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International Collaboration Publishes New Precise Measurement of Universe's Expansion Rate, Confirms Persistent Hubble Tension

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A New Precision in the Cosmos: Why the Universe's Expansion Still Doesn't Add Up

A comprehensive analysis combining decades of independent measurements has produced a new, high-precision value for the local universe's expansion rate, confirming a long-standing discrepancy with predictions based on the early universe. The findings, published in the journal Astronomy & Astrophysics on April 10, were produced by the H0 Distance Network (H0DN) Collaboration.

The comparison between the late and early-universe value of [the Hubble constant] tests basic physics on cosmological scales, and it tells us that something's missing. — Study co-author Richard Anderson

The Measurement

The collaboration measured the Hubble constant (H₀), which describes the universe's current expansion rate, at 73.50 ± 0.81 kilometers per second per megaparsec. This result corresponds to a precision of just over 1% and is described by the researchers as the most precise direct measurement of the local expansion rate to date.

The Hubble Tension

Astronomers use two primary methods to calculate the Hubble constant:

  • The Local Universe Method: This method measures distances to stars and galaxies in the nearby universe.
  • The Early Universe Method: This method uses the cosmic microwave background (CMB), the earliest light from approximately 380,000 years after the Big Bang, to predict the current expansion rate based on the standard model of cosmology.

These two approaches have consistently yielded different values. The CMB method calculates the Hubble constant at approximately 67-68 km/s/Mpc, while local universe methods produce values around 73 km/s/Mpc. This persistent discrepancy, which exceeds statistical uncertainty, is known as the Hubble tension.

Methodology: The Local Distance Network

The H0DN Collaboration created a framework called the Local Distance Network, which integrates multiple, overlapping techniques for measuring cosmic distances into a single unified analysis. This differs from the traditional "distance ladder" method used for nearly a century.

The network includes observations of:

  • Cepheid variable stars
  • Red giant stars (using the Tip of the Red Giant Branch, or TRGB)
  • Mira variables
  • Megamasers
  • Type Ia and Type II supernovae
  • Surface brightness fluctuations
  • The Tully-Fisher relation
  • The Fundamental Plane

The analysis incorporates data from dozens of observatories, including:

  • NSF Cerro Tololo Inter-American Observatory (CTIO) in Chile
  • NSF Kitt Peak National Observatory (KPNO) in Arizona
  • The Hubble Space Telescope
  • The Dark Energy Spectroscopic Instrument (DESI)

By linking these probes simultaneously through full covariance weighting, the network accounts for correlations and shared uncertainties. The collaboration reported that removing any single measurement technique from the analysis changed the overall result only minimally, indicating the measurement's robustness.

Findings and Implications

The authors concluded that the "analysis effectively rules out explanations for the Hubble tension based on a single overlooked error in local distance measurements."

Study co-author John Blakeslee, director of research and science services at NOIRLab, noted one possible explanation: "Primordial magnetic fields, which could change the scale of the structure seen in the CMB."

If the tension is confirmed, it may indicate that the standard model of cosmology is incomplete. Potential explanations involve new physics beyond the current model, which could relate to:

  • The nature of dark energy
  • The existence of new particles
  • Modifications to the theory of gravity

Context and Future Work

The H0DN Collaboration's effort was launched at the "What's under the H0od?" Breakthrough Workshop at the International Space Science Institute (ISSI) in Bern, Switzerland, in March 2025.

The collaboration has made its methods, data, and analysis code publicly available to establish a foundation for future research. The modular framework is designed to incorporate data from next-generation observatories, which are expected to provide even more precise measurements.

Additional Measurement

A separate study, published in The Astrophysical Journal, used gravitational waves from the neutron star merger GW170817 to provide a new estimate of the Hubble constant. This measurement yielded a value between 61 and 70 km/s/Mpc, which is more consistent with the CMB-based method than the local universe method, though it remains four times less precise than the leading local measurements.