eli5 So if the universe is constantly expanding and there is no void then what happens to light? Does it just get to the furthest expansion and wait for it to expand more? Does the expansion of the universe “stretch” light enough where it loses too much energy and stops?

Question

Basically, where does light end up? I know that there is no void and that the universe is all that there is and it is constantly expanding, but what happens to light? Wouldn’t it reach the edge? Does cosmological redshift eventually cause the light to stop after long enough?

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Answer 1

Light does not have a specific “destination” in the expanding universe. It continues to travel in a straight line, but its wavelength is stretched and its frequency is lowered due to the expansion of the universe. This is known as the cosmological redshift.

As the universe expands, the space between galaxies also expands, causing the light to be stretched and redshifted. This means that the light will continue to travel, but it will become increasingly redshifted and dimmer over time. Eventually, after a very long time, the light will become so redshifted and dim that it will be difficult to detect.

However, it is important to note that the universe is not expanding into a “void” or emptiness. The universe is everything that exists, and it is expanding within itself. There is no edge or boundary to the universe, so light does not have to reach an edge or stop at any specific point. It simply continues to travel through the expanding universe.