PRELIMINARY-RESULTS FROM THE COBE DIFFERENTIAL MICROWAVE RADIOMETERS - LARGE ANGULAR SCALE ISOTROPY OF THE COSMIC MICROWAVE BACKGROUND

The COBE Differential Microwave Radiometers (DMR) instrument has produced preliminary full sky microwave maps at 31.5, 53, and 90 GHz with two independent channels at each frequency. At millimeter wavelengths, the cosmic microwave background (CMB) dominates the sky brightness with an intensity more than a thousand times the foreground Galactic emission. Emission from our Galaxy is seen unambiguously at all three frequencies. The dipole anisotropy, attributed to the motion of our solar system with respect to the CMB reference frame, shows strongly in all six sky maps and is consistent with a Doppler-shifted thermal spectrum. The best fitted dipole has amplitude 3.3 +/- 0.2 mK in the direction (alpha, delta) = (11.2h +/- O.2h,-7-degrees +/- 2-degrees) (J2000) or (l, b) = (265-degrees +/- 2-degrees, 48-degrees +/- 2-degrees). There is no clear evidence in the maps for any other large angular scale features. We place a limit of DELTA-T/T0 < 3 x 10(-5) (T0 = 2.735 K) for the rms quadrupole amplitude; for monochromatic fluctuations DELTA-T/T0 < 4 x 10(-5); and for Gaussian fluctuations DELTA-T/T0 < 4 x 10(-5). All limits are given at the 95% confidence level and reflect currently estimated limits on systematic effects. These measurements place the most severe constraints to date on many potential physical processes in the early universe, such as anisotropic expansion of the universe, very long wavelength gravitational waves, cosmic strings, and large-scale primordial fluctuations.