Edwin Hubble’s pioneering work in the 1920s showed that every one of the billions of galaxies out there are all moving away from each other at some incredible speed. Our name for that speed is not “fifty-five-miles-per-hour” or “Warp 7.2”; instead our name for that speed is the “Hubble Constant”. Over the years there’s been big debate on what the exact number of the Hubble Constant is; nailing down that exact number was one of the main reasons for building the Hubble Space Telescope in the first place. But whatever the final Hubble Constant number turned out to be, we all thought it didn’t alter what the main question of cosmology: is the Universe open or closed? Our mental model of cosmology was that if everything is moving away from everything else, then an explosion from a point source (the Big Bang) must have been what got the motion started. In that case, whether the Universe went on expanding forever (was open) or instead eventually reversed motion to collapse back in on itself (was closed) depended on the initial amount of matter created and its initial outward speed. Too little matter created or too high an initial velocity, gravity forces between the galaxies would be too weak to stop their outward expansion. Too much matter created or too slow an initial velocity, eventual collapse was a sure thing. It all depended on just how much matter got created in the Big Bang and the true value of the Hubble Constant.
Or so we thought. Perlmutter’s work showed that the Hubble Constant is changing with time in a way we never expected. For decades the main cosmological debate has been whether or not gravity forces were strong enough to affect the Hubble Constant and make it smaller and thereby slow down the expansion of the universe. Oops, wrong question. Perlmutter showed the Hubble Constant is getting bigger – the expansion of the universe is speeding up. What kind of Big Bang explosion has the debris still speeding up billions of years after the initial blast is over? That’s the NEW main question in cosmology. Gravity can’t explain such a thing – it’s a force that can only slow things down. So cosmologists have invented a new force called dark energy to explain what is speeding things up. But so far “dark energy” is only a name, an empty shell – just what IS this stuff and HOW does it work? Whoa, good question. As the Physics Today overview article concludes, “…Dark energy is one of the deepest and most exciting puzzles in all of science. It is likely that a crazy new idea is needed to explain cosmic speedup and resolve the cosmological constant problem. (That does not mean every crazy idea is a solution.) The payoff will be well worth the effort: We will gain new insights into the nature of matter, space, and time, and shed light on our cosmic destiny.”
Dark energy is definitely a mystery – though it is vaguely related to the prevailing “inflationary” theory of the early Big Bang (in a sense we seem to be going through a new phase of inflation). The interesting thing to me is that this dark energy now accounts for 2/3 of the mass/energy in the universe, and most of the remaining 1/3 is “dark matter” that we also know almost nothing about. Anybody who thinks science has discovered just about everything is pretty far off :)
An interesting article on a discusison of this recently at the American Museum of Natural History in NY.