minutes. These have a redshift of 10^10.

You can only go another ten minutes further back in distance / history. That's it. No more.

More to the point, if you could get back to the exact "zero" point, the radiation would be infinitely redshifted.

In fact, that point is recognised as an Event Horizon. Apparently it actually emits Unruh radiation. It perfectly describes the edge of the "Visible Universe". This is "our universe".

There are good theoretical reasons to believe that the big bang created space billions, trillions, quadrillions, etc... times bigger than the Visible Universe, which has the same physical laws as us. But, for all we know, the universe one micron "further away" than the event horizon could be dragons packed nose to tail. We would have no way to know.

Ironically, if there is an intelligence which exists 99% of the distance to the edge of our visible universe, they would see a sphere the same size as our visible universe, but centred on them.

The visible universe is centred on the observer. To an utterly irrelevant extent, people on the opposite sides of the Earth perceive slightly different edges to their visible universes.