Dark matter is the invisible scaffold that holds galaxies together. But how did it come to be? The standard answer: particles of dark matter once stopped colliding and "froze out" in the amount we see today. However, there's a trickier path — UFO, or ultra-fast freeze-out.
Here, everything is like coffee that cools down and then heats up again as you carry it through a cold corridor. In the usual picture, dark matter freezes out at a temperature roughly ten times smaller than its own mass. But in the UFO scenario, the Universe's temperature behaved unusually: it didn't drop smoothly but in jumps, as if a radiator kept turning on and off. Because of this, particles could not just freeze out but also start being born again, as if someone splashed boiling water into the coffee.
The authors generalized this mechanism to any possible cooling rate. It turned out that depending on how fast the Universe expanded after the Big Bang, dark matter could be either very light or super-heavy — up to 10 million proton masses. And if, at the very beginning, gravity itself produced particles from the vacuum, then the usual constraints almost disappear. This scenario was named GUFO — gravitationally enhanced ultra-fast freeze-out.
This approach links the properties of dark matter to what happened in the very first moments after the Big Bang. If future observations point to super-heavy particles, GUFO could become the leading explanation.
as written by the authors arXiv:2606.25033
We investigate ultrarelativistic freeze-out (UFO) in the context of generic reheating scenarios. While the standard WIMP dark matter paradigm has been extensively studied, UFO has so far only been analyzed within the specific reheating channel $$\phi \rightarrow f\bar{f}$$. Unlike in the standard WIMP mechanism, where dark matter can only be diluted after freeze-out at $$T_\mathrm{FO} \sim m_\chi/\mathcal{O}(10)$$, UFO dark matter can undergo freeze-in like phases following the initial freeze-out, driven by the non-trivial temperature evolution. The exact temperature evolution then needs to be accounted for, as a change in the temperature scaling can modify the IR/UV nature of UFO, impacting the relic abundance. We first generalize UFO to an arbitrary temperature profile $$T \sim a^{-\xi}$$, making explicit the UV and IR regimes for a thermally averaged cross section $$\langle \sigma v \rangle \sim T^n / \Lambda^{n+2}$$. Then, as a concrete example, we consider the minimal scenario in which gravitational particle production at the onset of reheating sources an initial radiation abundance, and show that this early hot bath changes the UFO parameter space. We refer to this effect as GUFO. Specializing to $$n = 2$$, we find that matter-like reheating ($$V \sim \phi^2$$) accommodates dark matter masses up to $$10^7~\mathrm{GeV}$$ for $$\Lambda \lesssim 10^9~\mathrm{GeV}$$ as thermalization becomes less stringent, while radiation-like reheating ($$V \sim \phi^4$$) is compatible with GUFO across all reheating channels only if gravitational processes are taken into account.
In the GUFO scenario, dark matter can be so heavy that a single particle weighs as much as 10 million protons — about like a tiny speck of dust, but completely invisible.
In Peter Watts's novel Blindsight, invisible matter turns out to be a form of alien intelligence hidden from view, but here, it may be born from gravity itself.