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COSMOLOGY

Tensions between the early and late Universe

Nature Astronomy volume 3, pages 891–895 (2019)Cite this article

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A Kavli Institute for Theoretical Physics workshop in July 2019 directed attention to the Hubble constant discrepancy. New results showed that it does not appear to depend on the use of any one method, team or source. Proposed solutions focused on the pre-recombination era.

Nearly a century of cosmological research has led us to a standard model of cosmology, the Λ cold dark matter model, or ΛCDM, with its six free parameters and several ansatzes. This model is dominated by dark components (energy and matter) with still uncertain physics. With some simplifying assumptions about the uncertain bits, the version of the model calibrated on physics prior to recombination (63% dark matter, 15% photons, 10% neutrinos and 12% atoms) is used to predict the physical size of density fluctuations in the plasma — that is, how far sound, or any perturbation in the photon–baryon fluid, could have travelled from the beginning of the Universe to recombination — the sound horizon and its overtones, as well as the primordial baryon density. By comparing the fluctuation spectrum predicted by the model to the angular spectrum observed in the cosmic microwave background (CMB), the six free parameters are set and the ansatzes are tested. An alternative to the use of the CMB for setting the sound horizon may be derived by relating measurements of the primordial deuterium abundance to the predicted baryon density. The evolving form of the model (68% dark energy, 27% dark matter and 5% atoms) is then used to predict the expansion history of the Universe from redshift z = 1,000 to z = 0. Uncalibrated high-redshift type-Ia supernovae (SNe) and baryon acoustic oscillations (BAOs) provide ‘guard rails’ between z ≈ 2 and 0; they do not tell us if we are on the ‘right road’ but they make sure we do not miss the curves in the model’s road (for instance, the cosmic acceleration must be consistent with w = –1, where the equation of state parameter w is model-dependent and w = –1 corresponds to a cosmological constant) along the way. The model calibrated on early-Universe observations predicts the present-day value of several cosmological parameters, some of which can be empirically measured locally (for z < 1) with little or no model dependence. In particular, the model calibrated with data from the Planck mission predicts the Hubble constant, today’s expansion rate, to a remarkable 1% precision, 67.4 ± 0.5 km s–1 Mpc–1. Is this whole story right?

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Fig. 1: Compilation of Hubble constant predictions and measurements taken from the recent literature and presented or discussed at the meeting.

courtesy of Vivien Bonvin and Martin Millon.

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Acknowledgements

We thank KITP for hosting and supporting the workshop ‘Tensions between the Early and the Late Universe’. We thank all of the participants for an extremely lively and productive workshop. We are also grateful to those who contributed to the scientific results presented at the meeting but could not attend in person. Finally, we are especially grateful to V. Bonvin and A. Shajib for producing and updating versions of Fig. 1 during the meeting, keeping up with the fast pace of the presentation of new results. L.V. acknowledges support from the European Union’s Horizon 2020 research and innovation programme ERC (BePreSySe, grant agreement 725327). T.T. acknowledges support from the Packard Foundation through a Packard Research fellowship, from the National Science Foundation through grant AST-1714953, and from NASA through grants 10158 HST-GO-12889 and HST-14254.

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Authors and Affiliations

  1. ICREA and ICCUB, University of Barcelona, Barcelona, Spain

    Licia Verde

  2. University of California, Los Angeles, Los Angeles, CA, USA

    Tommaso Treu

  3. Johns Hopkins University, Baltimore, MD, USA

    Adam G. Riess

  4. Space Telescope Science Institute, Baltimore, MD, USA

    Adam G. Riess

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  1. Licia Verde
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  2. Tommaso Treu
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  3. Adam G. Riess
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Correspondence to Licia Verde.

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Verde, L., Treu, T. & Riess, A.G. Tensions between the early and late Universe. Nat Astron 3, 891–895 (2019). https://doi.org/10.1038/s41550-019-0902-0

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