1932 with the Dutch physicist and astronomer Willem de Sitter, a model of a continuously expanding Universe with zero cosmological constant (Einstein–de Sitter spacetime). In 1998, two teams of astrophysicists, one led by Saul Perlmutter, the other led by Brian Schmidt and Adam Riess, carried out measurements on distant supernovae which showed that the speed of galaxies' recession in relation to the Milky Way increases over time. The universe is in accelerated expansion, which requires having a strictly positive . The universe would contain a mysterious dark energy producing a repulsive force that counterbalances the gravitational braking produced by the matter contained in the universe (see Standard cosmological model). For this work, Perlmutter, Schmidt, and Riess jointly received the Nobel Prize in physics in 2011. Equation The cosmological constant appears in the Einstein field equations in the form where the Ricci tensor , Ricci scalar and the metric tensor describe the structure of spacetime, the stress–energy tensor describes the energy density, momentum density and stress at that point in spacetime, and . The gravitational constant and the speed of light are universal constants. When is zero, this reduces to the field equation of general relativity usually used in the 20th century. When is zero, the field equation describes empty space (a vacuum). The cosmological constant has the same effect as an intrinsic energy density of the vacuum, (and an associated pressure). In this context, it is commonly moved to the right-hand side of the equation using . It is common to quote values of energy density directly, though still using the name "cosmological constant". The dimension of is generally understood as length. Using the values known in 2018 and Planck units for = and the Hubble constant 0 = = , has the value of where is the Planck length. A positive vacuum energy density resulting from a cosmological constant implies a negative pressure, and vice versa. If the energy density is positive, the associated negative pressure will drive an accelerated expansion of the universe, as observed. (See Dark energy and Cosmic inflation for details.) (Omega sub Lambda) Instead of the cosmological constant itself, cosmologists often refer to the ratio between the energy density due to the cosmological constant and the critical density of the universe, the tipping point for a sufficient density to stop the universe from expanding forever. This ratio is usually denoted by and is estimated to