Dark energy – the mysterious force that drives the acceleration of the universe – changes over time, controversial new calculations suggest. If true, the work rules out Einstein’s notion of a “cosmological constant” and suggests dark energy, which now repels space, once drew it together. Astronomers invoked the concept of dark energy to explain supernovae observations in the late 1990s that the universe is not only expanding but accelerating. The supernovae appeared dimmer – and therefore more distant – than expected, given their red shift, which measures how much their light has been stretched by the expansion of space.
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