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 4π. This allows us to determine the cosmological constant which drives the late time acceleration, to be Λ L_P^2 = 3 \exp(-24π^2 μ) where μ\ is a number of order unity. This expression leads to the observed value of cosmological constant for μ ~ 1.19. The implications are discussed.