Hubble Constant Press Release

PHYSICS NEWS UPDATE

The American Institute of Physics Bulletin of Physics News Number 270 May 9, 1996 by Phillip F. Schewe and Ben Stein

TWO EFFORTS TO MEASURE THE HUBBLE CONSTANT are converging somewhat. Wendy Freedman of the Carnegie Institution reported at a NASA press conference today that she and her colleagues were finding that values for the Hubble constant (H), a measure of the expansion of the universe, hovered in the range 68 to 78 km/sec/Mpc. (In 1994, they reported a preliminary value of 80.) A separate group led by Allan Sandage, also of Carnegie, recently reported a Hubble constant of 57. Freedman's team is midway through a 3-year program of measuring the distance to 20 distant galaxies by observing Cepheid variable stars, whose intrinsic brightness is related to the rate at which their luminosity varies. These observations in turn can be used to calibrate other means for determining distances to objects at even larger scales where local gravitational interactions have a lesser impact on a calculation of H. The secondary yardstick methods include the determination of the peak brightness of type-Ia supernovas and the use of the Tully-Fisher relation, according to which a galaxy's luminosity is related to its rotation rate. The latest entry in Freedman's inventory is galaxy NGC1365 in the Fornax cluster, at a distance of 60 million light years. (NASA press release, 8 May 1996.)

THE OLDEST STARS IN THE MILKY WAY ARE 15 BILLION YEARS OLD.

An important adjunct to the debate over the Hubble constant is the notion that the universe cannot be older than its older stars, which appear to be those in globular clusters, spherical clumps of hundreds of thousands or millions of stars found near and around our galaxy. Don VandenBerg of the University of Victoria (davb@uvvm.uvic.ca, 614-721-7739) uses the Canada-France-Hawaii telescope to view the ancient, metal-poor stars (they largely lack the elements heavier than helium which many younger stars inherit from earlier supernova explosions ) in globular clusters. By plotting the stars' luminosities versus their colors, and by employing the standard model for stellar evolution, the age of the stars can be calculated. VandenBerg, speaking at last week's meeting of the American Physical Society in Indianapolis, said the oldest reliably dated stars, in globular cluster M92, were most likely 15 billion years old. Uncertainties in the determination of the distances to the clusters (effecting calculations of the stars' luminosities) might permit an age of 13 or even 12 billion years. But VandenBerg asserted that the ages could not be much younger than that. New observations of his in globular cluster M13 did not alter this assessment.

SKY & TELESCOPE NEWS BULLETIN MAY 10, 1996

HOW OLD THE UNIVERSE?

On May 9th two teams of scientists issued a "midterm report" on the expansion rate of the universe, one of the long-term Key Projects being undertaken by the Hubble Space Telescope. Both groups use HST to pick out Cepheid variable stars in distant galaxies. These stars pulsate at rates related to their brightness, so they serve as "standard candles" that allow distances to be measured accurately. It's then possible to calculate how fast the universe is flying apart. One team, led by Wendy Freedman, finds that this controversial Hubble "constant" lies between 68 and 78 kilometers per second per megaparsec. That value, combined with the usual assumption about the large-scale density of matter, implies that the universe is only 8 or 9 billion years old. The second team, led by Allan Sandage, studied Cepheids only in galaxies in which Type Ia supernovae have been seen. That enables them to gauge other supernova-hosting galaxies hundreds of millions of light-years away. Their Hubble value is only 57 km/sec/Mpc, implying an age of about 12 billion years.

The crux of all this is that globular star clusters in our Milky Way are 12 to 16 billion years old. So can the universe be younger than the stars it contains? No, but the results may mean that ours is a low-density universe whose expansion is only gradually slowed by gravity. Or Einstein's infamous "cosmological constant," which imbues empty space with antigravity-like properties, also might be involved.