How Fast Is the Universe Really Expanding?

Cosmologists are wrestling with one of their biggest mysteries — and it just keeps getting larger. We've known for nearly a century that the universe is expanding. But in the nineteen-nineties, a shocking discovery turned that understanding upside down: the expansion isn't slowing down, it's speeding up. Something must be driving that acceleration. The leading answer is dark energy, an invisible force permeating space — but it remains deeply mysterious, even as Quanta Magazine examines how it has upended our understanding of cosmology.

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