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Applying the first law of thermodynamics to the apparent horizon of aFriedmann-Robertson-Walker universe and assuming the geometric entropy given bya quarter of the apparent horizon area, we derive the Friedmann equationsdescribing the dynamics of the universe with any spatial curvature. Usingentropy formulae for the static spherically symmetric black hole horizons inGauss-Bonnet gravity and in more general Lovelock gravity, where the entropy isnot proportional to the horizon area, we are also able to obtain the Friedmannequations in each gravity theory. We also discuss some physical implications ofour results.Comment: LaTex 15 pages, no figur

First Law of Thermodynamics and Friedmann Equations of Friedmann–Robertson–Walker Universe