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How Old is the Universe?

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
Alpha 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

2/(3 Ho)

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:

1/Ho

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!

Overview
Matter
Expansion
Age
Shape
Acceleration
Fate

Last updated: Friday, 03-19-2004

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Overview

Matter

Expansion

Age

Shape

Acceleration

Fate