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The Question
(Submitted July 06, 1996)
Data collected by COBE (Cosmic Background Explorer) is very useful.
Can you tell me how was it used to calculate the temperature of the
cosmic background? Where can I find more information about COBE's
explorations? (and what about data collected by COBE?)
The Answer
(This answer was supplied to us by Dr. Al Kogut, of the Infrared
Astrophysics Branch in the Laboratory for
Astronomy and
Solar Physics at NASA-Goddard)
COBE measured the temperature of the cosmic
microwave
background using one of the three instruments on board, the Far Infrared
Absolute Spectrophotometer (FIRAS). FIRAS measures the frequency spectrum
(change in intensity with respect to observing frequency) of radiation
in deep space, and compares this spectrum to observations of an on-board
blackbody target whose temperature is well known. There are three ways
to determine the absolute temperature:
- Vary the target temperature until the target produces the same spectrum
as the sky. Read off the temperature from thermometers buried inside
of the target.
- Observe the frequency at which the cosmic microwave background (CMB)
is brightest. The CMB follows a Planck spectrum, for which
there is a well-known relation between frequency and intensity.
By measuring the peak frequency, the "color temperature"
can be calculated.
- Observe the
Doppler
shift induced by the motion of the Earth about the Sun. This creates
a characteristic dipole pattern on the sky, whose spectrum (the
derivative of a Planck spectrum with respect to frequency) depends on the
temperature of the CMB. FIRAS measured this dipole and was thus able to
calculate the CMB temperature.
The final value for the CMB temperature (T = 2.728 +/- 0.004 K) is a
weighted average of the 3 methods. Further information on COBE and the
cosmic microwave background is available on the COBE home page,
http://www.gsfc.nasa.gov/astro/cobe/cobe_home.html (http://space.gsfc.nasa.gov/astro/cobe/)
COBE data are publicly available via anonymous ftp from the National Space
Science Data Center (NSSDC), or check their web page at
http://nssdc.gsfc.nasa.gov/
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