The universe is expanding—but not at the rate we once expected. Recent measurements have confirmed that the universe is expanding faster than what theoretical models predict, challenging our current understanding of cosmology and even suggesting that existing models may be broken. This phenomenon, which has come to be known as the “Hubble tension,” is becoming an increasingly urgent puzzle for physicists.
A recent study published in The Astrophysical Journal Letters has added even more weight to the evidence that the universe’s expansion rate, measured by the Hubble constant, is much faster than predicted. For years, there has been a growing divide between two major ways of measuring the expansion, and the new results have only amplified the mystery, leading some researchers to describe the situation as a “crisis” in cosmology.
The Search for the Hubble Constant: The Universe’s Expansion Rate
The concept of the expanding universe was first discovered by Edwin Hubble in 1929. He showed that galaxies were moving away from us, and the further away they were, the faster they receded. This groundbreaking discovery hinted at the expansion of the cosmos itself, ultimately leading to the theory that the universe began as a singularity in an event we call the Big Bang.
In modern cosmology, determining the exact rate at which the universe expands, known as the Hubble constant (denoted as H₀), has been one of the key scientific pursuits. It provides us with essential information about the size, age, and fate of the universe. Imagine it as measuring the growth of the universe, like charting how much a seed, representing the universe’s early moments, has sprouted over billions of years into its present, vastly expanded form. And therein lies the problem: this growth curve doesn’t seem to add up.
“The tension now turns into a crisis,” said Dan Scolnic, a lead researcher from Duke University and a key figure behind recent findings regarding H₀.
Scolnic’s analogy helps us understand the challenge: we know what the universe looked like as a baby at the time of the Big Bang, and we also know what it looks like today. But how the universe got from that tiny, hot origin to its current vastness—if the models are correct—is still an open question. Current observational measurements don’t connect those dots seamlessly.
The Growing Divide in Expansion Rates
Historically, scientists have measured the Hubble constant using two primary methods. The first uses measurements of the early universe, which study the Cosmic Microwave Background (CMB)—the faint remnant radiation from the Big Bang. Using data from missions like the Planck satellite, these measurements suggest that the expansion rate of the universe is around 67.4 km/s per megaparsec. A megaparsec is roughly 3.26 million light-years, and this value predicted by CMB observations has become widely accepted and forms the foundation of the cosmological model we use today.
The second method involves the “local” universe, where astronomers measure galaxies’ movement directly, through methods such as observing Cepheid variable stars and Type Ia supernovae in nearby galaxies. These methods provide much more accurate distance measurements. When measurements taken closer to the current era (on cosmic scales) are used, they estimate the expansion rate to be significantly faster, around 73-76 km/s per megaparsec—much higher than the CMB predictions.
This discrepancy between the two methods, known as the Hubble tension, has been puzzling the scientific community for nearly a decade. For years, there were heated debates about whether the difference could be due to measurement errors or some fundamental flaw in our models.
Scolnic, whose work has challenged the standard interpretation of these measurements, commented, “We’re at a point where we’re pressing really hard against the models we’ve been using for two and a half decades, and we’re seeing that things aren’t matching up.”
A New Measurement of the Hubble Constant
Scolnic, a professor at Duke University, and his collaborators are working to solve this mystery by refining measurements of the Hubble constant. Their strategy relies on enhancing something called the “cosmic distance ladder,” which is a methodical approach used by astronomers to measure cosmic distances.
The cosmic ladder is made up of different “rungs,” each using a different technique to measure distances—each one built on the previous rung. For example, parallax measurements (which rely on shifts in a star’s position as Earth moves around the Sun) provide accurate distance measures of nearby objects, which are then used to calibrate methods for more distant objects, like Cepheid stars and supernovae. But, like a ladder missing a rung, if one of these measurements is not accurate, it can distort the entire ladder, leading to incorrect cosmological calculations.
Scolnic’s team focused on anchoring the cosmic distance ladder more accurately. By using precise measurements from the Dark Energy Spectroscopic Instrument (DESI), which surveys over 100,000 galaxies each night from Kitt Peak National Observatory in Arizona, Scolnic saw an opportunity to solve the problem. He realized that the first step toward solving the issue involved a better understanding of the distance to the Coma Cluster, one of the nearest galaxy clusters to Earth. By using data from 12 Type Ia supernovae (a particularly useful tool for measuring cosmic distances), the team was able to more precisely determine the distance to the Coma Cluster—measuring it as roughly 320 million light-years away.
Type Ia supernovae are used because they have a known luminosity, or intrinsic brightness. Astronomers can compare their observed brightness to the known luminosity to calculate the distance to the supernova. This measurement fit squarely within the expected range, giving the researchers a solid foundation to improve the cosmic ladder.
“This measurement isn’t biased by how we think the Hubble tension story will end,” said Scolnic, referring to how critical it was for their work to come from a completely independent measurement. “This cluster is in our backyard, it has been measured long before anyone knew how important it was going to be.”
With that first rung of the ladder calibrated, the team went on to refine the rest of the ladder, ultimately calculating a Hubble constant of around 76.5 km/s per megaparsec. This new value corresponds closely to the previously measured expansion rates in our local universe—those measured from Cepheid variable stars, supernovae, and galaxy motions—and far exceeds predictions from the early universe based on the CMB.
The Broader Implications of the Hubble Tension
For decades, cosmological models have relied on the assumption that the universe’s expansion rate should be the same whether measured through the ancient, distant universe (via the CMB) or locally. The discrepancy between these two independent approaches suggests that something in the standard model of cosmology may be amiss. The more measurements point to this faster expansion rate, the more scientists begin to question whether our current understanding of cosmology—based on the ΛCDM model (Lambda-Cold Dark Matter model)—is fundamentally flawed.
“If we are consistently getting this number, if it keeps showing up across many independent measurements, then it’s pointing to the fact that something isn’t right with our models,” said Scolnic. “There might be a missing piece that we haven’t yet discovered—something new we haven’t accounted for.”
This discrepancy is far more than just a minor miscalculation or a bug in the measurement methods. It could signal new physics waiting to be discovered. This could be a manifestation of new, unknown forces, additional particles, or subtle tweaks in the fundamental constants of physics. Perhaps new theories of gravity or dark energy are needed to explain this unprecedented speed. It’s also possible that we simply haven’t accounted for some effects of the universe’s expansion that we don’t fully understand yet.
Moving Forward: An Exciting Frontier in Cosmology
As scientists refine the measurements further, the hunt for the true Hubble constant could lead to an even deeper understanding of the nature of the universe. If Scolnic and his team are correct, this “crisis” in cosmology may turn into a transformative opportunity—an opening for profound discoveries that could reshape how we think about the universe itself.
“This is an exciting time in cosmology,” Scolnic concludes. “There are still surprises left, and who knows what the next breakthrough might be.”
Though much work remains to be done, one thing is certain: the resolution of the Hubble tension will not only solve a long-standing puzzle but might open entirely new areas of exploration, broadening our understanding of the universe in ways we can’t yet predict.
Reference: Daniel Scolnic et al, The Hubble Tension in Our Own Backyard: DESI and the Nearness of the Coma Cluster, The Astrophysical Journal Letters (2025). DOI: 10.3847/2041-8213/ada0bd