Early results from the far infrared absolute spectrophotometer (FIRAS)

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The Far Infrared Absolute spectrophotometer (FIRAS) on the Cosmic Background Explorer (COBE) mapped 98% of the sky, 60% of it twice, before the liquid helium coolant was exhausted. The FIRAS covers the frequency region from 1 to 100 cm⁻¹ (10 mm to 0.1 mm wavelength) with a 7° angular resolution. The spectal resolution is 0.2 cm⁻¹ for frequencies less than 20 cm⁻¹ and 0.8 cm⁻¹ for higher frequencies. Preliminary results include: a limit on the deviations from a Planck curve of 1% of the peak brightness from 1 to 20 cm⁻¹, a temperature of 2.735±0.06 K, a limit on the Comptonization parameter y of 10⁻³, on the chemical potential parameter µ of 10⁻², a strong limit on the existence of a hot smooth intergalactic medium, and a confirmation that the dipole anisotropy spectrum is that of a Doppler shifted blackbody. Although there are many unresolved issues about the data processing and analysis of systematic errors, some of which are described here, the instruments is expected to achieve its goal of an accuracy and rms sensitivity for I better than 10⁻⁹ W/m²sr, for each 7° beamwidth and each 5% spectral resolution element, for frequencies from 1 to 20 cm⁻¹.