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Possibly—but current evidence does not show that modified gravity is the explanation. The idea is one way to account for the universe’s accelerated expansion, alongside dark energy within general relativity. DESI observations have prompted discussion about whether dark energy changes over time, but the reported evidence was not definitive and does not by itself identify a new law of gravity.
What does “modified gravity” mean in this debate?
In the standard cosmological account, the universe’s accelerated expansion is associated with a cosmological constant or another component called dark energy. Modified-gravity theories instead ask whether gravity behaves differently on the largest cosmic scales than it does in the settings where it has been tested most precisely. If it does, that change could affect how cosmic expansion is interpreted.
These are broad categories, not two perfectly separate boxes. Some proposals combine features of modified gravity and unusual forms of matter or energy. The 2016 review Dark Energy Versus Modified Gravity by Austin Joyce, Lucas Lombriser, and Fabian Schmidt discusses ways the approaches can differ in their observable signatures and in how they treat equivalence principles.
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What has DESI reported about dark energy changing over time?
An Associated Press report published November 19, 2024, and updated November 20, described DESI’s map of galaxy clustering across more than 11 billion years of cosmic history. The findings could indicate that dark energy is changing or weakening, but the report emphasized that the result was not definitive and that more observations and analysis were needed.
That distinction matters: the observations concern the distribution of galaxies and its cosmological interpretation. They are not a direct measurement of a new gravity law. Even if later evidence establishes that dark energy evolves, that would not alone show whether the cause is a changing energy component, modified gravity, or another viable model.
How do the main explanations differ?
| Question | Dark energy within general relativity | Modified gravity |
|---|---|---|
| What changes in the model? | An added component, such as a cosmological constant or an evolving form of dark energy, accounts for accelerated expansion. | The model changes gravity’s behavior on cosmological scales, potentially changing the inferred cause of acceleration. |
| What can observations test? | The model’s predicted expansion history and structure growth. | The model’s predicted expansion history and structure growth, along with whether it remains consistent with other gravity tests. |
| Can one observation identify the explanation? | Not necessarily; multiple viable models may fit the same evidence. | Not necessarily; modified-gravity proposals can also resemble other viable models. |
The comparison reflects the framework discussed by Joyce, Lombriser, and Schmidt (2016) and the assessment by Chris Smeenk and James Owen Weatherall (2024). It is not a claim that every theory in either category makes the same predictions.
What would it take to distinguish the explanations?
Cosmologists need to compare each candidate’s predictions with observations, rather than infer a mechanism from a possible change in dark energy alone. The key questions include:
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- Structure growth: Does it correctly describe how cosmic structure develops, as well as the expansion itself?
- Other gravity tests: Can the proposed change on cosmological scales remain consistent with constraints from other tested regimes?
- Distinctiveness: Do the observations separate this candidate from alternatives, or can several viable models produce similar effects?
Smeenk and Weatherall’s 2024 analysis cautions that viable dark-energy and modified-gravity proposals have not been cleanly separated by observational comparisons. They also discuss the roughly 14 orders of magnitude between the best-tested gravity regimes and cosmological scales—a challenge of extrapolation, not a direct measurement of a change in gravity.
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What can be said about the evidence now?
The careful conclusion is that modified gravity could help explain cosmic acceleration and is a candidate framework for interpreting a changing dark-energy signal. The DESI-related report does not establish that dark energy is weakening, that general relativity has failed, or that any particular alternative theory is correct. A 2024 paper by Salvatore Capozziello, Anupam Mazumdar, and Giuseppe Meluccio specifically scrutinizes whether nonlocal gravity could address the dark-energy problem; it is a theoretical investigation, not observational confirmation.
For scale, Smeenk and Weatherall’s 2024 article reports that the cosmological constant accounts for roughly 70% of the total mass-energy density in the standard model. That is contextual reporting in their article, not a new measurement by its authors.
Quick Recap
Sources
- Associated Press, “The dark energy pushing our universe apart may not be what it seems, scientists say,” first published November 19, 2024; updated November 20, 2024.
- Austin Joyce, Lucas Lombriser, and Fabian Schmidt, “Dark Energy Versus Modified Gravity,” Annual Review of Nuclear and Particle Science 66 (2016), 95–122.
- Chris Smeenk and James Owen Weatherall, “Dark Energy or Modified Gravity?,” Philosophy of Science 91 (2024), 1232–1241; published online in 2023.
- Salvatore Capozziello, Anupam Mazumdar, and Giuseppe Meluccio, “Can nonlocal gravity really explain dark energy?,” Physics of the Dark Universe 45 (July 2024), article 101517.
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