Dimensional reduction , truncations , constraints and the issue of consistency 1 Josep

A brief overview of dimensional reductions for diffeomorphism invariant theories is given. The distinction between the physical idea of compactification and the mathematical problem of a consistent truncation is discussed, and the typical ingredients of the latter –reduction of spacetime dimensions and the introduction of constraints– are examined. The consistency in the case of of group manifold reductions, when the structure constants satisfy the unimodularity condition, is shown in a clear way together with the associated reduction of the gauge group. The problem of consistent truncations on coset spaces is also discussed and we comment on examples of some remarkable consistent truncations that have been found in this context. Talk presented at the NEB XII conference Recent Developments in Gravity, June 29th July 2nd 2006, Nafplion, Greece.