Dating the cosmos from super stars ... and relic radiation.

The Hubble constant provides a measure of the expansion of the universe, indicating the age and size of the cosmos (SN: 10/8/94, p.232). Next week, astronomers using the repaired Hubble Space Telescope will make a long-awaited announcement -- their estimated value for the constant. In the meantime, researchers using ground-based telescopes have reported a flurry of new measurements.

Obtaining a value for the Hubble constant requires astronomers to gauge accurately the distance to objects far from the Milky Way. Several teams use a succession of "standard candles," the celestial equivalent of lightbulbs of known wattage, to calibrate, step-by-step, distances in the universe. But Robert P. Kirshner of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and an international team of colleagues use an approach that provides direct distances to objects several hundred million light-years from Earth.

He and his coworkers rely on observations of the brilliant remnants of type II supernovas, the rapidly expanding shells of hydrogen-rich gas hurled into space when the cores of massive stars collapse. By measuring the color and brightness of the expanding gas, the team deduces how big a patch of sky the exploded star occupies at different times. Comparing the size of the patches with the expansion velocity yields the distance to the supernova.

In the Sept. 1 ASTROPHYSICAL JOURNAL, Kirshner and his colleagues report the results of their recent observations of five type II supernovas. Combined with findings from their previous studies of 13 similar objects, these results show that the Hubble constant has a value ranging from 67 to 79 kilometers per second per megaparsec. This would make the cosmos no more than about 14 billion years old, younger than many theorists believe but perhaps old enough to allow for the evolution of the most ancient known groups of stars.

... and relic radiation

Many methods of measuring the Hubble constant are at least partly flawed because they don't peer deeply enough into the universe. John P. Huchra of Harvard University notes that just because the Hubble constant appears to have a certain value a few hundred million light-years from Earth, it doesn't mean that number can be applied to the rest of the cosmos.

Several new strategies enable astronomers to measure the Hubble constant directly at larger distances. Although the methods don't require calibration from a nearby galaxy, some of the techniques force astronomers to make unproved assumptions about the shape of galaxies and galaxy clusters.

One of these strategies relies on a phenomenon known as the Sunyaev-Zel'dovich effect. This effect involves the interaction between hot, X-ray emitting gas found in galaxy clusters and the cosmic microwave background, the relic radiation left over from the Big Bang.

Some of the microwave background photons passing through a cluster get kicked to a higher energy by electrons in the hot gas there. A radio telescope measuring the microwave background near a cluster would thus see a decrease in the number of microwave photons. This would make it appear that the glow from the radiation has a slightly lower temperature near a cluster than elsewhere in the universe.

Combining the perceived drop in the microwave background temperature with data on the intensity of X rays in the cluster, astronomers can calculate the length of the galaxy cluster along the line of sight to Earth. At this point, they typically make a simplifying assumption: They model the cluster as a sphere.

If a cluster takes the form of a sphere, its length is equal to its width. By comparing the cluster's true width with its diameter on the sky as observed from Earth, scientists can calculate how far away the cluster must lie. Armed with the distance to a far-off cluster, astronomers can then obtain a more accurate value for the Hubble constant.

Richard Saunders, Michael Jones, and their colleagues at the University of Cambridge in England used the Ryle telescope, a group of five 13-meter radio dishes in Cambridge, to observe the Sunyaev-Zel'dovich effect in the galaxy cluster Abell 2218. The team's latest results, reported in the most recent ASTROPHYSICAL LETTERS AND COMMUNICATIONS (vol. 29, no. 5), suggest the Hubble constant is low -- between 25 and 55. This implies that the universe is 18 to 20 billion years old. Preliminary observations of another cluster, Abell 1413, give a similar value, Saunders says.

Saunders asserts that had he assumed these clusters were highly elongated -- cigar-shaped rather than spherical -- the Hubble constant his team inferred would only have been lower, indicating an even older age for the universe. However, other groups observing the Sunyaev-Zel'dovich effect find different values for the constant.