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HIGHLIGHTS OF 2019

# The expansion of the Universe is faster than expected

### Subjects

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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## Change history

• ### 10 January 2020

The Competing interest statement is added as it was missing from the previous version.

## References

1. 1.

Riess, A. G. et al. Large Magellanic Cloud Cepheid standards provide a 1% foundation for the determination of the Hubble constant and stronger evidence for physics beyond ΛCDM. Astrophys. J. 876, 85 (2019).

2. 2.

Pietrzyn´ski, G. et al. A distance to the Large Magellanic Cloud that is precise to one per cent. Nature 567, 200–203 (2019).

3. 3.

Reid, M. J., Pesce, D. W. & Riess A. G. An improved distance to NGC 4258 and its implications for the Hubble constant. Astrophys. J. Lett. 886, L27 (2019).

4. 4.

Wong K. C. et al. H0LiCOW XIII. A 2.4% measurement of H0 from lensed quasars: 5.3σ tension between early and late-Universe probes. Preprint at: https://arxiv.org/abs/1907.04869 (2019).

5. 5.

Shajib, A. J. et al. STRIDES: A 3.9 per cent measurement of the Hubble constant from the strong lens system DES J0408-5354. Preprint at: https://arxiv.org/pdf/1910.06306 (2019).

6. 6.

Freedman, W. L. et al. The Carnegie-Chicago Hubble Program. VIII. An independent determination of the Hubble constant based on the tip of the red giant branch. Astrophys. J. 882, 34 (2019).

7. 7.

Yuan, W. et al. Consistent calibration of the tip of the red giant branch in the Large Magellanic Cloud on the Hubble Space Telescope photometric system and a re-determination of the Hubble constant. https://10.3847/1538-4357/ab4bc9 (2019).

8. 8.

Huang, C. D. et al. Hubble Space Telescope observations of Mira variables in the type Ia supernova host NGC 1559: an alternative candle to measure the Hubble constant. Astrophys. J. in the press.

9. 9.

Verde, L., Treu, T. & Riess, A. G. Tensions between the early and the late Universe. Nat. Astron. 3, 891–895 (2019).

10. 10.

Knox, L & Millea, M. The Hubble hunter’s guide. Preprint at: https://arxiv.org/abs/1908.03663 (2019).

11. 11.

Wu, H. Y. & Huterer, D. Sample variance in the local measurements of the Hubble constant. Mon. Not. R. Astron. Soc. 471, 4946–4955 (2017).

12. 12.

Kenworthy, W. D., Scolnic, D. & Riess, A. G. The local perspective on the Hubble tension: local structure does not impact measurement of the Hubble constant. https://10.3847/1538-4357/ab0ebf (2019).

Authors

## Ethics declarations

### Competing interests

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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