Everpresent $\Lambda$

A variety of observations indicate that the Universe is dominated by “dark energy” with negative pressure, one possibility for which is a cosmological constant. If the dark energy is a cosmological constant, a fundamental question is, why has it become relevant at so late an epoch, making today the only time in the history of the Universe at which the cosmological constant is of the order of the ambient density. We explore an answer to this question drawing on ideas from unimodular gravity, which entails fluctuations in the cosmological constant, and causal set theory, which predicts a specific magnitude for the fluctuations. The resulting ansatz provides a cosmological “constant” which fluctuates about zero, remaining always comparable to the ambient energy density.