Thursday, March 18, 2004
The cost of w
Out here in the desert today we were worrying about dark energy. Seventy percent of the energy in the universe is some mysterious stuff whose density, as far as we can tell, remains constant as the universe expands. (In contrast to, say, ordinary matter, which dilutes away as a result of the expansion.)
The leading candidate for dark energy is vacuum energy, or the cosmological constant, which is exactly constant throughout space and time. But it's not an especially attractive candidate, so we're looking for alternatives anywhere we can -- some persistent but nevertheless dynamical field, or even a modification of gravity on large scales.
To tell if the dark energy is vacuum energy, we try to see whether it's changing or absolutely constant. The parameter we go out and measure is w; its value is -1 for pure vacuum energy. We know that w is pretty close to -1; if it's a little bit greater (like -0.9) the dark energy is gradually diminishing, while if it's less than -1 the density is actually increasing. (The amount of dark energy per cubic centimeter is going up everywhere in the universe.) That seems crazy, which was what my talk this morning was about. It is crazy, but it can't be completely ruled out, so we should keep an open mind.
The first direct evidence for dark energy came from using supernovae as standard candles (objects whose intrinsic brightness is known, so their distance can be inferred from their aparent brightness). Many of the talks today were devoted to current and future supernova searches. There was much discussion between the observers, who wanted to know just what kind of deviation from w=-1 we should expect, and theorists (like myself) who kept admitting that we have no idea. I don't think they believed us. Unfortunately it's a question we have to keep asking, since it costs money to do these observations; money is tight, and we have to decide which experiments are most deserving of our efforts.