Dark Matter Hunters Need Fresh Answers

There should be five times as much dark matter around us as normal everyday matter, the stuff of stars, planets and people. So the failure to find this missing chunk of the universe—seen most recently in new results from the XENONnT experiment (in which I participated)—has sent physicists scrambling for an explanation. But most such attempts simply tweak an existing theory or turn to battle-worn alternatives just to propose more of the same: huge experimental efforts to build the most sensitive instruments ever, only for them to look at nothing.

If astrophysicists don’t take big, new and different swings at dark matter, the search will only stagnate until it atrophies. Fortunately, the alarming reality that we’re on the wrong track has led some back the drawing board to seek out new avenues for discovery rather than retreading old ground. These more ambitious, sometimes radical and farfetched, ideas, whether they succeed or not, at least attempt to reckon with the dire situation.

A Fuzzier Possibility

As Earth-bound detectors fail, desperate physicists have begun looking to the sky for answers. One new kind of dark matter search would, for example, look for galaxies with distortions symptomatic of “fuzzy dark matter.” This theory considers dark matter particles as so incredibly lightweight that they act like waves instead of particles. Rather than a tiny, localized bundle of mass shooting through the Earth like an invisible bullet, a fuzzy dark matter particle is everywhere in the galaxy at once, like how you can tune into the same satellite radio station in both New York and L.A.

On a galactic scale, fuzzy dark matter forms an underlying bath of undulating waves, buzzing together like a quiet static fuzz. Their combined gravity still holds the galaxy together, but the diffuse, fuzzy particles never smash into an atom like traditional particles, so you can never see them in a detector on Earth—which would explain our experimental failures thus far.

Versions of this idea have floated around for decades, but new energy has poured in recently as theorists realized that this kind of dark matter could have characteristic effects on galaxies that we can look for with the right kind of telescope. For instance, this wavy fuzz would form natural peaks and valleys across the galaxy as a result of the interference of so many wave-particles piled together. By carefully tracking the motion of stars in galaxies, we may be able to see signs of these variations, proving the rippling nature of the galactic glue.

These astronomical signatures make fuzzy dark matter especially attractive in an era of dwindling hope for underground dark matter searches.

Gravitational Detectors

Some ambitious experimentalists are starting to explore the use of modern quantum mechanical sensors to register the tiny gravitational pull of individual dark matter particles zooming through Earth. Dark matter’s gravity, they fear, could be its only connection to the world of regular matter.

But we can only measure particles in a lab if they have another, much stronger interaction than gravity. Since gravity exerts a far gentler tug than any other force, the gravity of individual particles is too tiny to notice. You can only measure it in aggregate, like when all the particles of dark matter in a galaxy pull on a star in unison. This makes gravity-only dark matter a kind of nightmare scenario, as it would be impossible to detect directly. Or so we thought.

Now scientists are trying to think of new technologies that would be sensitive to a much tinier gravitational pull than conventional detectors can see. Current experimental quantum mechanical sensors may only work if dark matter particles happen to be exceptionally heavy. But in the face of the scary but growing possibility that gravity-only dark matter is real, exploring this avenue of research is a necessity that might birth new, inventive technologies that can detect even lighter particles.

Back to Basics

A small but stalwart faction in physics has long held that it’s gravity we don’t understand, not the makeup of the universe. This idea is gaining new traction as dark matter particles evade detection and our main evidence remains gravitational.

One fresh idea in this arena comes from the Dutch theoretical physicist Erik Verlinde, who challenges the idea that gravity is a force at all. Instead, he starts with the observation that the second law of thermodynamics demands that the “entropy” of the universe, a number quantifying how “mixed up” things are, tends to increase. Verlinde claims that the phenomenon we call “gravity” is merely a result of this tendency—an apple on the ground somehow constitutes a more stirred-together world than an apple in a tree.

To explain this, he turns to a gravitational wonder: the black hole. We’ve known since the 1970s that gravity and entropy are related, thanks to black holes. As objects fall into a black hole and it grows larger, its entropy increases. But Verlinde really ran with this gravity-entropy connection, generalizing it to show that by cleverly rearranging the laws of entropy, you can get the familiar laws of gravity.

According to Verlinde, our mistaking of the laws of entropy for a separate force has led us to misunderstand the motion of stars around galaxies. His theory predicts all that movement without recourse to dark matter. The idea has been polarizing; critics have pointed out that the “entropic gravity” theory’s predictions were incompatible with quantum measurements of very cold neutrons, only for other theorists to come to Verlinde’s defense. But in an era of dwindling hope for dark matter, polarizing is good; more people should be thinking big and drawing heat like Verlinde.

Dark matter experiments of the past decade were built because of the beautiful simplicity of the hypothesis they hoped to prove. When they instead ruled it out, a cottage industry sprung up to invent more complicated versions of the hypothesis to explain its evasion of the current detectors, and justify their successors. The motivation is no longer simplicity, but inertia.

This is the nature of science; it’s easier to pivot than completely shift gears. But at some point, it’s time to go big or go home. Our best hope is to push in new directions, and even tear down long-standing pillars of theory, in order to resolve the most embarrassing hole in our picture of the universe.

This is an opinion and analysis article, and the views expressed by the author or authors are not necessarily those of Scientific American.