What happens when a photon reaches the "edge" of the universe?

As I understand it, there are several models for what the universe could look like from the “outside”. I don’t know if any of these are outdated, so if they are please tell me if they are.

1. Expanding bubble. This model says the universe will never stop expanding, and the universe will die because the energy all turns to photons and gets spread out so much.


2. Bubble that will eventually collapse. In this model the universe will stop expanding, and collapse. Everything merges into super-massive black holes, and the universe dies with fire.


3. The universe is a torus. I'm not clear whether the size of the universe in this model is changing, but the central idea is that space is donut-shaped.

Those are the major ones that I have heard of. My question involves what I have heard to be the most popular of the three, #2. In this model, eventually something will "touch" the edge of the universe. Obviously, the first thing to "touch" it would be photons as they travel at, well, the speed of light. This involves all forms of energy in the universe, but I thought it might be easier to explain for something without mass.

What happens if some form of energy "touches" the edge of the universe? Logically, the energy cannot be lost without violating the law of conservation of energy. The only thing I can think of, is that it would "bounce" or be refracted off the edge.

I realize that it sounds silly, however if model #2 is correct then this situation will arise some number of billions of years from now.

I can only assume that there is either a new model that accounts for this, there is some complex explanation, or that model #2 is incorrect.

Answer

Your question has been addressed before, see the questions:

But this is a subtle issue and one that's worth using a few more of the Physics SE's electrons to discuss.

You describe the universe as a bubble or a torus, and the implication of these descriptions is that the universe is embedded in some higher dimension in the way the 2D surface of an expanding ballon is embedded in 3D space. But it's important to be clear that the universe is not embedded in a fourth spatial dimension. There are only three space and one time dimensions and the universe isn't expanding into anything. This means the universe has no edge so there is no edge for photons to reach.

There are three types of universe, open, flat and closed. The open and flat universes are infinite while the closed universe is finite but has no edge (more on this below). The evidence is that our universe is flat so it's infinite. In this context infinite means you can draw a straight line outwards away from you and the line will never reach any point where it can't be continued smoothly outwards.

It's possible for a flat universe to be finite if it has some large scale closed topology. This is what we mean by saying the universe could have the topology of a torus (though this would be a 3-torus not a doughnut - a doughnut is a 2-torus). If the universe is closed in this way there is still no edge but the line described above would eventually return to its starting point. However if our universe is closed in this way the length scale for the closure is much larger than the size of the observable universe or we'd see evidence for it in the cosmic microwave background.

The nearest we get to an edge is the closed universe, though I must emphasise that the evidence suggests our universe is not closed, so this is of intellectual interest only - fun maths but not physically relevant.

In a closed universe the universe expands outwards from a Big Bang but then the expansion stops and the universe starts to contract again, and eventually everything collapses into a Big Crunch. This universe is finite in time (the time between the Big Bang and Big Crunch is finite), and it's finite in space. However photons in a closed universe still can't reach an edge. In fact every possible particle trajectory, whether it's a photon or a massive particle, starts at the Big Bang and ends at the Big Crunch. So there is a sense in which the Big Bang and Big Crunch are the edges of a closed universe.