International Collaboration Helps Pinpoint Universe’s Expansion Rate - The Latest in NASA Science News

The universe is expanding, but the exact speed at which it’s doing so – a figure known as the “Hubble constant” – remains a mystery. Astronomers’ calculations have become far more precise over time, thanks to data from NASA’s Hubble and, more recently, James Webb space telescopes. But different approaches continue to offer slightly different results. Now, an international collaboration of astronomers has developed a framework that incorporates a range of methods into one analysis, to produce a more precise measurement.

To measure the Hubble constant, astronomers need to know the distance to various objects, and they use different techniques to measure that distance depending on how far away the objects are. The connection and calibration of one technique to the next is referred to as the “cosmic distance ladder.” Now, the new analysis incorporates a variety of these different distance markers, taking into account their relative robustness to minimize the impact of uncertainties on the distance ladder and deliver a more exact figure for the Hubble constant. The findings, published April 10 in the journal Astronomy & Astrophysics, indicate an expansion rate for the nearby universe of about 73.5 kilometers per second per megaparsec. Other estimates have typically ranged from 73 to 76 kilometers per second per megaparsec.

“The power of this work is that it doesn’t depend on any single method,” said Adam Riess, coauthor of the paper at the Johns Hopkins University and Space Telescope Science Institute in Baltimore. “When multiple, independent measurements all point to the same answer, it strengthens the case that we’re seeing a real feature of the universe, not a flaw in one technique. Right now, those measurements suggest the universe today is expanding faster than we would expect based on how it looked shortly after the big bang.”

One of NASA’s key goals in building Hubble decades ago was to determine the Hubble constant, which was theorized at the time to be different than observations of the early universe indicated. That disagreement is known as the Hubble tension. NASA will take the next step as early as this fall with the launch of the agency’s Nancy Grace Roman Space Telescope, which will deliver a vast wealth of data to improve distance indicators and further investigate the Hubble tension – a missing puzzle piece in our understanding of the cosmos.

This graphic provides an overview of the Local Distance Network, the research team’s new tool to integrate diverse measurements of the Hubble constant into a coherent, rig-orous framework.

Credit: Fabio Crameri (ISSI Bern), based on the original by Richard I. Anderson and the H₀DN Collaboration (2025). 

Source: International Collaboration Helps Pinpoint Universe’s Expansion Rate - NASA Science