The rest of the universe bar our galazy ceases to exist. When would we know about it?

[dtilque]

@Chronos

And the period of 550,000 years is in the first paragraph of that Wikipedia link. On that kind of time scale, you’ve got to worry about not only Proxima’s motion, but also the motion of other stars in our vicinity. There are a lot of red dwarfs out there that are on their own, and occasionally one of them will wander close to the Sun.

According to the paper I linked to, Proxima’s period is 591,000 years. It’s near apastron (furthest point in its orbit from the other two). As for other stars passing close, see Scholz’s Star.

@Darren_Garrison

The Andromeda Galaxy wouldn’t “blink out” but fade away over tens of thousands of years, from the nearest stars to the furtherest ones. It would be like see in an MRI of a galaxy.

This of course would apply to whatever galaxy we decide is closest but not part of the Milky Way. These kinds of questions are always poorly defined, since as Chronos pointed out, it doesn’t say what happens to the light outside our galaxy that’s already on the way here.

@watchwolf49

I hope there’s other arguments … because the logic you give could also be applied to Andromeda … we are on collision course right now …

Maybe, although my understanding is that we aren’t actually in orbit around Andromeda. The two galaxies have not actually circled each other (the universe isn’t old enough for that to have happened). We’re just falling towards each other.

Consider what is part of the Solar System. Perhaps the best definition is “the Sun and everything in orbit around it”. Can you think of a better definition (with the appropriate changes) for what is part of the galaxy?

[Chronos]

We certainly can’t say that our Galaxy and the Andromeda galaxy are on a collision course. They might be, but we can’t tell. All we can actually tell is that we’re currently getting closer to each other. But then, when you’re on the freeway and look at cars on the other side of the median, they’re getting closer to you, too. That doesn’t mean that you’re going to collide.

[Kimble]

@Chronos

We certainly can’t say that our Galaxy and the Andromeda galaxy are on a collision course. They might be, but we can’t tell. All we can actually tell is that we’re currently getting closer to each other. But then, when you’re on the freeway and look at cars on the other side of the median, they’re getting closer to you, too. That doesn’t mean that you’re going to collide.

Has that changed since 2012? I thought these three papers showed that Andromeda’s transverse motion was small enough that the collision was inevitable. (Wikipedia, Nature)