The present rate of the expansion of our Universe, the Hubble constant, can be predicted from the cosmological model using measurements of the early Universe, or more directly measured from the late Universe. But as these measurements improved, a surprising disagreement between the two appeared. In 2019, a number of independent measurements of the late Universe using different methods and data provided consistent results, making the discrepancy with the early Universe predictions increasingly hard to ignore.
The local or late Universe measurement of the Hubble constant improved from 10% uncertainty 20 years ago to less than 2% by 2019.
In 2019, multiple independent teams presented measurements with different methods and different calibrations to produce consistent results.
These late Universe estimations disagree at 4\(\sigma \) to 6\(\sigma \) with predictions made from the cosmic microwave background in conjunction with the standard cosmological model, a disagreement that is hard to explain or ignore.
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The author declares no competing interests.
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Riess, A.G. The expansion of the Universe is faster than expected. Nat Rev Phys 2, 10–12 (2020). https://doi.org/10.1038/s42254-019-0137-0
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