For over a decade, cosmology has been stuck on a stubborn contradiction. The universe's expansion rate—how fast it's actually stretching—gives two completely different answers depending on which measurement method you trust.
Local observations of nearby supernovas suggest the expansion rate is about 73 kilometers per second per megaparsec. But measurements of the cosmic microwave background—the faint afterglow of the Big Bang itself—point to a slower rate of around 67 km/s/Mpc. It's a gap that shouldn't exist, and it's become one of modern physics' biggest headaches. If the disagreement is real and not just a measurement mistake, it means our understanding of how the universe works is fundamentally incomplete.
Now a team of astronomers has approached the problem sideways, using a method that sidesteps the usual measurement pitfalls entirely. Their findings suggest the Hubble tension—as physicists call it—might actually reflect something real about the universe, not just sloppy data.
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Start Your News DetoxA Different Way to Measure the Universe
Traditionally, astronomers measure cosmic expansion by building a distance ladder. They start with nearby stars whose brightness they understand well, use those to calibrate supernovas, then use those supernovas to gauge distances across the universe. It works, but small errors at each step compound. Critics have long suspected these accumulated mistakes might explain the Hubble tension.
The new study bypasses this entire ladder using something called time-delay cosmography. It exploits one of gravity's strangest tricks: gravitational lensing.
Imagine a massive galaxy sitting between Earth and a distant quasar. The galaxy's gravity is so strong it bends the quasar's light around itself, creating multiple images of the same object. Each light path takes a slightly different route, so each image arrives at Earth at a different time—sometimes separated by weeks or months.
The researchers studied eight such lens systems, watching for moments when the background quasar brightened or dimmed. Those brightness changes appeared in all the multiple images, but each one arrived at a different moment. By precisely measuring these delays, the team could calculate how long each light path actually was. Combined with models of how mass is distributed inside the lensing galaxies—which determines how light bends—they could work backward to the universe's expansion rate.
They used some of the world's sharpest telescopes for the job, including the James Webb Space Telescope. The result: a measurement of the Hubble constant with about 4.5% precision. That measurement aligns with the higher expansion rate (~73 km/s/Mpc) from local studies, suggesting the Hubble tension reflects real physics rather than measurement error.
The Work Ahead
But the researchers aren't ready to declare the standard cosmological model broken. Uncertainties remain, particularly in how mass actually distributes inside lensing galaxies—even small deviations can shift the final numbers. And eight lens systems, while promising, aren't enough. To decisively settle whether new physics is needed, the team needs to reach 1–2% precision. They're currently at 4.5%.
"Right now our precision is about 4.5%, and in order to really nail down the Hubble constant to a level that would definitively confirm the Hubble tension, we need to get to a precision of around 1–2%," said Eric Paic, one of the researchers at the University of Tokyo.
The next phase is straightforward: collect more lens systems, sharpen the images, and eliminate remaining sources of error. With new powerful telescopes now coming online, the team is confident the method can soon deliver the precision needed. If they're right, we may finally know whether the universe's expansion truly behaves differently than our best theories predict—or whether we've simply been measuring it wrong all along.
