‘Something’s missing’: The most thorough study of the universe to date proves we still can’t explain how the universe is expanding

There is a serious crisis in cosmology. Different measurements give different values ​​for the rate of expansion of the universe. Now, a comprehensive analysis combining decades of independent measurements suggests that this discrepancy is not due to error or uncertainty. Rather, it is a potential pathway to new physics beyond standard cosmological models.

Astronomers calculate the expansion rate of the universe, or the Hubble constant, in two ways. One method is to use measurements of the distance to the earliest light, the cosmic microwave background (CMB), which spread just 380,000 years after the Big Bang. The second method is to study the expansion of the local universe using “standard candles,” observations of nearby stars of known brightness. That light is stretched, or redshifted, as it reaches us.

The first method calculates the Hubble constant to be about 67 or 68 kilometers per second per megaper second, while the second method gives a value of about 73 kilometers per second per megaper second. (1 megaparsec is approximately 3.26 million light years.)

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Although this seems like a small discrepancy, it is much larger than the statistical uncertainty can explain, and points to a puzzling discrepancy known as the Hubble tension. So a large symposium of astronomers was held to vote on the best methods and data to constrain the Hubble constant and determine whether tension actually exists.

In the resulting paper, published April 10 in the journal Astronomy & Astrophysics, the authors derive the most accurate Hubble constant to date and find that the tension persists, suggesting that current cosmological models are incomplete.

“That’s why the Hubble tension is so interesting,” study co-author Richard Anderson, an astrophysicist at the University of Göttingen, told Live Science in an email. “Comparison of values ​​in the late universe and early universe [the Hubble constant] It tests fundamental physics on a cosmological scale and tells us we’re missing something. ”

The most comprehensive review of the expanding local universe

Previous cosmological calculations relied on the creation of a cosmic distance ladder. Its rungs include increasingly distant objects, such as Cepheid variable stars pulsating within the Milky Way and supernovae further afield, whose distances can be calculated from the difference between their intrinsic brightness and the brightness they appear to us after their light passes through expanding space.

But this recent community effort, launched at the International Institute for Space Science’s Breakthrough Workshop in Bern, Switzerland, in March 2025, extends the cosmic distance ladder to a comprehensive survey of nearby space called the Local Distance Network, achieving a lofty goal that was considered “likely to be out of reach” a decade ago.

“This is not just a new value for the Hubble constant,” the researchers explained in a statement from the National Science Foundation’s NOIRLab. “This is a community-built framework that brings together decades of independent distance measurements in a transparent and accessible way.”

The unified framework combines decades of independent research using different techniques that may overlap in observations to achieve “redundancy,” a valuable technique for mitigating systematic errors and statistical anomalies.

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For example, this enabled researchers to perform a series of “ignore me” analyses. We find that by excluding certain techniques, such as Cepheid-based calculations, there is minimal change in the overall results for the newly constrained Hubble constant.

Basics of space network

Local distance networks are based on anchors. An anchor is a celestial body whose distance is determined geometrically by methods such as parallax, an apparent change in the object’s position caused by a change in perspective. Access to space telescopes may be limited, but you can recreate parallax yourself by holding your finger at arm’s length, closing one eye, then the other, and watching the position appear to shift.

Therefore, the researchers used multiple local cosmic anchor points, including the galaxy NGC 4258, which is more than 20 million light-years away. The Magellanic Clouds are a pair of dwarf galaxies about 200,000 light years away. A large number of variable stars in the Milky Way galaxy.

It also included a number of objects within the measurement range, including old dying red giant stars and megamasers, extremely bright cosmic lasers produced in the accretion disks of supermassive black holes.

The researchers also included more than 7,500 galaxies observed more than 1 billion light-years away by facilities such as the Hubble Space Telescope and the Dark Energy Spectrograph.

As a result, the local distance network developed in this study represents the most accurate direct measurement of the Hubble constant in the local universe. 73.50 kilometers per second per megapar second, relative uncertainty is 1.09%. What is the conclusion? The Hubble tension is real, similar to previously measured values, and not just an artifact.

The fact that this discrepancy still exists may suggest that measurements from the early universe need to be reevaluated at a deeper level as well.

“One of the more interesting, relatively new, and perhaps more natural ideas involves primordial magnetic fields that can change the scale of the structures seen in the CMB,” study co-author John Blakeslee, NOIRLab’s director of research and scientific services, explained in an email.

Interestingly, this study further supports the idea that new physics is needed to uncover dark energy and other forces driving the expansion and ultimate fate of the universe. And because this framework is modular, future techniques and data from the next generation of observatories may ultimately resolve the Hubble strain. But then again, that’s what cosmologists have been expecting for more than a decade.