The universe is expanding — but we’re still not sure how quickly! With past measurements of this expansion rate causing yielding conflict and debate, a new study investigates whether we can resolve the evident tension.
Conflicting Measurements

Estimated values of the Hubble constant, 2001–2018. Data marked with circles show local, distance-ladder-calibrated measurements; data marked with squares indicate global measurements from the CMB and baryon-acoustic oscillations. Click to enlarge. [Kintpuash]
Though we’ve come a long way since our initial, imprecise measurements of H0, today the two primary methods of measuring the Hubble constant remain in tension:
- Local measurements can be made by determining the distances and recession speeds of visible objects in the universe; Type Ia supernova surveys provide the standard candles needed for these measurements. Using this approach, scientists obtain an H0 value around us of ~74 (km/s)/Mpc.
- Global measurements are made by estimating the Hubble constant from measurements of the cosmic microwave background (CMB), relic radiation from the Big Bang. By fitting a multi-parameter model to Planck-mission observations of the CMB, scientists obtain a slower expansion estimate of ~68 (km/s)/Mpc for H0.
Inhomogeneity to the Rescue?
This discrepancy of nearly 9% between the two measurements — which cannot be brought into agreement by the measurements’ error bars — remains puzzling. Is one or the other group of astronomers making a mistake, or underestimating their errors? Could there be new physics at play in the cosmological model used to interpret the CMB results?

Expansion rate (left) and density (right) of a simulated inhomogeneous anisotropic universe. [Macpherson et al. 2018]
To determine whether the tension between the two types of H0 measurements can be explained by such an inhomogeneous universe, a team of scientists led by Hayley Macpherson (Monash University) has explored the behavior of a simulated universe.
A Simulated Universe

The global measurement (Planck measurement; blue solid line) and local measurement (Riess et al. measurement; red solid line) of the Hubble constant can’t be brought into agreement by local deviations in the Hubble constant due to inhomogeneities (blue data showing distribution of various local spheres). [Macpherson et al. 2018]
The authors find that, in their simulated universe, local measurements of the Hubble constant differ by less than 1% compared to the global value. An inhomogeneous universe therefore cannot explain the nearly 9% difference we measure between the CMB-inferred global and supernova-measured local values of the Hubble constant.
What’s next? It’s back to the drawing board — the mystery of our expanding universe continues to elude us. Here’s hoping that high-precision measurements from future surveys will help us to further refine our understanding!
Citation
“The Trouble with Hubble: Local versus Global Expansion Rates in Inhomogeneous Cosmological Simulations with Numerical Relativity,” Hayley J. Macpherson et al 2018 ApJL 865 L4. doi:10.3847/2041-8213/aadf8c
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