Dark energy bombshell sparks race to find a new model of the universe

In the wake of bombshell findings that suggest dark energy might be weakening as the universe expands, physicists are considering replacing the standard cosmological model of the universe with exotic new theories that say gravity works differently to how we thought. Ideas involving string theory, a new fundamental force or even a form of gravity that changes over time are all options on the table.

Our best model of the universe is called lambda-cold dark matter (LCDM), which splits the cosmos into three parts: the matter we can see, the matter we can’t see that still has a gravitational pull – known as dark matter – and dark energy, a persistent repulsive element that is forcing the universe to expand increasingly quickly. In equations first devised by Albert Einstein, the acceleration of the universe is thought to have a fixed rate known as the cosmological constant, represented by the Greek letter lambda.

This fits almost all of our observations of the universe so far, but the Dark Energy Spectroscopic Instrument (DESI) in Arizona, which has built the largest 3D map of the cosmos by tracking millions of galaxies across the sky, has called the model into question. DESI precisely measures the distances between galaxies at different times in the universe’s history, allowing cosmologists to calculate how quickly the universe is expanding.

Last year, it found the first hints that dark energy isn’t a constant and that the universe may be accelerating less quickly over time. These initial results were tentative, but a second release of DESI findings in March, covering three years’ worth of data, strengthened those hints, though it still fell short of the required statistical certainty needed for a conclusive discovery. “Everybody was watching this data release from DESI, and it was pretty shocking,” says Yi-Fu Cai at the University of Science and Technology of China.

“This is exciting – it might actually be putting the standard model of cosmology in danger,” says Yashar Akrami at the Autonomous University of Madrid in Spain.

While researchers from the DESI group suggested tweaking LCDM to fit their findings by adding new adjustable numbers to it, Akrami and his colleagues argue that these tweaks aren’t grounded in any physical reality. They instead found that they could better describe the distance changes that DESI is seeing by describing dark energy as a field, similar to the ones that describe light or the nuclear forces, but smoothly changing over time.

One advantage of this idea, called a quintessence field, is that its existence can be derived from more complete models of the universe, such as certain string theory models. “If you prove that quintessence is dark energy, it’s very good for [string theorists],” says Akrami. “That’s why the string theory community is really excited now.”

It is also good news for other theories of quantum gravity, which have long grappled with the prediction based on LCDM that the universe’s expansion will accelerate forever, says Cai. Many of these theories say that, in a constant-acceleration scenario, quantum ripples can expand over time and eventually become as large as the universe itself. This is incompatible with a complete description of the universe, but a changing dark energy would make many of these theories more viable, he says.

Akrami and his team’s quintessence model doesn’t interact with gravity in any meaningful way, but other researchers have found that models of dark energy that do interact with gravity fit the data better. Gen Ye at Leiden University in the Netherlands and his colleagues have suggested a model called thawing gravity, which says that gravity as described by Einstein’s general relativity will begin to work differently at some point in time through its interactions with the changing dark energy field. This can help explain a problematic new implication from DESI that the energy density of the universe must increase over time if dark energy becomes weaker, says Ye.

Pedro Ferreira at the University of Oxford and his colleagues have found that a different quintessence model of dark energy, one that also interacts with gravity, describes what DESI appears to be measuring. “It seems to be, in my view, the only way that can be done,” he says.

This would be a radical change to how we think about the universe, says Ferreira. “We’ve always grown up thinking about the universe as having the gravitational force, and gravity fuels everything,” he says. “But now there’s going to be an additional fifth force, which is due to the dark energy, which also fuels everything.”

One major problem with this adjustment, however, is that we should have seen evidence of this fifth force in precise measurements such as planetary orbits and tests of relativity on Earth. So we would also need to invent a physical principle that prevents us from measuring the effects of that fifth force in these systems, says Ferreira. “Physics ends up being even more complicated than we thought it could have been, and that kind of makes you wonder, why do you want to go down that route?”

Unfortunately, the current DESI data isn’t precise enough to discriminate between the different models of dark energy interacting with gravity, says Ferreira. “Two or three or four or five people come up with different theoretical explanations for it, and there will be no way using this data to distinguish between these theoretical proposals,” he says. “What we’ll end up doing is having these debates based on opinion on what’s the right theory, and we’ll never resolve it.”

Ye is hopeful that there will be a way to determine which idea is correct. If gravity is altered in subtle ways by dark energy, then it might show up in other observations, such as weak lensing surveys, which measure how light is bent by galaxies as it travels through the universe. “If thawing gravity is correct, it will change how light is bent by big masses,” he says. “The bending effect will be different from general relativity.”

But concluding that dark energy is changing at all is still premature, most cosmologists agree, until we get more data showing this. Some clarification will come from DESI data, which is due in the next couple of years, and further information will come from other dark energy surveys, such as the European Space Agency’s Euclid satellite.

Mysteries of the universe: Cheshire, England

Spend a weekend with some of the brightest minds in science, as you explore the mysteries of the universe in an exciting programme that includes an excursion to see the iconic Lovell Telescope.