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The Economist

The universe is peculiar. But it may soon become less so

New theories and experiments should shine light on string theory

Never say physicists have no sense of humour. A topic that can dub fundamental particles "strange", "charm", "truth" and "beauty", and propose that such objects are stuck together with bits of sub-atomic adhesive called gluons, cannot be accused of taking itself too seriously. But the true masterpiece of quirky physics nomenclature is, perhaps, the "swampland".

The swampland is the here-be-dragons territory beyond the edges of string theory, a mathematical model of reality in which vibrating strings are the building blocks of nature. As is often true of such models, string theory has many answers. In its case each describes a particular hypothetical universe. Those answers, of which early estimates counted more than 10500, are known collectively as "the landscape".

To contrast explicitly with this conceptual terra firma Cumrun Vafa, of Harvard, coined, in 2005, the term swampland to describe universes beyond the scope of string theory. A big existential question from a physics point of view is thus whether the universe people call home is part of the landscape or part of the swamp. Proving it was on dry land would suggest that string theory might be true. Which would be nice, because a criticism often thrown at this theory is that it is elegant but untestable—at least with tools that humanity might conceivably build today.

Dr Vafa and a group of like-minded enthusiasts have therefore spent the past two decades trying to devise a way to tackle this question. They call their endeavour the Swampland programme. In doing so, they believe they have discovered a manageable subset of string theory’s possible universes that resemble humanity’s, and could thus include it among them.

Hypothetical universes

One of the most annoying unsolved problems in physics is combining quantum mechanics (which describes the very small) and general relativity (which describes gravity as it applies to larger objects). For string theory, this is not an issue. In the hypothetical universes of its landscape, quantum mechanics and gravity are tied together in a combination called quantum gravity. For those in the swampland, by contrast, the two are impossible to reconcile. That is why physicists are so keen to understand what happens where the landscape ends and the swampland begins. By mapping this terrain with their theories, Dr Vafa’s cosmic cartographers are attempting to reveal something of gravity’s essential nature.

Central to their findings is a phenomenon called dark energy. This was discovered in 1998, when astronomers found that the expansion of the universe (itself discovered in the 1920s) was not slowing down, as expected, but speeding up. Something—for which the moniker "dark energy" seemed appropriate—is actively pushing the universe apart.

The initial suspicion was that the density of this dark energy is the same across the whole of spacetime. But in 2024 the Dark Energy Spectroscopic Instrument (DESI) experiment, a piece of kit attached to a telescope in Arizona, reported evidence to the contrary. Its measurements indicated that dark energy is changing.

Banished to the swampland

For mainstream cosmologists, such a thought is hard to stomach—and the conclusion that dark energy is evolving does, indeed, remain contested. But those who had spent their time wading through the swampland were delighted. In 2018 Dr Vafa and his colleagues had put forward a proposal that has become known as the de Sitter conjecture. This says that any and all possible universes with a fixed density of dark energy driving their expansion—the simplest of which, de Sitter spacetime, is called after the Dutch astronomer who proposed the idea—are banished to the swampland. To sit in the landscape, a universe’s dark energy has to be changing. Which is what DESI suggests is happening in humanity’s universe.

Another fact about dark energy is that it is weak. That may sound like an odd description for the thing that is pushing the fabric of the universe apart. But its very ubiquity means it is spread thin, so that at any given point there is not much of it around. The swamplanders have ideas about this, too. Another of their predictions, the distance conjecture, says that a universe whose dark energy is weak should also contain an infinite number of related particles of small mass.

Swamp things

Dr Vafa and his colleagues got to work determining what these particles could be. They found that all other matter should feel the presence of these particles only via the force of gravity, and not any other force of nature. Which is promising, because the observable universe is stuffed with something which fits the bill. It is known as "dark matter". Its nature remains obscure, but its gravity is, among other things, responsible for holding galaxies together, so few doubt its existence. And its total mass is more than five times that of the familiar matter which atoms are made from, so it is clearly abundant.

Dr Vafa’s proposal is that the two swampland conjectures, paired with measurements of dark energy, point to a special region in string theory’s landscape: one that would wrap together three of the universe’s biggest mysteries—dark energy, dark matter and quantum gravity—into a single, neat package.

Ten dimensions

The link between the three depends on one of string theory’s odder predictions: that the universe has ten dimensions rather than just the familiar three of space and one of time. The other six are supposedly "curled up" so tightly as to be imperceptible. To match the dark-energy data, Dr Vafa and his colleagues claim string theory permits only one possibility: one of its six extra dimensions has to be a little less coiled than the rest.

Dr Vafa calls this slightly uncoiled space the "dark dimension". Its uncoiling allows gravitons—hypothetical particles responsible for the gravitational force—to pick up a mass as they pass through it. Those gravitons, he proposes, are the particles of dark matter. Dark energy therefore sets the size of the dark dimension, which in turn sets the mass of the dark matter, which is itself nothing but gravity seeping into the dark dimension. And, tantalisingly, the dimension’s uncoiled nature means it might be observable without the massive (and massively expensive) particle colliders that are the current preferred hardware of particle physics.

The contortionists

The equipment in question is in Austria. There, a team led by Armin Shayeghi of the Institute for Quantum Optics and Quantum Information are building a sophisticated torsion balance—a type of apparatus used in the 18th century to measure the gravitational attraction between two lead spheres. They are searching for departures from the predictions of Newton and Einstein as the range over which a gravitational field is acting shrinks. So far this has been measured down to 30 microns (about half the thickness of a human hair). Dr Shayeghi is trying to push that to ten microns. At that point, according to Dr Vafa’s calculations, the slightly uncoiled dark dimension’s effects might be detectable.

The experiments involved are incredibly sensitive, so Dr Shayeghi’s team are building theirs at the Conrad Observatory, an underground laboratory in the Alps packed with some of the world’s most precise seismic and magnetic instruments. So precise that, as Dr Shayeghi approaches the lab, he says, "They see me coming just from the magnetic field my car produces." He hopes to have findings to report within the next five years.

Not everyone is convinced—even among those who put their faith in strings. The swampland conjectures are still conjectures, and those with the most profound implications, like the de Sitter conjecture, are also the ones with least theoretical evidence. But proponents claim that, in the dark dimension, string theory has made a testable prediction. And for a topic that has been stuck in the mud for half a century, that should be lauded.

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