Using NASA’s orbiting Chandra x-ray telescope to measure the radiation of 26 galaxy clusters, an international team of astrophysicists has confirmed and strengthened the case that “dark energy” comprises the bulk of the mass of the universe, and that the expansion of the universe is accelerating.

The presence of dark energy was first indicated more than a decade ago by examination of distant supernova explosions, then later buttressed by observations of detailed “splotches” in the cosmic background radiation that is the afterglow of the “big bang” at the origin of our universe.

This third method produces results which agree with the two preceding approaches, indicating that about 75% of the total mass of the universe consists of dark energy, while “dark matter” makes up 21%, and visible matter just 4%. To attain this end, the astronomers measured the frequency and brightness of the x-rays emitted by the galaxy clusters—enormous aggregations of scores of galaxies filled with glowing gases heated to nearly 100 million degrees, hot enough to generate x-rays—at varying distances from 1 to 8 billion light years. This allowed the scientists to determine that, after an initial phase of deceleration for roughly the first half its existence, the universe began expanding at an accelerating rate some 6 billion years ago.

The real mystery, of course, is precisely what this “repulsive gravity”, dark energy is. Some theorize it could be due to extra dimensions of space, or that it could be an energy intrinsic within empty space like Einstein’s cosmological constant. Alternative explanations abound.
(New Scientist, BBC News)

– From the May 2004 Austin Review KB