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.
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