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A vast project observing dark energy suggests that the prevailing theory of the universe may be outdated. It is time to develop an alternative model.
A vast project observing dark energy suggests that the prevailing theory of the universe may be outdated. It is time to develop an alternative model.
If you imagine the history of the universe as a film in constant post production, cosmologists would be its obsessive editors, always working to refine the plot. The version they are currently working on is a stunning cinematic achievement: it begins with a bang, spacetime emerging from nothing and then unfolding majestically with the formation of stars and galaxies, shaped by the gravitational pull of visible matter and mysterious dark matter, while the cosmos continues to expand calmly thanks to an inscrutable force called dark energy.
But this cannot be the final version. The more we observe space, the more the story seems incomplete: there are small inconsistencies and some key elements remain elusive. For decades, cosmologists have struggled to perfect the script.
Now the cosmos is finally offering them new inspiration. A powerful telescope has mapped millions of distant galaxies, reconstructing the history of the expansion of the universe with unprecedented precision. The result suggests that dark energy behaves so unpredictably that it cannot be what we thought. If confirmed, it would be a dramatic twist. Theorists are considering completely revising the concept of dark energy. How it will turn out is far from clear. Many, however, are beginning to believe that we are about to write a much richer and more detailed story of the cosmos, one that could be completely different from the current version. “We are in an interesting moment”, says Adam Riess, an astrophysicist at Johns Hopkins University who shared the 2011 Nobel Prize in Physics for contributing to the discovery of dark energy. “I would not recommend going to the bathroom right now”.
Our best account of the origins and evolution of the universe has been assembled over a century. It all began in 1915 with Albert Einstein’s theory of general relativity, which describes gravity as the effect of the curvature of spacetime caused by massive bodies.
At the time, the universe was thought to be static, so Einstein added a stabilizing term to his equations, the cosmological constant. But in 1929 the astronomer Edwin Hubble observed that distant galaxies were moving away from each other rapidly, showing that the universe was expanding. This discovery led Einstein to abandon his constant.
Then came the big bang theory. Although today it is considered almost a dogma, the alternative steady state hypothesis was only definitively ruled out in the 1960s, when astronomers discovered a residual thermal imprint of the big bang, the cosmic microwave background, whose features matched predictions.
As our ability to observe space improved, the big bang theory began to fall short. In the 1980s astronomers discovered that the gravity of visible matter was not sufficient to hold galaxies together or explain the formation of galaxy clusters. The solution was to hypothesize the existence of invisible dark matter. A decade later, observations of distant supernovae by Riess and his colleagues revealed, unexpectedly, that the expansion of the universe was accelerating. The cosmological constant was reinstated, though with a new name: dark energy.
According to Riess, the fact that the data do not match is a strong indication that something in the Λcdm model does not work.
And this, essentially, is the current standard model of cosmology, known as Λcdm. The Greek letter lambda indicates the cosmological constant, while cdm stands for cold dark matter, which is thought to consist of heavy and slow particles. Combined with general relativity and some basic assumptions, especially that on large scales the universe appears the same in every direction, this model provides a convincing picture of how cosmic structures formed from quantum fluctuations in the early universe, following a brief but extremely rapid exponential expansion in its first moments.
The Λcdm model is considered one of the greatest triumphs of science. It combines elegance with extraordinary explanatory power: with just six parameters it describes the entire history of the cosmos, making precise predictions that have been confirmed by increasingly accurate observations. “It has been extraordinarily successful”, says Mike Turner, a theoretical cosmologist at the University of Chicago. “If I think about what we had when I started around 1980, it is more than we could have imagined”. Yet, Turner observes, today it is “much less than what we can accept”. Partly because science never stands still: even the most successful theories are approximations of a deeper understanding, and when tested with more precise observations, unresolved issues and cracks appear.
In the case of the Λcdm model, these unresolved issues are evident. Dark matter and dark energy have always been placeholders: they were used to explain observed phenomena without a physical explanation. Despite decades of research, physicists have not yet directly detected dark matter particles. And although dark energy is explained as vacuum energy resulting from quantum fluctuations of space, it remains theoretically problematic. Quantum theory predicts a value about 10 to the power of 120 times larger than what is needed to explain the observed expansion of the universe.
“Dark energy and dark matter are just patches”, Turner argues. Both play a specific role in the model and there is strong empirical evidence for their existence. “But they are simply phenomenological descriptions, and there must be something more fundamental behind them”.
Cracks have begun to appear in the model. The most well known is the so called Hubble tension. It is named because two different methods for measuring the expansion rate of the universe, known as the Hubble constant, give incompatible values. When cosmologists project forward data from the cosmic microwave background using the current model, they obtain about 67 kilometers per second per megaparsec. When astronomers measure the local universe directly using supernovae and variable stars, the value is around 73. According to Riess, the mismatch is strong evidence that something in the Λcdm model is not working.
Yet most cosmologists are not ready to abandon it. All proposed solutions to the Hubble tension disrupt the near perfect consistency of the current model with the cosmic microwave background and the large scale structure of the observable universe. It is also possible that subtle errors exist in the measurements underlying the tension. In particular, measuring the recent expansion of the universe relies on a complex chain of inferences, where each step depends on careful calibrations and assumptions about stars and galaxies. The suspicion is that with more data the tension may disappear. “There are too many variables to draw definitive conclusions”, says Pedro Ferreira, a cosmologist and astrophysicist at the University of Oxford.
Riess is not convinced. His measurements of recent expansion have been checked repeatedly, he says, and no errors have been found. “Ten years have passed since we discovered the Hubble tension and it has not gone away”, he says. “If anything, it has become more pronounced”. According to him, the real reason the scientific community is reluctant to abandon the Λcdm model is that scientists struggle to give up such a successful theory without a better one to replace it. “No one likes to wander in the desert”.
From this perspective, observations that more clearly indicate the right path are needed. Fortunately, a new generation of instruments has begun to deliver exceptional results. One of these is the Dark Energy Spectroscopic Instrument, Desi.
If you imagine the history of the universe as a film in constant post production, cosmologists would be its obsessive editors, always working to refine the plot. The version they are currently working on is a stunning cinematic achievement: it begins with a bang, spacetime emerging from nothing and then unfolding majestically with the formation of stars and galaxies, shaped by the gravitational pull of visible matter and mysterious dark matter, while the cosmos continues to expand calmly thanks to an inscrutable force called dark energy.
But this cannot be the final version. The more we observe space, the more the story seems incomplete: there are small inconsistencies and some key elements remain elusive. For decades, cosmologists have struggled to perfect the script.
Now the cosmos is finally offering them new inspiration. A powerful telescope has mapped millions of distant galaxies, reconstructing the history of the expansion of the universe with unprecedented precision. The result suggests that dark energy behaves so unpredictably that it cannot be what we thought. If confirmed, it would be a dramatic twist. Theorists are considering completely revising the concept of dark energy. How it will turn out is far from clear. Many, however, are beginning to believe that we are about to write a much richer and more detailed story of the cosmos, one that could be completely different from the current version. “We are in an interesting moment”, says Adam Riess, an astrophysicist at Johns Hopkins University who shared the 2011 Nobel Prize in Physics for contributing to the discovery of dark energy. “I would not recommend going to the bathroom right now”.
Our best account of the origins and evolution of the universe has been assembled over a century. It all began in 1915 with Albert Einstein’s theory of general relativity, which describes gravity as the effect of the curvature of spacetime caused by massive bodies.
At the time, the universe was thought to be static, so Einstein added a stabilizing term to his equations, the cosmological constant. But in 1929 the astronomer Edwin Hubble observed that distant galaxies were moving away from each other rapidly, showing that the universe was expanding. This discovery led Einstein to abandon his constant.
Then came the big bang theory. Although today it is considered almost a dogma, the alternative steady state hypothesis was only definitively ruled out in the 1960s, when astronomers discovered a residual thermal imprint of the big bang, the cosmic microwave background, whose features matched predictions.
As our ability to observe space improved, the big bang theory began to fall short. In the 1980s astronomers discovered that the gravity of visible matter was not sufficient to hold galaxies together or explain the formation of galaxy clusters. The solution was to hypothesize the existence of invisible dark matter. A decade later, observations of distant supernovae by Riess and his colleagues revealed, unexpectedly, that the expansion of the universe was accelerating. The cosmological constant was reinstated, though with a new name: dark energy.
According to Riess, the fact that the data do not match is a strong indication that something in the Λcdm model does not work.
And this, essentially, is the current standard model of cosmology, known as Λcdm. The Greek letter lambda indicates the cosmological constant, while cdm stands for cold dark matter, which is thought to consist of heavy and slow particles. Combined with general relativity and some basic assumptions, especially that on large scales the universe appears the same in every direction, this model provides a convincing picture of how cosmic structures formed from quantum fluctuations in the early universe, following a brief but extremely rapid exponential expansion in its first moments.
The Λcdm model is considered one of the greatest triumphs of science. It combines elegance with extraordinary explanatory power: with just six parameters it describes the entire history of the cosmos, making precise predictions that have been confirmed by increasingly accurate observations. “It has been extraordinarily successful”, says Mike Turner, a theoretical cosmologist at the University of Chicago. “If I think about what we had when I started around 1980, it is more than we could have imagined”. Yet, Turner observes, today it is “much less than what we can accept”. Partly because science never stands still: even the most successful theories are approximations of a deeper understanding, and when tested with more precise observations, unresolved issues and cracks appear.
In the case of the Λcdm model, these unresolved issues are evident. Dark matter and dark energy have always been placeholders: they were used to explain observed phenomena without a physical explanation. Despite decades of research, physicists have not yet directly detected dark matter particles. And although dark energy is explained as vacuum energy resulting from quantum fluctuations of space, it remains theoretically problematic. Quantum theory predicts a value about 10 to the power of 120 times larger than what is needed to explain the observed expansion of the universe.
“Dark energy and dark matter are just patches”, Turner argues. Both play a specific role in the model and there is strong empirical evidence for their existence. “But they are simply phenomenological descriptions, and there must be something more fundamental behind them”.
Cracks have begun to appear in the model. The most well known is the so called Hubble tension. It is named because two different methods for measuring the expansion rate of the universe, known as the Hubble constant, give incompatible values. When cosmologists project forward data from the cosmic microwave background using the current model, they obtain about 67 kilometers per second per megaparsec. When astronomers measure the local universe directly using supernovae and variable stars, the value is around 73. According to Riess, the mismatch is strong evidence that something in the Λcdm model is not working.
Yet most cosmologists are not ready to abandon it. All proposed solutions to the Hubble tension disrupt the near perfect consistency of the current model with the cosmic microwave background and the large scale structure of the observable universe. It is also possible that subtle errors exist in the measurements underlying the tension. In particular, measuring the recent expansion of the universe relies on a complex chain of inferences, where each step depends on careful calibrations and assumptions about stars and galaxies. The suspicion is that with more data the tension may disappear. “There are too many variables to draw definitive conclusions”, says Pedro Ferreira, a cosmologist and astrophysicist at the University of Oxford.
Riess is not convinced. His measurements of recent expansion have been checked repeatedly, he says, and no errors have been found. “Ten years have passed since we discovered the Hubble tension and it has not gone away”, he says. “If anything, it has become more pronounced”. According to him, the real reason the scientific community is reluctant to abandon the Λcdm model is that scientists struggle to give up such a successful theory without a better one to replace it. “No one likes to wander in the desert”.
From this perspective, observations that more clearly indicate the right path are needed. Fortunately, a new generation of instruments has begun to deliver exceptional results. One of these is the Dark Energy Spectroscopic Instrument, Desi.
Installed on a telescope in Arizona, Desi is equipped with a large mirror and five thousand robotically controlled optical fibers that automatically align with distant galaxies one after another, much faster than previous dark energy surveys. Since 2021, Desi has been observing millions of galaxies to measure their redshift, that is, how much the wavelength of their light has stretched due to cosmic expansion, an indicator of their distance from Earth. Because galaxies have different redshifts, comparing recurring distances in their distribution allows scientists to reconstruct how the expansion rate of the universe has changed over time.
To calibrate these distances, Desi also measures subtle imprints left by the early universe, known as baryon acoustic oscillations. Like ripples frozen in ice, these patterns in galaxy separation provide a standard ruler for measuring cosmic expansion. The goal is to build the most accurate three dimensional reconstruction of cosmic expansion ever achieved. The latest version, released in March 2025 and based on three years of data from 15 million galaxies, contained a finding that shook cosmology.
Researchers compared Desi observations with the latest supernova data and cosmic microwave background measurements. After checking consistency with the Λcdm model, they found that the current model does not fully hold, because it does not allow for the possibility that dark energy changes over time. The conclusion was clear: dark energy is weakening and does not appear to be a constant.
“It was really shocking”, says Will Percival, an astrophysicist at the University of Waterloo involved in Desi. The result has undergone rigorous checks. “But in many ways it is exactly what we had all been waiting for”.
Desi results also suggest that in the early universe dark energy may have dropped below the so called phantom divide, a threshold beyond which its repulsive force would have been much stronger than allowed by the cosmological constant, before rising again.
“The Desi results are wonderfully bizarre”, says Eric Linder, a physicist and cosmologist at the University of California, Berkeley. “They deviate from the cosmological constant in a way no one had considered”. At present, the results are not strong enough to be considered a definitive discovery. The analysis suggests evolving dark energy with a statistical significance of up to 4.2 sigma, still below the 5 sigma standard, so it could be disproven by future data. Evidence for crossing the phantom divide is even less certain. “I am undecided”, says Ferreira. “We have been through this too many times”.
Still, there are reasons to think the Desi results are different. According to Catherine Heymans, an astronomer at the University of Edinburgh, “their method for measuring cosmic expansion is one of the cleanest. It is much harder to argue against these results than the Hubble tension”.
Some have tried. In May 2025 George Efstathiou of the University of Cambridge argued that the evidence for evolving dark energy is weak for two reasons: the discrepancy appears only when supernova data are included, and the statistical analysis relies on prior assumptions that may unfairly favor variable dark energy models.
However, all agree that if confirmed, the Desi results would deal a severe blow to the Λcdm model. “We would have to rethink everything”, says Ferreira.
In an August 2025 paper, Riess and cosmologist Alexie Leauthaud wrote that we are witnessing the twilight of the Λcdm model and must prepare to move beyond it. For the first time in 25 years, there is a concrete hint of what a better model might look like.
This does not mean it will be easy. While Desi provides clues about the properties of dark energy, the resulting picture makes it difficult to find the exact formulation. One simple idea is that dark energy is not vacuum energy but a field similar to those describing light or nuclear forces. But such models require fine tuning to explain why dark energy increased in recent cosmic history. Moreover, they cannot alone explain the phantom crossing.
Many theorists focus on models where dark energy interacts with gravity rather than evolving independently. In these models, gravity changes behavior due to energy exchange between ordinary matter and dark energy. “This allows dark energy density to rise and then fall”, explains Alessandra Silvestri of Leiden University, whose model fits Desi data better than Λcdm.
Other models propose energy exchange between dark matter and dark energy, with the former slowly converting into the latter during cosmic expansion. This is appealing because it links the two biggest mysteries in cosmology.
However, such interacting models face problems. If correct, their effects should already have been observed, for example in planetary orbits, but they have not. Even if interactions are too small to detect, they might violate conservation laws. There are many ideas, but none fully solves the problem.
For Ferreira and Riess, the goal should not be to patch the concept of dark energy but to rethink the lessons of the Desi results. “We need to pause and reflect”, says Riess. If we are at the beginning of a major leap in understanding, cosmologists must carefully consider which direction to take.
Perhaps a new elegant theory will emerge. Or perhaps the explanation will be more complex, involving multiple dark energy fields, different types of dark matter, interactions between them or a new understanding of gravity on cosmic scales. “The focus on elegance comes from particle physics”, says Riess. “But who says it applies to the cosmos? The universe is quite complex, so we should keep an open mind”.
As always, observations will guide us. Desi continues to collect data, with a new release expected in 2027. Cosmologists also expect much from the Euclid space telescope and the Vera Rubin Observatory. These should provide more clarity on cosmic expansion. Or perhaps not. They will allow exploration of previously unobserved redshifts.
Ferreira is less optimistic. Because we can only observe a limited range of cosmic history, many theoretical models can produce similar results. Even with new data, “we will end up with a large family of models that are observationally indistinguishable”.
The risk is a situation similar to the Hubble tension, where cosmologists hesitate to abandon Λcdm without a better alternative. Such a theory may not come soon. Riess worries this could lead to inertia. “Pulling the sword from the stone is difficult work. But the sword is still there”.
According to Riess, the problem is that the scientific community gives too much weight to a model developed before new data and not enough to the data themselves. When tensions arise, failure to resolve them is used against new observations, leading to fixation on unknown errors. “Many people have built their careers on this model”, he says. “The idea that it may not explain everything is unsettling”.
This may simply be how paradigm shifts work. Change always brings conflict, and Λcdm will not disappear easily. But this is not necessarily bad. “It is right that defenders of the model scrutinize the data, and equally right that others are willing to look beyond”, says Linder. “It may seem conflictual, but it is actually beneficial”.
The fact that cosmologists are preparing for confrontation may mean we are on the verge of another revolution. The only certainty is that, after a long period of harmony, cosmology is entering a much more interesting era. “We are eagerly awaiting new data that I think will astonish us”, says Linder. “It is an incredibly exciting moment”.
Source: New Scientist
edited: Nicolas F. E.
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