Einstein's field equations include a cosmological constant (Lamda: "Λ"), that is related to an energy density of empty space. Depending on its sign, the cosmological constant can either slow (negative "Λ") or accelerate (positive "Λ") the expansion of the Universe. Although many scientists, including Einstein, had speculated that "Λ" was zero, recent astronomical observations of type Ia supernovae have detected a large amount of dark energy that is accelerating the Universe's expansion. Preliminary studies suggest that this dark energy is related to a positive "Λ", although alternative theories cannot be ruled out as yet.