The cosmological constant problem is a major unsolved issue in theoretical physics and cosmology, related specifically to the energy density of empty space, or "vacuum energy." It originates from the inconsistency between theoretical predictions of the vacuum energy density and the observed large-scale dynamics of the universe, particularly its accelerated expansion.
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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 , an ultraviolet cutoff generically gives contributions of order , whereas the observed dark-energy density is of order . 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.