An international team of astronomers has released one of the clearest measurements yet of how fast the nearby universe is expanding. Far from solving long-standing problems, the new findings make them even more difficult to ignore. The collaboration includes John Blakeslee of the NSF NOIRLab, which is funded by the National Science Foundation, and incorporates data from multiple NOIRLab telescopes.
Scientists have long relied on two main strategies to determine the rate of expansion of the universe. One approach focuses on the nearby universe, measuring the distances to stars and galaxies to calculate how fast everything is moving away. The other goes way back and uses the Cosmic Microwave Background radiation to estimate what the rate of expansion should be today based on the Standard Model of cosmology.
In theory, either method should give you the same answer. Actually not. Observations of the local Universe consistently point to a faster expansion rate of about 73 kilometers per second per megaparsec. Calculations based on the early Universe, on the other hand, suggest a slower speed of around 67 or 68. Although the gap between these values is small in absolute terms, it is too large to be ignored as a statistical coincidence. This discrepancy is known as the Hubble tension, and it shows up 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, provides the most accurate direct measurement of local spread rates to date. Their findings were published on April 10th. astronomy and astrophysicsputs the Hubble constant at 73.50 ± 0.81 kilometers/second/megaparsec, achieving an accuracy of slightly better than 1%.
The study, “Local Distance Networks: A Community Consensus Report on Measuring the Hubble Constant with ~1% Accuracy,” grew out of a large-scale collaboration initiated during the International Space Science Institute (ISSI) Breakthrough Workshop “What lies beneath H0od?” It will be held at ISSI in Bern, Switzerland in March 2025.
“This is not just a new value for the Hubble constant; it is a community-built framework that transparently and accessiblely integrates decades of independent distance measurements,” the collaboration says.
Data from ground and space observatories
NSF NOIRLab contributed both scientific expertise and important observations. John Blakeslee, director of research science services at NSF NOIRLab, is also part of the collaboration. This analysis includes data from the NSF Cerro Tololo Inter-American Observatory (CTIO) in Chile and the NSF Kitt Peak National Observatory (KPNO) in Arizona, both programs of the NSF NOIRLab. These observations were combined with data from other facilities, both on the ground and in space, to strengthen the overall findings.
Rather than relying on a single technology, the team built what they called a “remote network.” This system connects several overlapping methods used to measure distances in space. These include Cepheid variable stars that brighten and dim in a predictable manner, red giant stars of known brightness, Type Ia supernovae, and certain types of galaxies.
This multilayered approach allows researchers to cross-check results in multiple ways. If one method is flawed, removing it from the analysis will change the final answer. That didn’t happen. Excluding individual techniques did not change the overall results much. Consistency between methods increases the reliability of measured expansion rates.
“This study effectively rules out explanations for the Hubble tension that rely on a single overlooked error in local distance measurements,” the authors conclude. “If this tension is real, as growing evidence suggests, it could point to new physics beyond standard cosmological models.”
What Hubble tension means
The impact goes beyond measurement techniques. The slow expansion rate originating from the early universe relies on the Standard Model of cosmology, which explains how the universe has evolved since the Big Bang. If that model lacks details such as dark energy, unknown particles, and changes in gravity, predictions of today’s expansion may be off.
In that case, the Hubble tension may be indicative of a deeper problem than a simple measurement problem. This could mean that scientists need to rethink their understanding of how the universe works.
Looking ahead by observing the future
The newly developed distance network also provides a framework for future research. By making their methods and data public, the research team has created a system that can be improved as new observations become available. Future observatories are expected to make more precise measurements, which could help determine whether this discrepancy is finally resolved or continues to point in new directions for physics.
Detailed information
The research was published in a paper titled “Local Distance Network: Community Consensus Report on Measuring the Hubble Constant with ~1% Accuracy” astronomy and astrophysics.
The results will be published by the H0DN collaboration.
NSF NOIRLab, the National Science Foundation’s ground-based optical and infrared astronomy center, operates the International Gemini Observatory (a facility of NSF, Canada’s NRC, Chile’s ANID, Brazil’s MCTIC, Argentina’s MINCyT, and South Korea’s KASI), NSF Kitt Peak National Observatory (KPNO), NSF Cerro Tororo Inter-American Observatory (CTIO), and Community Science. Data Center (CSDC), and the NSF-DOE Vera C. Rubin Observatory (in collaboration with DOE’s SLAC National Accelerator Laboratory). It is managed by the Association of Universities for Astronomical Research (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 at Iorigam Duag (Kitt Peak) in Arizona, Mauna Kea in Hawaii, and Cerro Tololo and Cerro Pachón in Chile. We recognize and acknowledge the vital cultural role and respect of Iorigam Duagu for the Tohono O’odham Nation and Mauna Kea for the Kanaka Maoli (Native Hawaiian) community.
