3 K - The Temperature of the Universe
The sun and stars emit
thermal radiation covering all wavelengths; other objects in the sky, like the
great clouds of gas in the Milky Way, also emit thermal radiation but are much
cooler. These objects are best detected by infrared and radio telescopes -
telescopes whose detectors are sensitive to the longer wavelengths.
In 1965, Arno Penzias
and Robert Wilson were
conducting a careful calibration of their radio telescope at the Bell Laboratory
at Whippany, New Jersey. The found that their receiver showed a "noise" pattern
as if it were inside a container whose temperature was 3K - i.e. as if it were
in equilibrium with a black body at 3 K. This "noise" seemed to be coming from
every direction. Earlier theoretical predictions by
George Gamow and other
astrophysicists had predicted the existence of a cosmic 3 K background. Penzias'
and Wilson's discovery was the observational confirmation of the isotropic
radiation from the Universe, believed to be a relic of the "Big Bang". The
enormous thermal energy released during the creation of the universe began to
cool as the universe expanded. Some 12 billion years later, we are in a universe
that radiates like a black body now cooled to 3 K. In 1978 Penzias and Wilson
were awarded the Nobel prize in physics for this discovery.
A black body at 3 K emits most of its energy in the microwave wavelength
range. Molecules in the earth's atmosphere absorb this radiation so that from
the ground, astronomers cannot make observations in this wavelength region. In
1989 the Cosmic Background Explorer
(COBE) satellite, developed
by NASA's Goddard Space Flight
Center, was launched to measure the diffuse infrared and microwave radiation
from the early universe. One of its instruments, the Far Infrared Absolute
Spectrophotometer (FIRAS) compared the spectrum of the cosmic microwave
background radiation with a precise blackbody. The cosmic microwave background
spectrum was measured with a precision of 0.03% and it fit precisely with a
black body of temperature 2.726 K. Even though there are billions of stars in
the universe, these precise COBE measurements show that 99.97% of the radiant
energy of the Universe was released within the first year after the Big Bang
itself and now resides in this thermal 3 K radiation field.
A more detailed explanation of the origin of the microwave background
radiation, and its possible anisotropy, may be found
here. A new mission
selected by NASA is the Microwave Anisotropy
Probe (MAP) will measure the small fluctuations in the background radiation
and will yield more information on the details of the early universe. The
European Space Agency has a similar
mission
planned.
Summary
The concept of temperature is as fundamental a physical concept as
the three fundamental quantities of mechanics - mass, length, and time. Through
the study of such practical problems as how to make a highly efficient steam
engine, fundamental physical theories emerge, including the concepts of the
quantum theory and the two laws of thermodynamics. The second law, with its
irreversibility requirement, predicts an inevitable evolution from other forms
of energy into heat. It is the second law alone that provides an "arrow" for the
concept of time.
We can record
events (illustration from
Low Temperature Laboratory of
Helsinki University of Technology)that cover 18 orders of magnitude in the
temperature range, and we have one clearly defined lower limit to the
temperature, absolute zero. Because of this 10-with-18-zeros-behind-it range in
temperatures, there are many different kinds of thermometers developed to
explore it and many different fields of research.
One of the beauties of "publishing" on the web is the interactive element it
offers. Joachim Reinhardt
has written to point out that the highest temperatures that are accessible on
earth (only surpassed by the early stages of the big bang) occur in high-energy
collisions of particles (in particular of heavy ions), during which one sees a
"fireball" with a temperature of several hundred MeV (which corresponds to a
temperature of 10 to the 12th power k). This fireball cools down by expanding
and by radiating off particles, mostly pions, quite similar to the thermal
black-body radiation.
Thermal physics is a field rich in theoretical and practical applications.
Acknowledgments
I would like to thank Rick Ebert of IPAC for his help in locating
some of the infrared files used here and Dave Leisawitz of NASA Goddard for his
very careful editing of the article and for his assistance with the COBE
results. Joachim
Reinhardt generated the
pictures of
most of the scientists. Thanks to Seth Sharpless for scanning Galen's
picture. Carl Mungan provided advice on low-temperature thermodynamics, and very
generously served as an "expert" reviewer.
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