The Milky Way Will Be It

As the universe's expansion accelerates, other galaxies will someday fall off the observable horizon—leaving ours as lonely as scientists before Einstein used to think it was.

Question: Based on our current picture of the universe, what do you predict its long-term fate will be?
Robert Kirshner: Right.  Well, we're very good at observing the past and telescopes really let you see light that was emitted 10 billion years ago.  We do not see so clearly into the future, so if you make a prediction about what the universe is going to be like a billion years from now, it's a little harder to test it. 
Nevertheless, if the dark energy is really just the cosmological constant and there are no surprises, or another way to say it is, if the latest picture that we have in the last 10 years is absolutely perfect, which, you know, a certain kind of modesty is probably a better policy.  But never mind that.  If we've got the whole story and we know exactly what the dark energy is, then you can predict what the expansion is going to be in the future.  And it will be literally exponential expansion.  That means the rate of expansion will depend on how big it is.  It will go faster, and faster, and faster, and it's like compound interest, or any of those things that kind of get out of hand.  The expansion of the universe will get out of hand.  And what that means, for astronomy is that distant galaxies, which are moving away from us at a fairly large fraction of the speed of light, will be moving away faster and faster, their light will get red shifted.  That means that the light gets stretched out to longer wave lengths and they get dimmer and eventually we just won't be able to see them. 
So what will happen is that as time goes by, the distant galaxies will kind of go beyond the horizon for us.  We won't be able to see them.  Another way to say this is the piece of the universe that we can see will kind of shrink in and we will have fewer and fewer things in it until finally -- well finally -- until, if you follow this idea far enough, it will just be us and the Andromeda Galaxy and we'll be whirling around and eventually we'd collide with one another and there'd just be one galaxy, and that would be the whole universe. 
And what's so funny about it is that that is the picture that Einstein started with.  That the Milky Way galaxy was the whole universe.  We will have come full circle.  Also, it will be impossible at that date in the long distant future, 50 or 100 billion years in the future, it will be impossible for astronomers to sort this out because there won't be any galaxies for them to observe.  So, that's why we have to do this now.  That's why increasing astronomy budgets in the coming decade is really an important thing because in 100 billion years, it will be impossible in principle to make these measurements.Recorded on February 17, 2010Interviewed by Austin
Allen