= Cosmological constant problem
{wiki}
The <cosmological constant problem> is the unexplained smallness of the gravitationally observable <vacuum energy> compared with the contributions expected from <quantum field theory>. In units with $\hbar=c=1$, an <ultraviolet cutoff> $\Lambda_{\mathrm{UV}}$ generically gives contributions of order $\Lambda_{\mathrm{UV}}^4$, whereas the observed dark-energy density is of order $10^{-47}\,\mathrm{GeV}^4$. The <Einstein equation> depends on the sum of a bare <cosmological constant> and these contributions; subtracting a constant from a nongravitational <Hamiltonian> does not remove its gravitational effect. Exact global <supersymmetry> gives zero energy to a <supersymmetric vacuum>, but <supersymmetry breaking> generally reintroduces a much larger positive scale.
Back to article page
