Hubble constant: controversy continues.

Like contestants in a never-ending tennis match, teams of cosmologists continue to argue back and forth about the numerical value of the Hubble constant. This hotly debated number, which relates the recession velocity of a galaxy to its distance from Earth, provides a measure of the age and size of the expanding universe.

Last fall, Wendy L. Freedman of the Carnegie Observatories in Pasadena, Calif., and her colleagues announced a new distance to the galaxy M100, a member of the Virgo cluster of galaxies, based on Hubble Space Telescope observations. From this measurement, they calculated a Hubble constant of 80 kilometers per second per megaparsec. This indicates a cosmos between 8 and 12 billion years old (SN: 10/8/94, p.232).

That poses a predicament because it makes the universe appear younger than its oldest stars, believed to be 16 billion years old.

Last week, cosmologists heard from Allan R. Sandage of the Carnegie Observatories, a longtime proponent of a smaller Hubble constant and an older universe. He announced that two new observations with the Hubble telescope have each yielded a Hubble constant of about 50. The findings, which agree with his team's earlier results, would make the cosmos about 20 billion years old, avoiding any conflict with the age of its elderly stars.

"I firmly believe there is no [cosmological] crisis," Sandage asserted at the Particles, Strings, and Cosmology symposium in Baltimore.

In their study, Sandage and his collaborators employed a special strategy to measure the distance to two galaxies, NGC 4496A and NGC 4536. Using one type of distance indicator, youthful pulsating stars known as Cepheid variables, the team calibrated the true brightness of another indicator in the same galaxies, exploded stars known as type 1a supernovas.

Critics argue that Sandage and his coworkers underestimate the Hubble constant because they don't account for the possibility that some of their 1a supernovas may be intrinsically brighter than average (SN: 2/18/95, p.106). But Sandage argues it's unlikely that all five of the exploded stars his team has examined are unusually bright.

However, Mark M. Phillips of the Cerro Tololo Inter-American Observatory in La Serena, Chile, says that's a distinct possibility. Sandage's team relies on 1a supernovas in galaxies that contain lots of Cepheids. Such galaxies may tend to harbor 1a supernovas with higher intrinsic brightnesses, Phillips notes. He adds that published data on two of the stellar explosions, one in 1895 and another in 1960, aren't reliable.

Meanwhile, Freedman says that her team has corroborated their initial finding and expects much more data. "It feels like we're drinking from a firehose. Within a year to 18 months we'll have much more to say about the Hubble constant."