COBE:

COBE

COBE's scientific payload consisted of three instruments shielded from the adverse thermal and radiation effects of the Sun and Earth. Two cryogenically cooled instruments observed the sky at infrared wavelengths. DIRBE, the diffuse infrared background experiment, mapped the sky at 10 wavelengths over 1–300 μm in search of the cumulative uniform glow from earliest luminous objects. FIRAS, the far-infrared absolute spectrophotometer, measured the average temperature of the background with unprecedented precision: it showed the spectrum of the microwave background to be a perfect black body curve characteristic of a temperature of 2.735 ± 0.06 K. FIRAS was turned off after 10 months of flight when the helium coolant ran out.

The third detector, the DMR (differential microwave radiometer), consisted of six differential radiometers observing the sky at three radio wavelengths – 3.3, 5.7, and 9.5 mm. They made whole-sky surveys and were designed to detect any fluctuations in the brightness of the microwave background.

The microwave background is not completely uniform, showing a slight dipole anisotropy with an enhancement of one part in a thousand in the direction of the Galaxy's motion and an equal and opposite deficit in the opposite part of the sky. This results from the real motion of the Galaxy relative to the fixed background. It was announced in April 1992 that careful statistical analysis of the COBE measurements also revealed even weaker temperature fluctuations of one part in one hundred thousand on scales of ten degrees and larger. These are the imprints of quantum fluctuations in the early Universe. Further study of such fluctuations by such missions as the Wilkinson Microwave Anisotropy Probe have served to constrain cosmological models that determine how the early inhomogeneities collapsed to form the large-scale structure we see in today's Universe.