Dark matter’s origin is still a puzzle. A new study by Professor Joachim Kopp from Johannes Gutenberg University Mainz (JGU) and the PRISMA++ Cluster of Excellence in cooperation with Dr. Azadeh Maleknejad from Swansea University, suggests that gravity itself may have created dark matter and other faintly interacting particles in the early Universe.
The researchers point out that cosmic perturbations naturally break a symmetry in certain particles called Weyl fermions. This led them to ask: could these perturbations, especially in the form of random gravitational waves, have produced dark matter?
Earlier studies hinted at this idea, but this team shows convincingly that the answer is yes.
Gravitational waves, spacetime ripples, are usually born from violent cosmic events. But stochastic gravitational waves are different: they arise from early‑Universe phenomena that don’t involve massive objects. Their weaker signals merge with the background “noise” of spacetime, yet they are often incredibly ancient. Many of these waves were found to generate during the Universe’s first moments, for example, during matter phase transitions after the Big Bang or through primordial magnetic fields.
Researchers have studied several mechanisms behind their formation, including inflationary gauge fields, cosmic phase transitions, primordial magnetic fields, preheating, and cosmic strings. For decades, such stochastic gravitational wave backgrounds have been a hot topic of research, with their detection prospects widely discussed.
Yet, their possible role in the freeze-in process of dark matter, in which particles slowly accumulate without ever reaching thermal equilibrium, has remained largely untouched. This new study highlights that gap and suggests it may be a promising new avenue.
In this study, the researchers calculated the energy gain of Weyl fermions in a stochastic GW background using the 1-loop in-in formalism. Their results suggest that these gravitational waves could have produced massless or nearly massless fermions. If those particles later acquired mass, they could naturally serve as today’s dark matter.
Professor Joachim Kopp from Johannes Gutenberg University Mainz (JGU) emphasized, “This leads to a new mechanism of dark matter production that has not been researched before.”
In this study, the team introduced a simple phenomenological broken-power-law model for the GW spectrum to study the dark matter production mechanism. Their model captures the behavior observed in simulations of scenarios such as phase transitions and primordial magnetic fields. This allowed researchers to analytically estimate how such waves could have seeded dark matter.
The authors noted, “We expect that our result is generic, but accurately estimating the resulting fermion energy density for other sources of primordial GWs typically requires advanced modeling and simulations, which we leave for future work.”
Kopp said, “Another avenue for future research is the investigation of further possible effects of gravitational waves in the early universe. One example for this would be a mechanism that could account for the well-known difference in particles and antiparticles produced.”
Journal Reference:
- A. Maleknejad, J. Kopp, Gravitational-wave induced freeze-in of fermionic dark matter, Physical Review Letters 136: 131501, 31 March 2026, DOI: 10.1103/lr69-45v8