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How Old is the Universe?

February 06, 2001

Most astronomers would agree that the age of the Universe - the time
elapsed since the "Big Bang" - is one of the "holy grails of
cosmology".

Despite great efforts during recent years, the various estimates of
this basic number have resulted in rather diverse values. When derived
from current cosmological models, it depends on a number of
theoretical assumptions that are not very well constrained by the
incomplete available observational data. At present, a value in the
range of 10-16 billion years is considered most likely.

But now, an international team of astronomers has used the powerful
ESO Very Large Telescope (VLT) and its very efficient spectrograph
UVES to perform a unique measurement that paves the way for a new and
more accurate determination of the age of the Universe. They measured
for the first time the amount of the radioactive isotope Uranium-238
in a star that was born when the Milky Way, the galaxy in which we
live, was still forming. It is the first measurement ever of uranium
outside the Solar System.

This method works in a way similar to the well-known Carbon-14 dating
in archaeology, but over much longer times. Ever since the star was
born, the Uranium "clock" has ticked away over the eons, unaffected by
the turbulent history of the Milky Way. It now reads 12.5 billion
years. Since the star obviously cannot be older than the Universe, it
means that the Universe must be older than that.

Although the stated uncertainty is still about 25% or about +-3
billion years, this is only to a minor extent due to the astronomical
observation. The main problem is the current absence of accurate
knowledge of some of the basic atomic and nuclear properties of the
elements involved. However, further laboratory work will greatly
improve this situation and a more accurate value for the age of the
star and implicitly, for the Universe, should therefore be forthcoming
before long.

European Southern Observatory (ESO)