Hubble Completes Eight-Year Effort To Measure Expanding Universe

"Before Hubble, astronomers could not decide if the universe was 10billion or 20 billion years old. The size scale of the universe had arange so vast that it didn't allow astronomers to confront with anycertainty many of the most basic questions about the origin andeventual fate of the cosmos," said team leader Wendy Freedman, of theObservatories of the Carnegie Institution of Washington. "After allthese years, we are finally entering an era of precision cosmology.Now we can more reliably address the broader picture of the universe'sorigin, evolution and destiny."

The team's precise measurements are the key to learning about theexpansion rate of the universe, called the Hubble constant. Measuringthe Hubble constant was one of the three major goals for NASA's HubbleSpace Telescope before it was launched in 1990.

For the past 70 years astronomers have sought a precise measurement ofthe Hubble constant, ever since astronomer Edwin Hubble realized thatgalaxies were rushing away from each other at a rate proportional totheir distance, i.e. the farther away, the faster the recession. Formany years, right up until the launch of the Hubble telescope - therange of measured values for the expansion rate was from 50 to 100kilometers per second per megaparsec (a megaparsec, or mpc, is 3.26million light-years).

The team measured Hubble's constant at 70 km/sec/mpc, with anuncertainty of 10 percent. This means that a galaxy appears to bemoving 160,000 miles per hour faster for every 3.3 million light-years awayfrom Earth.

"The truth is out there, and we will find it," said Dr. Robert Kirshner,of Harvard University. "We used to disagree by a factor of 2; now we arejust as passionate about 10 percent. A factor of two is like beingunsure if you have one foot or two. Ten percent is like arguing aboutone toe. It's a big step forward."

Added Dr. Robert Kennicutt of the University of Arizona, a co-leader ofthe team: "Things are beginning to add up. The factor of two controversyis over."

The team used the Hubble telescope to observe 18 galaxies out to 65million light-years. They discovered almost 800 Cepheid variable stars,a special class of pulsating star used for accurate distancemeasurement. Although Cepheids are rare, they provide a very reliable"standard candle" for estimating intergalactic distances. The team usedthe stars to calibrate many different methods for measuring distances.

"Our results are a legacy from Hubble telescope that will be used in avariety of future research," said Dr. Jeremy Mould, of the AustralianNational University, also a co-leader of the team. "It's exciting to seethe different methods of measuring galaxy distances converge, calibratedby the Hubble Space Telescope."

Combining the Hubble constant measurement with estimates for the densityof the universe, the team determined that the universe is approximately12 billion years old - similar to the oldest stars. This discoveryclears up a nagging paradox that arose from previous age estimates. Theresearchers emphasize that the age estimate holds true if the universeis below the so-called "critical density" where it is delicatelybalanced between expanding forever or collapsing. Or, the universe ispervaded by a mysterious force pushing the galaxies farther apart, inwhich case the Hubble measurements point to an even older universe.

The universe's age is calculated using the expansion rate from precisedistance measurements, and the calculated age is refined based onwhether the universe appears to be accelerating or decelerating, giventhe amount of matter observed in space. A rapid expansion rate indicatesthe universe did not require as much time to reach its present size, andso it is younger than if it were expanding more slowly.

The Hubble Space Telescope Key Project team is an international group of27 astronomers from 13 different U.S. and international institutions.The Space Telescope Science Institute is operated by the Association ofUniversities for Research in Astronomy, Inc. for NASA, under contractwith NASA's Goddard Space Flight Center in Greenbelt, MD.

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