Is that so? My understanding is that it doesn't expend into something. And that it expend so fast that the edge of it becomes unobservable.
There are ongoing debates whether the actual Entire Universe is infinite or not.
Trying to see beyond the edge would be like trying to peer out of a black hole. It would probably look blue, like Cherenkov radiation. (but I'm biased, due to having blue eyes).
If the Entire Universe is infinite, then it's eternal in time. And then we get philosophical again.
https://cds.cern.ch/images/CERN-HOMEWEB-PHO-2022-023-1
Observing the edge is effectively looking back in time, to see the conditions of the universe closer to the time of the Big Bang.
New telescopes keep expanding that edge, and new particle colliders (such as those at CERN or Fermilab) keep "bashing 2 rocks together to make fire" - recreating the conditions of the Big Bang to see what comes off.
What I'm not sure about is whether the speed of light (assumed to be constant) is correlated with the size of the observable universe. Perhaps a physicist could shed some light on that question. Relativity means that galaxies that are moving at the speed of light away from one another (one travelling at c, another travelling at -c) have a relative velocity of higher than the speed of light (|c| + |-c| = 2c).
There's also the theory of the One Electron Universe, which I quite like (though that reveals my bias as an electronic systems engineer). Perhaps what we see is the One photon universe. https://en.wikipedia.org/wiki/One-electron_universe
Hopefully this rambling makes sense to someone!
The equation is on Wikipedia. https://en.wikipedia.org/wiki/Lorentz_factor
The terms are: v is the relative velocity between inertial reference frames, c is the speed of light in vacuum, β is the ratio of v to c, t is coordinate time, τ is the proper time for an observer (measuring time intervals in the observer's own frame).
If we could compare the time as we know it (based on the SI unit of seconds using an atomic clock) against the time at the singularity at the centre of the universe, we could figure out whether we're in a black hole, whether we're at the event horizon, or whether we're outside.
But we would have to assume space is a vacuum, which isn't entirely true.
I never considered time expanding (and accelerating?).
Is that even possible? What does that imply?
The question starts to become very philosophical if there is a backup system for this universe. Everything being saved, for eternity, in infinite time. It would require very advanced computational power and storage, but it would probably work in binary (but that's just the kind of thing a computer engineer would say).
Maybe, though, the observable universe is rotating clockwise around a centre that is in the unobservable universe, and time is just a measure of how many rotations have been made since the Big Bang.
The observable universe is the only true sphere.
The observable universe is expanding into the unobservable universe.
What would be interesting is to run a diff on the cosmic microwave background and the pictures from the James Webb space telescope to figure out where the true centre of the universe is, and derive the poles from there.