For less than a century, we have known that the universe is expanding. Only 20 years ago, data suggested that the expansion was accelerated. To explain this, astronomers proposed the existence of dark energy, a mysterious substance responsible for the accelerated expansion.

However, we have yet to find dark energy. Now, a team of researchers from the University of Canterbury, New Zealand, suggest that dark energy might not exist at all. They propose a concept called “timescape cosmology”, where dark energy doesn’t exist. Instead, galaxies might be seeing the expansion of the universe as accelerated or decelerated depending on their location.

The paper, published in the Monthly Notices of the Royal Astronomical Society, says that a flaw in the dark energy hypothesis is the reliance on Friedmann’s equation, the formula that describes the expansion of the universe as a whole.

One requirement of this is called the cosmological principle. This is the idea that on a large scale, the universe is the same everywhere and looks the same in every direction. Obviously, at small scales, we have galaxies, planets, and capybaras, but the universe as a whole appears to be statistically homogeneous and isotropic, according to data. The cosmological principle seems to be respected in broad terms.

But there are a number of observations that conflict with the principle, so maybe the observation and equation only approximate what’s actually going on in the universe. "The past debate missed an essential point; if dark energy does not exist, then a likely alternative is that the average expansion law does not follow Friedmann's equation," senior author Professor David Wiltshire said in a statement.

The researchers claim that the timescape model fits the expansion model just as accurately as the dark energy model, known as Lambda-CDM. The researchers state that even if this model turns out not to be the correct one, it could be useful in testing for selection bias that might not have been take into account in other analyses.

Dark energy is expected to make up 70 percent of the energy content of the universe. To understand if it exists (and its properties), we require not only more observations, but a better understanding of how the objects we use in those observations, like supernovae, actually work.