“If you were blind and I told you that there are ‘magical’ particles of light called ‘photons’ all around you,” ponders Swati Singh, “what evidence would convince you that these photons are real?”
It’s this kind of question that has inspired Singh, a quantum optical physicist and engineer at the University of Delaware, in Newark, and others, to search for novel ways to detect a different particle that is veiled from all our views. Astronomers have inferred that this so-called “dark matter” likely makes up 85 percent of all matter in the universe. “We are probably swimming in a sea of dark matter,” says Singh; yet nobody has had any noticeable experience of interacting with it directly, and its identity remains a mystery.
Astronomers first posited the existence of an unknown and invisible kind of matter in 1933, when Fritz Zwicky examined the super-fast motion of galaxies within the distant Coma galaxy cluster. The galaxies appeared to be zooming around so quickly that they should have flown out of the cluster; the fact that they hadn’t suggested that they were strongly bound by gravity holding them in. But adding up the pull from the visible stars and matter in the cluster could not account for the huge amount of gravity needed. Zwicky postulated that the galaxies must contain some other kind of dunkle Materie, or dark matter.
Almost a century on, astronomers monitoring the motion of galaxies, clusters and other astrophysical features have gathered ever more evidence that dark matter is real. But while most are certain that there is a lot of the weird substance around, they do not know its form.
“I find it incredibly humbling that we don’t know what most of the universe is made of,”
Particle physicists at the world’s largest accelerator, the Large Hadron Collider, near Geneva in Switzerland, are busy smashing beams of particles together at high energies, in part in the hope of creating dark matter. In July 2023, the Euclid spacecraft launched, carrying a space telescope to make more detailed observations in an effort to elucidate the nature of the dark universe. But while these billion-dollar experiments are hugely valuable, there is no guarantee they’ll solve the dark matter puzzle, given the huge spectrum of identities the elusive particle might take. The range of possible values for its mass, from the smallest to the largest predicted size, spans 50 orders of magnitude (or 1 followed by 50 zeros), notes Andrew Geraci, a physicist at Northwestern University in Evanston, Illinois. “There simply isn’t one device or one experiment that can test all this range at once,” he says.
That means that there is room for other physicists to venture along more modest avenues to try and detect the presence of dark matter in the lab.
“If it really does surround us, dark matter can only interact extremely weakly with normal visible matter, otherwise we would have encountered it directly already,” says Singh. That means physicists need highly sensitive apparatus that is capable of sensing tiny effects. In recent years, physicists have been developing ever more precise measurement techniques using table-top experiments. The trick is repurposing such sensors to tackle the dark matter conundrum. “You could accuse us of having a hammer and then seeing everything as a nail,” laughs Singh. “But I think it is amazing that with a small team of postdocs and students, you may be able to build a detector that could one day find dark matter.”
Geraci, for instance, has been on the hunt for a kind of ultra-light dark matter candidate, trillions of times smaller than an electron, that has a wave-like character, just like light. A flow of these particles in the vicinity of ordinary matter would create a faint fluctuation in the mass of ordinary matter. “There would be a wiggle on top of the mass of the electron, which would affect the radius of an ordinary atom,” says Geraci.
“It’s like the atoms are breathing,”
he adds, which would cause a material to gently expand and contract.
To search for signs of these breaths, the team uses a tube of glass bounded at both ends by mirrors. Laser light shone into this cavity will bounce back and forth between the mirrors. If the glass starts to breathe, the cavity will subtly change length, affecting the path length of the light in a detectable manner. The tough part is that other effects, like heat in the room, might also cause expansion or make the mirrors move, so the experiment must be carried out in a liquid helium cryostat. “Even the cooling system itself, the cryogenics, create vibrations, so the engineering is not trivial,” says Geraci.
Geraci’s team is hoping to get preliminary data in the next few months. Meanwhile, partially inspired by Geraci’s early work in the area, Singh and colleagues have proposed other tests for dark matter. One uses an object known as an optomechanical resonator that works a little like a tuning fork, with prongs made of two different materials. Dark matter would pull on one prong more than the other, because the atoms of the two materials have different mass. This would create a detectable vibration, which is currently being searched for in multiple experimental groups, including at the University of Arizona, in Tucson, led by physicist Dalziel Wilson.
Another of Singh’s proposals works on the principle that superfluid helium would be stretched or pulled if dark matter particles pass through it. The HeLIOS experiment is underway at the University of Alberta, in Edmonton, led by physicist John Davis, and preliminary results are imminent.
Singh notes, however, that just spotting signs of deformation due to one passing particle won’t be enough to seal the deal and declare a detection. Instead, any team must find and analyze multiple detections, and look for patterns in the timing between the particles. For instance, astronomers know that when another galaxy—presumably carrying its own dark matter particles—merged into ours in the past, it would have triggered a stream of its own dark matter particles through our galaxy. “If dark matter does not interact much with normal matter, then it will hold on to the memories of its creation and journey through the cosmos a lot longer than normal matter,” says Singh. “This information will be encoded in the timing between detector clicks when we finally see it.”
Of course, the experiments may not find any signs of dark matter. But even that could provide profound insights, notes Singh, because it will tell physicists more about what dark matter is not.
That will leave the door open for new ideas for experiments from the younger generation. “What’s clear is that to solve this we will need intellects from across different fields: optics, astrophysics, engineering, computer science, and more,” she says.
Geraci agrees with Singh’s call. “We need diverse perspectives to come together to test the huge range of possible forms that dark matter may take,” he says. “There are a lot of rocks that have yet to be turned over.”