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 \Lambda , 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 \approx 1.19 .
The implications are discussed .