I was reading a thread on immortality and dimly remembered a book which ended in the characters ? species ? encoding themselves as microwaves which would allow them to continue to exist as the universe expanded and cooled towards absolute zero. The idea was that they would continue to think, just more and more slowly. I thought that it was by Gregory Benford but it doesn't seem to be Timescape, Against Infinity or the Galactic Centre series (all of which I have forgotten but can look at the last few pages). Any ideas?