Cosmological constant problem on the horizon

We revisit the quantum cosmological constant problem and highlight the important roles played by the de Sitter (dS) horizon of zero-point energy. We argue that fields which are light enough to have a dS horizon of zero-point energy comparable to the Friedmann-Lemaître-Robertson-Walker (FLRW) Hubble radius are the main contributors to dark energy. On the other hand, the zero-point energy of heavy fields develop nonlinearities on sub-Hubble scales and cannot contribute to dark energy. We speculate that our proposal may provide a resolution for both the old and new cosmological constant problems by noting that there exists a field, the (lightest) neutrino, which happens to have a mass comparable to the present background photon temperature. The proposal predicts multiple transient periods of dark energy in the early and late expansion history of the Universe, yielding a higher value of the current Hubble expansion rate which can resolve the H₀ tension problem.