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Posted 9/10/97
Is the universe younger than its stars? Two Case Western Reserve University physicists, in collaboration with colleagues at Yale and Arizona, have closed the gap between the age discrepancies with new estimates, finding the stars' ages as almost three billion years younger than previously thought.
New information gathered by the Hipparcos satellite, combined with a reanalysis of other distance data, has enabled the researchers to refine the lower age limit of the universe to 9.6 billion years.
The researchers were Lawrence Krauss, chair and the Ambrose Swasey Professor of Physics and professor of astronomy, and Peter Kernan, CWRU postdoctoral fellow in physics, along with colleagues Brian Chaboyer at Steward Observatory and Pierre Demarque at Yale University
For years, physicists have been perplexed about the long-standing problem of how globular clusters have appeared older than the age of the universe. Measurements of the expansion rate of the universe. Measurements of the expansion rate of the universe, otherwise known as the Hubble Constant, suggest an upper limit for the age of the universe of between 8-11 billion years.
Astrophysical Journal will publish the findings by Krauss's team in early 1998.
The researchers recalculated former data, incorporating new distance measures from Hipparcos and other sources, to determine the luminosity of stars in 17 old globular clusters in the halo of the Milky Way galaxy. This halo is older than the disk of the galaxy, which consists of the concentration of younger stars, gases, and dust clouds revolving around the Milky Way's galactic center somewhere near the constellation Sagittarius.
The distance of these star clusters remains one of the keys to solving the age dilemma, according to Kernan. Krauss added that the group made a special effort to reconsider not only the new most-likely value for these distances, but also the possible uncertainties which remain at the present time.
In 1996, this research team, using a Monte Carlo computer simulation of more than four million stellar models, estimated the age limits as 12-15 billion years. Science published their first study in February 1996.
"Our findings now overlap the Hubble age and are consistent with an open universe, or a flat, matter-dominated universe," said Krauss.
Their new results now place the globular clusters' average age at 11.5 billion years and confirm that the star systems are farther away from Earth and younger than previously thought, which was suggested as early Hipparcos data began to be released in February.
The Hipparcos satellite provided parallaxes, the angle by which nearby stars move in relation to background stars when viewed from Earth over the course of the year. The satellite was able to measure many more, and smaller, parallaxes than had been measured before.
Krauss explained that this technique is the same one which surveyors had regularly used to measure the distance between two objects on earth. The trickier part of the data interpretation involves relating these measured star distances to those in the much farther globular clusters.
"If the stars in globular clusters are actually farther away than we thought, they must also be brighter than we thought. If brighter, the stars are burning faster. This means the stars would evolve more quickly and thus the globular clusters would be younger than we originally thought," added Krauss.
When the research team first ran the calculations on the age of the universe, it took almost three months of computer calculations to run all the variables, according to Kernan. They then separately performed the part of the calculation related to the distance to globular clusters, because it involved by far the largest remaining uncertainty, Krauss added. Thus, future measurements of this quantity could be easily accommodated in revised estimates.
This time the new calculations took about 24 hours of computer time to establish the new age estimate because an extensive stellar evolution database had already been created, according to Kernan.
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