Beginning in 2014, measurements of the Hubble constant — the present-day expansion rate of the universe — began to disagree. Measurements taken from the distant universe were about 10% off from measurements taken from the nearby universe. While that doesn't sound like a lot (and it isn't, considering the considerable feats of science needed to make these measurements in the first place), the uncertainties in those measurements were only about 2%.
A 10% difference with a 2% uncertainty is statistically significant and something worth investigating. Since 2014, there have been over 300 proposals for solutions to this "crisis in cosmology." None of these proposals is universally agreed upon by cosmologists, and as measurements continue, the crisis just keeps getting worse.
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