The Graviton as a Bound State and the Cosmological Constant Problem

The graviton is pictured as a bound state of a fermion and anti-fermion with the spacetime metric assumed to be a composite object of spinor fields, based on a globally Lorentz invariant action proposed by Hebecker and Wetterich. The additional degrees of freedom beyond those of the graviton are described by Goldstone boson gravitational degrees of freedom. If we assume that the fermion is a light neutrino with mass $m_nu\sim 10^{-3}$ eV, then we obtain the effective vacuum density ${\bar\rho}_\lambda\sim (10^{-3} eV)^4$, which agrees with the estimates for the cosmological constant from WMAP and SNIa data.