Until recently, astronomers estimated that the Big Bang occurred
between 12 and 14 billion years ago. To put this in perspective, the Solar
System is thought to be 4.5 billion years old and humans have existed as
a species for a few million years. Astronomers estimate the age of the universe
in two ways: 1) by looking for the oldest stars; and 2) by measuring the
rate of expansion of
the universe and extrapolating back to the Big Bang; just as crime detectives
can trace the origin of a bullet from the holes in a wall.
Older Than the Oldest Stars?
Astronomers can place a lower limit to the age of the universe by studying
globular clusters. Globular clusters are a dense collection of roughly a
million stars. Stellar densities near the center of the globular cluster
are enormous. If we lived near the center of one, there would be several
hundred thousand stars closer to us than Proxima Centauri, the star nearest
to the Sun.
Text Link to the HST press release describing this image
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.
All of the stars in a globular cluster formed at roughly the same time,
thus they can serve as cosmic clocks. If a globular cluster is more than
20 million years old, then all of its hydrogen burning stars will be less
massive than 10 solar masses. This implies that no individual hydrogen burning
star will be more than 1000 times brighter than the Sun. If a globular cluster
is more than 2 billion years old, then there will be no hydrogen-burning
star more massive than 2 solar masses.
The oldest globular clusters contain only stars less massive than 0.7
solar masses. These low mass stars are much dimmer than the Sun. This observation
suggests that the oldest globular clusters are between 11 and 18 billion
years old. The uncertainty in this estimate is due to the difficulty in determining
the exact distance to a globular cluster (hence, an uncertainty in 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 finer details of stellar
evolution. Presumably, the universe itself is at least as old as the oldest
globular clusters that reside in it.
Extrapolating Back to the Big Bang
An alternative approach to estimating is the age of the universe is to
measure the “Hubble constant”. The Hubble constant 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 on the history of the expansion rate which in
turn depends on the current 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 the universe is
where Ho is the value of the Hubble constant.
If the universe has a very low density of matter, then its extrapolated age is larger:
If the universe contains a form of matter similar to the cosmological constant, then the inferred age can be even larger.
Many astronomers are working hard to measure the Hubble constant using
a variety of different techniques. Until recently, the best estimates ranged
from 65 km/sec/Megaparsec to 80 km/sec/Megaparsec, with the best value being
about 72 km/sec/Megaparsec. In more familiar units, astronomers believe that
1/Ho is between 12 and 14 billion years.
An Age Crisis?
If we compare the two age determinations, there is a potential crisis.
If the universe is flat, and dominated by ordinary or dark matter, the age
of the universe as inferred from the Hubble constant would be about 9 billion
years. The age of the universe would be shorter than the age of oldest stars.
This contradiction implies that either 1) our measurement of the Hubble constant
is incorrect, 2) the Big Bang theory is incorrect or 3) that we need a form
of matter like a cosmological constant that implies an older age for a given
observed expansion rate.
Some astronomers believe that this crisis will pass as soon as measurements
improve. If the astronomers who have measured the smaller values of the Hubble
constant are correct, and if the smaller estimates of globular cluster ages
are also correct, then all is well for the Big Bang theory, even without
a cosmological constant.
WMAP Can Measure the Age of the Universe
Measurements by the WMAP satellite can help resolve this crisis. If current ideas about the origin of large-scale structure are correct, then the detailed structure of the cosmic microwave background fluctuations will depend on the current density of the universe, the composition of the universe and its expansion rate. WMAP has been able to determine these parameters
with an accuracy of better than 5%. Thus, we can estimate the expansion age
of the universe to better than 5%. When we combine the WMAP data with complimentary
observations from other CMB experiments (ACBAR and CBI), we are able to determine
an age for the universe closer to an accuracy of 1%.
The expansion age measured by WMAP is larger than the oldest globular clusters, so the Big Bang
theory has passed an important test. If the expansion age measured by WMAP
had been smaller than the oldest globular clusters, then there would have
been something fundamentally wrong about either the Big Bang theory or the
theory of stellar evolution.
Either way, astronomers would have needed to rethink many of their cherished
ideas. But our current estimate of age fits well with what we know from other
kinds of measurements: the Universe is about 13.7 billion years old!
Last updated: Tuesday, 03-01-2005