In 1927, the American astronomer Edwin Hubble showed the Universe is expanding: distant galaxies are receding from each other. This confirmed the so-called Standard Model of Cosmology, that the universe, on the largest scale, is evolving according to a Friedman-Robertson-Walker spacetime. The starting assumption in this model is the Cosmological Principlethat on the largest scale, we are not in a special place in the universe-that the universe is homogeneous and isotropic about every point like

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... the FRW spacetime. In 1998, more accurate measurements of the recessional velocity of distant galaxies based on new Type 1a supernova data, made the astounding discovery that the Universe was actually accelerating relative to the standard model. So the Standard Model is incorrect. The explanation for the Anomalous Acceleration of the Galaxies is one of the great open problems of physics. Recent supernova data have tested the dependence of the Hubble constant on time, and the results don't fit standard model... Dark energy is non-classical Negative pressure Anti-gravity effect Standard Model for Dark Energy Leads to: Assume k = 0 FRW: Assume Einstein equations with a cosmological constant: ds 2 = −dt 2 + R(t) 2 dr 2 + r 2 dΩ 2