The Hubble Tension Deepens: Why the Universe’s Expansion Rate Remains a Cosmological Mystery

There is a number at the very heart of cosmology that measures how fast the universe is expanding. It is called the Hubble constant. And right now, our two best ways of measuring it flatly disagree.

One method looks at the early universe and gives one answer. Another looks at the nearby universe and gives a value about eight percent higher.

That gap should not exist. Both methods are precise, both are independent, and both have been checked exhaustively. The disagreement has only grown sharper with time.

This is the Hubble tension — one of the deepest unsolved problems in modern physics. It may be a hidden error. Or it may be the first crack in our entire model of the cosmos.

This article explains what the Hubble constant is, how each side measures it, why the gap has only grown, and what it might finally reveal about the universe.

What Is the Hubble Constant?

In the 1920s, Edwin Hubble and Georges Lemaître found that distant galaxies are moving away from us — and the farther a galaxy is, the faster it recedes. The universe is expanding.

The Hubble constant, written H₀, is the rate of that expansion. It is measured in kilometres per second per megaparsec — the speed a galaxy recedes for every megaparsec of distance.

A megaparsec is about 3.26 million light-years. So a Hubble constant of 70 means a galaxy one megaparsec away recedes at roughly 70 kilometres every second.

This single number is fundamental. It sets the age of the universe, its size, and how it will evolve — which is exactly why a disagreement over its value is so serious.

Two Ways to Measure the Universe

There are two fundamentally different routes to the Hubble constant, and the tension lives in the gap between them.

The first works forward from the early universe. It takes the physics of the infant cosmos, measured with great precision, and predicts what the expansion rate should be today.

The second works directly in the nearby universe. It measures the actual distances and speeds of galaxies around us and reads off the expansion rate we truly observe.

In a healthy theory, both should give the same answer. They do not — and understanding why requires looking at each method in turn.

The Early-Universe Value: Planck and the CMB

The early-universe measurement comes from the cosmic microwave background — the faint afterglow of the Big Bang, released when the universe was about 380,000 years old.

The European Space Agency’s Planck satellite mapped this radiation across the whole sky with exquisite precision. Its tiny temperature ripples encode the composition and geometry of the young universe.

Feeding that data into the standard model of cosmology yields a Hubble constant of 67.4 kilometres per second per megaparsec, with an uncertainty of well under one percent.

This is not a direct measurement of today’s expansion. It is a prediction — what H₀ must be if the standard model is correct and we extrapolate forward 13.8 billion years.

Crucially, the same value emerges from an entirely separate early-universe probe: baryon acoustic oscillations, the frozen sound waves imprinted in the distribution of galaxies. The early-universe answer is robust.

The Late-Universe Value: The Cosmic Distance Ladder

The nearby measurement uses a chain of distance markers called the cosmic distance ladder. Each rung calibrates the next, reaching ever farther out.

The first rung is parallax — the tiny shift in a nearby star’s position as Earth orbits the Sun. It gives direct, geometric distances to stars in our own galaxy.

The second rung is Cepheid variable stars. These pulsate at a rate tied precisely to their true brightness, so measuring their pulse reveals their distance.

The third rung is Type Ia supernovae — exploding white dwarfs that all reach nearly the same peak brightness. They are visible across billions of light-years, extending the ladder deep into the expanding universe.

The SH0ES team, led by Nobel laureate Adam Riess, has spent years refining this ladder with the Hubble Space Telescope. Their result is a Hubble constant of about 73 kilometres per second per megaparsec.

That is a direct measurement of the real, present-day expansion — and it sits stubbornly above the value the early universe predicts.

How Big Is the Disagreement?

The gap between roughly 67 and 73 might sound small. In cosmology, it is enormous.

The significance of the disagreement now stands at around five standard deviations, often called five sigma. That is the traditional threshold physicists use to declare a genuine discovery.

In plain terms, the odds that this is a random statistical fluke are less than one in a million. The two numbers are not going to drift into agreement.

Either there is a subtle, shared error hiding in one of the methods — or the standard model of cosmology is missing something real about how the universe works.

A Tension That Refuses to Fade

When the discrepancy first surfaced in the early 2010s, most cosmologists expected it to melt away. Early measurements are noisy, and small gaps often close as data improve.

The opposite happened. As both the early- and late-universe measurements grew more precise, their error bars shrank — but the values themselves did not budge toward each other.

The significance climbed steadily, from around two sigma a decade ago to roughly five sigma today. Better data made the problem worse, not better.

That trajectory is the opposite of what a statistical fluke or a simple error usually does. It is the signature of a real, physical effect that our current model cannot accommodate.

Did the James Webb Telescope Solve It?

For years, the leading suspicion was that the Cepheid rung of the ladder was flawed. In crowded galaxies, nearby stars might blur together and bias the brightness measurements.

The James Webb Space Telescope offered a clean test. Its sharper infrared vision can separate Cepheids from their neighbours far better than Hubble could.

The verdict, reported by Riess and colleagues, was decisive. Webb’s measurements confirmed the Hubble Cepheid distances almost exactly — the crowding was not causing the tension.

This was a turning point. It closed off the most popular “boring” explanation and pushed the field toward taking the tension seriously as a physical puzzle. The telescope behind that test is covered in our article on the James Webb Space Telescope.

The Coma Cluster Result

A recent measurement has sharpened the puzzle further. A team led by Dan Scolnic of Duke University and Adam Riess of Johns Hopkins targeted the Coma Cluster, a dense swarm of over a thousand galaxies about 320 million light-years away.

The Coma Cluster is close enough to reach with the distance ladder yet far enough that the pure expansion of space dominates its motion. That makes it an excellent independent anchor.

Using Type Ia supernovae in the cluster, the team measured its distance precisely. The result pointed to a high Hubble constant, in line with the other local measurements.

Far from easing the tension, the Coma result reinforced it. Yet another independent handle on the nearby universe landed on the high side of the divide.

Independent Referees: Lensing, Masers, and Red Giants

If only two methods disagreed, one might simply be wrong. What makes the tension so hard to dismiss is that many independent techniques have weighed in.

Strong gravitational lensing measures the expansion using the time delays between multiple images of a lensed quasar. It lands near the high, local value — and is fully independent of the distance ladder. The method is explained in our article on gravitational lensing.

Water megamasers — natural microwave lasers in distant galaxies — give geometric distances with no ladder at all. They too favour a high Hubble constant.

One method sits in between. The tip of the red-giant branch, an alternative distance marker championed by Wendy Freedman, has yielded intermediate values around 70 — neither fully confirming nor resolving the split.

This scatter is itself informative. The nearby universe consistently gives values at or above 70, while the early universe insists on 67 — a stand-off across many methods.

Could It Be a Mistake?

The most cautious explanation is that a systematic error lurks somewhere, unnoticed. Science has been fooled by hidden biases before.

But the room for such an error keeps shrinking. Webb ruled out Cepheid crowding, multiple ladder-free methods agree with the local value, and the early-universe result is backed by two separate probes.

For a systematic error to explain everything, it would have to be shared across techniques that have almost nothing in common. That is possible, but increasingly hard to imagine.

Each year the “mistake” explanation grows less comfortable, and the “new physics” explanation grows harder to avoid.

The Standard Model Under Strain

The framework connecting the two measurements is called Lambda-CDM — the standard model of cosmology. It describes a universe built from ordinary matter, cold dark matter, and dark energy.

This model has been extraordinarily successful. It explains the cosmic microwave background, the abundance of light elements, and the large-scale web of galaxies with remarkable accuracy.

The early-universe Hubble value is not measured directly — it is derived by assuming Lambda-CDM and extrapolating forward. That assumption is precisely what the tension calls into question.

If the local measurements are right and the model is sound, the two numbers should match. Their stubborn disagreement suggests a missing ingredient somewhere between the Big Bang and today.

That is why physicists no longer treat the tension as a nuisance to be explained away. They increasingly treat it as a clue — a place where the standard model may be quietly breaking down.

Could It Be New Physics?

If both measurements are right, then the model connecting them must be wrong. This is where the tension becomes genuinely exciting.

A leading idea is early dark energy — a burst of extra repulsive energy in the infant universe that faded quickly, changing the early-universe calibration and raising the predicted H₀.

The appeal of early dark energy is that it targets the right moment. By acting only in the first fraction of the universe’s history, it can nudge the predicted expansion rate upward without spoiling later successes.

But it is not a clean fix. Adding early dark energy tends to worsen agreement with other data, such as the detailed clustering of galaxies, creating fresh tensions as it relieves the old one.

Other proposals invoke new lightweight particles, subtle changes to the properties of neutrinos, or modifications to gravity itself on cosmic scales.

None of these is confirmed, and each brings its own difficulties. But the fact that serious theorists are exploring them shows how far the mainstream has moved.

Resolving the tension with new physics would be the biggest change to our cosmic model in decades — potentially as significant as the discovery of dark energy itself.

The DESI Result: Is Dark Energy Changing?

The tension has recently been joined by a second cosmological surprise. The Dark Energy Spectroscopic Instrument, or DESI, is mapping the positions of tens of millions of galaxies across a third of the sky.

DESI measures how cosmic structure has grown over time using baryon acoustic oscillations — the same primordial sound waves that anchor the early-universe measurement.

Its recent data releases have hinted at something startling: dark energy may not be constant after all. Its strength appears to have changed subtly over cosmic history.

If confirmed, evolving dark energy would overturn a core assumption of the standard model — and could be deeply connected to the Hubble tension. The nature of that force is explored in our article on dark energy, the invisible force pushing the universe apart.

The DESI hint is not yet a confirmed discovery, and the significance depends on how the data are combined. But together with the Hubble tension, it suggests our cosmic model may be fraying at more than one seam.

The Next Generation of Instruments

A wave of new observatories is arriving to attack the problem from every side.

The Vera C. Rubin Observatory will catch thousands of Type Ia supernovae, sharpening the local ladder. The European Euclid mission is mapping dark energy’s grip on cosmic structure across billions of years.

Gravitational-wave detectors add a completely new tool. Merging neutron stars act as “standard sirens,” giving distances directly from the physics of the wave, with no ladder at all.

The first such measurement came in 2017, from the neutron-star merger GW170817. It yielded a Hubble constant near the middle of the range, though with large uncertainty from a single event.

The power of sirens is that they sidestep every rung of the traditional ladder. They do not rely on Cepheids, supernovae, or the early-universe model — an entirely fresh line of evidence.

As detectors improve and catch more mergers, their combined precision will sharpen. Within a few years, standard sirens may deliver a decisive, independent verdict on the tension.

That is the real hope of the coming decade: not one better measurement, but many independent ones, converging until the universe is forced to reveal which number is right.

Why the Hubble Tension Matters

The Hubble tension is not a squabble over a decimal place. The Hubble constant underpins our entire picture of the cosmos.

It sets the age of the universe, its size, and the balance between its ingredients — ordinary matter, dark matter, and dark energy. A shift in H₀ ripples through all of it.

The invisible components most affected are explored in our articles on dark matter and on the vast structures that shape the cosmos.

Most importantly, moments like this are how physics advances. Small, stubborn discrepancies have repeatedly cracked open into revolutions — from the orbit of Mercury to the birth of quantum theory.

The universe is telling us, through two numbers that will not agree, that our understanding is incomplete. Following that clue is exactly how the next chapter of cosmology will be written.

For now, the honest answer is that no one knows which value is correct, or whether both may need revising. That uncertainty is not a failure of science — it is science working exactly as it should.

A number first sketched by Edwin Hubble a century ago has become the sharpest edge in modern cosmology. Wherever the resolution leads, it promises to reshape how we understand the size, age, and destiny of everything.

Frequently Asked Questions

Sources

• Planck Collaboration (2020). “Planck 2018 results. VI. Cosmological parameters.” Astronomy & Astrophysics, 641, A6 (DOI: 10.1051/0004-6361/201833910).
• Riess, A. G., et al. (2022). “A Comprehensive Measurement of the Local Value of the Hubble Constant (SH0ES).” The Astrophysical Journal Letters, 934, L7 (DOI: 10.3847/2041-8213/ac5c5b).
• Riess, A. G., et al. (2023). “Crowded No More: The Accuracy of the Hubble Constant Tested with JWST.” The Astrophysical Journal Letters, 956, L18 (DOI: 10.3847/2041-8213/acf769).
• Freedman, W. L., et al. (2019). “The Carnegie-Chicago Hubble Program: A Measurement of H₀ from the Tip of the Red Giant Branch.” The Astrophysical Journal, 882, 34 (DOI: 10.3847/1538-4357/ab2f73).
• Pesce, D. W., et al. (2020). “The Megamaser Cosmology Project. XIII. Combined Hubble Constant Constraints.” The Astrophysical Journal Letters, 891, L1 (DOI: 10.3847/2041-8213/ab75f0).
• Scolnic, D., Riess, A. G., et al. (2025). Distance to the Coma Cluster and a local measurement of H₀, The Astrophysical Journal Letters. See NASA / HubbleSite news releases.
• DESI Collaboration — Dark Energy Spectroscopic Instrument data releases (evolving dark energy).