Our present understanding of the evolution of the universe relies upon the Friedmann- Robertson- Walker cosmological models. The accelerating universe is the observation that the universe appears to be expanding at an increasing rate. In formal terms, this means that the cosmic scale factor a(t) has a positive second derivative, so that the velocity at which a distant galaxy is receding from us should be continuously increasing with time. The gravity of all the mass in the universe would be expected to pull everything back inward, so scientists call whatever force is counteracting gravity "dark energy." Dark energy is the name given to whatever is causing the universe's expansion to speed up. The new measurement doesn't just tell scientists how fast the universe is expanding, but helps shed light on the mystery of why this expansion is accelerating.By combining the new value of the Hubble Constant with observations of the universe by NASA's Wilkinson Microwave Anisotropy Probe (WMAP), the scientists were able to make an independent calculation of the strength of dark energy, which is battling against gravity to pull the universe outward.