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

Applying the first law of thermodynamics to the apparent horizon of a FriedmannRobertson-Walker universe and assuming the geometric entropy given by a quarter of the apparent horizon area, we derive the Friedmann equations describing the dynamics of the universe with any spatial curvature. Using entropy formulae for the static spherically symmetric black hole horizons in Gauss-Bonnet gravity and in more general Lovelock gravity, where the entropy is not proportional to the horizon area, we are also able to obtain the Friedmann equations in each gravity theory. We also discuss some physical implications of our results. e-mail address: [email protected] e-mail address: [email protected]