The 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.
Discovery 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.
Today, the CMB radiation is very cold, only 2.725° above absolute zero, thus this radiation shines primarily in the microwave portion of the electromagnetic spectrum,
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.
Why study the Cosmic Microwave Background?
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.
The Origin of the Cosmic Microwave Background
One of the basic predictions of the Big Bang theory is that the universe is expanding.
This 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.
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 34 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
“Surface of Last Scattering”
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 COBE and 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.
Last updated: Tuesday, 03-01-2005