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Equation of state

Another ratio that is used by scientists is the equation of state, usually denoted w, which is the ratio of pressure that dark energy puts on the universe to the energy per unit volume. This ratio is w = −1 for the cosmological constant used in the Einstein equations; alternative time-varying forms of vacuum energy such as quintessence generally use a different value. The value w = −1.028 ± 0.032, measured by the Planck Collaboration (2018) is consistent with −1, assuming w does not change over cosmic time.