The most precise measurement of how fast the Universe is expanding

Most thorough-ever study of the cosmos expansion confirms that we still can't explain how it's expanding  

The most precise measurement yet of how fast the Universe is expanding shows we have a real and serious problem on our hands. The international H0DN Collaboration, a community consensus report on the Hubble constant, has remapped the markers we use to measure cosmic expansion, creating a framework which pins the rate at 73.5 km's/second/megaparsec for the local Universe, with a certainty of 7 sigma. The problem is that independent measurements still return a rate of 67.24 km's/second/megaparsec for the early Universe, and these new efforts have brought us no closer to resolving the discrepancy, known as the Hubble tension. A comprehensive new study combines decades of research to reveal that we're missing an essential component in our understanding of how the universe works. New research confirms, with the most thorough dataset ever, that something still doesn’t add up in our standard model of cosmology. There's a central crisis in cosmology: Different measurements yield different values for the expansion rate of the universe. Now, a comprehensive analysis combining decades of independent measurements suggests that this discrepancy is not due to error or uncertainty; instead, it's a potential pathway to new physics beyond the standard cosmological model.

Astronomers calculate the universe's expansion rate, or Hubble constant, in two ways. One method is to use measurements of the distance to the cosmic microwave background (CMB), the earliest light that spread out just 380,000 years after the Big Bang. The second method is to study the expansion of the local universe, using observations of "standard candles," nearby stars of a known brightness whose light gets stretched, or redshifted, as it reaches us. The first method's calculations yield a Hubble constant of around 67 or 68 km's/second/megaparsec, while the latter yield a value of approximately 73 km's /second/megaparsec. (One megaparsec is about 3.26 million light-years.). Although this seems like a diminutive discrepancy, it is far greater than statistical uncertainty can explain, presenting a puzzling disagreement known as the Hubble tension. So a large symposium of astronomers convened to vote on the best methods and data for constraining the Hubble constant and determining if the tension actually exists. Our Universe burst into existence some 13.8 billion years ago and has been expanding ever since. The rate at which it does so is known as the Hubble constant, or H0, and it's one of the fundamental measurements that we use to understand the cosmos around us. The Hubble constant helps calculate the age and size of the Universe. It helps us understand the influence of the mysterious dark energy which drives the Universe's expansion. It's one of the values required to calculate intergalactic distances.

In the resulting paper, published in the journal Astronomy & Astrophysics, the authors derived the most precise Hubble constant yet and found that the tension persists, suggesting that our current cosmological model is incomplete. "That's why the Hubble tension is so interesting," study co-author Richard Anderson, an astrophysicist at the University of Göttingen, said. "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." Astronomers have several very precise tools for determining the rate of H0, and this is where the problems start. Previous cosmological calculations relied on the creation of a cosmic distance ladder. Its rungs comprise increasingly distant celestial objects, including pulsating Cepheid variable stars within the Milky Way and more distant supernovas, whose distances can be calculated from the difference in their intrinsic brightness versus how bright they appear to us after their light has traveled through expanding space. Yet this recent community effort, launched at the International Space Science Institute Breakthrough Workshop in Bern, Switzerland, in March 2025, expanded the cosmic distance ladder into a comprehensive survey of the nearby universe called the Local Distance Network, achieving a lofty goal which was considered "potentially unreachable" a decade ago.

"This isn't just a new value of the Hubble constant," the researchers explained in a statement from the National Science Foundation's NOIRLab; "it's a community-built framework that brings decades of independent distance measurements together, transparently and accessibly." The unified framework combined decades of independent research using various techniques that may overlap in observations to achieve "redundancy", an invaluable technique to reduce systematic errors and statistical anomalies. For example, it allowed the researchers to perform a series of "leave me out" analyses: By excluding a specific technique, such as Cepheid-based calculations, they found a minimal change in the overall results of their newly constrained Hubble constant. However, these two epochs can't be brought into agreement with each other, suggesting we're missing something important. The H0DN collaboration approached the problem by focusing on the local Universe. To measure H0 in local space, astronomers rely on something known as the cosmic distance ladder, where each of the rungs on the ladder represents a different measurement technique. The first rung is parallax, which is the apparent shift in position of distant objects when viewed from different vantage points. As Earth moves around the Sun, the parallax of stars tells us how far away they are. The second rung is stars of known brightness, such as Cepheid variables. The third rung is Type Ia supernovae, which have a known brightness peak.

The Local Distance Network is founded on anchors, celestial objects whose distances have been determined geometrically through methods like parallax, an apparent change in an object's position which occurs with a change in perspective. Space telescope access may be limited, but you can reproduce parallax yourself by holding a finger at arm's length and seeing it seemingly shift positions by closing one eye and then the other. Accordingly, the researchers used multiple local-universe anchor points, including the galaxy NGC 4258, located more than 20 million light-years away; the Magellanic Clouds, which are a pair of dwarf galaxies about 200,000 light-years away; and numerous variable stars within the Milky Way. Then, they included a multitude of objects of measured distances, including dying old red giant stars and "megamasers," the intensely bright cosmic lasers generated in the accretion disks of supermassive black holes. The researchers also included more than 7,500 galaxies, observed by facilities such as the Hubble Space Telescope and the Dark Energy Spectroscopic Instrument, out to a distance of more than 1 billion light-years.

As a result, the Local Distance Network developed in this study represents the most precise direct measurement of the Hubble constant in the local universe: 73.50 km's/second/megaparsec, with a relative uncertainty of 1.09%. The conclusion? The Hubble tension is real, similar to previously measured values, and not just an artifact. One possible explanation for the Hubble tension is that there may have been a miscalculation in one of the rungs of the distance ladder, which was carried through to the final measurement. To address this, the collaboration built, not a ladder, but a distance network built from many overlapping techniques for measuring distance, including Cepheid variables, stars at the tip of the red giant branch, Mira variables, megamasers, Type Ia and Type II supernovae, surface brightness fluctuations, the Tully-Fisher relation, and the Fundamental Plane. All these give accurate measurements to nearby stars and galaxies, some of which overlap with each other. Crucially, the researchers rigorously stress-tested their results. They tried removing, by turn, several of the methods and telescopes to see if taking one out changed the result, which would have indicated a flaw in that method. The fact that this discrepancy persists may hint that early-universe measurements need to be similarly reassessed on a deeper level.

"One interesting, relatively new, and perhaps more natural idea involves primordial magnetic fields, which could change the scale of the structure seen in the CMB," study co-author John Blakeslee, director of research and science services at NOIRLab, explained. Excitingly, this research further supports the idea that new physics are needed to illuminate dark energy and the other forces driving the expansion and ultimate fate of the universe. And because this framework is modular, upcoming methods and data from next-generation observatories may finally resolve the Hubble tension, but then again, that's what cosmologists have been hoping for more than a decade. They also tried using different datasets and changing the assumptions on which their analysis was based. The needle barely moved. This is the most stringent examination of H0 at the local level to date, and it survived everything the H0DN Collaboration could throw at it. But measurements of H0 in the distant Universe are also robust, and consistently hovering around the 67 km's/second/megaparsec mark. In recent years, some efforts have focused on overturning the Hubble tension on the basis that our measurements may be wrong. Generally, if our two options are human error and unknown physics, the culprit tends to end up being the former, so that's not an unreasonable expectation. However, this new research strongly indicates that the problem is indeed real, and may require new physics to resolve. "Rather than serving solely to constrain dark energy models, as envisioned a decade ago, the improved accuracy of H0 now exposes a broader inconsistency within the standard cosmological framework and strengthens the case for new physics or a deeper reassessment of early-Universe inferences," the H0DN Collaboration explains. "The evolving role of H0 has already reshaped our understanding of precision cosmology, and further surprises may lie ahead."