Scattering at the boundary of universe

Question

Suppose that we have a particle moving at $0.8 c$ (or even a photon) and it is just, say $2 times 10^{5} m$ away from the boundary of the universe (whatever that boundary be).
Since the expansion speed of universe is less than that of our particle then it has to reach the boundary (right?), what happens then? Does it bound back? Does it continue to be on the boundary?

Andrew · Accepted Answer

As far as we know, the Universe does not have a boundary. But, if it does, then we will need to know what are the boundary conditions in order to answer your question of what a particle will do when it reaches it. For example...

The Universe could have reflecting boundary conditions, in which case the photon would bounce back from the boundary.
The Universe could have transmitting boundary conditions, in which case the photon would leave the Universe forever.
The Universe could have periodic boundary conditions, in which case the photon will effectively "circle around" the Universe.

Other boundary conditions are possible as well.
I would personally find the first two options are very weird, and the last one possible but also somewhat unexpected. But, this maybe just shows my own biases.

Scattering at the boundary of universe

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