COBE was launched in 1989 and mapped the Cosmic Microwave Background (the CMB) over whole sky during the 4-year period 1989-1993. The CMB, the residual heat left over from the Big Bang itself, was discovered by Arno Penzias and Robert Wilson in 1964, a feat for which they were awarded the 1978 Nobel Prize in Physics. In its first 9 minutes of operation John Mather's instrument, the Far Infrared Astronomical Spectrometer, showed that the intensity of the CMB as a function of wavelength (its spectrum) follows exactly the theoretical prediction of Max Planck (in 1900) for the spectrum of a perfect black-body, an object in perfect thermodynamical equilibrium with its radiation field. The full 4 years of COBE/FIRAS data confirmed the conclusion from the first 9 minutes: any deviation from an exact black-body curve is extremely small, less than about one part in 10,000. This requires the early universe to be almost uniform. A global equilibrium between matter and radiation as precise as that observed could not have been established later than about 1 month after the Big Bang itself.

George Smoot's instrument, the Differential Microwave Radiometer, was designed to map small variations in the temperature of the CMB about its mean value of 2.73 degrees above absolute zero. After one year of operation Smoot and his colleagues were able to demonstrate for the first time that such variations exist at a level of about one part in one hundred thousand. Their presence had been predicted by Russian and American theorists more than 20 years before COBE was launched. They can be thought of as reflecting weak variations in the density of matter and radiation in the early Universe. Since the time at which we now observe them (about 400,000 years after the Big Bang) they have grown through the action of gravity to turn into all the structures we see around us today. COBE's DMR thus gave us the first real picture of what the Universe looked like before there were any stars or galaxies.

The COBE experiment took more than 15 years to design, construct and complete and it required the effective collaboration of about 1000 scientists and engineers. Tight groups of scientists worked together intensely under the leadership of John Mather and George Smoot to interpret the data from FIRAS and from DMR. In addition, John Mather acted as Lead Scientist for the COBE project as a whole, coordinating the entire enterprise. Scientific advances of the kind recognised by this year's Nobel Prize require major effort from large numbers of extremely talented and dedicated individuals.

It is interesting that in the 15 years since COBE, no subsequent experiment has been able to improve upon FIRAS's spectrum measurement. Further information of fundamental importance should be hidden in the details of the CMB spectrum, but technical advances to date have not yet brought them within reach. On the other hand, there have been great efforts to improve on DMR's measurements of structure in the microwave background, and technical advances here have led to further spectacular results, most notably from NASA's WMAP satellite launched in 2001. It appears that structure in the early universe has precisely the statistical properties expected if all structure originated as quantum fluctuations of the vacuum itself during a very early period of accelerated "inflationary" expansion. In addition, these new data confirm that the current contents of the Universe are dominated by as yet unidentified forms of Dark Matter and Dark Energy. The next major advance is expected to come from the European Space Agency's Planck satellite, for which launch is exected in 2008. As the principal German partner within the Planck project, the Max Planck Institute for Astrophysics will be heavily involved in the interpretation of these new data.

Simon White