The Physical Principle that determines the Value of the Cosmological
  Constant
  Observations indicate that the evolution of our universe can be divided into
three epochs consisting of early time inflation, radiation (and matter)
domination and the late time acceleration. One can associate with each of these
epochs a number N which is the phase space volume of the modes which cross the
Hubble radius during the corresponding epoch. This number turns out to be
(approximately) the same for the cosmologically relevant ranges of the three
epochs. When the initial de Sitter space is characterized by the Planck length,
the natural value for N is 4\pi. This allows us to determine the cosmological
constant which drives the late time acceleration, to be \Lambda L_P^2 = 3
\exp(-24\pi^2 \mu) where \mu\ is a number of order unity. This expression leads
to the observed value of cosmological constant for \mu ~ 1.19. The implications
are discussed.
