Dark energy is central to our modern understanding of cosmology. In the standard model, dark energy is what drives the expansion of the Universe. In general relativity, it’s described by a cosmological constant, making dark energy part of the structure of space and time. But as we’ve gathered more observational evidence, there are a few problems with our model. For one, the rate of cosmic expansion we observe depends on the observational method we use, known as the Hubble tension problem. For another, while we assume dark energy is uniform throughout the cosmos, there are some hints suggesting that might not be true. Now a new study argues we’ve got the whole thing wrong. Dark energy, the authors argue, doesn’t exist.
Let’s start with what we know. When we look out across the billions of light-years of cosmic space, we see that matter is clumped into galaxies, and those galaxies are groups into clusters so that the Universe has clumps of matter separated by great voids. On a small scale, this means that the distribution of matter is uneven. But as we go to larger scales, say a billion light-years or so, the average distribution of matter evens out. On a large scale, the cosmos is homogeneous and not biased in a particular direction. This means we can broadly describe the Universe as the same everywhere. This is known as the principle of homogeneity. By applying this principle to cosmic expansion, we can model the Universe by the Friedmann–Lemaître–Robertson–Walker (FLRW) metric, where dark energy is a cosmological constant.
Opponents of the standard model argue that the principle can’t be applied to cosmic expansion. Some even argue that the basic principles of general relativity can’t be applied on cosmic scales. In one such model, known as the Timescape model, it’s argued that dark energy would violate the principle of equivalence. Since the principle equates inertial energy and gravitational energy, there is no way to distinguish cosmic expansion as a real effect. Furthermore, since we know that gravitational fields affect the rate of time, the Timescape model argues that the Universe can’t be homogeneous in time. Basically, the model argues that within the gravitational well of a galactic cluster, clocks would run more slowly than they would within the vast empty cosmic voids. Over the billions of years of cosmic history, this difference would build up, creating a variance of time throughout the Universe. It is this time divergence that would give the appearance of cosmic expansion.
In this latest study, the authors use the Pantheon+ dataset of Type Ia supernovae to see if it better fits the standard cosmological model or the Timescape model. The main difference between the two models is that cosmic expansion must be uniform in the standard model, while in the Timescape model, cosmic expansion can’t be uniform. What the team found was that while the Pantheon+ supports both models, the data is a slightly better fit to the Timescape model. In other words, the best fit of the data suggests that dark energy is an illusion, but the fit is not strong enough to disprove the standard model.
If future observations continue to support the Timescape model, it would revolutionize our understanding of the Universe. But there are reasons to be cautious. To begin with, the Timescape model is only one of many proposed alternatives to the standard model, which this study doesn’t address. The Timescape model also has some internal issues of its own that would need to be resolved to become the new cosmological model. But it is clear now that we can’t ignore the fact that the standard model may be wrong. We are entering an exciting period of astronomy where our knowledge of the Universe will increase significantly in the near future.
Reference: Seifert, Antonia, et al. “Supernovae evidence for foundational change to cosmological models.” Monthly Notices of the Royal Astronomical Society: Letters 537.1 (2025): L55-L60.
Reference: Wiltshire, David L. “Cosmic clocks, cosmic variance and cosmic averages.” New Journal of Physics 9.10 (2007): 377.