A thermodynamic description of cosmic evolution from the Big Bang: an explanation of dark energy based on the entropy of the cosmological horizon

This article explores a possible thermodynamic–holographic explanation for the origin of dark energy and the observed evolution of the Universe. Within this framework, two complementary derivations have been presented. The first derivation, based on the holographic principle and the Bekenstein–Hawking entropy of the Hubble horizon, leads naturally to the relation ρΛ=βH2. It provides a unified description of the successive cosmological regimes characterized by the effective equations of state w=1/3,w=0,w=−1, corresponding respectively to the radiation, matter, and dark-energy dominated eras. The second derivation applies the first law of thermodynamics to the entropy associated with the cosmological horizon. It yields the equation of state p=−ρΛ,w=−1,which governs the late-time cosmological dynamics through the continuity and Friedmann equations offering a novel interpretation of the origin of dark energy.