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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:

  1. 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.
  2. 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.
  3. 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, (

COBE data are publicly available via anonymous ftp from the National Space Science Data Center (NSSDC), or check their web page at

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