Hubble Tension Explained: Reconciling the Universe's Expansion Rate

In a concise review, this video traces the history of the expanding universe from Hubble's 1929 observations to the modern Hubble constant tension. It explains two main measurement methods used to determine the expansion rate today: the cosmic distance ladder based on redshift and distances to distant galaxies, and the early-universe approach using the cosmic microwave background data from missions like WMAP and Planck. The presentation highlights the current discrepancy between about 73.5 km/s per megaparsec from redshift measurements and about 67.4 km/s per megaparsec from CMB data, and discusses the profound implications for the standard cosmological model. It also surveys potential resolutions, from new physics to a controversial slow rotation of the universe, and ends with a cautious note about not throwing away established cosmology too quickly.

Overview

The video examines the Hubble constant tension, the mismatch between expansion rates inferred from the early universe and those measured from the present day, and why this matters for our cosmological model.

Historical Background

Starting with Edwin Hubble's 1929 discovery that galaxies are receding, the talk explains how Cepheid variables and Leavitt's law enabled distance measurements, expanding our view of the cosmos and underpinning the Big Bang framework. The initial Hubble constant was much higher than today, but over time refinements brought it into a narrower range.

Two Main Measurement Approaches

The late-universe method, often called the cosmic distance ladder, uses redshift data to gauge current expansion, yielding approximately 73.5 km/s per megaparsec. The early-universe method relies on the cosmic microwave background (CMB) to rewind to the early universe, predicting about 67.4 km/s per megaparsec. The Planck satellite’s CMB analysis is the benchmark against which redshift measurements are compared, and the tension between these values persists with improving data.

Independent Measurements and the Crisis Deepens

Various independent studies have reported higher values: mega-maser galaxies gave 73.9 km/s per megaparsec, the DESI survey with Type Ia supernovae around the Coma cluster yielded 76.5, and recent work using mirror-variable stars in our galaxy aligned with roughly 73.0. All these results sit well above Planck’s estimate, reinforcing the notion that something is missing from the standard cosmological model.

Possible Explanations

Proposed remedies range from radical to conservative. Some researchers suggest new physics such as decaying dark matter, a changing dark energy equation of state, or modifications to general relativity. A particularly intriguing idea gaining traction is a very slow rotation of the entire universe. In the 2025 study by Balash Andre Zigetti, Istvan Zapudi, Imre Fyres Bane, and Gurgli Garbor Manifaldi, a rotation once every 500 billion years could reconcile the two measurements without violating known physics, though it remains a preliminary result requiring extensive modeling and cross-checks against other observations.

Next Steps

The video emphasizes cautious optimism: even if the rotating model proves viable, it would imply a rotating variant of cosmology with shifted parameters rather than a wholesale rewrite of physics. Ongoing work will need to test this hypothesis against multiple datasets and develop self-consistent simulations. The takeaway is that the Hubble tension is a sign that our understanding may be incomplete, not that the entire framework must be discarded.

Conclusion

Whether the tension will be resolved by refined measurements or a new piece of physics remains to be seen, but the pursuit highlights the dynamic nature of cosmology and the continual refinement of our picture of the universe.