How do we measure the size and the age of the
Astronomers estimate that the Big Bang occurred between 10 and 20
ago. They estimate the age of the Universe in two ways: (a) by
for the oldest stars; and (b) by measuring the rate of expansion of the
Universe and extrapolating back to the Big Bang.
The Oldest Stars
Astronomers can figure out the ages of some of the oldest stars in the
Universe by studying globular clusters.
A globular cluster is a dense collection of close to a million stars, all
of which formed at roughly the same time.
The density of stars near the center of a globular cluster is enormous. If
we lived near the center of a globular cluster, there would be
several hundred thousand stars closer to us than Alpha Centauri,
our current nearest stellar neighbor.
The life cycle of a star depends upon its mass. High mass stars are much
brighter than low mass stars; thus they rapidly burn through their supply of
hydrogen fuel. A star like the Sun has enough fuel in its core to burn at its
current brightness for approximately 9 billion years. A star that is
twice as massive as the Sun will burn through its fuel supply in only 800
million years. A 10 solar mass star (a star that is 10 times more massive than the Sun) burns nearly a thousand times brighter and
has only a 20 million year fuel supply. Conversely, a star that is half as
massive as the Sun burns slowly enough for its fuel to last more than 20 billion years.
Since all of the stars in a globular cluster formed at roughly the same
these clusters can serve as cosmic clocks. If a globular cluster is more than 10
years old, then all of its hydrogen burning stars will be less massive
than 10 solar masses. This implies that no individual hydrogen burning
be more than 1000 times brighter than the Sun. If a globular cluster is
than 2 billion years old, then there will be no hydrogen-burning star
massive than 2 solar masses.
The oldest globular clusters contain only stars less massive than 0.7
masses. These low mass stars are much dimmer than the Sun. This
suggests that the oldest globular clusters are between 11 and 18
billion years old. The uncertainty in this estimate is due to the
determining the exact distance to a globular cluster (hence, an
the brightness (and mass) of the stars in the cluster). Another source
of uncertainty in this estimate lies in our ignorance of some of the
details of stellar evolution.
Extrapolating Back to the Big Bang
Another way to estimate the age of the Universe is to
"Hubble constant". The
(H0) is a measure of the current
expansion rate of the Universe. Cosmologists use this measurement to
extrapolate back to the Big Bang. This extrapolation depends upon the
density of the Universe and on the composition of the Universe.
If the Universe is flat and composed mostly of matter, then the age of
Universe is 2/(3 H0). If the Universe has a very low density of matter,
its extrapolated age is larger: 1/H0. If the theory of general
modified to include a cosmological constant, then the inferred age can
Many astronomers are working hard to measure the Hubble constant using a
of different techniques. The current best estimates of H0 range from 50
kilometers/sec/Megaparsec to 100 km/s/Megaparsec. In more
familiar units, astronomers believe that 1/H0 is between 10 and 20
If we compare the two age determinations, there is a potential crisis.
astronomers who estimate that 1/H0 is as small as 10 Billion years are
correct, then the age of the Universe would be shorter than the age of
stars. This contradiction implies that either the Big Bang theory is
or that we need to modify general relativity by adding a cosmological constant.
Some astronomers believe that this crisis will pass as soon as our
improve. If the astronomers who have measured the larger values of
correct and the smaller estimates of globular cluster ages are
also correct, then all may be well for the Big Bang theory.
Thank you to the MAP project for contributing to this article. Find out
about the Microwave Anisotropy Probe at http://map.gsfc.nasa.gov/.