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Investigating Our Expanding Universe

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inhomogeneous universe
Could natural inhomogeneity in our universe lead to different local and global measurements of how fast it’s expanding? [Adapted from Macpherson et al. 2018]

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

Hubble constant

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]

The first sign that the universe around us is expanding was found in the late 1920s, when astronomers first recorded evidence that distant galaxies appear to be moving away from us at ever faster rates, the more distant they are. This led to the development of the Hubble constant (H0), a value used to quantify this observed rate of expansion — and its value has been debated ever since.

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:

  1. 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.
  2. 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?

inhomogeneous universe

Expansion rate (left) and density (right) of a simulated inhomogeneous anisotropic universe. [Macpherson et al. 2018]

Some scientists have proposed an alternative explanation: what if the global expansion rate of the universe is not the same as the local rate? One possibility is that we live in a local void, an underdense region of the universe that expands faster than does the universe overall.

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

Local deviations in H0

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]

Macpherson and collaborators simulated the growth of large-scale cosmological structures using numerical relativity. Starting with an inhomogeneous universe, the authors evolved random density fluctuations of the universe from its birth to today, and then investigated what effect these inhomogeneities have on local measurements of the Hubble constant.

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