CLEVELAND — 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 consisten 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
suggested — based 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 cosmology — the age of the
universe, the measurement of its geometry and the
determination of large scale structure — all 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.