A natural cosmological constant from chameleons
  We present a simple model where the effective cosmological constant appears
from chameleon scalar fields. For a Kachru-Kallosh-Linde-Trivedi
(KKLT)-inspired form of the potential and a particular chameleon coupling to
the local density, patches of approximately constant scalar field potential
cluster around regions of matter with density above a certain value, generating
the effect of a cosmological constant on large scales. This construction
addresses both the cosmological constant problem (why $\Lambda$ is so small,
yet nonzero) and the coincidence problem (why $\Lambda$ is comparable to the
matter density now).
