Two teams of astronomers measure the same thing: the rate at which the Universe is expanding. They use state-of-the-art instruments, impeccable methods. And yet, they obtain two numbers that stubbornly refuse to coincide. This disagreement, far from fading with time, sharpens. Welcome to one of the most beautiful enigmas of contemporary cosmology.
The topic seems technical, almost abstract. It nonetheless touches a vertiginous question: do we truly understand the structure of our cosmos? Because if two reliable methods contradict each other, perhaps our theory of the Universe must be revised from the ground up.
When Two Numbers Supposed to Be Identical Tear Cosmology Apart
Hubble constant describes the rate at which the Universe expands, expressed in kilometers per second per megaparsec. In plain terms, the farther away a galaxy is, the faster it recedes. This principle, known for almost a century, seems straightforward.
Except that a problem has appeared. Two major families of measurements, expected to yield the same result under the standard model, give strikingly different values. On one side, about 67 km/s/Mpc. On the other, around 73 km/s/Mpc. A gap that, in a field where precision is chased to the decimal, becomes frankly embarrassing.
Scientists have given a name to this puzzle: the Hubble tension. For a long time, there was hope that a measurement error would reconcile them. But as instruments sharpen, the gap only widens. And that is where the excitement begins.
The Cosmic Microwave Background, that luminous echo of the Big Bang
The first method traces back to the origins. About 380,000 years after the Big Bang, the Universe became transparent, releasing a light that still reaches us today. This fossil radiation bears a name: the cosmic microwave background. It is, in a sense, the oldest photograph of our cosmos.
By analyzing the minute temperature variations of this radiation, astronomers reconstruct the state of the very young Universe. From there, by applying the standard cosmology model, they calculate the speed at which the Universe should be expanding today. The verdict: around 67.2 km/s/Mpc.
This value is solid, almost carved in stone. It rests on well-understood physics and observations of remarkable precision. The catch? It describes a primitive Universe, extrapolated to today. Yet, when measuring directly the present expansion, in our vicinity, the number changes. And then, everything becomes more complex.
Cepheids and supernovae, those local beacons that contradict the primordial sky
The second approach does not look back to the origins: it surveys the neighboring cosmos. It relies on Cepheids, stars whose brightness pulses at a regular rhythm, allowing precise distance calculations. These stars serve as beacons, true lighthouses in the interstellar darkness.
By combining these measurements with those of supernovae, we obtain a local value for the expansion. Recently, a large international collaboration delivered the most precise direct measurement to date: 73.50 km/s/Mpc, with an uncertainty of just over 1%. To pierce through the dust that had clouded earlier observations, astronomers mobilized the infrared vision of the James Webb Space Telescope.
The result is stubborn. Cepheids, red giants, carbon-rich stars: several independent methods converge toward the same answer, around 73. Dust was ruled out as the culprit. The gap with the primordial figure only grows more evident, again and again.
Measurement error or new physics: the path that could rewrite everything
So, a simple blunder or a revolution in the making? Statistics heavily tilt toward the latter hypothesis. The gap between the two values is now solidified at more than 5 sigma. In physics, this threshold is the one that allows speaking of a discovery: the chances that such a discrepancy is due to random fluctuations are less than one in 3.5 million.
In other words, the disagreement can no longer be explained by a measurement error. It becomes an independent datum. And that points toward a fascinating possibility: our standard model of the Universe would be incomplete. Some unknown physics could lurk between the primitive cosmos and the present cosmos, a new component still invisible to our equations.
To settle the matter, the scientific community does not stand idly by. Projects are tackling the question head-on, pushing methodologies to their limits to uncover potential biases. The Euclid mission, for its part, is drawing an independent map of the structure of the Universe, offering a third perspective on this tension. Could this help resolve the two camps?
The Hubble tension is therefore not a mere whim of numbers. It pits two visions of cosmic expansion against each other, one inherited from the cosmic microwave background, the other arising from local observations, with neither willing to yield. And if, rather than a measurement anomaly, this disagreement were the first crack toward a completely new understanding of our Universe?
