Energy, general relativity, and the Big Bang

Sean Carroll dissects an anti-Big-Bang petition in New Scientist, and explains why energy isn't conserved in general-relativistic cosmology. He's right on both counts, and his explanations are well worth reading. When I first read, in Edward Harrison's great and thoughtful (if rather out-of-date by now) textbook Cosmology: The Science of the Universe, that energy wasn't conserved in general relativity, I thought he was nuts. It's true, though; energy conservation is best seen as a special case of something more general that holds in GR. That's why a universe with a vacuum energy density, such as exists in inflationary cosmology and in dark-energy scenarios, can keep on inflating.