Step on it. X-ray observations (left) of clusters of galaxies such as Abell 2390 confirm that the expansion of the universe is accelerating.

Galaxy Clusters Affirm Acceleration

No matter how you gauge it, the expansion of the universe is speeding up, reports a team of astronomers and astrophysicists. Measurements of x-rays from far-flung clusters of galaxies bolster previous estimates from supernovae and the cosmic background radiation. But the new results depend on an assumption that skeptics may question.

In 1998, two independent research teams reached the stunning conclusion that some distant exploding stars were farther away than they would be if expansion were constant or slowing down as expected (ScienceNOW, 26 February 1998. The case for accelerating expansion gained strength last year. Researchers working with the Wilkinson Microwave Anisotropy Probe satellite measured tiny spatial variations in the cosmic microwave background and deduced that the universe must consist mainly of space-stretching "dark energy" (ScienceNOW, 11 February 2003). But the analysis did not directly measure expansion.

Now, x-rays from clusters of thousands of galaxies provide more direct evidence of accelerated expansion, report researchers working with NASA's orbiting Chandra X-ray Observatory. Led by astronomer Steven Allen of the University of Cambridge, U.K., the team used the energy spectra of the x-rays from 26 clusters to infer the amount of hot gas in each cluster. That in turn determined the amount of x-rays the cluster emits. The researchers deduced the distance to a cluster by noting how bright it appeared in x-rays, just as a driver might judge the distance to another car by noting the brightness of its taillights.

The researchers then compared the results to how far away each cluster would be assuming constant expansion, a distance determined by how red the cluster appears in visible and infrared light. The comparisons confirmed that the universe began accelerating 6 billion years ago, they reported today at a press conference at NASA Headquarters in Washington, D.C.

However, the analysis assumes that hot gas accounts for the same fraction of matter in every cluster, no matter how old. That isn't obviously true, says astrophysicist Neta Bahcall of Princeton University in New Jersey, especially for the oldest, most distant clusters. Alastair Edge, an astrophysicist at the University of Durham, U.K., agrees but says most researchers will be persuaded in the end. "Everyone's going to scratch their heads for a few months," he says, "and then they'll probably accept it."