The cosmic microwave background is the "hottest" topic in cosmology at the moment. It is a remnant of the hot early universe. When the temperature of the universe was more than 4000K, the universe was not transparent, and photons interacted strongly with matter. However, at about 4000K, free electrons and protons recombined to form neutral hydrogen. Photons were therefore no longer strongly coupled and have just free streamed ever since. Due to the expansion of the universe, this photon background has now cooled to 2.7K, which corresponds to microwave emission. This emission was first detected in 1965, and was a brilliant confirmation of the big bang model. At NORDITA, research is in the theoretical aspects of CMBR ( S. Hannestad ). The most obvious way to use the CMBR data is to measure the curvature and expansion rate of the universe. However, it turns out that the CMBR anisotropies hold so much information that they can also be used to study particle physics [ S. Hannestad ]. [ A. Jokinen ] research interests are cosmology and astroparticle physics. Using the latest CMB data from the WMAP satellite along with information from supernaovae type Ia and galaxy surveys, one can study and constrain non-standard cosmological models arising e.g. from extra dimensional constructions.