Key takeaways
- One theory proposes a hidden physical realm with its own versions of particles and forces that gave birth to tiny, stable black hole–like objects that would account for all the dark matter observed today.
- The other theory explores whether dark matter could be a product of the universe’s own expansion, created by quantum radiation near the cosmic horizon during a brief but intense post-inflation phase.
- Both theories offer testable, self-contained frameworks based on known physics, continuing UC Santa Cruz’s legacy of linking particle theory with cosmic-scale phenomena to tackle one of the biggest mysteries in science.
Two recent studies by Professor Stefano Profumo at the University of California, Santa Cruz, propose theories that attempt to answer one of the most fundamental open questions in modern physics: What is the particle nature of dark matter?
Science has produced overwhelming evidence that the mysterious substance that accounts for 80% of all matter in the universe exists. Dark matter’s presence explains what binds galaxies together and makes them rotate. Findings such as the large-scale structure of the universe and measurements of the cosmic microwave background also prove that something as-yet undetermined permeates all that darkness.
What remains unknown are the origins of dark matter, and hence, what are its particle properties. Those weighty questions primarily fall to theoretical physicists like Profumo. And in two recent papers, he approaches those questions from different directions, but both centered on the idea that dark matter might have emerged naturally from conditions in the very early universe—rather than dark matter being an exotic new particle that interacts with ordinary matter in some detectable way.
Shadowy origins
The most recent study, published on July 8, explores whether dark matter could have formed in a hidden sector—a kind of “mirror world” with its own versions of particles and forces. While completely invisible to humans, this shadow sector would obey many of the same physical laws as the known universe.
The idea draws inspiration from quantum chromodynamics (QCD), the theory that describes how quarks are bound together inside protons and neutrons by the strong nuclear force. UC Santa Cruz has deep roots in this area: Emeritus physics professor Michael Dine helped pioneer theoretical models involving the QCD axion, a leading dark matter candidate, while research professor Abe Seiden contributed to major experimental efforts probing the structure of hadrons—particles made of quarks—in high-energy physics experiments.
In Profumo’s new work, the strong force is replicated in the dark sector as a confining “dark QCD” theory, with its own particles—dark quarks and dark gluons—binding together to form heavy composite particles known as dark baryons. Under certain conditions in the early universe, these dark baryons could become dense and massive enough to collapse under their own gravity into extremely small, stable black holes—or objects that behave much like black holes.
These black hole–like remnants would be just a few times heavier than the Planck mass—the fundamental mass scale of quantum gravity—but if produced in the right quantity, they could account for all the dark matter observed today. Because they would interact only through gravity, they would be completely invisible to particle detectors—yet their presence would shape the universe on the largest scales.
This scenario offers a new, testable framework grounded in well-established physics, while extending UC Santa Cruz’s long-standing exploration of how deep theoretical principles might help explain one of the biggest open questions in cosmology.
On the horizon
Profumo’s other recent study, published in May, explores whether dark matter might be produced by the universe’s expanding “cosmic horizon”—essentially, the cosmological equivalent of a black hole’s event horizon.
This paper asks, if the universe underwent a brief period of accelerated expansion after inflation—something less extreme than inflation, but still expanding faster than radiation or matter would allow—could that phase itself have “radiated” particles into existence?
Using principles from quantum field theory in curved spacetime, the paper shows that a wide range of dark matter masses could result from this mechanism, depending on the temperature and duration of this phase. Importantly, Profumo said this doesn’t require any assumptions about how the dark matter interacts—only that it is stable and produced gravitationally. The idea is inspired by the way observers near cosmic horizons, like those of a black hole, perceive thermal radiation due to quantum effects.
“Both mechanisms are highly speculative, but they offer self-contained and calculable scenarios that don’t rely on conventional particle dark matter models, which are increasingly under pressure from null experimental results,” said Profumo, deputy director for theory at the Santa Cruz Institute for Particle Physics.
One could say Profumo wrote the book on the quest to understand the nature of dark matter. His 2017 textbook An Introduction to Particle Dark Matter presents lessons that he personally learned and used in his research work from state-of-the-art techniques that scientists have developed over the years to build and test particle models for dark matter.
The book describes the “paradigm of dark matter” as “one of the key developments at the interface of cosmology and elementary particle physics,” and is intended for anyone interested in the microscopic nature of dark matter as it manifests itself in particle physics experiments, cosmological observations, and high-energy astrophysical phenomena.
Connection to UC Santa Cruz
Researchers here have played a key role in cosmology for decades, contributing to the development of the standard Lambda-Cold Dark Matter model — still the best fit to all cosmological data — and to the theoretical and observational study of how structure forms in the universe. In addition, UC Santa Cruz has long supported a close interplay between theory and observation, with strengths in particle physics, astrophysics, and early universe cosmology.
Profumo said these recent publications continue in that tradition, exploring ideas that connect the deepest questions in particle physics with the large-scale behavior of the cosmos. “And they do so in a way that remains rooted in known physics — whether quantum field theory in curved spacetime, or the well-studied properties of SU(N) gauge theories — while extending them to new frontiers,” he said.
Both studies appeared in Physical Review D, the American Physical Society’s premier venue for theoretical particle physics.