Cosmic dissonance: are new physics or systematics behind a short sound horizon?

¹ DARK, Niels-Bohr Institute, Lyngbyvej 2, 2100 Copenhagen, Denmark
e-mail: nikki.arendse@nbi.ku.dk
² Physics Department UC Davis, 1 Shields Ave., Davis, CA 95616, USA
³ STAR Institute, Quartier Agora – Allée du six Août, 19c B-4000 Liège, Belgium
⁴ Exzellenzcluster Universe, Boltzmannstr. 2, 85748 Garching, Germany
⁵ Ludwig-Maximilians-Universität, Universitäts-Sternwarte, Scheinerstr. 1, 81679 München, Germany
⁶ Laboratoire d’Astrophysique, École Politechnique Fédérale de Lausanne (EPFL), Obs. de Sauverny, 1290 Versoix, Switzerland
⁷ Kavli IPMU (WPI), UTIAS, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan
⁸ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85748 Garching, Germany
⁹ Physik-Department, Technische Universität München, James-Franck-Straße 1, 85748 Garching, Germany
¹⁰ Academia Sinica Institute of Astronomy and Astrophysics (ASIAA), 11F of ASMAB, No.1, Sect. 4, Roosevelt Rd, Taipei 10617, Taiwan
¹¹ Department of Physics and Astronomy, University of California, Los Angeles, CA 90095, USA
¹² Kavli Institute for Particle Astrophysics and Cosmology and Department of Physics, Stanford University, Stanford, CA 94305, USA
¹³ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands

Received: 17 September 2019
Accepted: 7 May 2020

Abstract

Context. Persistent tension between low-redshift observations and the cosmic microwave background radiation (CMB), in terms of two fundamental distance scales set by the sound horizon r_d and the Hubble constant H₀, suggests new physics beyond the Standard Model, departures from concordance cosmology, or residual systematics.

Aims. The role of different probe combinations must be assessed, as well as of different physical models that can alter the expansion history of the Universe and the inferred cosmological parameters.

Methods. We examined recently updated distance calibrations from Cepheids, gravitational lensing time-delay observations, and the tip of the red giant branch. Calibrating the baryon acoustic oscillations and type Ia supernovae with combinations of the distance indicators, we obtained a joint and self-consistent measurement of H₀ and r_d at low redshift, independent of cosmological models and CMB inference. In an attempt to alleviate the tension between late-time and CMB-based measurements, we considered four extensions of the standard ΛCDM model.

Results. The sound horizon from our different measurements is r_d = (137 ± 3^stat. ± 2^syst.) Mpc based on absolute distance calibration from gravitational lensing and the cosmic distance ladder. Depending on the adopted distance indicators, the combined tension in H₀ and r_d ranges between 2.3 and 5.1 σ, and it is independent of changes to the low-redshift expansion history. We find that modifications of ΛCDM that change the physics after recombination fail to provide a solution to the problem, for the reason that they only resolve the tension in H₀, while the tension in r_d remains unchanged. Pre-recombination extensions (with early dark energy or the effective number of neutrinos N_eff = 3.24 ± 0.16) are allowed by the data, unless the calibration from Cepheids is included.

Conclusions. Results from time-delay lenses are consistent with those from distance-ladder calibrations and point to a discrepancy between absolute distance scales measured from the CMB (assuming the standard cosmological model) and late-time observations. New proposals to resolve this tension should be examined with respect to reconciling not only the Hubble constant but also the sound horizon derived from the CMB and other cosmological probes.