Cosmic Microwave Background experiments — past, present and future

The cosmic microwave background (CMB) is the oldest electromagnetic radiation to reach the earth. Measurements of this radiation play a central role in modern Cosmology. In this presentation, I will describe the information that can be obtained from measurements of the CMB. The importance of these measurements is such that there have been two rounds of Nobel Prizes. The field has been advanced by small ground- and balloon-based measurements followed by space missions including COBE and WMAP. The Planck mission will be launched next year. A new generation of ground-based observations is now beginning, which may lead to yet another space mission. The requirements for such measurements have strongly driven the development of direct detection systems at millimeter and submillimeter wavelengths. The physical and practical issues that generally favor the use of coherent receivers (now based on HEMT amplifiers) at frequencies below -90 GHz and bolometric detectors at higher frequencies will be discussed. The development of this field will be illustrated by describing generic types of technology developments and of experiments.