Will 2024 be remembered as a banner year in the quest to understand the universe, or just an average one? That depends on whether a result from this spring turns out to be real.
In April, physicists detected a hint of a signal suggesting that dark energy, the mysterious energy of space itself, may be weakening. “Hint” is the preferred term because the sign in the heavens isn’t quite robust enough to be called “evidence,” to say nothing of “discovery.” Astrophysicists used the Dark Energy Spectroscopic Instrument (DESI) to map millions of galaxies at different distances in space and time, and from this map they inferred how the universe has expanded over its history. The data confirmed — as we’ve known since 1998 — that the cosmos’s expansion is accelerating, driven by what we call dark energy. But DESI’s data hints that the rate of acceleration has been dropping.
If dark energy is an energy source that can get diluted, it would upend and deepen physicists’ understanding of the fundamental laws of the universe. “If true, it would be the first real clue we have gotten about the nature of dark energy in 25 years,” Adam Riess, one of the Nobel Prize–winning discoverers of dark energy, told Quanta. Theoretical physicists are busy trying to explain why dark energy might change, while DESI logs more data for a more definitive assessment in the coming years.
Kouzou Sakai for Quanta Magazine
Dark Matter Is Dead, Long Live Dark Matter
In the search for the invisible components of the universe, dark matter reached a discouraging milestone. (Fuzzy on the difference between dark energy and dark matter? Read our Fundamentals newsletter from May.) Experimenters hunting for hypothesized dark matter particles known as WIMPs — heavy, inert particles that were long considered the top candidate for the nonreflective stuff floating in and around galaxies — hit a limit. Detectors have become so sensitive that they’re now picking up the glow of neutrinos from the sun, which blinds them to any subtler signals. “So that’s kind of the end of the WIMP detection era,” the Stanford University physicist Natalia Toro told us.
She and other dark matter hunters have switched gears and now seek new dark matter candidates, especially lightweight but abundant particles that would come in multiple species. “The most common hypothesis is that this is somehow simple. Why on Earth should we expect that?” said Philip Schuster, also a Stanford physicist, voicing an increasingly common sentiment among specialists.
Lest you suspect that dark matter is the Ptolemaic epicycle of the 21st century — a long-believed but convoluted and ultimately erroneous model of the universe — astronomers discovered a new reason to think it’s really out there. The finding, an object called MACS J0018.5, has proved so compelling that people are referring to it as the new Bullet Cluster. In the original Bullet Cluster — long considered one of the single most persuasive pieces of evidence for dark matter’s existence — we see two enormous clusters of galaxies crashing together. The colliding gas glows brightly in the center of the crash site, but most of the matter has sailed right through, forming heavy, light-distorting blobs on either side. That’s how dark matter particles would behave, because they don’t (or barely) interact.
MACS J0018.5 is similar, except the galaxy clusters are merging along our line of sight. Researchers effectively pointed a radar gun at them and found that their visible gas has slowed as it collides while the majority of the mass moves faster, unimpeded by the collision.
These merging clusters are hard to explain without invoking the kind of invisible particles we’re looking for.
Kristina Armitage/Quanta Magazine
Astronomical Discoveries
The night sky holds many secrets. The flagship of modern astronomy, the James Webb Space Telescope, beamed down a few more this year, particularly in its observations of faraway objects from the universe’s first billion years. Banana-shaped galaxies, little red dots, grape-like clusters, shockingly big young black holes: Astrophysicists are reveling in the “beautiful confusion” of that formative epoch of cosmic history.
The Webb telescope also enabled a precise new measurement of the universe’s expansion rate, deepening a puzzle known as the Hubble tension. Meanwhile, other telescopes revealed the largest magnetic fields in the universe, hidden organic molecules and the clumpiness of the cosmos itself.
Ibrahim Rayintakath and RuiBraz for Quanta Magazine
Happy Days in the Lab
Moving from the largest stage to the very smallest one, physicists who manipulate atoms, molecules and crystals in the lab have also spent 2024 in the throes of discovery, having achieved astonishing levels of precision and control over their quantum quarries. A team in Innsbruck created a long-predicted exotic state of matter called a supersolid, and even imaged the hallmark “quantum tornadoes” that formed when they stirred an otherwise rigid crystal of dysprosium atoms. Astrophysicists suspect that this supersolid phase might arise inside incredibly dense, fast-spinning stars called pulsars.
Meanwhile, condensed matter physicists studying two-dimensional materials — that is, crystalline sheets of atoms — discovered three new kinds of superconductivity this year, while also mulling over a strange quantum phase of matter in which emergent particles possessing fractions of charge flow around the crystal’s edge. No telling yet whether these phases will prove technologically useful, but that’s always the dream.
Other labs made progress in encoding and manipulating information in arrays of atoms. Once an underdog approach to quantum computing, these so-called neutral-atom quantum computers seem to have suddenly shot to the front of the pack. The ascendant devices yielded a landmark result in November, achieving a noise-resistant, or “fault-tolerant,” logical computation.
Moreover, for decades, physicists have sought to pinpoint the energy of a special nuclear transition in thorium, knowing it could serve as a tool to probe the fundamental forces that bind the universe. This year, three different groups finally succeeded in measuring this “nuclear clock” transition, which they plan to monitor to look for variations in the strength of those fundamental forces.
Señor Salme for Quanta Magazine
A Peek Beneath Space-Time
Theoretical physicists have made progress of a more abstract kind. They’ve developed a new geometric language for predicting the outcomes of particle interactions. Traditionally, they use equations that describe these interactions as dynamical events playing out in space and time according to quantum rules. Using the new method, answers seem to flow from sets of curves on surfaces. These breakthrough insights are part of an effort to discover the fundamental underpinnings of space and time themselves — the subject of “The Unraveling of Space-Time,” a nine-part special issue we published in September.
For another deep dive into a deliciously profound subject, check out our multimedia exploration of entropy, which examines how the evolving understanding of this quantity has reframed the purpose of science and our role in the universe.
Kouzou Sakai for Quanta Magazine
All Riled Up
Physics-related discussions on X (formerly known as Twitter) are pale shadows of what they used to be, but lively chatter did ensue from one bit of physics news, when Scientific American reported that a quantum physics experiment had detected evidence of “negative time.” What, exactly, is that supposed to mean? Had something really taken less than no time at all? Not exactly. In the quantum world, words often fail.
What happened was that physicists at the University of Toronto shot photons toward a cloud of rubidium atoms. Each photon might excite an atom in the cloud, or go straight through without interacting, or both. These quantum possibilities interfered like two waves. Then the researchers could determine that some photons went through the atom cloud faster when they got absorbed and reemitted than when they didn’t, implying a “negative dwell time,” as if these photons excited the atoms for a negative amount of time — but again, these are just words. “We are measuring a duration, not something finishing before it starts,” one of the researchers involved tried to explain on X.
Eyebrows also shot up in October when the 2024 Nobel Prize in Physics went to pioneers of artificial intelligence — a technology that seems, on its face, unrelated to the laws of nature. “I’m flabbergasted,” one of the recipients, the computer scientist Geoffrey Hinton, told Science. Yet in the 1980s, he and the other winner, John Hopfield, closely modeled their rudimentary artificial neural networks on systems in statistical physics.
Some statistical physicists were pleased by the attention given to their obscure research on the behavior of systems of many parts. “For us, it’s super-great,” Aurélien Decelle told Science. “It’s recognition at the broader level that what we’re doing matters a lot.”
