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Dark energy keeps Einstein’s general relativity and adds a component or field whose negative pressure drives accelerated expansion. Modified gravity changes the gravitational laws themselves, especially on cosmological distances, so the acceleration could arise without a separate dark-energy substance.
That distinction is conceptually clear but observationally difficult. Measurements directly map expansion, matter clustering and the bending of light; they do not label the cause as “energy” or “gravity.” Recent survey results have produced interesting, combination-dependent hints of evolving dark energy, but neither evolving dark energy nor modified gravity has been established as a discovery.
What each explanation changes
Dark energy: add a component, retain general relativity
In the standard interpretation, the universe is governed by general relativity and contains matter, radiation and an additional component called dark energy. Its defining cosmological effect is sufficiently negative pressure to make expansion accelerate rather than slow solely under gravity.
The simplest version is the cosmological constant, written as Λ. It has a constant energy density that does not dilute as the universe expands. In the field equations, Λ can be treated as part of the energy budget while the gravitational framework remains unchanged. This is the dark-energy term in the familiar Lambda-CDM model.
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A cosmological constant is sometimes identified with vacuum energy. The difficulty is that a straightforward theoretical estimate of vacuum energy is enormously larger than the small effective density inferred from cosmological observations. NASA describes this mismatch as the cosmological-constant problem. The problem does not disprove Λ; it shows that its physical interpretation is incomplete.
Another possibility is a changing field, commonly called quintessence. Unlike a cosmological constant, quintessence could vary in amount and distribution across time and space. Such fields are proposed explanations, not detected substances. A time-dependent dark-energy equation of state can reproduce an expansion history that differs slightly from the constant-Λ prediction.
Modified gravity: change the gravitational framework
Modified-gravity theories start from a different premise: general relativity may be an excellent local description but incomplete on the largest scales. Instead of adding a dark-energy component, they alter the relationship between matter, spacetime curvature and gravitational fields. The altered dynamics can produce accelerated expansion or change how cosmic structures grow.
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Any viable theory faces a demanding requirement. General relativity has passed precision tests in the Solar System and other local environments, so a cosmological alternative must reproduce those successes while departing from general relativity where observations could reveal a difference. It must also be mathematically consistent and agree with multiple cosmological measurements.
The Particle Data Group review of dark energy notes an important limitation for constrained f(R) models, a class in which the curvature term in the gravitational action is replaced by a function of curvature. Some versions are forced so close to general relativity by observational and theoretical constraints that they cannot generate acceleration on their own; a separate dark-energy component is still required. This is a warning against treating “modified gravity” as a single, unconstrained explanation.
Why the boundary is not perfectly sharp
Cosmology often describes observations through an effective expansion history. The same history can sometimes be written as a changing dark-energy component inside Einstein gravity or as altered gravitational dynamics with no independent dark-energy fluid. A mathematical rearrangement of field equations can move terms from the “geometry” side to the “energy” side without changing the predicted distances.
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Consequently, an expansion curve by itself may not identify the underlying theory. To distinguish the categories, scientists compare expansion with structure growth and gravitational lensing. Those measurements test whether matter responds to gravity exactly as general relativity predicts, not merely whether the scale factor follows a particular trajectory.
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Type Ia supernovae, baryon acoustic oscillations (BAO), cosmic microwave background (CMB) observations and other probes establish how the universe’s distances and expansion rate change over time. Within the standard cosmological interpretation, the expansion is accelerating. That empirical statement is stronger than any claim about the mechanism.
In a Friedmann-equation description, the expansion rate depends on the total energy content and its pressure. A component with sufficiently negative pressure accelerates the scale factor under general relativity. Modified gravity changes the equations that connect the expansion rate to matter and curvature, allowing a similar distance-redshift relation to arise from different underlying physics.
This is why two models can fit the same supernova or BAO catalogue while making different predictions for the motion of matter, the depth of gravitational potential wells and the deflection of light. A model that matches only distances has not yet passed the stronger test of explaining the full set of observables.
The observations used to separate the explanations
| Evidence axis | What it measures | Why it helps |
|---|---|---|
| Expansion history | Distances and expansion rate over cosmic time, using probes such as BAO and Type Ia supernovae | Tests whether a cosmological constant is sufficient or whether a time-varying effective equation of state fits better. |
| Growth of structure | How galaxies and matter cluster over time | Modified gravity may change the growth rate relative to general-relativity predictions even when expansion histories look similar. |
| Gravitational lensing | How foreground matter bends light from more distant sources | Tests the relationship between matter, gravitational potentials and observed structure. |
| Cross-probe consistency | Whether CMB, BAO, supernova, clustering and lensing measurements agree under one model | Requires an explanation to fit several independent kinds of evidence rather than one data set. |
Expansion history
BAO provide a standard ruler: the preferred separation scale left by early-universe sound waves can be measured at different redshifts. DESI tracks this pattern across cosmic time. Supernovae provide a complementary distance indicator. Together, these probes test whether the effective dark-energy density behaves like a constant or changes with redshift.
Expansion data are powerful but intrinsically degenerate. A modified-gravity model can be tuned to produce nearly the same distance-redshift relation as a dynamical-dark-energy model. A preference in an expansion-only fit therefore points to a mismatch with a constant cosmological constant, not automatically to a new field or altered gravity.
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Growth of structure
After small density variations began to grow, dark matter and ordinary matter assembled into galaxies, clusters and the cosmic web. Surveys can measure how clustering amplitude changes with time. In general relativity, the growth history is linked to the expansion history and the matter content. Modified gravity can alter that link, causing structure to grow faster or slower than the general-relativity prediction for the same expansion curve.
Growth measurements therefore provide a discriminant that distances alone lack. They also bring additional modelling challenges, including galaxy bias and nonlinear structure formation, so a claimed discrepancy must survive those uncertainties.
Gravitational lensing
Mass bends light. Weak-lensing surveys measure tiny, coherent distortions in the apparent shapes of distant galaxies, while other lensing observations probe stronger deflections. Comparing lensing with the amount of matter inferred from clustering tests the gravitational potentials directly.
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If an altered theory changes how those potentials are produced or how light responds to them, lensing can disagree with the relation expected in general relativity even when BAO and supernova distances look acceptable. Lensing is thus complementary to both expansion and growth rather than a replacement for either.
Cross-probe consistency
A credible explanation must fit the CMB, BAO, supernovae, clustering and lensing together with one set of parameters and assumptions. Combining probes can reveal tensions hidden in any individual survey, but it can also make the result sensitive to calibration, modelling choices and the assumed class of theories. Significance values must always be read with the exact combination that produced them.
What the DESI results show
DESI’s three-year report
The Dark Energy Spectroscopic Instrument’s 2025 report used nearly 15 million galaxies and quasars from its first three years. DESI’s BAO measurements by themselves were consistent with Lambda-CDM, the model with a cosmological constant and cold dark matter.
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When DESI was combined with CMB, supernova and weak-lensing data, the collaboration reported preferences for evolving dark energy ranging from 2.8 to 4.2 sigma, depending on which external data sets were included. The collaboration explicitly noted that the result had not reached the 5-sigma threshold commonly used for a discovery claim.
These numbers indicate that some combinations fit a time-varying dark-energy description better than a constant cosmological constant under the tested assumptions. They do not prove that dark energy changes, rule out Lambda-CDM, or select modified gravity as the alternative. A three-sigma-level preference is suggestive and worth testing with additional observations, but it is not a confirmed physical detection.
Dark Energy Survey Year 6
The Dark Energy Survey’s Year 6 cosmology results provide a separate multi-probe assessment. DES reports a 2.2-sigma deviation from a cosmological constant for its own Year 6 combination. Adding DESI DR2 BAO gives 2.3 sigma; adding primary CMB information to that combination gives 3.0 sigma.
DES characterizes these findings as support for a weak preference for evolving dark energy. The values depend on the selected data sets and the model assumptions, so they should not be compared as interchangeable measurements with one another or with the DESI range. They are separate analyses that happen to address a related question.
Where modified gravity enters the current discussion
A 2025 Physical Review Letters paper proposed a nonminimally coupled gravity model as an interpretation of the DESI 2024 BAO result and described the outcome as a hint of modified gravity under that analysis. This is a model-specific theoretical interpretation of a data set, not evidence that the field has established modified gravity as the cause of acceleration.
The distinction matters because many theories can be adjusted to fit an anomalous-looking expansion history. To become a compelling explanation, a modified-gravity proposal must also survive local tests of general relativity, remain theoretically consistent and predict the observed growth and lensing signals. A fit to BAO alone cannot meet all of those requirements.
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How to read sigma values correctly
A sigma value describes how far a result lies from a reference model under a specified statistical analysis. It is not a probability that the reference model is true or false, and it is not a measure of how large a physical effect must be.
- 2.2 sigma: the DES Year 6 multi-probe deviation reported against a cosmological constant.
- 2.3 sigma: the DES Year 6 combination after adding DESI DR2 BAO.
- 2.8–4.2 sigma: the range reported by DESI’s 2025 combinations with CMB, weak lensing and supernova data.
- 3.0 sigma: the DES Year 6 and DESI combination after adding primary CMB information.
- 5 sigma: the discovery threshold cited in the DESI account; the reported DESI preferences were below it.
Changing the data combination, nuisance-parameter treatment or model family can change the sigma value. A result also needs independent confirmation and a physical model that explains all relevant observables. For that reason, the current safe description is “a modest, combination-dependent preference for evolving dark energy,” not “dark energy has been proven to evolve.”
Theoretical strengths and problems
What the cosmological-constant model does well
Lambda-CDM is economical: one constant term added to general relativity accounts for the observed late-time acceleration while preserving the many successful predictions of the standard framework. Its weakness is conceptual rather than a demonstrated failure of its predictions. The vacuum-energy interpretation has a severe naturalness problem, and a constant density may not be the final description if future data establish genuine time evolution.
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A quintessence-like field introduces additional physics and parameters. Its density and pressure can evolve, potentially matching the preferences seen in some combined data sets. The field would still operate inside general relativity, so its predictions must be checked against structure growth, lensing and early-universe constraints rather than distances alone.
What modified gravity must accomplish
Modified gravity offers a route to acceleration without postulating a separate dark-energy substance, but it inherits a stricter consistency burden. It must recover general relativity in regimes where experiments already test it precisely, produce a stable and mathematically coherent theory, and fit expansion, growth and lensing simultaneously. The Particle Data Group’s discussion of constrained f(R) models illustrates how those requirements can leave too little departure from general relativity to generate acceleration independently.
Dark energy is not dark matter
Dark matter and dark energy are distinct concepts. Dark matter behaves gravitationally like an additional clustering matter component and helps form galaxies and large-scale structure. Dark energy is invoked to explain the accelerated expansion and is generally modelled as smooth on the scales relevant to that acceleration. Both are poorly understood, but replacing one term with the other creates a category error.
Modified gravity is different again: it changes the law that relates matter and spacetime rather than simply adding either a clustering matter component or a smooth energy component. A complete cosmological model must still account for the evidence usually attributed to dark matter while addressing the acceleration problem.
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- Identify the reference model. Check whether the claim compares against Lambda-CDM with a constant cosmological constant, another dark-energy model or general relativity itself.
- Separate the observable from the interpretation. Ask whether the result concerns distances, expansion rate, clustering, lensing or several of these at once.
- Read the data combination. Record which survey, release and external data sets produced the quoted significance. DESI alone and DESI combined with other probes answer different statistical questions.
- Check the threshold and uncertainty. A value below 5 sigma is not a discovery under the threshold used in the DESI account, and a sigma value does not by itself identify the physical cause.
- Look for cross-probe predictions. A dark-energy and a modified-gravity model that fit the same expansion history should make different predictions for growth and lensing. Those tests are essential.
- Check local consistency. A modified-gravity proposal must explain why Solar-System and other precision tests continue to agree with general relativity.
- Wait for independent confirmation. A persistent signal across future releases and independent surveys is more informative than a single combination-dependent preference.
What is established—and what is not
- Cosmic expansion is accelerating within the standard cosmological interpretation.
- Dark energy and modified gravity are broad categories, not single fully specified theories.
- The cosmological constant is the simplest dark-energy model, but its vacuum-energy interpretation has a major theoretical problem.
- Some modified-gravity models face constraints so strong that they cannot produce acceleration without an additional dark-energy component.
- Expansion measurements alone cannot reliably distinguish a changing dark-energy component from altered gravity.
- DESI and DES have reported modest preferences for evolving dark energy in selected combinations, below the cited discovery threshold.
- No observation summarized here establishes evolving dark energy, modified gravity or the failure of Lambda-CDM.
The most accurate current conclusion is therefore cautious: dark energy and modified gravity differ in what they change—an added component versus the gravitational law—but observations have not yet selected between those explanations. The decisive evidence will come from whether one framework consistently explains expansion, structure growth and lensing across independent data sets.
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