The Hubble Tension Explained: Why the Universe’s Expansion Measurements Still Disagree

Imagine asking two highly experienced teams to measure the same distance. Both have advanced instruments, carefully calibrated equipment and years of experience. Both repeat their measurements. Both check their calculations. Yet when they present their final numbers, the answers disagree by an amount that neither team can easily explain.

Now imagine that the distance being measured is not across a city or between two mountains, but across the expanding Universe itself.

This is the situation facing modern cosmology. Astronomers have spent decades improving their understanding of how quickly the Universe expands. Their measurements have become extraordinarily precise, their telescopes have become more powerful, and new methods have opened independent ways of examining cosmic distances. Nevertheless, two major approaches continue to produce different answers.

The disagreement is known as the Hubble tension, and it has become one of the most important unresolved problems in astronomy.

The question sounds straightforward: how fast is the Universe expanding today? Yet the answer depends on how astronomers reconstruct distances, interpret ancient light and connect observations from different periods in cosmic history.

One approach measures nearby and relatively recent galaxies using stars and exploding supernovae. Another studies the faint radiation released when the Universe was extremely young, then uses a cosmological model to calculate what the expansion rate should be today.

The first approach generally produces a value around 73 kilometres per second per megaparsec. The second traditionally produces a value closer to 67.

That difference might appear small. In everyday life, a disagreement of six units could be insignificant. In precision cosmology, however, it is substantial because the uncertainty attached to each measurement is much smaller than the disagreement itself.

And the story became even more interesting in 2026. An international collaboration called the H0 Distance Network reported a local measurement of 73.50 ± 0.81 kilometres per second per megaparsec. Its comparison with an early-Universe determination produced a discrepancy of approximately 7.1 standard deviations.

Meanwhile, other researchers have challenged the traditional way of dividing the measurements into early-Universe and late-Universe camps. A September 2026 study examined 88 different measurements and suggested that the disagreement may be more accurately described as a difference between distance-ladder measurements and other techniques.

So is the Universe expanding faster than our theories predict? Are astronomers overlooking an error? Or are two different groups discovering that our standard picture of cosmic history is incomplete?

To understand the answer, we need to examine what the Hubble constant actually measures, how the competing techniques work and what the strongest available evidence tells us.

1. What the Hubble Constant Actually Means

Before examining the disagreement, it is important to understand what astronomers are trying to measure.

The Universe is expanding. On sufficiently large scales, distant galaxies generally appear to move away from one another as space itself expands. The farther away a galaxy is, the greater its average recession speed tends to be.

This relationship is described by the Hubble-Lemaître law:

v = H₀ × d

Recession velocity equals the Hubble constant multiplied by distance.

Here, H₀ represents the present-day value of the Hubble parameter, while distance is usually expressed in megaparsecs.

A megaparsec is approximately 3.26 million light-years. The standard unit for the Hubble constant is kilometres per second per megaparsec, written as km/s/Mpc.

Understanding the numbers

Suppose the Hubble constant is 70 km/s/Mpc. In a simplified illustration, a galaxy one megaparsec away would have an average recession velocity of approximately 70 kilometres per second because of cosmic expansion.

A galaxy 100 megaparsecs away would have an expected recession velocity of approximately 7,000 kilometres per second.

If the Hubble constant were instead 73.5 km/s/Mpc, the corresponding simplified expectation at that distance would be 7,350 kilometres per second.

The difference becomes increasingly important when astronomers study enormous distances across the observable Universe.

These calculations describe the large-scale expansion relationship. Individual galaxies also move because of local gravitational interactions. Such additional motion is called peculiar velocity, and it must be accounted for when astronomers estimate cosmic expansion from nearby galaxies.

Is the Hubble constant really constant?

This is one of the most common misunderstandings.

The name can make it sound as though the Universe has always expanded at precisely the same rate. That is not what the constant means.

The Hubble constant is the value of the Hubble parameter at the present cosmic epoch. Cosmologists often write the more general expansion rate as H(t), because its value changes throughout cosmic history.

In the early Universe, the expansion rate was different from what it is today. Radiation, ordinary matter, dark matter and dark energy have influenced the expansion history in different ways.

Therefore, measuring H₀ is not simply measuring how quickly a galaxy happens to travel through space. It is determining a fundamental parameter describing the present expansion of the cosmic scale itself.

Why a small difference matters

Consider the difference between 67.24 and 73.50 km/s/Mpc. The absolute difference is 6.26 km/s/Mpc, corresponding to approximately 9.3 percent of the lower value.

That is a substantial disagreement when both research approaches claim uncertainties close to one percent or less.

It is also important because H₀ influences several other cosmological calculations, including the relationship between distance and redshift, estimates of cosmic age and the parameters describing the evolution of the Universe.

Changing H₀ does not automatically change every cosmological conclusion by the same percentage. The age of the Universe, for example, depends on the entire expansion history rather than on H₀ alone.

The central problem is therefore not that astronomers disagree over a minor numerical detail. They disagree over a parameter that connects observations of the nearby Universe with our understanding of its earliest stages.

2. Two Different Ways to Measure the Universe

The Hubble tension exists because astronomers can approach the same fundamental question from very different directions.

One technique begins with objects relatively close to Earth and gradually builds a network of cosmic distances. Another begins with the Universe's ancient radiation and reconstructs its later development using mathematical models.

Both methods have strong scientific foundations. Neither is simply an educated guess.

The direct, local approach

The local approach uses observable astronomical objects whose distances can be calibrated through other measurements.

It connects several types of objects, including variable stars and supernovae, to establish a distance scale. Once astronomers know how far away galaxies are, they compare those distances with the redshift of their light.

The result is an estimate of the present expansion rate.

This method is often described as direct because it uses relatively recent cosmic observations rather than relying primarily on a model that begins with the early Universe.

However, direct does not mean assumption-free. Every stage requires calibration, statistical treatment and corrections for possible observational errors.

The early-Universe approach

The second approach studies the cosmic microwave background, or CMB.

This radiation is an extraordinary scientific record. It was released when the Universe was approximately 380,000 years old, after it cooled enough for electrons and atomic nuclei to combine into neutral atoms. Photons could then travel much more freely through space.

Today, this ancient radiation reaches telescopes as a faint microwave glow with an average temperature of approximately 2.7 degrees above absolute zero.

Its tiny temperature variations contain information about the density, composition and geometry of the early Universe.

Researchers compare those observations with predictions from a cosmological model. Under the standard six-parameter Lambda cold dark matter model, usually called ΛCDM, they infer the present-day Hubble constant.

The important distinction is that the CMB measurement does not directly watch a nearby galaxy moving away from Earth today. It measures ancient radiation and uses a model to infer the present expansion rate.

This distinction becomes essential when evaluating the competing results.

3. The Local Cosmic Distance Ladder: Measuring How Far Away Galaxies Really Are

Imagine trying to measure the distance between two cities without having a single instrument capable of measuring the entire journey.

You might begin with a short, accurately measured road, use it to calibrate a longer route and then use the longer route to establish distances across an entire country.

Astronomers face a similar challenge.

Different astronomical objects are useful at different distances. Some provide reliable measurements relatively close to Earth, while others are bright enough to be observed across much larger regions of space.

The resulting system is called the cosmic distance ladder.

First rung: geometric measurements

The foundation of the ladder is geometry.

One important technique is stellar parallax. As Earth moves around the Sun, nearby stars appear to shift slightly against the much more distant background.

By measuring this apparent movement and knowing the size of Earth's orbit, astronomers can calculate stellar distances using geometry.

Space missions such as ESA's Gaia have transformed this field by measuring the positions and distances of enormous numbers of stars with remarkable precision.

Other geometric anchors include detached eclipsing binary stars and water megamasers in galaxies. In suitable systems, astronomers can use orbital properties and physical geometry to establish distances independently of the usual stellar brightness relationships.

These measurements provide reference points for calibrating more distant astronomical objects.

Second rung: Cepheid variable stars

One of the most important objects in the distance ladder is the Cepheid variable star.

Cepheids are stars that periodically expand and contract. Their brightness changes in a regular pattern, making them particularly useful for astronomical distance measurements.

The crucial discovery is that their pulsation periods are related to their intrinsic luminosities.

In simple terms, astronomers can observe how long a Cepheid takes to complete a brightness cycle and use that information to estimate how bright it really is.

They can then compare its actual luminosity with its apparent brightness from Earth.

A nearby bright star and a distant bright star can look similar to an observer. But if their intrinsic luminosities are known, their apparent brightness provides information about their distances.

This relationship is known as the period-luminosity relation.

Once astronomers calibrate the relationship using nearby stars with independently measured distances, they can use Cepheids in more distant galaxies as reliable distance indicators.

Third rung: Type Ia supernovae

Cepheids are useful, but their brightness limits how far away astronomers can measure them reliably.

Type Ia supernovae provide another step.

These are extremely luminous stellar explosions associated with white dwarfs in particular binary systems or other suitable progenitor configurations.

Although they are not perfectly identical events, Type Ia supernovae can be standardized using relationships between their light curves, colors and peak brightness.

That makes them powerful distance indicators.

Astronomers first identify galaxies containing both measurable Cepheids and Type Ia supernovae. Cepheids help establish the distance to those galaxies, allowing researchers to calibrate the supernova brightness scale.

Once calibrated, Type Ia supernovae can be observed in much more distant galaxies.

Their measured brightness provides distances, while their spectral redshifts help researchers determine how much the Universe has stretched their light.

Combining the two measurements produces an estimate of H₀.

What could go wrong?

Each rung has potential sources of uncertainty.

For Cepheids, astronomers must consider metallicity, dust extinction, stellar crowding and the calibration of their period-luminosity relationship.

Metallicity matters because stars with different chemical compositions can behave differently. Dust can make stars appear dimmer than they really are. Crowding occurs when several stars appear too close together to be cleanly separated in an image.

For supernovae, researchers must examine differences between stellar environments, selection effects, light-curve corrections and the calibration of their intrinsic brightness.

There are also uncertainties in the geometric anchors used to establish the first rung.

The scientific response is not to ignore these problems. It is to identify them, quantify their effects and compare independent methods.

Other distance indicators provide additional checks

The Cepheid-supernova route is not the only way to construct a cosmic distance scale.

Researchers also use the tip of the red giant branch, or TRGB. This method relies on a relatively predictable maximum luminosity reached by certain older stars before they undergo helium ignition.

Another approach uses stars in the J-region asymptotic giant branch, known as JAGB stars.

Other techniques include Mira variable stars, surface brightness fluctuations, the Tully-Fisher relation, the fundamental plane of elliptical galaxies and Type II supernovae.

These techniques have different strengths and different vulnerabilities.

Some depend on stellar population properties. Others rely on galaxy rotation, stellar velocity distributions or statistical properties of unresolved stellar populations.

Their importance lies in the possibility of checking the expansion rate without relying entirely on one calibration chain.

4. Reading the Universe's Ancient Afterglow

The early-Universe approach begins with an extraordinary observation: the oldest light we can observe directly fills the sky.

The cosmic microwave background is not ordinary starlight. It is radiation left over from a much younger, hotter and denser Universe.

Its patterns provide a record of physical processes that occurred long before galaxies, planets or stars existed.

How sound waves shaped the early Universe

Before neutral atoms formed, the Universe contained a hot plasma made of electrons, nuclei and photons.

Gravity pulled matter toward slightly denser regions. Radiation pressure resisted compression. Together, these effects produced waves that travelled through the plasma.

These are known as baryon acoustic oscillations, or BAO.

When the Universe cooled sufficiently for electrons and nuclei to combine into neutral atoms, photons could travel freely. The pressure-supported sound waves effectively stopped propagating through the plasma.

Their effects did not disappear. They left characteristic patterns in the distribution of matter.

Those patterns can still be detected in the large-scale arrangement of galaxies today.

The acoustic patterns also influenced the temperature variations in the cosmic microwave background.

What Planck actually measured

The European Space Agency's Planck mission mapped the CMB with extraordinary precision.

Rather than treating the ancient radiation as a simple photograph, scientists analyzed its angular power spectrum: a statistical description of how temperature variations are distributed across different angular scales.

The positions and relative heights of the acoustic peaks reveal information about the quantities of ordinary matter, dark matter, radiation and the geometry of the early Universe.

Researchers then fit these observations using cosmological models.

Under the standard base-ΛCDM model, the Planck Collaboration obtained a Hubble constant of approximately 67.4 ± 0.5 km/s/Mpc in its 2018-results analysis, published in 2020.

The precision is impressive. However, the inferred Hubble constant depends on the cosmological framework used to interpret the observations.

Why baryon acoustic oscillations matter

Another important source of evidence comes from galaxy surveys.

The Dark Energy Spectroscopic Instrument, known as DESI, measures the positions and redshifts of enormous numbers of galaxies and quasars.

By studying the characteristic separation of galaxy pairs, scientists can identify the surviving imprint of the ancient acoustic waves.

This provides a standard ruler for studying cosmic distances.

Imagine having a ruler whose physical length is known. If you observe it from far away, its apparent size helps you estimate the distance to the object.

BAO measurements work in a related way, except the ruler is an enormous pattern imprinted in the distribution of matter.

Researchers compare its apparent size at different redshifts to reconstruct the expansion history.

The method is particularly valuable because it provides information that can be combined with CMB observations and supernova measurements.

DESI's official scientific documentation explains how the acoustic scale is used to measure the relationship between distance and redshift and investigate the evolution of dark energy.

The hidden assumption

There is an important scientific distinction here.

The CMB and BAO do not independently hand researchers a single universally agreed value of H₀ without further interpretation.

The conversion from their observed patterns into a present-day expansion rate requires assumptions about the physical history of the Universe.

Under standard ΛCDM, those assumptions produce a remarkably successful description of many cosmological observations.

But if an important physical ingredient is missing from that model, the inferred Hubble constant could change.

This is one reason why the Hubble tension is so interesting: it may be revealing something about the relationship between the early and late Universe that existing models do not fully capture.

5. The Numbers Behind the Hubble Tension

The disagreement has developed over several generations of observations. New instruments have improved the precision of individual methods, while collaborations have attempted to combine different datasets into more reliable measurements.

As of 5 November 2026, several important results illustrate why the discussion remains active.

Values are in km/s/Mpc. The rows represent different datasets and analysis assumptions; they should not be treated as interchangeable direct measurements.

The 2026 Local Distance Network result

One of the most important developments came from the H0 Distance Network Collaboration.

Published in Astronomy & Astrophysics in April 2026, the study brought together specialists working with different astronomical distance indicators.

Instead of depending on a single distance ladder, the researchers constructed a broader network incorporating measurements involving:

• Parallaxes and geometric distance anchors.
• Cepheid variable stars.
• The tip of the red giant branch.
• Mira variable stars.
• JAGB stars.
• Type Ia and Type II supernovae.
• Surface brightness fluctuations.
• Galaxy-based distance indicators.
• Water megamasers.

One of the central challenges was accounting for correlations between measurements. Two studies may appear independent because they use different telescopes, but they may share calibration data or rely on the same underlying astronomical reference.

Simply averaging their results could therefore create an artificially precise answer.

The collaboration used covariance-weighted methods to account for these relationships.

Its baseline network produced:

H₀ = 73.50 ± 0.81 km/s/Mpc.

The authors reported a relative uncertainty of approximately 1.1 percent for the baseline result. A broader combination of estimators achieved approximately 0.9 percent relative uncertainty.

They also performed tests by removing important components of the network. Removing either Cepheid measurements or TRGB measurements had little effect on the central value.

This is significant because it suggests that the high local result is not entirely dependent on one particular distance indicator.

The collaboration reported a 7.1-sigma discrepancy relative to its early-Universe plus ΛCDM reference value of 67.24 ± 0.35 km/s/Mpc. Its comparison with a BBN-plus-BAO reference value of 68.51 ± 0.58 produced a difference of approximately 5 sigma.

These are substantial statistical disagreements, although their interpretation remains dependent on the datasets and models being compared.

A different result from the Chicago-Carnegie Hubble Program

Not every measurement produces a value near 73.

In a 2025 paper published in The Astrophysical Journal, Wendy Freedman and colleagues presented updated James Webb Space Telescope observations for the Chicago-Carnegie Hubble Program.

The team examined three independent stellar distance indicators:

• Cepheid variables.
• Tip of the red giant branch stars.
• J-region asymptotic giant branch stars.

Its highest-precision TRGB-based estimate was:

H₀ = 70.39 km/s/Mpc, with uncertainties reported separately as ±1.22 statistical, ±1.33 systematic and ±0.70 associated with supernova scatter.

The researchers also reported JWST-only results of 68.81 km/s/Mpc using TRGB stars and 67.80 km/s/Mpc using JAGB stars, again with separate statistical and systematic uncertainties.

These results demonstrate why the debate cannot be reduced to a simple argument between two fixed numbers.

Different indicators, calibrations and samples can produce different estimates. Their uncertainties and correlations must be considered before drawing conclusions.

The CCHP findings do not establish that the Hubble tension has disappeared. They do, however, provide important alternative evidence that researchers must include in any serious assessment of the problem.

6. Five Misconceptions About the Hubble Tension

The Hubble tension is frequently presented in headlines as a straightforward scientific crisis. But several popular explanations oversimplify what astronomers have actually discovered.

Misconception 1: One team must have made a mistake

It is tempting to imagine that one group has simply calculated the wrong answer.

Scientific measurements certainly contain errors. Even the most advanced instruments have limitations, and astronomers must consider calibration uncertainties, statistical fluctuations and possible systematic effects.

However, the situation is more complicated than a single calculation error.

The local distance ladder has been tested using different stars, supernova samples, geometric anchors and independent observational programmes.

The early-Universe result has also been checked using several CMB experiments and complementary cosmological observations.

The 2026 Local Distance Network study is particularly important because it combines many local indicators rather than relying on one research group's preferred technique.

This does not mean every possible systematic error has been eliminated. It means that explaining the entire disagreement through one simple observational mistake has become increasingly difficult.

Misconception 2: The James Webb Telescope has solved the problem

James Webb is sometimes described as the instrument that will finally settle the debate.

Its infrared capabilities are extremely valuable, especially when studying stars in crowded galaxies.

But a telescope does not automatically resolve a cosmological disagreement simply because it produces sharper images.

Webb has strengthened confidence in important distance measurements, particularly Cepheid observations. It has also enabled alternative measurements using other types of stars.

Those contributions are significant, but the full Hubble tension involves several connected physical and observational questions.

Improving one part of the distance ladder does not automatically settle the calibration of every other part.

Misconception 3: The early-Universe measurement is completely independent of theory

The CMB is directly observed. Its temperature fluctuations are real measurements, not predictions invented by a mathematical model.

However, the Hubble constant inferred from those fluctuations depends on the cosmological framework used to interpret them.

The Planck value of 67.4 km/s/Mpc is associated with the standard base-ΛCDM model.

If the early expansion history, the behaviour of additional particles or the physics of recombination were different from the standard assumptions, the inferred value could change.

This is why scientists distinguish between measured quantities and parameters inferred under a particular theoretical model.

Misconception 4: A five-sigma or seven-sigma result proves new physics

Statistical significance is important, but it does not automatically identify the cause of a disagreement.

A high sigma value indicates that the observed difference would be difficult to explain as a random statistical fluctuation under the assumptions of the analysis.

It does not prove that a particular new particle exists, that gravity behaves differently or that the standard cosmological model is fundamentally wrong.

Systematic uncertainties, shared calibrations and model assumptions must also be investigated.

A discovery of new physics requires more than a discrepancy. Scientists need a physical explanation that makes testable predictions and survives independent observations.

Misconception 5: The entire Universe is suddenly expanding faster

The phrase faster expansion can create the impression that astronomers have detected a recent acceleration of the Universe.

That is not what the Hubble tension establishes.

It is a disagreement between estimates of the present expansion parameter obtained through different methods.

Accelerated cosmic expansion is a separate phenomenon associated with dark energy and was established through observations of distant supernovae and other cosmological evidence.

The Hubble tension concerns the precise value of the current expansion rate and how that value connects to the Universe's earlier history.

7. What James Webb Has Actually Confirmed

The James Webb Space Telescope has become an important part of the Hubble tension debate because it offers an opportunity to test one of the most discussed possible sources of error in local distance measurements.

The issue is stellar crowding.

The problem of stars blending together

Imagine trying to photograph a single streetlight in a crowded city at night.

If the surrounding lights are too close together, their brightness may blend into the same image. You could mistakenly measure several lights as one brighter source.

A similar problem can occur when astronomers observe distant Cepheid stars.

Galaxies contain enormous numbers of stars. As observations become more distant, individual stars become harder to distinguish, and light from neighbouring objects can contaminate measurements.

If a Cepheid appears brighter than it truly is because of blended light, its calculated distance could be incorrect.

Even a small systematic error repeated across many galaxies could influence the final value of H₀.

Why infrared observations make a difference

Webb observes the Universe primarily through infrared instruments.

Its sharp infrared vision helps astronomers distinguish Cepheids from neighbouring stars. Infrared observations can also reduce some of the complications associated with interstellar dust.

In September 2023, NASA announced that Webb observations supported the accuracy of important Hubble Cepheid measurements.

The work was extended through observations covering the distance range used by the SH0ES collaboration.

NASA reported that the combined Hubble and Webb observations examined five supernova-host galaxies containing approximately 1,000 Cepheids, reaching the galaxy NGC 5468, approximately 130 million light-years away.

The findings were published in The Astrophysical Journal Letters in February 2024.

The result weakened the argument that stellar crowding in Hubble images could by itself explain the high local expansion measurement.

What Webb has not proved

It is important not to exaggerate this achievement.

Webb's observations support the reliability of the Cepheid measurements examined in the study. They do not eliminate every possible uncertainty throughout the cosmic distance ladder.

Researchers must still investigate:

• How different geometric distance anchors are calibrated.
• Whether Cepheid populations have been fully characterized.
• How metallicity influences stellar brightness relationships.
• How Type Ia supernovae are standardized across different environments.
• Whether selection effects influence the observed samples.
• How uncertainties are correlated across independent studies.

There is also a distinction between checking the Cepheid measurements and validating the entire chain used to determine H₀.

The first is an important observational achievement. The second requires multiple independent methods and continued testing.

NASA's account of the Hubble and Webb collaboration is available in its report, Webb and Hubble Telescopes Affirm Universe's Expansion Rate, Puzzle Persists.

8. Why Independent Measurements Complicate the Picture

One of the most interesting developments in 2026 was a study published in Physical Review D on September 3.

Researchers Ioannis Pantos and Leandros Perivolaropoulos examined 88 measurements of the Hubble constant that did not rely on the conventional sound-horizon calibration in the same way.

Their analysis divided the measurements into four categories:

• Distance-ladder measurements using local astronomical calibrators.
• Local ΛCDM measurements using an assumed standard expansion history.
• Purely local measurements that do not depend on the assumed shape of the expansion history.
• CMB-based measurements that avoid using the sound-horizon scale.

A surprising difference between measurement groups

The authors reported a combined value of approximately 72.73 ± 0.39 km/s/Mpc for 30 distance-ladder measurements.

The 58 distance-ladder-independent or sound-horizon-free measurements produced approximately 69.37 ± 0.34 km/s/Mpc.

After accounting for realistic correlations between measurements, the authors estimated a disagreement of approximately 3.9 to 4.3 sigma.

Their preferred conservative estimate was 3.9 sigma.

This is an important result because it challenges a simplified explanation of the Hubble tension.

The disagreement may not be exclusively between the early and late Universe. Instead, the distance ladder may systematically produce higher values than several other approaches, including methods that do not depend on the same early-Universe sound horizon.

Why this interpretation matters

Consider two hypothetical research teams.

The first uses nearby stars to establish distances and then calibrates supernovae. The second uses a completely different approach to establish distances and infer expansion.

If the second team also obtains a lower value, the problem may involve something specific to the distance-ladder methodology rather than simply a disagreement between observations of the young and old Universe.

The 2026 study does not establish that the distance ladder is wrong. It provides a different way of organizing the evidence and identifies additional disagreements that need investigation.

The researchers also found that local ΛCDM measurements produced approximately 67.61 ± 0.96 km/s/Mpc, while their purely local category yielded approximately 71.03 ± 0.69.

These differences show that the choice of methodology and cosmological assumptions matters.

Importantly, combining measurements requires careful treatment of correlations. Several results may share observations, calibrations or theoretical assumptions. Treating every published number as completely independent would exaggerate the statistical strength of a combined result.

The study is therefore best understood as a significant investigation of the structure of the disagreement, not a final declaration that one side has won.

9. Could New Physics Explain the Disagreement?

If careful observations continue to produce inconsistent values, researchers must consider whether something is missing from the standard model of cosmology.

The most important possibility is that the Universe evolved differently from what ΛCDM predicts during a period that connects its early and later history.

Several explanations have attracted attention.

Early dark energy

One proposal suggests that an additional component of energy briefly influenced the expansion of the young Universe.

This idea is known as early dark energy.

In standard cosmology, the sound horizon represents the characteristic distance travelled by sound waves in the early plasma before those waves effectively stopped propagating.

That distance acts as a reference scale when interpreting CMB and BAO observations.

If an additional energy component changed the expansion rate before recombination, the distance travelled by sound waves could become smaller.

This would change the relationship between the observed acoustic pattern and the inferred present expansion rate.

In some models, the adjustment allows a higher value of H₀ while maintaining agreement with important CMB observations.

However, the idea creates additional challenges.

A successful model must also explain the effects on the CMB acoustic peaks, the distribution of matter, galaxy clustering and other measurements.

It is not enough to change the sound horizon and obtain a preferred Hubble constant.

The model must remain consistent with the rest of cosmology.

Additional relativistic particles

Another possibility involves particles or radiation that influenced the expansion of the early Universe.

Neutrinos already play an important role in cosmology. They are extremely light particles that interact weakly with ordinary matter and contribute to the energy density of the early Universe.

Some theoretical proposals investigate whether additional relativistic particles or changes in neutrino interactions could alter the expansion history before recombination.

Such changes might influence the sound horizon and therefore the value of H₀ inferred from CMB observations.

But these proposals must satisfy other constraints, including measurements of the cosmic microwave background and predictions from primordial nucleosynthesis.

Modified gravity

Einstein's general theory of relativity provides the foundation for modern cosmology.

It describes how matter and energy influence the geometry of spacetime.

Some researchers have investigated whether gravity behaves differently under certain cosmic conditions.

Modified-gravity models could potentially alter how the Universe expands, how structures form and how the relationship between matter and geometry evolves.

Such theories are scientifically interesting, but they face a demanding test.

General relativity has passed many observational tests, from the Solar System to gravitational-wave observations. Any replacement or extension must reproduce those successful results while explaining the cosmological discrepancy.

Changing dark energy over time

Dark energy is associated with the accelerated expansion of the Universe.

In the simplest ΛCDM model, dark energy behaves like a cosmological constant, meaning its energy density remains constant as space expands.

However, some observations and theoretical models have motivated investigations into whether dark energy could evolve over time.

If its behaviour differs from a constant energy density, the relationship between cosmic distance and redshift could also change.

This might influence measurements of H₀ obtained through supernovae and BAO.

It is important to distinguish a possible variation in dark energy from an established discovery. Researchers have not demonstrated that a changing dark-energy component is the solution to the Hubble tension.

What the latest reviews tell us

A 2026 review by Thárcio Cerqueira and Cássio Pigozzo examined early-dark-energy models and their observational constraints.

The authors discussed how the viability of these models depends on the combination of CMB datasets, local Hubble measurements, large-scale structure observations and other cosmological information.

They emphasized that some models can reduce the tension but face difficulties involving structure formation, theoretical consistency and remaining disagreements with other datasets.

No proposed extension has yet emerged as a universally accepted replacement for the standard cosmological model.

This is a crucial point. Scientists are investigating new physics, but the Hubble tension has not yet identified which physical explanation, if any, is correct.

10. Can Gravitational Waves Provide the Answer?

There is another way to measure cosmic distances that does not rely on Cepheid stars or the traditional distance ladder.

It involves gravitational waves.

When two neutron stars or black holes orbit one another and eventually merge, they produce ripples in spacetime.

These ripples carry information about the physical properties of the merging objects.

By studying the gravitational-wave signal, scientists can estimate the luminosity distance to the source.

If researchers can also determine the source's redshift, they can use the event to constrain the Hubble constant.

Standard sirens

Gravitational-wave distance measurements are often called standard sirens, a name inspired by standard candles such as Type Ia supernovae.

The advantage is that the waveform itself contains information about the distance.

In August 2017, the LIGO and Virgo collaborations detected the neutron-star merger GW170817.

Scientists also observed electromagnetic signals associated with the event and identified its host galaxy, NGC 4993.

Combining the gravitational-wave distance with the galaxy's recession information provided an independent estimate of the Hubble constant.

The original measurement was approximately 70 km/s/Mpc, although its uncertainty was large.

It demonstrated that gravitational waves could provide an entirely different route to measuring cosmic expansion.

What the latest gravitational-wave results show

In 2026, the LIGO-Virgo-KAGRA collaboration reported a new analysis using gravitational-wave observations from its fourth observing run.

The study used information from 142 gravitational-wave sources and combined statistical dark-siren measurements with the bright-siren result from GW170817.

The reported Hubble constant was:

H₀ = 76.6 with a 68-percent credible interval of +13.0 and -9.5 km/s/Mpc.

This result is independent of the traditional Cepheid-supernova distance ladder, but its uncertainty remains much larger than those of the leading optical methods.

Consequently, it is consistent with a broad range of possible values and cannot yet distinguish decisively between the competing measurements.

Why this technique could become important

Gravitational-wave astronomy is still developing.

As detectors become more sensitive and more compact-object mergers are observed, researchers will have access to larger samples of potential standard sirens.

Some events will have clearly identified electromagnetic counterparts. Others will be analyzed statistically using galaxy catalogues and the population properties of merging objects.

Each additional independent measurement can help determine whether the discrepancy originates in a particular distance-calibration method or reflects a deeper issue with the cosmological model.

Gravitational waves are not yet the solution, but they provide an especially valuable independent test.

11. What Could Finally Resolve the Mystery?

The next stage of the investigation will depend on combining different types of evidence rather than relying on one telescope or one research group.

James Webb and improved stellar calibration

Webb will continue to help astronomers examine stellar populations in distant galaxies.

Its observations can improve the measurement of Cepheids and provide additional information about alternative distance indicators.

Better infrared imaging also helps researchers investigate the influence of dust and stellar crowding.

As the number of independently calibrated galaxies increases, astronomers can test whether the local distance scale remains stable.

DESI and large-scale galaxy surveys

DESI is creating an increasingly detailed three-dimensional map of the Universe.

Its measurements of baryon acoustic oscillations help researchers reconstruct the relationship between cosmic distance and redshift.

The second data release, known as DESI DR2, has provided additional cosmological information. The collaboration released further analyses using its first three years of observations in July 2026.

These observations are important because they provide independent information about cosmic expansion and the possible evolution of dark energy.

Future analysis will help determine whether the standard expansion history remains adequate when confronted with increasingly precise data.

Euclid and the Nancy Grace Roman Space Telescope

ESA's Euclid mission is investigating the large-scale distribution of galaxies and the effects of dark energy and dark matter.

NASA's Nancy Grace Roman Space Telescope is designed to carry out wide-field astronomical surveys and investigate questions involving dark energy, galaxy evolution and cosmic structure.

Both observatories will contribute additional information about the expansion history and the growth of cosmic structures.

Their observations could help distinguish between different theoretical explanations.

Better statistical methods

More observations are valuable, but the way scientists analyze them matters just as much.

The 2026 Local Distance Network illustrates why accounting for correlations is essential.

Future studies will need transparent statistical methods, clearly documented calibration chains and independent cross-checks.

Researchers must also avoid treating several related measurements as though they were completely independent experiments.

Improved statistical methods can reveal whether an apparent discrepancy is robust or whether it becomes smaller when previously overlooked relationships between datasets are considered.

Combining all the evidence

The most convincing resolution will probably involve several independent measurements agreeing within their uncertainties.

If local distance indicators, gravitational-wave standard sirens, BAO measurements and CMB observations eventually converge on one value under a consistent cosmological framework, confidence in that result will increase substantially.

If they continue to disagree despite improvements in observational accuracy, the case for new physics will become more compelling.

Either outcome would represent important progress.

12. Why the Hubble Tension Matters Beyond One Number

It might be tempting to regard the Hubble tension as a technical disagreement that matters only to professional astronomers.

In reality, the consequences extend across our understanding of the Universe.

The age of the Universe

The expansion history is closely connected to the age of the cosmos.

By reconstructing how quickly the Universe expanded at different periods, cosmologists can estimate how much time has passed since its early hot, dense state.

The standard cosmological model, using Planck observations and other evidence, gives an age of approximately 13.8 billion years.

A different expansion history could influence that estimate.

However, it would be incorrect to assume that increasing H₀ by nine percent automatically makes the Universe nine percent younger. The age calculation depends on the full evolution of the expansion rate, not simply its present-day value.

The evolution of galaxies

Cosmic expansion influences the distances and timescales used to reconstruct galaxy evolution.

Astronomers use cosmological models to estimate when galaxies formed, how quickly structures developed and how the distribution of matter changed over billions of years.

If the standard expansion history needs modification, some of those calculations may also require refinement.

This does not mean that observations of galaxies suddenly become invalid. It means that the framework connecting those observations could need improvement.

The nature of dark energy

Dark energy represents one of the greatest unanswered questions in modern physics.

Although the standard model describes its effects successfully, scientists still do not know what dark energy fundamentally is.

If the Hubble tension reveals that the expansion history is more complicated than expected, it could provide clues about the nature of dark energy or another missing ingredient in cosmology.

Testing the limits of scientific knowledge

There is also a broader lesson.

Science becomes stronger when independent methods produce results that can be compared and tested.

When two approaches disagree, researchers have an opportunity to discover hidden assumptions, improve instruments or develop a more complete explanation of nature.

The Hubble tension is a particularly powerful example because both sides are supported by sophisticated observations and well-developed scientific methods.

Rather than making cosmology less reliable, the disagreement is forcing researchers to examine its foundations more carefully.

13. Conclusion: The Universe Has Not Given Us Its Final Number

For decades, astronomers have worked to determine how quickly the Universe expands. The result is one of the great achievements of modern observational cosmology: a detailed picture of cosmic history supported by satellites, ground-based observatories, stellar measurements and mathematical models.

Yet the Hubble tension shows that even a successful scientific framework can contain unresolved problems.

The local distance ladder continues to produce relatively high estimates. The early-Universe approach, when interpreted through standard ΛCDM cosmology, produces lower values. The 2026 Local Distance Network has strengthened the case that the high local result is supported by multiple distance indicators.

At the same time, independent TRGB and JAGB measurements, alternative statistical analyses and gravitational-wave observations demonstrate that the evidence is more complicated than a simple contest between two teams.

James Webb has helped eliminate an important proposed source of observational error. DESI continues to improve our understanding of the large-scale distribution of matter. Gravitational-wave detectors are creating new opportunities to measure cosmic distances independently.

None of these developments has yet produced a universally accepted solution.

Perhaps the explanation lies in a systematic uncertainty that has not been fully identified. Perhaps the early Universe contained physical ingredients missing from our current models. Or perhaps the connection between cosmic expansion at different epochs is more complicated than scientists have assumed.

What makes this mystery remarkable is that the disagreement is emerging from increasingly precise observations rather than from a lack of information.

In astronomy, better measurements do not always make the Universe easier to understand. Sometimes they reveal that the questions we thought we had answered were only the beginning.

And until independent observations converge on a consistent result, the Hubble tension will remain one of the most important clues to what we still do not understand about the cosmos.

Frequently Asked Questions About the Hubble Tension

1. What is the Hubble tension in simple terms?

The Hubble tension is the disagreement between different methods of measuring the present expansion rate of the Universe. Local distance measurements generally produce a higher value, around 73 km/s/Mpc, while values inferred from the cosmic microwave background using standard ΛCDM cosmology are closer to 67 km/s/Mpc.

2. What is the current value of the Hubble constant?

There is no universally agreed single value that resolves the disagreement. The 2026 Local Distance Network reported 73.50 ± 0.81 km/s/Mpc. The Planck 2018 analysis reported 67.4 ± 0.5 km/s/Mpc under base-ΛCDM. These numbers describe results obtained using different observational approaches and assumptions.

3. Has James Webb solved the Hubble tension?

No. The James Webb Space Telescope has strengthened confidence in important Cepheid measurements by reducing problems associated with stellar crowding and dust. It has also enabled additional distance measurements using other stellar populations. However, the disagreement between the principal approaches remains unresolved.

4. Why do scientists use Cepheid variable stars?

Cepheids are pulsating stars whose pulsation periods are related to their intrinsic luminosities. By measuring their periods and apparent brightness, astronomers can estimate their distances. They are particularly useful for calibrating the distances to galaxies that also contain Type Ia supernovae.

5. What is the cosmic microwave background?

The cosmic microwave background is ancient radiation released approximately 380,000 years after the Big Bang, when the Universe cooled enough for neutral atoms to form. Its tiny temperature variations preserve information about the early Universe and allow cosmologists to constrain fundamental parameters.

6. Does the Hubble tension prove that Einstein's theory of gravity is wrong?

No. General relativity remains an extremely successful theory supported by many observations. Modified-gravity theories are among the proposed explanations for the Hubble tension, but no such theory has been established as the solution.

7. What is early dark energy?

Early dark energy is a hypothetical component that could have influenced the expansion of the Universe during an early period before becoming less important. Some models suggest that it could reduce the sound horizon and help explain the higher local estimates of H₀. However, these models must also satisfy other cosmological observations.

8. What does a seven-sigma disagreement mean?

A seven-sigma discrepancy indicates that two results are very difficult to reconcile as random statistical fluctuations under the assumptions of the analysis. It does not identify the physical cause or automatically prove that new physics exists. Systematic uncertainties and model assumptions still need to be examined.

9. Can gravitational waves measure the Hubble constant?

Yes. Gravitational-wave signals from merging compact objects can provide estimates of their distances. When combined with information about their recession velocities or probable host galaxies, these events can constrain H₀. The technique is called the standard-siren method, and its current uncertainties remain relatively large.

10. What is the difference between H₀ and the expansion rate in the past?

H₀ is the present-day value of the Hubble parameter. The expansion rate has changed throughout cosmic history because the relative influences of radiation, matter and dark energy have evolved. Cosmologists use H(t) when discussing the expansion rate at different cosmic epochs.

11. Why is the 2026 Local Distance Network result important?

The collaboration combined numerous astronomical distance indicators using statistical methods designed to account for correlations between measurements. Its baseline result of 73.50 ± 0.81 km/s/Mpc provides a broad local measurement that is less dependent on any single distance indicator.

12. Do all local measurements support a value near 73?

No. Some independent techniques produce lower values. The Chicago-Carnegie Hubble Program reported a TRGB-based estimate of 70.39 km/s/Mpc in 2025, with separate statistical, systematic and supernova-scatter uncertainties. Its JWST-only TRGB and JAGB results were lower still. These findings illustrate why different calibration methods must be examined carefully.

13. Could the Hubble tension disappear with better telescopes?

It is possible that improved observations will reveal previously unrecognized systematic uncertainties. However, better telescopes could also strengthen the disagreement by confirming the existing measurements. Future observations from Webb, Euclid, Roman, DESI and gravitational-wave detectors will help distinguish between these possibilities.

14. Does the Hubble tension mean the Universe is expanding faster than light?

No. The Hubble tension concerns the estimated present expansion parameter, not a claim that individual galaxies are locally travelling through space faster than light. In cosmology, the distance between sufficiently separated objects can increase because space itself expands. This is different from an object moving locally through space.

15. When will scientists solve the Hubble tension?

There is no reliable date for a solution. The answer will depend on new observations, independent distance measurements, better statistical analysis and the development or rejection of alternative cosmological models. The problem may require several generations of observations before a convincing explanation emerges.

Sources and Further Reading

The following scientific publications and institutional resources provide the observational results, technical explanations and research context discussed in this article.

1. H0 Distance Network Collaboration (2026). The Local Distance Network: A Community Consensus Report on the Measurement of the Hubble Constant at Approximately 1% Precision. Astronomy & Astrophysics, Volume 708, Article A166.
2. Planck Collaboration (2020). Planck 2018 Results. VI. Cosmological Parameters. Astronomy & Astrophysics, Volume 641, Article A6.
3. Pantos, I. and Perivolaropoulos, L. (2026). Dissecting the Hubble Tension: Insights from a Diverse Set of Sound-Horizon-Free H₀ Measurements. Physical Review D, Volume 114, 063507.
4. Freedman, W. L. and colleagues (2025). Status Report on the Chicago-Carnegie Hubble Program: Measurement of the Hubble Constant Using the Hubble and James Webb Space Telescopes. The Astrophysical Journal, Volume 985, Article 203.
5. NASA (2024). NASA's Webb, Hubble Telescopes Affirm Universe's Expansion Rate, Puzzle Persists.
6. NASA Hubble Mission. Hubble Constant and Tension.
7. Cerqueira, T. and Pigozzo, C. (2026). Early Dark Energy: A Review of Theoretical Models and Updated Observational Constraints. Brazilian Journal of Physics, Volume 56, Article 208.
8. Dark Energy Spectroscopic Instrument (DESI). The DESI Science Mission: Baryon Acoustic Oscillations and Cosmic Expansion.
9. DESI Collaboration. DESI Data Release 2 Cosmology Publications.
10. LIGO Scientific Collaboration, Virgo Collaboration and KAGRA Collaboration (2026). GWTC-4.0: Constraints on the Cosmic Expansion Rate and Modified Gravitational-Wave Propagation. The Astrophysical Journal Letters.
11. LIGO Scientific Collaboration. New Constraints on the Cosmic Expansion and General Relativity with Gravitational Waves.
12. Hubble and the Inverse Distance Ladder (2026). On the Stability of H₀ and the Inverse Distance Ladder. Monthly Notices of the Royal Astronomical Society.

Editorial note: Numerical comparisons in this article identify the observational method and cosmological assumptions behind each result. Statistical uncertainties from different studies are not interchangeable, and quoted sigma discrepancies depend on the datasets, correlations and reference models used. The Hubble tension remains an open scientific question as of 5 November 2026.