Cosmologists from Case Western Reserve University and Dartmouth College have continued efforts to refine the age of the universe by using new information from a variety of sources to calculate a new lower age limit that is 1.2 billion years higher than previous age limits. The new information lends new support to the potential presence of a strange new form of energy that dominates approximately 95 percent of the universe and causes its expansion to accelerate.

In a paper published on January 3 in Science, Lawrence M. Krauss, the Ambrose Swasey Professor and chair of physics at Case Western Reserve University, and Brian Chaboyer of the department of physics and astronomy at Dartmouth College establish that with 95 percent confidence the age of the universe is between 11.2 and 20 billion years old.

Their estimates were derived from updated information about clusters of the oldest stars in the Milky Way galaxy and refined parameter estimates for their star evolution.

Prior estimates by Krauss and a team of researchers in 1996 and later in 1997 placed the a lower limit of approximately 10 billion years, which marginally was consistent with the possibility of a flat, matter-dominated universe.

Dating the age of the universe has evolved since 1929 when Edwin Hubble's discovery that the universe is expanding suggestedbased on his earliest measurements that the universe was only 1.5 billion years old. Even at that time, it was in obvious contradiction with the age of the Earth, which was even then known to be several billion years old. In the 1980s, estimates of stellar ages suggested that the universe had to be at least 16-20 billion years old. The inconsistency with the Hubble age provided motivation to reintroduce the cosmological constant first proposed by Albert Einstein in 1916. However, refined estimates of stellar ages, performed by Krauss and Chaboyer, among others, later resolved this apparent inconsistency.

This was the right time to reexamine stellar age estimates, says Krauss, because of refined possibilities for dating globular star clusters, in light of new measurements of the redshift versus distance for supernovae and new information about cosmic microwave background.

The new comparison of the lower limit on the age of the oldest stars in our galaxy with the upper limit on the age of the universe itself, determined by refined measurements of the expansion rate produces independent evidence for dark energy, said Krauss.

He added that as a result, for the first time the three fundamental measurements of cosmologythe age of the universe, the measurement of its geometry and the determination of large scale structureall point independently toward exactly the same ultimate model of the cosmos.

The globular clusters used in the analysis exist in the halo of the Milky Way galaxy, thought to have formed well before primordial gases collapsed to form its present disk structure. Each of the clusters is a compact group of up to one million stars. A determination of the brightness of stars in each cluster as a function of their color allows one to estimate their age. The new estimates are based on new distance determinations to the clusters, allowing a better determination of the intrinsic brightness of the stars.

The Monte Carlo simulation techniques used by Krauss and Chaboyer, in which thousands of different stars were evolved on computers and compared to the observed distributions of stars in globular clusters, complement other recent techniques used to estimate stellar ages. Radioactive dating of stars has been performed using measurements of the abundance of thorium and uranium in several of the clusters' stars. The cooling of white dwarf stars, which are stars near the end of their lives where luminosities begin to fade, also allows lowers limits on stellar ages to be derived. Detailed pictures from the Hubble Space Telescope have enabled observations of fainter stars for a better age estimate.

The technique used by Krauss and Chaboyer relies on the main sequence turnoff time-scale of the stars based on the star's surface temperature and luminosity as hydrogen in the star's core is burnt up over the life of the star and the star begins to dim.

The new estimated distance to the globular star clusters are an essential feature in the new results, obtained by using white dwarfs, the main sequence stars, so-called horizontal branch stars and a subclass of the horizontal branch stars called RR Lyra stars, all of which can be used as "standard candles" to calibrate the intrinsic luminosity of stars in the cluster.

The researchers also updated other critical factors determining the rate of stellar evolution, including the abundance of oxygen, the treatment of convection within the stars, the primordial helium abundance, helium diffusion, stellar opacities and the transformation from theoretical temperatures and luminosities to observed colors and magnitudes.

While the research focused on the age limits of the universe, Krauss stressed that this program is part of a broad scale effort to pin down the fundamental parameters of cosmology.

"We are living in a golden age of observational cosmology, where our fundamental picture of the universe has been revolutionized in the last decade. At the same time, we are establishing the essential features of the cosmos that will serve as the datum at the basis for fundamental physics in the 21st century and beyond," says Krauss.