Diluting Cosmological Constant via Large Distance Modification of Gravity

We review a solution (Ref. [1]) of the cosmological constant problem in a braneworld model with infinite-volume extra dimensions. The solution is based on a nonlinear generally covariant theory of a metastable graviton that leads to a largedistance modification of gravity. From the extra-dimensional standpoint the problem is solved due to the fact that the four-dimensional vacuum energy curves mostly the extra space. The fourdimensional curvature is small, being inversely proportional to a positive power of the vacuum energy. The effects of infinite-volume extra dimensions are seen by a brane-world observer as nonlocal operators. From the four-dimensional perspective the problem is solved because the zeromode graviton is extremely weakly coupled to localized four-dimensional sources. The observable gravity is mediated not by zero mode but, instead, by a metastable graviton with a lifetime of the order of the present-day Hubble scale. Therefore, laws of gravity are modified in the infrared above the Hubble scale. Large wave-length sources, such as the vacuum energy, feel only the zero-mode interaction and, as a result, curve space very mildly. Shorter wave-length sources interact predominantly via exchange of the metastable graviton. Because of this, all standard properties of early cosmology, including inflation, are intact. Based on talks given by the authors at various recent conferences: 2002 Aspen Winter Conference on Particle Physics Current and Upcoming Discoveries, February 2002, Aspen, Colorado; Continuous Advances in QCD 2002/Arkadyfest, May 17-23, 2002, Minneapolis, MN, USA; ICTP Summer School on Astroparticle Physics and Cosmology, June 17-July 5 2002, Trieste, Italy; Payresq Physics Workshop, June 22-30, Payresq, France; Third International Sakharov Conference, June 24-29, Moscow, Russia; XVIIIth IAP Colloquium on the Nature of Dark Energy, July 2002, Paris, France; PIMS Workshop on Brane World and Supersymmetry Frontiers in Mathematical Physics, UBC, July 22–August 2, 2002, Vancouver, Canada. 2 Address after September 1, 2002: Theory Division, CERN, CH-1211, Geneva 23, Switzerland