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The Expansion of the Universe is Faster than Expected

Adam G. Riess

Abstract

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. We review key advances realized by 2019: -- The local or late Universe measurement of the Hubble constant improved from 10% uncertainty twenty years ago to under 2% by the end of 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$σ$ to 6$σ$ with predictions made from the Cosmic Microwave Background in conjunction with the standard cosmological model, a disagreement that is hard to explain or ignore.

The Expansion of the Universe is Faster than Expected

Abstract

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. We review key advances realized by 2019: -- The local or late Universe measurement of the Hubble constant improved from 10% uncertainty twenty years ago to under 2% by the end of 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 to 6 with predictions made from the Cosmic Microwave Background in conjunction with the standard cosmological model, a disagreement that is hard to explain or ignore.

Paper Structure

This paper contains 2 sections, 3 figures.

Table of Contents

  1. Key advances
  2. Supplementary Materials

Figures (3)

  • Figure 1: Figure. 1 |. Disagreement between the Hubble constant prediction from the cosmological model using measurements of the early Universe, and the more direct measurements from the late Universe. The discrepancy between the early and late Universe central values represented by the dashed lines at $67.4 \mathrm{~km} \mathrm{~s}^{-1} \mathrm{Mpc}^{-1}$ and $73.0 \mathrm{~km} \mathrm{~s}^{-1} \mathrm{Mpc}^{-1}$ is of $4 \sigma$ to $6 \sigma$. The labels denote different measurements: a Planck stands for the ESA Planck Satellite Cosmic Microwave Background observations; b BBN+BAO refers to Big Bang nucleosynthesis and Baryon Acoustic Oscillations (BAO); c SH0ES refers to the SH0ES (Supernovae $H_{0}$ for the Equation of State) project measurements; d and e refer to the H0LiCOW ( $H_{0}$ Lenses in COSMOGRAIL's Wellspring) team and STRIDES (STRonglensing Insights into Dark Energy Survey) collaboration estimates, respectively; f and g refer to TRGB (Tip of the Red Giant Branch) estimates; $h$ refers to the estimation using Miras stars; i refers to the estimation using water masers; j SBF refers to the surface brightness fluctuation method; WMAP+BAO refers to measurements using a combination of data from Wilkinson Microwave Anisotropy Probe (WMAP) and BAO; ACTPol+BAO refers to measurements using a combination of data from the Atacama Cosmology Telescope Polarization camera (ACTPol) and BAO; SPT-SZ+BAO refers to measurements using a combination of data from the South Pole Telescope SZ camera (SPT-SZ) and BAO.
  • Figure 2: Late Universe
  • Figure 3: Early & Late Universe