Abstract. An effective equation of state parameter for the cosmic fluid of radiation, matter and dark energy of the form: ω_eff = -[1+(z/(ρR))ρ_d]/3, where z is a constant is shown a beautiful expression for representing our flat Universe after its shift to the phase of dark era that is believed to have occurred about 5 billion years back. The model shows how complicated the Universe becomes by the emergence of dark energy and how it gradually decays. It also finds the range of size of the present Universe.

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