The Problem of Vacuum Energy and Cosmology (A lecture presented at the 4th Colloque Cosmologie, Paris, June, 1997; to be published in the Proceedings)

Quantum field theory predicts that vacuum energy (or what is the same, cosmological constant) should be 50-100 orders of magnitude larger than the existing astronomical limit. A very brief review of possible solutions of this problem is presented. A mechanism of adjustment of vacuum energy down to (almost) zero by the back-reaction of massless vector or second rank tensor fields is discussed.