Key Takeaways

• The Hubble Tension persists as a statistically significant discrepancy of 5-sigma as of late 2025.
• Data from the James Webb Space Telescope confirmed that previous Cepheid measurements were accurate.
• New research initiatives like RedH0T are investigating potential flaws in the standard cosmological model.

The expansion of the universe is a fundamental concept in modern astrophysics. This expansion implies that the cosmos has evolved from a dense, hot beginning known as the Big Bang. The rate of this expansion is quantified by a parameter called the Hubble constant, or H0. For decades, astronomers have worked to determine the precise value of this number, as it dictates the age, size, and fate of the universe.

As of December 2025, the scientific community faces a persistent and perplexing challenge known as the Hubble Tension. Two highly precise methods for measuring the cosmic expansion rate yield incompatible results. Measurements derived from the local universe suggest a rapid expansion, while predictions based on the early universe indicate a slower rate.

Expectations that advanced technology would resolve this discrepancy have not been met. Instead, data from the James Webb Space Telescope (JWST) and the Dark Energy Spectroscopic Instrument (DESI) have reinforced the disagreement. The tension has evolved from a potential measurement error into a significant crisis in physics, suggesting that the current understanding of the universe may require revision.

The Expanding Universe: A Century of Discovery

To understand the current impasse, it is necessary to review the discovery of cosmic expansion. Before the 1920s, the consensus was that the universe was static. This view changed with the work of Edwin Hubble at the Mount Wilson Observatory.

The Great Debate and Island Universes

In the early 20th century, astronomers debated the nature of spiral nebulae. Some argued these were clouds of gas within the Milky Way, while others contended they were distant galaxies, or “island universes.”

Hubble resolved this debate by identifying Cepheid variable stars in the Andromeda nebula. Cepheids pulsate with a regularity directly related to their intrinsic brightness. By measuring the pulse period, Hubble calculated the star’s true brightness and compared it to its apparent brightness on Earth to determine distance. His calculations proved that Andromeda was far outside the Milky Way.

The 1929 Discovery

Hubble and his colleague Milton Humason subsequently measured the distances to various galaxies and their redshifts. Redshift describes how light from an object moving away stretches toward the red end of the spectrum. They observed a linear relationship: galaxies farther away receded faster. This relationship is now called the Hubble-Lemaitre Law.

The slope of this relationship represents the Hubble constant. A higher value indicates a faster expansion and a younger universe. A lower value implies a slower expansion and an older universe.

The Variable Constant

Throughout the mid-20th century, estimates for the Hubble constant varied significantly. Early values suggested a universe younger than Earth, which was physically impossible. By the 1990s, the launch of the Hubble Space Telescope allowed for the resolution of this uncertainty. The “Key Project,” led by Wendy Freedman , established a value of roughly 72 kilometers per second per megaparsec (km/s/Mpc) by 2001.

The Physics of Expansion

The Hubble constant describes the stretching of space itself rather than the movement of galaxies through static space. This expansion is uniform on large scales.

The Metric of Space

In General Relativity, a metric defines the geometry of space-time. As the universe expands, the distance between gravitationally unbound objects increases. The Hubble constant measures this rate of increase at the present epoch. The unit “kilometers per second per megaparsec” indicates that for every megaparsec (approximately 3.26 million light-years) of distance, the recession velocity increases by a specific amount.

For example, if the constant is 70 km/s/Mpc, a galaxy one megaparsec away recedes at 70 km/s, while a galaxy 10 megaparsecs away recedes at 700 km/s.

Dark Energy and Acceleration

In 1998, astronomers observing distant supernovae discovered that cosmic expansion is accelerating. This acceleration is attributed to Dark Energy , which constitutes roughly 68% of the universe in the standard Lambda-CDM model.

To test this model, scientists measure H0 using two distinct approaches:

• Late Universe Probes: Direct measurements of expansion in the local universe.
• Early Universe Probes: Observations of the infant universe extrapolated forward to the present day using the standard model.

The incompatibility of these two methods creates the Hubble Tension.

Method One: The Cosmic Distance Ladder

The direct measurement method involves constructing a “distance ladder” extending from Earth into the cosmos.

Rung 1: Geometric Anchors

The first step relies on geometry. Astronomers use parallax to measure distances to nearby stars. The Gaiamission has provided precise parallax measurements for over a billion stars, anchoring the distance scale within the Milky Way. Water masers in galaxies like NGC 4258 also provide geometric distances independent of stellar brightness.

Rung 2: The Calibrators

The second rung connects local anchors to external galaxies using standard candles.

Cepheid Variables: These stars are the primary tool for this rung. Their period-luminosity relationship allows for precise distance calculations.

Tip of the Red Giant Branch (TRGB): This method uses red giant stars that reach a consistent maximum brightness. TRGB serves as an independent check on Cepheid results.

Rung 3: Type Ia Supernovae

The final rung reaches deep into the universe using Type Ia supernovae . These stellar explosions have consistent peak luminosities. By calibrating the brightness of nearby supernovae using Cepheids, astronomers can measure distances to supernovae billions of light-years away.

The SH0ES Project

The SH0ES (Supernova, H0, for the Equation of State of Dark Energy) team, led by Adam Riess , leads this effort. Using data from the Hubble Space Telescope and JWST, the SH0ES team reports a Hubble constant of approximately 73.0 km/s/Mpc as of late 2025.

Method Two: The Early Universe and the Inverse Ladder

The second method analyzes the universe shortly after the Big Bang and predicts the current expansion rate.

The Cosmic Microwave Background (CMB)

The Cosmic Microwave Background is radiation released 380,000 years after the Big Bang. The Plancksatellite mapped minute temperature fluctuations in this radiation. These fluctuations reveal the precise composition of the early universe.

Using the Lambda-CDM model, cosmologists project these initial conditions forward by 13.8 billion years. The Planck collaboration consistently predicts a Hubble constant of 67.4 km/s/Mpc.

Baryon Acoustic Oscillations (BAO)

Sound waves traveling through the early universe left an imprint on the distribution of matter known as Baryon Acoustic Oscillations. This imprint acts as a standard ruler. The Dark Energy Spectroscopic Instrument (DESI) maps millions of galaxies to measure this scale.

Results released by DESI in 2024 and 2025 largely align with the Planck predictions. When combined with CMB data, these measurements favor a value near 68.5 km/s/Mpc.

The Tension: 67 vs. 73

The discrepancy between the early universe prediction (67.4) and the local measurement (73.0) is approximately 9%. This difference has reached a statistical significance of 5-sigma, meaning there is less than a one in 3.5 million chance that it is a random fluctuation.

Searching for Errors: The Role of JWST

A leading hypothesis for the discrepancy was measurement error in the Cepheid data. Critics suggested that Cepheids in distant galaxies might be obscured by dust or crowded by other stars, leading to inaccurate brightness measurements by the Hubble Space Telescope.

The James Webb Space Telescope was tasked with testing this hypothesis. In 2024 and 2025, the SH0ES team utilized JWST’s superior resolution to observe the same Cepheids. The JWST data matched the Hubble data, effectively ruling out crowding or dust as the source of the tension. The local measurement of 73 km/s/Mpc is considered robust.

The TRGB Debate

The Chicago-Carnegie Hubble Program (CCHP) focused on the TRGB method as an alternative to Cepheids. While earlier results suggested a middle ground, 2025 analyses using updated JWST calibrations show TRGB results ranging between 69 and 72 km/s/Mpc. While slightly lower than Cepheids, these results do not bridge the gap to the Planck value of 67.

Theoretical Solutions: New Physics?

With measurement errors appearing increasingly unlikely, theoretical physicists are exploring modifications to the standard cosmological model.

Early Dark Energy (EDE)

Early Dark Energy proposes that a distinct form of dark energy existed briefly before the universe became transparent. This energy would have accelerated the early expansion, altering the calibration of the cosmic ruler used in CMB and BAO calculations. This modification could theoretically raise the predicted H0 to match local measurements.

Late-Time Solutions

Other theories suggest modifications to gravity or dark energy in the recent universe. Some models propose that dark energy interacts with dark matter or that the strength of dark energy increases over time.

The DESI 2025 Clue

Data from DESI in 2024 and 2025 provided evidence that dark energy may not be a constant. The results hinted at a dynamic form of dark energy. While this discovery would revolutionize physics, it does not immediately resolve the numerical difference between 67 and 73.

New Initiatives: RedH0T and Beyond

The scientific community has launched new projects to address the crisis. The RedH0T (Red teaming the H0 Tension) project, funded by the European Research Council, employs a “red team” approach to challenge measurement methodologies aggressively. This project seeks to identify any subtle biases that may have been overlooked.

The Euclid mission, operated by the European Space Agency , is also gathering data. Euclid provides an independent map of the universe’s structure, offering a third perspective on the tension when its primary results are published in 2026.

Summary

As of December 2025, the Hubble Tension remains the most significant puzzle in cosmology. The James Webb Space Telescope has verified local measurements, while Planck and DESI have solidified early universe predictions. The persistence of this disagreement suggests that the standard model of the universe may be incomplete. Whether the resolution lies in a complex systematic error or a fundamental discovery of new physics remains to be seen.

Appendix: Top 10 Questions Answered in This Article

What is the Hubble Tension?

The Hubble Tension is a major discrepancy in cosmology where measurements of the universe’s expansion rate differ depending on the method used. Local measurements suggest a rate of 73 km/s/Mpc, while early universe predictions suggest 67 km/s/Mpc.

Did the James Webb Space Telescope solve the Hubble Tension?

No, the James Webb Space Telescope did not solve the tension; it deepened it. JWST observations confirmed that the Hubble Space Telescope’s measurements of Cepheid stars were accurate, ruling out measurement error caused by dust or crowding as the source of the discrepancy.

What is the current value of the Hubble Constant as of late 2025?

There is no single agreed value. Measurements from the local universe (SH0ES team) indicate a value of approximately 73 km/s/Mpc, while measurements based on the early universe (Planck/DESI) indicate a value of approximately 67-68 km/s/Mpc.

What is the significance of the “5-sigma” discrepancy?

A 5-sigma discrepancy means the statistical probability of the disagreement being a random fluke is one in 3.5 million. This level of confidence indicates that the difference between the measurements is real and likely caused by physical differences or systematic errors, not chance.

What is the difference between early and late universe probes?

Late universe probes measure the expansion rate directly using stars and supernovae existing in the universe today. Early universe probes measure the conditions of the infant universe (via the Cosmic Microwave Background) and use a model to predict what the expansion rate should be today.

What role does the DESI instrument play in this debate?

The Dark Energy Spectroscopic Instrument (DESI) measures Baryon Acoustic Oscillations to map the expansion history. Its 2024 and 2025 results largely support the lower Hubble constant value found by the Planck satellite but also hint that dark energy might change over time.

What is the “RedH0T” project?

Launched in late 2025, RedH0T is a research initiative funded by the European Research Council. It uses a “red team” approach inspired by cybersecurity to aggressively stress-test and challenge the methodologies used to measure the Hubble constant, aiming to find hidden errors.

What is Early Dark Energy?

Early Dark Energy is a theoretical solution to the tension proposing that a burst of dark energy existed briefly in the infant universe. This would alter the expansion rate prior to the formation of the Cosmic Microwave Background, potentially realigning the early predictions with local measurements.

Why are Cepheid variables important to the Hubble Constant?

Cepheid variables act as “standard candles” because their pulsation periods correlate directly with their true brightness. This allows astronomers to calculate precise distances to nearby galaxies, forming a important rung on the cosmic distance ladder used to measure the expansion rate.

What is the implication if the Hubble Tension is real?

If the tension cannot be explained by measurement error, it implies that the standard model of cosmology is broken or incomplete. This would require new physics, such as new particles, modified gravity, or a more complex understanding of dark energy.

Appendix: Top 10 Frequently Searched Questions Answered in This Article

Why is the universe expanding?

The universe is expanding due to the initial impulse of the Big Bang and the ongoing stretching of space-time described by General Relativity. Additionally, a mysterious force called dark energy is currently causing this expansion to accelerate rather than slow down.

How do scientists measure the speed of the universe?

Scientists measure the expansion speed by calculating the Hubble constant. They do this by comparing the distances of galaxies (found using stars like Cepheids or supernovae) with how fast those galaxies are moving away from us (found by measuring their redshift).

Is the universe 13.8 billion years old?

The age of 13.8 billion years is derived from the standard model of cosmology using the lower Hubble constant value (approx 67 km/s/Mpc). If the higher value (73 km/s/Mpc) is correct, the universe could be slightly younger, around 13 billion years old.

What is the difference between Dark Matter and Dark Energy?

Dark matter is an invisible substance that has mass and pulls things together via gravity, holding galaxies intact. Dark energy is a repulsive force that permeates space and pushes the universe apart, driving the acceleration of cosmic expansion.

Can the James Webb Telescope see the Big Bang?

The James Webb Telescope cannot see the Big Bang itself, as the universe was opaque at that time. However, it can see galaxies that formed just a few hundred million years after the event, providing data on the early eras that connect to the Hubble tension debate.

What is the Lambda-CDM model?

Lambda-CDM is the standard mathematical model of the Big Bang cosmology. Lambda represents dark energy, and CDM stands for Cold Dark Matter; together with ordinary matter, they describe the evolution and structure of the universe.

Who was Edwin Hubble?

Edwin Hubble was an American astronomer who, in the 1920s, proved that other galaxies exist outside the Milky Way and discovered that the universe is expanding. The Hubble Space Telescope and the Hubble constant are named in his honor.

What is a standard candle in astronomy?

A standard candle is an astronomical object, such as a Cepheid variable star or Type Ia supernova, whose true brightness is known. By comparing this true brightness to how bright it looks from Earth, astronomers can calculate its exact distance.

Will the universe expand forever?

Current data, especially the presence of dark energy, suggests the universe will continue to expand forever. The expansion is accelerating, meaning galaxies will eventually move away from each other so fast that they will disappear from view, leading to a “Big Freeze.”

What is the Crisis in Cosmology?

The “Crisis in Cosmology” refers to the Hubble Tension – the inability of current science to reconcile the two different measured speeds of the universe’s expansion. It suggests that our fundamental understanding of the physics governing the cosmos might be incorrect.