eli5: when a star leaves the observable universe due to expansion, does it get fainter over time from our perspective, or it just appears to turn off, like a flashlight?

Question

eli5: when a star leaves the observable universe due to expansion, does it get fainter over time from our perspective, or it just appears to turn off, like a flashlight?

In: 30

Answer 1

When light leaves a star, it spreads out. When it spreads out, it covers a larger area and the luminosity declines as a result. The same amount of light is spread thin across more space.

For stars that are very far away from us, an awful lot of spreading has been done by the time their light reaches us. By that token, stars get dimmer to us as the universe expands and they get farther away.

If we were able to pintpoint a start that we knew was going to become so far and so dim that we’d fail to be able to detect or observe it, it would get fainter until that happened.