DudeWithAnAxeToGrind t1_jcnjzmm wrote on March 18, 2023 at 3:11 AM

Let assume that photon is emitted from Earth, just so we can have convenient frame of reference. The same would be true if it was emitted from anywhere else.

Assuming nothing ever absorbs it, the photon would just keep going forever. However, even if the Universe itself is not infinite, and has an edge, it will never get to that edge. Not even close. I.e. as far as your question goes, the difference between infinite and finite Universe is irrelevant. The final fate of that photon and how far it can get is the same.

As it travels, its frequency will get lower and lower because the space it is traveling through is expanding; it will redshift more and more until its energy becomes so low to be undetectable.

It will only be able to reach a region of space that is finite distance from where it started its journey. This furthest point in space that it can reach is within our currently observable Universe. However, by the time that photon reaches it (after it was traveling for infinite amount of time), that point in space will be far outside of our future observable Universe. Again because space is expanding, and the space beyond observable Universe is moving away from us much faster than the speed of light, it can never get to those regions of space (it actually can't even reach the edge of currently observable Universe, because that region of space is also already moving away from us faster than the speed of light).

This also means that there is a sphere around the Earth from which the photons emitted right now towards us, Earth's current location in space is the furthest they will ever be able to get to. By the time they get here after travelling for trillions of years, they'll be so much red shifted as to be undetectable.

House13Games t1_jco64y1 wrote on March 18, 2023 at 7:18 AM

But where do they go?

DudeWithAnAxeToGrind t1_jcqwb8m wrote on March 18, 2023 at 9:32 PM

Are you trolling me here? Assuming you are not, the above answers exactly where they'll go. It'll simply take them forever to get there, because space they have to travel through is itself expanding.

To make an analogy, imagine an ant on the surface of a balloon trying to get from the bottom to the top of the balloon. Imagine you can just keep inflating this balloon indefinitely, making it bigger and bigger. If you are inflating balloon fast enough so that it increases in size faster than ant can move over its surface, the ant will make a progress in its journey, but it will never be able to reach the top of the balloon. If ant can keep going infinitely, there's a spot on the balloon it will eventually reach after infinite amount of time has passed.

This is what happens to a photon traveling through Universe. The universe is expanding, and the far regions of space are receding from this photon faster than the photon can travel through space.