The two bright faces of dark matter
Theo M. Nieuwenhuizen
IPhT and U. Amsterdam
Wed, Dec. 15th 2010, 14:15
Salle Claude Itzykson, Bât. 774, Orme des Merisiers
It is widely believed that dark matter is composed of heavy supersymmetric particles called WIMPs, referred to as Cold Dark Matter (CDM). Combined with a cosmological constant, $\Lambda$, the standard model for cosmology $\Lambda$CDM explains the WMAP cosmic microwave background and large scale structures. \par Still, not all is well with $\Lambda$CDM. For decades WIMP searches have been carried out all over the world, now also at the LHC, without unambiguous detection. By now, almost every month there come observations of cosmic structures which pose questions to $\Lambda$CDM. The best known of these is the missing satellite problem, in which $\Lambda$CDM predicts 10,000 satellites of our Galaxy, of which one is observed. The recently discovered galaxy at redshift z = 8.6 (600 Myr after the Big Bang) should not be visible, so that additional assumptions are needed. The many heavy, bright galaxies that existed when the Universe was 1.5-2 billion years old require a new formation theory. \par A new approach to strong and weak lensing data of the galaxy cluster Abell 1689 has shown compatibility with dark matter made up of 1.5 eV neutrinos. This case will be tested in a few years in the KATRIN tritium decay experiment. It would bring 10\% dark matter from standard (active) neutrinos, and could bring another 10\% from ``sterile'' neutrinos, if the standard model of elementary particles is extended by a Majorana mass matrix. \par Such neutrinos are too light to explain large scale structures. But gravitational hydrodynamics does not only predict the formation of Jeans clumps at the decoupling of photons and matter at redshift 1100, it also predicts that they fragment into earth mass micro brown dwarfs (MACHOs), thereby becoming Jeans clusters of MACHOs. There is support for Jeans clusters in galaxy merging and for MACHOs in planetary nebula; they may constitute the observed Cosmic Microwave Foregrounds, and their merging explains mysterious radio events. They were observed in quasar microlensing, and I will give reasons why they may have been missed in the EROS, MACHO and OGLE microlensing searches. \par In this picture, dark matter has two faces: neutrino dark matter in clusters and MACHO dark matter in galaxies.