The Hubble Tension: Why the Universe's Expansion Doesn't Add Up
Two of the best measurements of how fast the universe is expanding disagree by about 8 per cent, and after JWST the easy explanations are running out. Here is what the Hubble tension is, why it will not go away, and the fresh idea that might ease it.
The Hubble tension is a stubborn disagreement between two ways of measuring how fast the universe is expanding. Readings from the early universe give a Hubble constant of about 67.4 kilometres per second per megaparsec, while measurements of the nearby universe give about 73, a gap of roughly 8 per cent that is far too large to be chance. The James Webb Space Telescope has ruled out the most convenient explanation (a measurement error in older data), so many cosmologists now suspect the standard model of the universe is missing something. A 2025 study even suggests a faint primordial magnetic field could be the culprit, though its authors call it a hint, not a solution.
Cosmology has a number it cannot agree with itself on. Two of the most careful measurements science can make, one looking back at the infant universe and one looking at galaxies nearby, give different answers for how fast space is expanding. The gap has survived better instruments, independent teams and years of scrutiny, and that refusal to disappear is why the Hubble tension has become one of the biggest open questions in physics.
Here is what the disagreement actually is, why the James Webb Space Telescope made it harder rather than easier to dismiss, and the ideas, including one from 2025, that might resolve it.
What is the Hubble tension?
The Hubble constant, written H0, measures how fast the universe is expanding: how much faster a galaxy appears to recede for every extra megaparsec (about 3.26 million light years) of distance from us. The tension is that the two main ways of pinning it down give answers that do not match.
- The early-universe value comes from the cosmic microwave background, the relic light from about 380,000 years after the Big Bang. Feeding the European Space Agency's Planck map into the standard model of cosmology predicts H0 = 67.4 kilometres per second per megaparsec, give or take 0.5.
- The local value comes from measuring distances to relatively nearby galaxies directly, using pulsating stars called Cepheids to calibrate exploding stars called Type Ia supernovae. The leading team here, SH0ES, led by Nobel laureate Adam Riess, gets H0 = 73.04, give or take about 1.
That is a difference of about 8.4 per cent. It sounds small, but the error bars on each number are tight enough that the two disagree at roughly the five-sigma level, the rough threshold physicists use to call something real rather than a fluke.
Why can't scientists just measure it once and settle it?
Because the two numbers are not rival attempts at the same measurement. They are answers to subtly different questions, and both are defensible.
The early-universe figure is really a prediction: take the baby universe as the CMB shows it, apply the standard model (ordinary matter, dark matter, dark energy and the known physics linking them), and calculate how fast it should be expanding today. The local figure is a direct measurement of how fast it actually is expanding now. When a prediction and a measurement disagree this firmly, either one of them has a hidden error, or the model connecting them is incomplete.
Did the James Webb telescope confirm the Hubble tension?
Mostly, and that is the important development, but not unanimously.
The obvious escape route was that the local measurement was wrong, specifically that crowded fields of stars were contaminating the older Hubble Space Telescope readings of Cepheids and nudging the distances off. In a paper published in February 2024, Riess and the SH0ES team used the sharper eyes of the James Webb Space Telescope to re-observe more than 1,000 Cepheids and reported that the older distances held up, ruling out that crowding explanation at the eight-sigma level. A joint NASA and ESA release that year summarised the upshot in Riess's words: "with measurement errors negated, what remains is the real and exciting possibility that we have misunderstood the universe."
Two cautions keep that honest. First, that eight-sigma figure refers to rejecting the crowding error, not to the strength of the tension itself, which remains around five sigma. Second, a rival JWST programme led by Wendy Freedman, using three independent distance indicators, reads a lower value closer to 70 and argues the data may still be consistent with the standard model without new physics. So JWST has demolished the easiest way out and strongly backed the SH0ES measurement, but it has not produced a single agreed local number, and that part of the debate is still live.
What could explain the Hubble tension?
If the measurements are sound, the model is the suspect. The mainstream candidates, surveyed in review papers such as "In the Realm of the Hubble Tension," cluster around changing the physics of the early universe so the predicted expansion rate rises to meet the local one.
| Proposed fix | The basic idea |
|---|---|
| Early dark energy | A brief burst of a dark-energy-like component before the CMB formed, shrinking a key cosmic yardstick and raising the inferred H0 |
| Extra relativistic particles | More, or differently behaving, neutrinos or "dark radiation" in the young universe |
| Modified gravity | Gravity itself behaving slightly differently than general relativity assumes |
| Modified recombination | New physics (including faint magnetic fields) changing how the first atoms formed |
The rough consensus is that a pre-recombination tweak, something that acted before that relic light was released, is the most promising class, but none is established.
The fresh idea: a primordial magnetic field
The most interesting recent development is a 2025 study by Karsten Jedamzik, Levon Pogosian and Tom Abel, published in Nature Astronomy. Modelling how a faint primordial magnetic field would have sped up the formation of the first atoms, and testing that model against the cosmic microwave background, galaxy-clustering and supernova data, they found that a field of around 5 to 10 picogauss would push the inferred Hubble constant higher, easing the tension while still fitting the data about as well as the standard model.
It is a genuinely intriguing candidate because such magnetic fields are independently suspected to exist and could explain other cosmic puzzles too. But the authors are careful: they describe it as a hint that needs testing against sharper CMB measurements over the next several years, not a resolution. It belongs in the "promising lead" column, not the "solved" one.
Why does this matter?
Because if the tension is real and the measurements are right, it means the standard model of cosmology, the framework that has fit almost everything for decades, is missing an ingredient. That is not a failure so much as a doorway: the last time cosmology ran into a surprise this stubborn, the resolution was something as large as dark energy. Whatever closes the roughly 8 per cent gap, whether early dark energy, exotic particles or a whisper of primordial magnetism, could turn out to be the next genuinely new piece of physics. For now, the honest status is that the universe is handing us two different answers and daring us to work out why.
Frequently asked questions
What is the Hubble tension in simple terms?
It is a disagreement between two good measurements of how fast the universe is expanding. Looking at the early universe gives about 67.4 kilometres per second per megaparsec; looking at nearby galaxies gives about 73. The roughly 8 per cent gap is too big to ignore.
Why don't scientists agree on how fast the universe is expanding?
Because the two methods answer slightly different questions: one predicts today's expansion from the infant universe plus the standard model, the other measures it directly now. When the prediction and the measurement disagree this firmly, either a measurement is flawed or the model is incomplete.
Did James Webb confirm the Hubble tension?
It confirmed the leading local measurement and ruled out the easy "it was a measurement error" explanation at high confidence. It did not, however, make every team agree: a rival JWST programme still reads a lower value, so the local number is strongly supported but not unanimous.
What is the Hubble constant's value?
The two contested figures are about 67.4 (early universe, from Planck) and about 73.04 (local, from the SH0ES team) kilometres per second per megaparsec. Other local methods land roughly in the 70 to 76 range.
Could the Hubble tension mean new physics?
Possibly. If the measurements are right, the standard model of cosmology may be missing something, with early dark energy, extra particles, modified gravity or primordial magnetic fields among the proposed additions. None is confirmed yet.
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