What began as a debate over astronomical measurements is on the verge of becoming a full-blown crisis in how we understand the cosmos. Two data sets—one from the newborn universe nearly 14 billion years ago, the other from stars as we see them today—are yielding contradictory answers to a deceptively simple question: How fast is the universe expanding?
The gap between answers is only 9 percent, but that far exceeds each data set’s estimated uncertainties. Researchers on each side of the gap call it “the tension,” and are digging in their heels about the validity of their observations. This tension is the stuff of scientific dreams—and nightmares. It hints that somewhere, somehow, our understanding of the laws of nature may be fundamentally flawed—with potentially profound implications for physics, and perhaps even the fate of all things.
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