The tension is getting uncomfortable
Across decades of careful observation, humanity has grown confident in its map of the cosmos — yet the universe refuses to confirm a single, coherent speed for its own expansion. A new study of 63 elliptical galaxies, using a technique that reads the texture of starlight itself, arrives at the same answer as other local measurements: the universe expands at roughly 73 kilometers per second per megaparsec, a number that sits in quiet but stubborn disagreement with what the earliest light in existence seems to say. The gap is small enough to invite dismissal, yet wide enough — and now confirmed by enough independent methods — to suggest that either our physics is incomplete or our confidence in our measurements is misplaced. The universe, it seems, is keeping a secret.
- Multiple independent teams, using entirely different tools, keep arriving at two irreconcilable numbers for how fast the universe is expanding — and the gap refuses to close.
- A fresh study of 63 nearby elliptical galaxies, analyzed through infrared surface brightness fluctuations, adds yet another precise local measurement of 73 km/s/Mpc, tightening the tension rather than relieving it.
- The competing figure of 68 km/s/Mpc, drawn from the faint afterglow of the Big Bang itself, represents a fundamentally different era of cosmic history — making the disagreement feel less like a rounding error and more like a fracture in the model.
- Researchers now face an uncomfortable fork: either unknown physics is hiding inside our best cosmological theories, or the error bars on decades of careful measurement are quietly lying.
- New ground and space observatories are approaching readiness, offering fresh measurement strategies that may finally force the universe to give a single, consistent answer — or deepen the crisis further.
The universe is expanding, and we cannot agree on how fast. That disagreement has quietly grown into one of the most unsettling puzzles in modern cosmology, and a new study in The Astrophysical Journal confirms it is not going away.
Researchers selected 63 elliptical galaxies within 330 million light-years of Earth and applied a technique called surface brightness fluctuation, which exploits the predictable light patterns of old, star-dense elliptical galaxies to measure cosmic distances with precision. Analyzing their infrared properties, the team calculated a Hubble constant — the number describing the universe's expansion rate — of approximately 73 kilometers per second per megaparsec. In plain terms: two points in space separated by a million parsecs are rushing apart at roughly 73 kilometers every second.
That result sits comfortably alongside other measurements of the nearby universe. It does not, however, sit comfortably alongside observations of the early universe, where the cosmic microwave background — the faint thermal echo of the Big Bang — consistently points to a value closer to 68 km/s/Mpc. Five kilometers per second may sound like a rounding error, but when independent teams using independent methods all land on the same side of the divide, the discrepancy becomes difficult to wave away.
Co-author Chung-Pei Ma of UC Berkeley described the surface brightness fluctuation method as a powerful new tool for this kind of measurement, and noted that this study represented the first large, homogeneous dataset assembled with it specifically to probe the Hubble constant. Even so, the careful work only sharpened the mystery. As Ma put it, assuming no one has underestimated their uncertainties, 'the tension is getting uncomfortable.'
Cosmology now faces a genuine fork in the road: either the standard model of the universe is missing some physics, or the measurements themselves carry hidden errors that have eluded detection across multiple techniques. New observatories, both on the ground and in orbit, are coming online and will bring fresh approaches to the problem. Until they do, the cosmos continues its expansion at a rate we cannot quite agree upon — a quiet reminder that even our most carefully constructed picture of reality still has room for surprise.
The universe is flying apart, and we can't agree on how fast. This simple fact has become one of cosmology's most vexing puzzles, and a new study published in The Astrophysical Journal confirms that the problem is not going away.
For years, astronomers have measured the expansion rate of the universe using different methods, and the results stubbornly refuse to align. When researchers look at the nearby cosmos—galaxies within a few hundred million light-years of Earth—they get one number. When they look back to the earliest moments after the Big Bang, encoded in the faint glow of the cosmic microwave background, they get a different one. The gap between these measurements is small enough to seem like it might be a rounding error, but it is large enough to suggest something fundamental is wrong with how we understand the universe.
A team of astronomers decided to test the waters with a fresh approach. They selected 63 elliptical galaxies, all within 330 million light-years of Earth, and applied a technique called surface brightness fluctuation, or SBF. The method works well for elliptical galaxies because they are old and densely packed with stars of similar age, making their light patterns predictable and measurable. By analyzing the infrared properties of these galaxies, the researchers calculated the Hubble constant—the number that describes how fast the universe expands—and arrived at a value of approximately 73 kilometers per second per megaparsec. To put that in concrete terms: if two galaxies sit one million parsecs apart, they are moving away from each other at roughly 73 kilometers per second, or about 45 miles per second.
This result aligns neatly with other measurements of the nearby universe. But it diverges sharply from observations of the early universe, which place the Hubble constant at around 68 kilometers per second per megaparsec. The difference—five kilometers per second—might sound trivial until you consider that multiple independent methods all point in the same direction. When different teams using different tools arrive at different answers, the problem becomes harder to dismiss as a fluke.
Chung-Pei Ma, a cosmologist at the University of California, Berkeley, and a co-author of the study, called the surface brightness fluctuation technique "a fantastic method" for measuring distances to galaxies out to 100 megaparsecs. She also noted that this was the first large-scale assembly of homogeneous data using the SBF method specifically to study the Hubble constant. Yet even this careful work only deepened the mystery. When asked about the persistent tension between measurements, Ma offered a measured response: the discrepancy might still fall within the error bars of current observations, but the margins are tightening. "Assuming everyone's error bars are not underestimated, the tension is getting uncomfortable," she said.
Cosmology now stands at a crossroads. Either the universe contains some physics we have not yet accounted for in our theories, or the measurements themselves are more uncertain than researchers believe. The jury, as Ma put it, remains out. What is certain is that the tension will not resolve itself. New observatories—both ground-based and space-based—are coming online, and they will offer fresh ways to measure the universe's expansion. Until then, the cosmos keeps expanding at a rate we cannot quite pin down, a reminder that even our most confident understanding of reality still harbors deep uncertainties.
Citações Notáveis
For measuring distances to galaxies out to 100 megaparsecs, this is a fantastic method. This is the first paper that assembles a large, homogeneous set of data on 63 galaxies for the goal of studying the Hubble constant using the SBF method.— Chung-Pei Ma, University of California, Berkeley
The jury is out. I think it really is in the error bars. But assuming everyone's error bars are not underestimated, the tension is getting uncomfortable.— Chung-Pei Ma, University of California, Berkeley
A Conversa do Hearth Outra perspectiva sobre a história
Why does it matter if the expansion rate is 68 or 73? Isn't that close enough?
Not really. These aren't rough estimates—they're measurements from independent teams using different methods. When they all disagree in the same direction, it suggests something is systematically wrong with either our theory or our tools. A five percent difference might sound small, but it could point to physics we don't understand yet.
So what could be missing from the theory?
That's the question keeping cosmologists up at night. Maybe there's a new type of particle or force we haven't discovered. Maybe dark energy behaves differently than we think. Or maybe the universe's geometry is more complex than our models assume.
And the other possibility—that the measurements are off?
That's actually harder to accept, because the measurements come from completely different methods. You have observations of nearby galaxies, observations of the early universe, and now this new technique using elliptical galaxies. They should disagree randomly if it were just measurement error. Instead, they cluster into two camps.
So we're stuck?
Not stuck, but humbled. The new observatories coming online will give us better data and tighter error bars. That will either shrink the gap or make it impossible to ignore. Either way, we'll learn something real about how the universe works.