Trust but verify. That little piece of strategy that Ronald Reagan applied to the measurement of nuclear stockpiles during the cold war is being used by astronomers.
When measuring the size and age of the universe, astronomers have relied on the "Hubble constant," named after the late American astronomer Edwin Hubble. The constant relates a galaxy's distance to the speed at which it's moving away from us.
Astronomers determine a galaxy's speed by looking at its light. The redder the light, the faster the speed. Divide that speed by the Hubble constant, and you get the galaxy's distance from Earth.
As Max Bonamente at NASA's Marshall Space Flight Center in Huntsville, Ala., has explained, "astronomers absolutely need to trust this number because we use it for countless calculations."
Attempts to verify that number, however, don't yet support that level of trust.
This high-stakes need for verification has energized a painstaking effort to refine the Hubble constant. Fritz Benedict and Barbara McArthur at the University of Texas in Austin led the international team that published their attempt to refine this number in this month's Astronomical Journal.
To verify the Hubble constant, astronomers compare the distance they get using the constant against the distance they get from an independent measurement based on Cepheid variable stars. A Cepheid's light varies with time. This variation reflects the star's inherent brightness. Once astronomers know this inherent brightness, or intrinsic luminosity, they can estimate the distance to the Cepheid by noting how dim it appears from Earth.
But the relationship between a Cepheid's variability and its intrinsic luminosity has not been known accurately enough to serve to verify the Hubble constant. The international team worked largely with nearby Cepheids whose distance could be measured directly by geometrical means. Comparing these absolute distances with those indicated by the Cepheid's apparent brightness helped refine this relationship.
The team had to take account of myriad errors. They include minute motions of the Hubble Space Telescope that made the observations. "We've been cranking on this since 1977," Dr. Benedict says. Now this picky picky work has produced a result that Benedict says "has excited me more than any [other result] in my 35-year career."
Astronomers now have a more accurate distance-measuring tool to use wherever they can find a Cepheid.
Last August, Dr. Bonamente and colleagues reported new distance measurements to 38 galaxy clusters ranging from 1.4 billion to 9.9 billion light years away. They used radio and X-ray observations to estimate the physical size of a galaxy cluster. Geometric triangulation then gave the cluster's distance. Their check of the Hubble constant confirmed its currently accepted value.
But Kris Stanek at Ohio State University in Columbus and colleagues found that value in error. They reported last August how they measured the intrinsic brightness of a binary star system in galaxy M33. Judging the stars' distance by how dim they appear from Earth, they found it to be 3 million light years away. The estimate based on the Hubble constant's accepted value was only 2.6 million light years. The true value of the constant may be 15 percent smaller – and the universe may be 15 percent larger and older – than we thought.
Astronomers will "trust" whatever they believe is the best value for the constant. But the need to verify remains.