Sunday, March 08, 2015

Far Horizons

You know that cute galaxy you saw in the NASA travel posters? Out at z = 1.8 redshift? (A few billion light years away). Were you planning to visit?

Well you can forget it. It's already gone. Over the the cosmic horizon. Already separating from us at more than the speed of light - so you can't send a message either. Oh, we will still get light from it for a while, but none of that light will come from its cosmic (co-moving) now. All the light (or anything else) we will ever get from it has already been emitted.

Of course that assumes that the dark energy really is an effective cosmic constant, and that the universe will continue (has continued) to accelerate its expansion. That galaxy, and all the others except for our local group will gradually fade out to infinite red shift over the horizon. But it's still not too late to book a trip to more or less anywhere in the local super cluster.