gamma ray bursts could cause mass extinctions or make planets inhospitable to life. The expansion of universe may help to avoid the effect of gamma rays.

In theory, Heavens explains, either the constant should be hundreds of orders of magnitude higher than it appears to be, or it should be zero, in which case the universe wouldn’t accelerate. But this would disagree with what astronomers have observed. “The small—but nonzero—size of the cosmological constant is a real puzzle in cosmology,” he says, adding that the research shows the number is consistent with the conditions required for the existence of intelligent life that is capable of observing it.