The universe has been expanding since the Big Bang, but astronomers have encountered a puzzle that could change how we understand its history. When scientists measure how quickly space is expanding today, they obtain a higher value than the one predicted from observations of the early universe using the standard cosmological model.
This discrepancy is known as the Hubble tension, and it remains one of modern cosmology’s most important unresolved problems.
The difference may point to overlooked measurement uncertainties, missing ingredients in our understanding of the universe or new physics involving dark energy, matter or gravity. Researchers are testing these possibilities using observations from the Hubble Space Telescope, the James Webb Space Telescope and large surveys of distant galaxies.
The mystery is not that the universe has suddenly begun expanding faster. Rather, measurements made using different methods do not yet agree on precisely how fast it expands today.
How Do Scientists Measure the Expansion of the Universe?
To understand the problem, it helps to know what astronomers mean by the Hubble constant.
The Hubble constant, written as Hâ‚€, describes the universe’s present-day expansion rate. In a simplified picture, more distant galaxies generally recede from us faster because space itself is expanding.
Astronomers express the Hubble constant in kilometres per second per megaparsec. A megaparsec is approximately 3.26 million light-years.
For example, a value of 70 kilometres per second per megaparsec means that the expansion contributes roughly 70 kilometres per second of additional recession speed for every megaparsec of distance. This is a description of cosmic expansion, not an assertion that every galaxy follows the same motion regardless of local gravity or other effects.
The difficulty is that scientists can estimate this rate in more than one way.
Method one: Measuring the nearby universe
One approach is to measure distances to relatively nearby galaxies and compare those distances with how much their light has been stretched by cosmic expansion.
Researchers build a sequence of distance measurements known as the cosmic distance ladder. They use objects such as Cepheid variable stars, whose changing brightness helps astronomers estimate their intrinsic luminosity. These measurements can calibrate distances to galaxies containing Type Ia supernovae, which are useful distance indicators because their brightness can be standardised.
By comparing galaxy distances with their redshifts, astronomers can calculate the present expansion rate.
In April 2026, the Hâ‚€ Distance Network collaboration reported a direct measurement of approximately 73.50 kilometres per second per megaparsec, with an uncertainty of about 0.81 kilometres per second per megaparsec. The result achieved a precision of just over one per cent.
The finding strengthened the evidence from local measurements that the expansion rate is around 73–74 in these units.
Source: Center for Astrophysics | Harvard & Smithsonian.
Method two: Reading the universe’s ancient light
A different approach begins with the cosmic microwave background, or CMB.
This faint radiation is the remnant of the early universe, released when the cosmos became transparent to light approximately 380,000 years after the Big Bang. It provides a detailed snapshot of conditions in the young universe.
The European Space Agency’s Planck mission measured tiny temperature variations in the CMB. Scientists use these observations, together with a cosmological model, to infer the universe’s properties and predict its present expansion rate.
Under the standard Lambda-CDM model, Planck’s observations imply a Hubble constant of approximately 67–68 kilometres per second per megaparsec.
That is significantly lower than the values obtained by several direct, local measurement methods.
The difference between these results is the Hubble tension.
NASA explains the two approaches and the unresolved discrepancy in its Hubble constant and tension overview.
Why Is the Hubble Tension So Important?
At first glance, a difference of several kilometres per second per megaparsec may appear small. But cosmologists use the expansion rate to connect observations of the early universe with its present state and future evolution.
If the measurements are accurate and the discrepancy persists, the standard model may be missing something important.
Lambda-CDM successfully explains many observations, including the cosmic microwave background, the large-scale distribution of galaxies and the abundance of light elements produced in the early universe. It combines ordinary matter, dark matter, a cosmological constant associated with dark energy and the equations of general relativity.
The Hubble tension raises the possibility that this framework may need refinement.
Scientists have considered numerous explanations, from subtle systematic errors in measurements to changes in the physics of the early universe. The challenge is finding a solution that explains the discrepancy without making other successful predictions of cosmology worse.
A proposed explanation must do more than produce a higher Hubble constant. It must also agree with independent observations.
Could Dark Energy Be the Answer?
Dark energy is the name given to whatever is responsible for the observed acceleration of the universe’s expansion within the standard cosmological framework.
In the simplest model, dark energy behaves like a cosmological constant, meaning its energy density remains constant as space expands.
But what if dark energy changed over time?
Some theories suggest that an evolving dark-energy component could alter the expansion history of the universe. Other models propose a brief period of additional energy in the early universe, sometimes called early dark energy.
Such a component could change how the universe evolved before the cosmic microwave background was released. That, in turn, could affect the inferred present-day expansion rate.
These ideas are under active investigation, but no particular explanation has been conclusively established as the solution to the Hubble tension.
The distinction matters because dark energy itself remains poorly understood. Evidence that the universe’s expansion is accelerating does not mean scientists have identified the underlying physical mechanism.
What Have Hubble and James Webb Revealed?
Astronomers initially questioned whether the discrepancy could be caused by errors in measuring the brightness of distant stars.
Cepheid variables can appear crowded together in distant galaxies. Light from neighbouring stars can interfere with measurements, potentially affecting estimated distances.
NASA’s James Webb Space Telescope can observe many of these stars with greater infrared sensitivity and resolution, helping researchers check the Hubble Space Telescope’s measurements.
In March 2024, NASA reported that Webb observations supported the accuracy of the relevant Hubble measurements. The findings made it harder to explain the entire tension through one particular source of stellar crowding.
This did not prove that all possible measurement errors had been eliminated. It strengthened the case that the disagreement deserves further investigation.
More recently, the Hâ‚€ Distance Network’s 2026 measurement provided an independent, high-precision estimate based on a community-built network of distance measurements.
These results do not settle the debate. They show why cosmologists are continuing to test both the measurements and the assumptions used to interpret them.
Sources: NASA’s Webb and Hubble findings and the 2026 Hâ‚€ Distance Network result.
What Did DESI Discover About Dark Energy?
The Dark Energy Spectroscopic Instrument, known as DESI, maps the three-dimensional distribution of galaxies and quasars. By measuring how cosmic structures are distributed across different distances, scientists can reconstruct aspects of the universe’s expansion history.
In March 2025, the DESI collaboration reported that combining its results with other cosmological datasets strengthened hints that dark energy might evolve over time.
These findings attracted considerable attention because an evolving dark-energy component could challenge the simplest version of Lambda-CDM.
However, the results did not establish that dark energy is changing. The strength of the evidence depends on which datasets are combined and how the results are analysed.
In July 2026, DESI reported new measurements using the distribution of intergalactic hydrogen traced through the Lyman-alpha forest. These measurements provide another way to constrain cosmological models and examine the expansion history.
The growing body of evidence is helping scientists test competing explanations, but it has not yet delivered a universally accepted resolution to the Hubble tension.
Sources: DESI’s 2025 dark-energy findings and DESI’s July 2026 Lyman-alpha results.
Could Our Understanding of Gravity Be Incomplete?
Another possibility is that gravity behaves differently from what the standard cosmological model assumes on the largest scales or under certain early-universe conditions.
Einstein’s general theory of relativity has passed many demanding tests. Nevertheless, physicists continue to explore extensions and alternatives that might explain observations that appear difficult to reconcile within the simplest framework.
Some models introduce additional fields or modify the behaviour of gravity in ways that change cosmic expansion.
These proposals face a demanding test: they must explain the Hubble tension while remaining consistent with observations of galaxies, gravitational lensing, the cosmic microwave background and the growth of large-scale structure.
No alternative theory has yet displaced the standard model as the broadly successful description of cosmology.
Is the Universe Expanding Too Fast for Its Own Age?
The Hubble tension does not mean that the universe is about to fly apart or that astronomers have discovered an imminent cosmic catastrophe.
The universe is estimated to be approximately 13.8 billion years old. That estimate comes from combining several observations within the standard cosmological framework, including measurements of the cosmic microwave background.
Different expansion rates can affect inferred estimates of the universe’s age, but the relationship is not a simple calculation using the Hubble constant alone. The universe’s changing expansion history, the amount of matter it contains and the behaviour of dark energy all matter.
The present discrepancy therefore does not automatically invalidate the estimated age of the cosmos.
Instead, it raises a more precise question: can a single physical model consistently explain the early universe, its subsequent evolution and the expansion rate measured today?
That is what scientists are trying to establish.
What Happens Next?
The coming years should bring more precise measurements from galaxy surveys, observatories studying the cosmic microwave background and improved distance-calibration methods.
Researchers will compare these independent datasets to determine whether the discrepancy persists and which proposed explanations remain viable.
Three broad outcomes are possible:
- Measurement uncertainties explain the difference. Improved calibration and new observations may bring the estimates into agreement.
- The standard model needs refinement. New ingredients, perhaps involving dark energy or early-universe physics, could explain the observations.
- A deeper theoretical change is required. If the discrepancy survives increasingly stringent tests, physicists may need to reconsider some assumptions about cosmic evolution.
These possibilities remain open. It is too early to declare that a new theory has solved the problem.
The Universe’s Expansion Remains One of Cosmology’s Biggest Mysteries
The Hubble tension is a reminder that scientific progress does not always make the universe simpler. Sometimes better measurements expose disagreements that earlier observations could not resolve.
Astronomers now have increasingly precise tools for measuring cosmic distances and studying the universe’s earliest light. Yet the expansion rate inferred from these approaches remains different.
Dark energy, early-universe physics, gravity and measurement uncertainties are all part of the investigation. The answer may involve one of these factors or a combination of several.
For now, scientists have not discovered a hidden force that is suddenly accelerating the universe beyond all expectations. They have identified a persistent discrepancy between measurements and the predictions of a successful cosmological model.
Resolving that discrepancy could improve our understanding of dark energy, refine the history of cosmic expansion and reveal whether the standard model needs a significant revision.
The universe is expanding. The outstanding mystery is why our best methods do not yet agree on exactly how fast.
