The theoretical details of the fluctuations and their consequenceshave been developed in the 30 years that cosmologists have knownabout the CMBR. Until 1992, no anisotropy (except for a separateeffect due to the motion of our Solar System in the cosmos) had beendetected in the CMBR, though many attempts had been made. Increasedreceiver sensitivity forced experimenters to develop increasinglysophisticated techniques to reduce systematic errors and the effectsof noise from other sources of microwave radiation. As measurementsbecame more sensitive and no anisotropy was found, the range of theoreticalmodels that could fit the observations became smaller and smaller.As models were increasingly constrained, many cosmologists, especiallythe theorists making predictions, became increasingly nervous overthe lack of detected fluctuations. So it was with great excitementthat the COBE science team announced in 1992 that it had detectedthe long-sought bumps in the CMBR. The illustration on the coverof this report is the resulting COBE map of the intensity of microwaveradiation arriving from various directions in the sky. The map containssome instrumental noise, but its lumps and bumps also show evidencefor the beginnings of structure in the universe.
Like the measurement of the CMBR spectrum, the COBE detection ofthe CMBR anisotropy could not have occurred without the experienceof earlier anisotropy experiments from the ground, from balloons,and from aircraft. As valuable experience was gained from the suborbitalmeasurements, the technology and the experimenters' understandingof how to avoid contamination from many bright local sources bothevolved. Balloons, rockets, and aircraft also provide important opportunitiesto follow up on satellite discoveries. Recently, data from an independentballoon experiment, using a frequency above that of the COBE's receivers,exhibited the same basic CMBR pattern as seen in the COBE data, thusconfirming the satellite result. Lowcost balloon experiments havealso enabled important first steps toward extending these resultsto smaller angular scales.
The detection of large-scale anisotropy has finally allowed the fieldof CMBR research to become established. Whereas previous noise-limitedmeasurements could exclude but not support certain theories, theanisotropy measured by the COBE satellite is approximately at thelevel needed for the origin of structure as predicted by theories.In addition, the manner in which the strength of the anisotropy varieswith the angular size of the bumps is consistent with Big Bang theory.Even the idea of an inflationary epoch in the early universe (seesection V) seems to fit with the COBE result, although this characteristicof the CMBR fluctuations is not yet well determined. Within the pastyear, both theoretical thinking and experiment planning have undergonean important transformation because of the COBE detection. Almostall areas of cosmology have been affected, and many now take themagnitude of the COBE anisotropy signal as a reference point fornew developments.
While the COBE anisotropy detection is extremely important, it wasmade at angular scales of more than 10 degrees on the sky, scalesmuch larger than those actually involved in the formation of galaxiesand clusters of galaxies. The best direct comparison between theprimordial seeds and present day structures awaits the reliable detectionand detailed mapping of CMBR anisotropy on smaller angular scales,comparable to the physical scale of superclusters of galaxies.
Medium-scale (0.5- to 10-degree) anisotropy measurements also probeimportant details of the decoupling process. Numerical calculationsshow that during the time that matter was combining into atoms andinteracting with the CMBR for the last time, there were acousticoscillations in the overdense bumps that should have left strong