How can the universe expansion be accelerating? Should not gravity be slowing it down (albeit not fast enough to overcome the acceleration factor)? Or am I confusing semantic terms?
No, you're not confusing semantic terms. You're just puzzled, but then, so are most astronomers!
This is in fact one of the most exciting stories in cosmology in the past few years—the discovery that the universe's expansion is apparently increasing over time, rather than decelerating for exactly the reason you state. To understand what is causing this requires, at a minimum, that you understand the field equations in general relativity—something which most of us (myself included) can't claim.
But I can, perhaps, convey something of the reason why such a conclusion has been reached from some independent evidence. One line of evidence comes from so-called Type Ia supernovae—supernovae that result from the accumulation of too much hydrogen on a white dwarf, from its companion star. As Chandrasekhar discovered analytically in the early years of the 20th century, once the mass of a white dwarf exceeds a certain limit, now called the Chandrasekhar Limit, it goes supernova. Since the limit is pretty stable (or so astronomers think), every Type Ia supernova should behave more or less the same. That's why they can be used as "standard candles" in determining the distance to faraway galaxies. You know how bright they ought to be in reality. Once you determine how dim a given one looks, you can figure how far away it must be to look that dim.
This works pretty well out to distances where the expansion of the universe hasn't affected the separation between us and a galaxy over the time that it takes light to get from there to here. For example, light from a supernova in a galaxy that is 100 million light-years away should take 100 million years to reach us. With a Hubble constant of H0 = 70 km/s per megaparsec (about 3.26 million light-years), the galaxy should be receding from us at about 2,100 km/s. Over 100 million years, that only amounts to about 700,000 light-years, a minuscule fraction of the total distance, so there isn't much difference between how far the supernova was when it first burst 100 million years ago, and how far it is now.
But for a really distant galaxy, there is a substantial difference between how far a supernova in it was back when it burst, as compared with how far it is now that we see it. If the universe were static and not expanding, then the supernova would appear with a certain brightness. But because it is expanding, its brightness is different. You can think of the expanding universe as spreading out everything in it, including the photons emitted by the supernova. With the photons spread out, fewer of them strike any detector—our eyes, a telescope, a piece of film, whatever—over a given period of time, and the supernova appears dimmer. So an expanding universe makes really distant supernovae look dimmer than they should, if the universe were static.
What's more, based on how much dimmer they appear, you can tell how fast the universe has been expanding since the supernova burst, and by looking at supernovae at different distances away from us, you can get a history of the expansion velocity over time. It is the way that the Type Ia supernova dimming varies over distance (and therefore over time) that tells us that the universe's expansion is accelerating.
Now, you have to be careful. We assume that Type Ia supernovae are consistent, but are they really? If they weren't, then we can't really draw the conclusion of an accelerating expansion. Or, there might be something else that makes the dimming vary the way it does. Maybe there's intervening dust, and that makes the distant supernovae dimmer than you'd expect, based on a steady expansion. If we think it's dust, and the dust is uniform throughout the universe, then the dimming varies one way. If it's really an accelerating expansion, it varies another way. (You could get them to agree by making the dust distribute itself unevenly, but that's called cooking the hypothesis to match the facts, and should be avoided.) By making careful measurements of supernova "light curves" against the red shift of their host galaxies, we can distinguish between the two hypotheses.
The problem is that until recently, measurements were too crude to make it clear which one was happening. In the past couple of years, it seems that the accelerating expansion explanation is what's really happening, and that dust is not the explanation for the dimming variation. The MAP results are an important corroboration of the supernova evidence.
Copyright (c) 2003 Brian Tung