Two research groups have recently reported finding evidence that the rate at which the universe is expanding is increasing, suggesting that galaxies are now flying away from one another at higher speeds than in the past.

        Up until now, cosmologists, astronomers who study the structure and evolution of the universe, had been trying to figure out how rapidly the expansion was slowing. If the universe's expansion were slowing rapidly enough, this would mean that eventually the expansion would halt and the universe would begin to contract, resulting in a "big crunch" when all the matter would be squashed into a tiny region much as it must have been in the original "big bang." If it were not slowing fast enough, then the expansion would continue forever, but with a gradually decreasing rate of expansion. The force responsible for the decrease in the expansion rate is assumed to be the mutual gravitational force exerted on all the matter in the universe by itself.

        There is convincing evidence for a great deal of matter in the universe that we have not been able to detect. Understanding this "missing mass," or dark matter, would help determine the rate at which the expansion is slowing. Consequently much effort is being put into identifying and detecting the dark matter. Now two research groups have found evidence that the expansion is not slowing at all, but instead is increasing.

        They have used telescopes around the world to determine the Hubble constant, a number that gives the expansion rate of the universe. This constant is determined simply by measuring the distance to a galaxy and then dividing its distance into its recession speed, the speed with which it is moving away. We learn its speed from the shift in its spectral lines. Of course this is not so easy as it sounds. Obtaining the spectrum of a very distant and, therefore, very faint galaxy is hard because there is so little light reaching us to spread into a spectrum. Determining the distance accurately is even harder. If we compare the brightness of something that we see with how bright it really is, its intrinsic brightness, then we can find out how far away it is because we know how the intensity of light diminishes as it spreads into space from its source.

        Astronomers seeking to measure the distances to galaxies have searched for objects whose intrinsic brightnesses are known -- "standard candles." Of course they must also be very bright to be seen at such great distances. The best known standard candles, Cepheid variables whose average luminosities are determined from their pulsation periods, can be seen to distances of a few hundred million light years. The distance in light years is a number that equals the duration of time light has spent reaching us from the emitting object and thus tells us how long ago the light left. Measuring the recession speed of galaxies several billion years ago means we must measure the distances to galaxies that are several billion light years away. This is too far to see Cepheid variables.

        The standard candle being used by these two research groups is a particular type of supernova, an exploding star whose brightness can rival the brightness of an entire galaxy for a few days or weeks. The intrinsic brightness of these explosions has been calibrated from those observed in nearer galaxies whose distances have been learned from other standard candles. The supernovae are then used as standard candles in more distant galaxies. They have yielded the astonishing result that when these explosions occurred more than 5 billion years ago the expansion rate of the universe was less than the expansion rate that has been measured for galaxies within a few hundred million light years of us. Theorists are scrambling, now, to see how their models could account for this discovery, a discovery as astonishing as Hubble's discovery of the expansion of the universe 70 years ago.