Einstein's field equations include a cosmological constant (Lamda: "�"), that is related to an energy density of empty space. Depending on its sign, the cosmological constant can either slow (negative "�") or accelerate (positive "�") the expansion of the Universe. Although many scientists, including Einstein, had speculated that "�" 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 "�", although alternative theories cannot be ruled out as yet.
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