Final answer:
The discovery of dark energy indicates that the recession speed of galaxies was slower in the past, contrary to the expectations that the universe's expansion would be decelerating due to gravity. Instead, it is now believed that dark energy is causing the expansion to accelerate.
Step-by-step explanation:
The discovery of dark energy relates to the observation that the recession speed of galaxies was unexpectedly faster in the past. According to Hubble's Law, the velocity of a galaxy's recession (v) is proportional to its distance (d) from us, which indicates a uniform expansion rate. However, astronomers initially expected that the expansion of the universe should be slowing down due to the gravitational attraction between galaxies. Contrary to these expectations, recent measurements have shown that the expansion may actually be accelerating, suggesting that the recession speed of galaxies was slower in the past and that dark energy could be fueling this acceleration.
The implications of such findings are staggering, as they suggest that a force, embodied by the concept of dark energy, is working against gravity to accelerate the expansion of the universe. The cosmological constant, a term introduced by Einstein, is often associated with dark energy and is used to explain the accelerated expansion observed in the current cosmological model.