Measurement of the cosmic microwave background spectrum by the COBE FIRAS instrument

The cosmic microwave background radiation (CMBR) has a blackbody spectrum within 3.4 X 10^-8^ ergs cm^-2^ s^-1^ sr^-1^ cm over the frequency range from 2 to 20 cm^-1^ (5-0.5 mm). These measurements, derived from the FIRAS instrument on the COBE satellite, imply stringent limits on energy release in the early universe after t~1 year and redshift z~3 x 10^6^. The deviations are less than 0.03% of the peak brightness, with an rms value of 0.01%, and the dimensionless cosmological distortion parameters are limited to |y| < 2.5 x 10^-5^ and |μ| < 3.3 x 10^-4^(95% confidence level). The temperature of the CMBR is 2.726 +/- 0.010 K (95% confidence level systematic).