A new model for dark energy rewrites the fate of the Universe

All observations show that the Universe is expanding at an ever increasing rate. In order to explain this accelerating expansion, astronomers utilize the concept of dark energy. Something is driving the acceleration of the expansion and it is not clear exactly what that is. So in order to make the dynamics of the Universe fit, dark energy needs to be introduced as a new component of the universe.

Obviously the concept itself is being explored intensively, as the nature of dark energy is one of the greatest scientific mysteries today. Here, it is important to distinguish between the expansion of the Universe itself, which is a natural consequence of the Big Bang – and on the other hand, the acceleration of the expansion – which requires an additional energy component thought to be intrinsic to space itself – dark energy.

Our collaborations’ work is still not definitive, but as more experiments, using entirely different data and methods, find results that point in the same direction, the interest and excitement in the scientific community is building up.

Dark energy differs from the other elements we are trying to fit into the puzzle

What we know about dark energy is that it is smoothly distributed in space and its density stays fairly constant in time. That’s very different from other components of the Universe, such as matter – the planets, stars, gas, galaxies etc, whose presence in space will “dilute” a lot as the Universe expands. You could say that a huge vacuum is building up in space, as the distances between everything in it grow.

In the Standard Cosmological Model, which has been the standard in the field over many decades and has been tested over a wide range of observations, the dark energy density is assumed to be exactly constant, known as a cosmological constant.

If dark energy density is exactly constant, it must be linked to the energy density of space itself, possibly to vacuum energy, so that as the universe expands and more space is created, its density stays constant. Judit Prat Martí explains: “Even in a perfect vacuum, space has a minimum energy. The uncertainty principle, one of the most fundamental laws of physics, forbids perfect emptiness and thus empty space retains an irreducible minimum energy. This energy is the same everywhere, constant and uniform across the entire universe even as the universe expands, and it could be what we observe as dark energy, driving the accelerating expansion of the universe." 

Vacuum energy – the cosmological constant, may not be so constant after all

“However,” Judit explains, “from a theoretical perspective we cannot explain at all the value of the vacuum energy that we measure from observations. It doesn’t match at all the theoretical predictions from quantum field theory. Thus scientists are considering alternative models in which dark energy density is allowed to evolve a little in time” – and when comparing this model with recent observations from the Dark Energy Survey, it seems the model with evolving dark energy fits the observations better.

If it is true that dark energy is actually evolving, so not being a cosmological constant, the implications would be profound. Such a finding would rule out that dark energy is only linked to the energy of empty space, and instead it would point to dark energy being a new kind of dynamic field, filling the cosmos and changing in strength as the universe ages. It would reshape our understanding of particle physics, possibly of gravity itself, and would mark one of the most significant shifts in cosmology in decades.

“Our collaborations’ work is still not definitive, but as more experiments, using entirely different data and methods, find results that point in the same direction, the interest and excitement in the scientific community is building up”, Judit explains.

Dark energy density over cosmic time

Implications for the fate of the Universe?

Our understanding of the fate of the universe depends on dark energy, so determining its evolution in time is crucial to predict the fate of the Universe:

1. If dark energy remains constant and drives the acceleration of the Universe, distant galaxies will recede from us faster and faster, until their light can no longer reach us. They’d be too distant and their light wouldn’t reach us.
2. If dark energy density starts going down, our cosmic fate would depend on how far that weakening goes. If the expansion keeps accelerating, just more gently, distant galaxies would still slip from view eventually, but over far longer timescales. But if dark energy faded enough to stop the acceleration altogether, we would keep them in sight, and gradually glimpse ever more of the cosmos, rather than less.
3. If dark energy density is rising, the expansion of the Universe will go so fast in the end that the fabric of space-time would “rip”.

What happens next?

Judit explains that this recent news piece is very much in its infancy and much is still unclear - but this is the way science works. You put forth a hypothesis based on your best assumptions from observations and models, and you hold it against other results in the area. These particular recent observations come from the Dark Energy Survey – the collaboration put together to exactly investigate this problem, so the interest is growing in the scientific community and more articles are likely to be published in the coming years. So this is an open ended story with sequels. Exactly like most science is.

Link to the scientific article: https://arxiv.org/abs/2605.27221