Evidence for an accelerating Universe: When using Type Ia supernovas as standard candles, data saw them as dimmer than expected. This is explained by assuming that they are further away than they would be if the Universe was expanding at the same rate. Evidence identifies this effect to have started when the Universe was about 2/3 of it's current size. (If the expansion of the Universe is accelerating due to a cosmological constant, then the expansion was slower in the past, and thus the time required to expand by a given factor is longer, and the distance now is larger. But if the expansion is decelerating, it was faster in the past and the distance now is smaller. Thus for an accelerating expansion, the supernovae at high redshifts will appear to be fainter than they would for a decelerating expansion because their current distances are larger.)
