The Universe is expanding too fast and scientists still can’t explain it

Scientists have long relied on two main strategies to determine the Universe's expansion rate. One approach focuses on nearby space, measuring distances to stars and galaxies to calculate how fast everything is moving apart. The other looks much farther back in time, using the cosmic microwave background to estimate what the expansion rate should be today based on the standard model of cosmology.

In theory, both methods should produce the same answer. In reality, they do not. Observations of the local Universe consistently point to a faster expansion rate of about 73 kilometers per second per megaparsec. Meanwhile, calculations based on the early Universe suggest a slower rate of roughly 67 or 68. The gap between these values is small in absolute terms, but far too large to dismiss as a statistical fluke. This mismatch is known as the Hubble tension, and it has appeared repeatedly across independent studies.

A Unified Approach Brings New Precision

To improve accuracy, researchers combined decades of observations into a single, coordinated framework. This effort, led by the H0 Distance Network (H0DN) Collaboration, produced the most precise direct measurement so far of the local expansion rate. Their findings, published on April 10 in Astronomy & Astrophysics, place the Hubble constant at 73.50 ± 0.81 kilometers per second per megaparsec, achieving a precision slightly better than 1%.

The study, "The Local Distance Network: a community consensus report on the measurement of the Hubble constant at ∼1% precision," grew out of a large collaborative effort launched during the International Space Science Institute (ISSI) Breakthrough Workshop, "What's under the H0od?" held at ISSI in Bern, Switzerland, in March 2025.

"This isn't just a new value of the Hubble constant," the collaboration notes, "it's a community-built framework that brings decades of independent distance measurements together, transparently and accessibly."

Data From Ground and Space Observatories

NSF NOIRLab contributed both scientific expertise and key observations. John Blakeslee, who serves as Director of Research and Science Services at NSF NOIRLab, is part of the collaboration. The analysis includes data from NSF Cerro Tololo Inter-American Observatory (CTIO) in Chile and NSF Kitt Peak National Observatory (KPNO) in Arizona, both Programs of NSF NOIRLab. These observations were combined with data from other facilities, both on the ground and in space, strengthening the overall findings.

Rather than depending on a single technique, the team built what they call a "distance network." This system connects several overlapping methods used to measure cosmic distances. These include Cepheid variable stars, which brighten and dim in predictable ways, red giant stars with known brightness, Type Ia supernovae, and certain galaxy types.

This layered approach allows researchers to cross-check results in multiple ways. If one method were flawed, removing it from the analysis would change the final answer. That did not happen. Even when individual techniques were excluded, the overall result remained largely unchanged. The consistency across methods strengthens confidence in the measured expansion rate.

"This work effectively rules out explanations of the Hubble tension that rely on a single overlooked error in local distance measurements," the authors conclude. "If the tension is real, as the growing body of evidence suggests, it may point to new physics beyond the standard cosmological model."

What the Hubble Tension Could Mean

The implications go beyond measurement techniques. The slower expansion rate derived from the early Universe depends on the standard model of cosmology, which describes how the Universe has evolved since the Big Bang. If that model is missing something, such as details about dark energy, unknown particles, or changes in gravity, its predictions for today's expansion could be off.

In that case, the Hubble tension may signal a deeper issue rather than a simple measurement problem. It could indicate that scientists need to revise their understanding of how the Universe works.

Looking Ahead With Future Observations

The newly developed distance network also provides a framework for future studies. By making their methods and data publicly available, the team has created a system that can be refined as new observations become available. Upcoming observatories are expected to deliver even more precise measurements, which may help determine whether the discrepancy will eventually be resolved or continue to point toward new physics.

More information

This research is presented in a paper titled "The Local Distance Network: A community consensus report on the measurement of the Hubble constant at ∼1% precision" to published in Astronomy & Astrophysics.

The results are presented by the H0DN Collaboration.

NSF NOIRLab, the U.S. National Science Foundation center for ground-based optical-infrared astronomy, operates the International Gemini Observatory (a facility of NSF, NRC-Canada, ANID-Chile, MCTIC-Brazil, MINCyT-Argentina, and KASI-Republic of Korea), NSF Kitt Peak National Observatory (KPNO), NSF Cerro Tololo Inter-American Observatory (CTIO), the Community Science and Data Center (CSDC), and NSF-DOE Vera C. Rubin Observatory (in cooperation with DOE's SLAC National Accelerator Laboratory). It is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with NSF and is headquartered in Tucson, Arizona.

The scientific community is honored to have the opportunity to conduct astronomical research on I'oligam Du'ag (Kitt Peak) in Arizona, on Maunakea in Hawai'i, and on Cerro Tololo and Cerro Pachón in Chile. We recognize and acknowledge the very significant cultural role and reverence of I'oligam Du'ag to the Tohono O'odham Nation, and Maunakea to the Kanaka Maoli (Native Hawaiians) community.