Hubble Tension Deepens: Final ACT Maps Reinforce Cosmic Puzzle

Final ACT Release Strengthens the Case for a Real Hubble Tension

The Atacama Cosmology Telescope (ACT) may have concluded observations, but its final dataset — the culmination of nearly two decades of work — delivers a precise new map of the distant universe that deepens the so‑called ‘Hubble tension’. ACT's high‑resolution measurements of the cosmic microwave background (CMB) polarization agree with previous early‑universe estimates and underscore a real discrepancy with local measurements of the expansion rate.

What Is the Hubble Tension?

In plain terms, the Hubble tension refers to a persistent mismatch in measurements of the Hubble constant, H0, the present‑day expansion rate of the universe. Local measurements that rely on distance ladders — most notably Type Ia supernovae as standard candles — return a higher value of H0. By contrast, measurements that infer H0 from the early universe, primarily via the CMB, return a lower value. ACT's final maps probe the latter and confirm that the early‑universe value remains distinct from local estimates.

How ACT Advances the Picture

ACT achieved this by producing high‑precision polarization maps of the CMB that complement the Planck mission's temperature maps. Where Planck exploited the advantages of space to map temperature fluctuations across the whole sky, ACT — operating from a high, dry site in the Atacama Desert of northern Chile at about 5,000 meters elevation — delivered finer angular resolution in polarization. The two independent CMB datasets now agree on the early‑universe H0, strengthening the conclusion that the discrepancy is not an artifact of a single instrument or analysis method.

“When we compare them, it’s a bit like cleaning your glasses,” said Erminia Calabrese of Cardiff University, a member of the ACT collaboration.

Colin Hill of Columbia University summarized the impact: the ACT polarization results align with Planck's temperature and polarization inferences, making the Hubble discrepancy even more robust. That agreement narrows the range of viable theoretical responses: many extended cosmological models that would force a single H0 across scales are now strongly constrained or ruled out.

Implications For Cosmology

Rather than representing a setback, ACT's confirmation is a scientific advance. By ruling out many proposed extensions to the standard Lambda Cold Dark Matter (LCDM) model, the new data shrink the ‘theoretical playground’ and focus efforts on a smaller set of promising explanations. Researchers can now target models that introduce new physics at particular epochs or scales, or reexamine potential systematic effects in local measurements, with greater confidence.

The ACT collaboration has released the results and analysis on the preprint server arXiv, accompanied by two companion papers that detail the methods and implications. Together with Planck, ACT provides a compelling, independent early‑universe measurement that will guide cosmological research for years to come.

Key Facts

• ACT operated from 2007 to 2022 and was sited at roughly 16,400 feet (5,000 meters) in northern Chile.
• ACT's polarization maps reach higher angular resolution than Planck's temperature maps and complement Planck's dataset.
• The ACT and Planck CMB inferences agree on the early‑universe H0, reinforcing the Hubble tension with local measurements.
• New observations rule out many extended LCDM models that attempted to erase the discrepancy.
• Full results and two companion papers are available on arXiv for independent scrutiny.