Big Bang theory
predicts that the early universe was a very hot place and that as it
expands, the gas within it cools. Thus the universe should be filled with
radiation that is literally the remnant heat left over from the Big Bang,
called the “cosmic microwave background radiation”, or CMB.
of the Cosmic Microwave Background
existence of the CMB radiation was first predicted by George Gamow in
1948, and by Ralph Alpher and Robert Herman in 1950. It was first observed
inadvertently in 1965 by Arno Penzias and Robert Wilson at the Bell
Telephone Laboratories in Murray Hill, New Jersey. The radiation was
acting as a source of excess noise in a radio receiver they were building.
Coincidentally, researchers at nearby Princeton University, led by Robert
Dicke and including Dave Wilkinson of the WMAP science team, were devising
an experiment to find the CMB. When they heard about the Bell Labs result
they immediately realized that the CMB had been found. The result was a
pair of papers in the Physical Review: one by Penzias and Wilson detailing
the observations, and one by Dicke, Peebles, Roll, and Wilkinson giving
the cosmological interpretation. Penzias and Wilson shared the 1978 Nobel
prize in physics for their discovery.
the CMB radiation is very cold, only 2.725° above
absolute zero, thus
this radiation shines primarily in the microwave portion of the
and is invisible to the naked eye. However, it fills the universe and can
be detected everywhere we look. In fact, if we could see microwaves, the
entire sky would glow with a brightness that was astonishingly uniform in
every direction. The picture at left shows a false color depiction of the
temperature (brightness) of the CMB over the full sky (projected onto an
oval, similar to a map of the Earth). The temperature is uniform to better
than one part in a thousand! This uniformity is one compelling reason to
interpret the radiation as remnant heat from the Big Bang; it would be
very difficult to imagine a local source of radiation that was this
uniform. In fact, many scientists have tried to devise alternative
explanations for the source of this radiation but none have succeeded.
the Cosmic Microwave Background?
Since light travels at a finite
speed, astronomers observing distant objects are looking into the past.
Most of the stars that are visible to the naked eye in the night sky are
10 to 100 light years away. Thus, we see them as they were 10 to 100 years
ago. We observe Andromeda, the nearest big galaxy, as it was three million
years ago. Astronomers observing distant galaxies with the Hubble Space
Telescope can see them as they were only a few billion years after the Big
Bang. (Most cosmologists believe that the universe is between 12 and 14
billion years old.)
The CMB radiation was emitted only a
few hundred thousand years after the Big Bang, long before stars or
galaxies ever existed. Thus, by studying the detailed physical properties
of the radiation, we can learn about conditions in the universe on very
large scales, since the radiation we see today has traveled over such a
large distance, and at very early times.
of the Cosmic Microwave Background
One of the basic predictions of the
Big Bang theory is
that the universe is
expansion indicates the universe was smaller, denser and hotter in the
distant past. When the visible universe was half its present size, the
density of matter was eight times higher and the cosmic microwave
background was twice as hot. When the visible universe was one hundredth
of its present size, the cosmic microwave background was a hundred times
hotter (273 degrees above absolute zero or 32 degrees Fahrenheit, the
temperature at which water freezes to form ice on the Earth's surface). In
addition to this cosmic microwave background radiation, the early universe
was filled with hot hydrogen gas with a density of about 1000 atoms per
cubic centimeter. When the visible universe was only one hundred millionth
its present size, its temperature was 273 million degrees above absolute
zero and the density of matter was comparable to the density of air at the
Earth's surface. At these high temperatures, the hydrogen was completely
ionized into free protons and electrons.
Since the universe was so very hot
through most of its early history, there were no atoms in the early
universe, only free electrons and nuclei. (Nuclei are made of neutrons and
protons). The cosmic microwave background photons easily scatter off of
electrons. Thus, photons wandered through the early universe, just as
optical light wanders through a dense fog. This process of multiple
scattering produces what is called a “thermal” or “blackbody” spectrum of
photons. According to the Big Bang theory, the frequency spectrum of the
CMB should have this blackbody form. This was indeed measured with
tremendous accuracy by the FIRAS experiment on NASA's COBE satellite.
figure shows the prediction of the Big Bang theory for the energy spectrum
of the cosmic microwave background radiation compared to the observed
energy spectrum. The FIRAS experiment measured the spectrum at 43 equally
spaced points along the blackbody curve. The error bars on the data points
are so small that they can not be seen under the predicted curve in the
figure! There is no alternative theory yet proposed that predicts this
energy spectrum. The accurate measurement of its shape was another
important test of the Big Bang theory.
Eventually, the universe cooled
sufficiently that protons and electrons could combine to form neutral
hydrogen. This was thought to occur roughly 400,000 years after the Big
Bang when the universe was about one eleven hundredth its present size.
Cosmic microwave background photons interact very weakly with neutral
behavior of CMB photons moving through the early universe is analogous to
the propagation of optical light through the Earth's atmosphere. Water
droplets in a cloud are very effective at scattering light, while optical
light moves freely through clear air. Thus, on a cloudy day, we can look
through the air out towards the clouds, but can not see through the opaque
clouds. Cosmologists studying the cosmic microwave background radiation
can look through much of the universe back to when it was opaque: a view
back to 400,000 years after the Big Bang. This “wall of light“ is called
the surface of last scattering since it was the last time most of the CMB
photons directly scattered off of matter. When we make maps of the
temperature of the CMB, we are mapping this surface of last scattering.
As shown above, one of the most
striking features about the cosmic microwave background is its uniformity.
Only with very sensitive instruments, such as
WMAP, can cosmologists detect
fluctuations in the
cosmic microwave background temperature. By studying these fluctuations,
cosmologists can learn about
the origin of galaxies and large scale structures of
galaxies and they can measure the basic
parameters of the Big Bang theory.
Courtesy of NASA