In the early 20th century Edwin Hubble discovered that the universe is expanding. Einstein's law of gravity would suggest that our expanding universe should be slowing down over time due to the gravitational attraction of the matter (i.e. galaxies, stars) that makes up the universe. Over the past several years a range of new observations has shown that the expansion of our universe is actually speeding up rather than slowing down! This universal acceleration poses an important new challenge to the Big Bang model. In a first step toward exploring this universal acceleration, scientists have begun examining a new component of our universe called Dark Energy. Dark Energy is different from matter in that gravity would lead a universe filled with Dark Energy to expand ever faster as opposed to slowing down. Another possibility is that there is a flaw in Einstein's law of gravity on cosmic distance scales. Currently, scientists are working to make precise measurements of how the universal expansion rate has changed over cosmic time with the hope that this additional information will allow us to differentiate between the Dark Energy hypothesis and a flaw in our understanding of gravity.