Cosmological constant problem on the horizon
  We revisit the quantum cosmological constant problem and highlight the
important roles played by the dS horizon of zero point energy. We argue that
fields which are light enough to have dS horizon of zero point energy
comparable to the FLRW Hubble radius are the main contributor to dark energy.
On the other hand, the zero point energy of heavy fields develop nonlinearities
on sub-Hubble scales and can not contribute to dark energy. Our proposal
provides a simple resolution for both the old and new cosmological constant
problems by noting that there exits a field, the (lightest) neutrino, which
happens to have a mass comparable to the present background photon temperature.
The natures of dark energy and dark matter are unified in this proposal in
which the zero point energy of light fields are the source of dark energy while
dark matter is sourced by the zero point energy of heavy fields. The proposal
predicts multiple transient periods of dark energy in early and late expansion
history of the universe yielding to a higher value of the current Hubble
expansion rate which can resolve the $H_0$ tension problem.
