contributors to the energy density of the universe are curvature, matter, radiation and dark energy. Each of the components decreases with the expansion of the universe (increasing scale factor), except perhaps the dark energy term. It is the values of these cosmological parameters which physicists use to determine the acceleration of the universe. The acceleration equation describes the evolution of the scale factor with time where the pressure is defined by the cosmological model chosen. (see explanatory models below) Physicists at one time were so assured of the deceleration of the universe's expansion that they introduced a so-called deceleration parameter . Current observations indicate this deceleration parameter being negative. Relation to inflation According to the theory of cosmic inflation, the very early universe underwent a period of very rapid, quasi-exponential expansion. While the time-scale for this period of expansion was far shorter than that of the current expansion, this was a period of accelerated expansion with some similarities to the current epoch. Technical definition The definition of "accelerating expansion" is that the second time derivative of the cosmic scale factor, , is positive, which is equivalent to the deceleration parameter, , being negative. However, note this does not imply that the Hubble parameter is increasing with time. Since the Hubble parameter is defined as , it follows from the definitions that the derivative of the Hubble parameter is given by so the Hubble parameter is decreasing with time unless . Observations prefer , which implies that is positive but is negative. Essentially, this implies that the cosmic recession velocity of any one particular galaxy is increasing with time, but its velocity/distance ratio is still decreasing; thus different galaxies expanding across a sphere of fixed radius cross the sphere more slowly at later times. It is seen from above that the case of "zero acceleration/deceleration" corresponds to is a linear function of , , , and . Evidence for acceleration To learn about the rate of expansion of the universe we look at the magnitude-redshift relationship of astronomical objects using standard candles, or their distance-redshift relationship using standard rulers. We can also look at the growth of large-scale structure, and find that the observed values of the cosmological parameters are best described by models which include an accelerating expansion. Supernova observation In 1998, the first evidence for acceleration came from the observation of Type Ia supernovae, which are exploding white dwarfs