Dark matter composition

Hot black matter and cold black matter Two main theories clash when they try to describe the nature of this dark matter: hot dark matter and cold dark matter. Those theories rely on the mass and speed of the particles composing the dark matter. In the case of the dark matter known as "hot", the particles have speeds close to light, while the particles of black matter known as "cold" would be more massive and thus slower.

The speed of these particles is crucial to the Big Bang model of cosmology and the order of formation of the Universe's great structures. If the Universe cmopostiion is primarly made of hot dark matter, the very high speed of the particles would initially prevent the formation of a structure smaller than the supercluster of galaxies dividing up in galaxy cluster then in galaxies, then in smaller structures. It is the scenario known as "up bottom", since the largest structures are formed initially, for then dividing.

The best candidate to constitute the hot dark matter is the neutrino.

On the other hand, if the cold dark matter is the main component of the Universe, the particles will go on a smaller distance and thus will erase the density's fluctuations on extents smaller than in the case of hot dark matter. The ordinary matter would then gather to form galaxies (starting from gas clouds and smaller structures), which themselves will gather in cluster, then supercluster. It is the scenario known as " bottom up". The candidates for cold dark matter are WIMP and MACHO.

These two theories were defended by Yakov Borisovitch Zeldovitch for the hot dark matter, and James Peebles for the cold dark matter. Currently, it is the cold dark matter model which seems to be the more consistent. Indeed, the galaxies are in a dynamic balance, which shows that they were created before the clusters (all do not seem stable yet). However, the current theory introduces a little bit of hot dark matter as it is necessary to explain the formation of galaxy clusters.

Website navigation:
Introduction First observational evidences The galactic rotation problem
Dark matter within galaxies Dark matter between galaxies Dark matter Composition
Baryonic Nonbaryonic Neutrino WIMP String theory