A balloon inflates on its own, even though no one is blowing it up. That's roughly how the accelerated expansion of the universe looks, and to explain it, scientists usually invoke dark energy, an invisible and poorly understood force. But what if the balloon expands because new air molecules are continuously appearing inside it? This work explores exactly that mechanism: dark matter particles (the invisible substance that makes up most of the mass of galaxies) are born directly from the vacuum and, along the way, exchange energy with another cosmic fluid.
The authors examined six different scenarios for this exchange — something like six recipes for how energy flows between two fluids. It turns out that a stable state, where both fluids coexist and expansion accelerates, arises only in three cases: when the exchange is controlled by the density of dark matter and energy flows from it to the second fluid. If the exchange depends on the properties of the second fluid or on mixed parameters, the universe eventually reaches a different outcome — domination by dark matter, but still with acceleration.
This approach is reminiscent of trying to explain a car's acceleration not by a mysterious tailwind, but by the fuel in the tank synthesizing itself. The key takeaway: dark energy might not be a separate entity, but rather a side effect of particle creation and energy redistribution in the cosmos. This doesn't eliminate the mysteries, but it provides a new tool for solving them — without invoking invisible entities.
as written by the authors arXiv:2604.21770
This article opens new window to obtain accelerating scaling attractors without any need of dark energy. We study cosmological dynamics in a two-fluid system where pressureless dark matter (DM) undergoes adiabatic particle creation and exchanges energy with a barotropic fluid. Considering six widely used interaction prescriptions, we formulate the corresponding autonomous systems in a compact phase space and perform a unified dynamical analysis. We find that accelerating scaling attractors, namely late-time states where both fluids coexist with fixed energy fractions, arise only when the interaction is controlled by the DM density and energy flows from DM to the second fluid. Such attractors appear in the global and local DM-based interactions, and in the global mixed case, but are entirely absent when the interaction depends on the second fluid or on local mixed terms, which instead drive the universe to a DM-dominated accelerating phase. These results clarify the unique conditions under which matter creation can mimic dark-energy-like behaviour without introducing a dark-energy component.
If particle creation in space were slightly more active, we would live in a world where galaxies fly apart even faster, and the night sky would look completely different.
In Poul Anderson's novel "Tau Zero," a spaceship accelerates so much that particle creation from the vacuum begins to affect the entire universe — much like in this work.