Four years ago, cosmologists reported the most startling finding about the universe since the late 1920s, when Edwin P. Hubble discovered that the cosmos is expanding. Most researchers assumed that gravity was slowing this expansion, but in 1998 two teams presented evidence that the universe is revving up its expansion rate. By measuring the brightness of distant supernovas, they concluded that some mysterious source, dubbed dark energy, must be opposing gravity’s braking action (SN: 3/21/98, p. 185).
Now, using entirely different evidence, another team has also found that cosmic expansion is accelerating.
George Efstathiou of the University of Cambridge in England and his colleagues examined the clustering pattern of 175,000 galaxies in nearby reaches of the universe. They then compared that pattern with the tiny fluctuations in temperatures of the cosmic microwave background, the radiation left over from the Big Bang. According to the leading theory of cosmology, those ancient temperature fluctuations provided the seeds for the clustering of galaxies seen today.
That supposition works, but only if the universe contains one-third the matter needed to make it perfectly flat, Efstathiou’s team found. However, other observations of the microwave background indicate that the universe has enough material to be flat.
Accepting the existence of dark energy is the way out of the conundrum, Efstathiou and his collaborators say. Its reality would make up for the rest of the material needed to make a flat universe. The researchers reported their findings in the Feb. 21 Monthly Notices of the Royal Astronomical Society and at a cosmology meeting last month in Cozumel, Mexico.
The finding “is compelling not because it is better than the supernova result, but because . . . it relies on different methodology and assumptions,” says Craig J. Hogan of the University of Washington in Seattle. But David N. Spergel of Princeton University calls the finding “only incremental progress,” because other studies had already indicated that the universe contains only one-third as much matter as needed for flatness and supported the case for dark energy.
