Physicists at the University of Minnesota have published research that challenges one of the foundational assumptions in cosmology: that dark matter must be cold when it first separated from the early universe. The study, published in Physical Review Letters, suggests dark matter particles could have been moving near the speed of light shortly after the Big Bang, only to cool down later and still play their important role in building galaxies.
For the past four decades, scientists have operated under the assumption that dark matter had to be cold—meaning slow-moving—when it "froze out" from the hot radiation filling the young universe. This became accepted wisdom because fast-moving particles would have smoothed out the distribution of matter in the early cosmos, preventing galaxies from ever forming. The new research opens a window onto a much earlier period in cosmic history and suggests this assumption may have been too restrictive.
Background
To understand why this matters, it helps to know what scientists mean by the early universe's timeline. After the Big Bang came a period of rapid expansion called inflation. When inflation ended, the universe entered what physicists call the reheating phase—a chaotic era when the inflaton field, which drove inflation, began decaying into particles. The universe was still extremely energetic during this time, with temperatures far exceeding anything we can create in laboratories.
During reheating, the universe filled with radiation and particles at extremely high energies. Dark matter particles would have been present in this hot soup, interacting with other particles until the universe cooled enough for dark matter to decouple and go its own way. This decoupling process is what scientists refer to as "freezing out."
The conventional understanding held that dark matter must be cold—moving slowly—when this freezing out happened. If the particles were moving too fast, their motion would have smoothed out density variations in the early universe. These variations were essential seeds for the formation of galaxies and the large-scale structures we observe today.
"Dark matter is famously enigmatic. One of the few things we know about it is that it needs to be cold. As a result, for the past four decades, most researchers have believed that dark matter must be cold when it is born in the primordial universe. Our recent results show that this is not the case; in fact, dark matter can be red hot when it is born but still have time to cool down before galaxies begin to form." – Stephen Henrich, lead author and graduate student at University of Minnesota
Key Details
The Minnesota research team focused specifically on dark matter production during post-inflationary reheating. Rather than assuming dark matter had to be cold from the start, they examined what would happen if dark matter particles decoupled while ultrarelativistic—meaning they were moving at speeds close to the speed of light.
The critical insight is that reheating provides time. As the universe expanded after inflation ended, it cooled down gradually. The researchers demonstrated that dark matter particles could separate from other matter while still moving at relativistic speeds, then cool down naturally as the universe expanded—all before galaxies began to form. This cooling process happens because the expansion of space itself reduces the energy of particles moving through it.
The team also explored how this scenario works with different types of dark matter interactions. They examined what happens when dark matter couples to a heavy particle called a Z' mediator. Under these conditions, dark matter could freeze out while ultrarelativistic and still end up cold enough to serve its role in galaxy formation.
Broadening the Possibilities
This research has practical implications for how physicists search for dark matter candidates. By allowing dark matter to start hot, the work opens up a broader range of possible dark matter models. It creates a conceptual bridge between weakly interacting massive particles, known as WIMPs, and feebly interacting massive particles, called FIMPs. These represent different strengths of interaction between dark matter and ordinary matter.
The findings also suggest that dark matter properties could serve as a probe of the reheating era itself—a period of cosmic history very close to the Big Bang that remains poorly understood. By studying what dark matter tells us, physicists might learn more about the universe's earliest moments.
What This Means
This research does not overturn the observation that dark matter appears to be cold today. Galaxies and galaxy clusters move in ways consistent with cold dark matter. What the study changes is our understanding of when and how dark matter became cold.
The implications extend beyond dark matter physics. The reheating phase remains one of the least-explored periods in cosmology. Understanding dark matter production during this era could unlock information about how the inflaton field decayed, what temperatures the universe reached, and how quickly the universe transitioned from the inflationary phase to the radiation-dominated era that followed.
For cosmologists, this work represents a shift in thinking about constraints on dark matter models. Rather than requiring dark matter to be cold at the moment of freezing out, the field can now consider models where dark matter is born hot but cools naturally. This flexibility could help resolve tensions between different observations and theoretical predictions that have puzzled physicists in recent years.
The research also highlights how much remains unknown about the universe's first moments. While scientists have made tremendous progress understanding cosmic inflation and the structure of the universe today, the transition between these two states—reheating—remains relatively mysterious. Dark matter may provide the key to unlocking that mystery.
