Nonstandard Leptogenesis via Gravitational Particle Production: Testing with Relic Gravitational Waves ✦
The coincidence of energy scales between inflation and right-handed neutrinos is explored in seesaw models. Predictive inflationary models, verifiable by new cosmic microwave background surveys, are shown to generate right-handed neutrinos in the amount needed to explain the observed baryon asymmetry. The resulting scenario is testable via gravitational waves: primordial ones from inflation, and secondary ones from particle creation. This paves the way for a unified understanding of two fundamental puzzles in modern cosmology.
Context
The origin of the baryon asymmetry of the universe remains one of the biggest puzzles in modern physics. Despite the success of the Standard Model, it doesn't explain why after the Big Bang matter dominates over antimatter. Traditional thermal leptogenesis faces challenges, including very high temperature scales and a lack of falsifiability. A new approach, based on nonthermal production of heavy Majorana neutrinos via gravitational effects during inflation, sidesteps these limitations and forges a link between cosmology, particle physics, and gravitational waves.
Methods
The study used numerical simulations of cosmological gravitational particle production in an inflation model with a quadratic potential, transitioning into a phase of kinetic domination (kination). The equations of motion for Majorana fermions were solved in an expanding spacetime, yielding their density after inflation. Then, a system of Boltzmann kinetic equations, including decays, inverse decays, and scatterings that violate lepton number, was used to compute the final baryon asymmetry. A key parameter is the inflation scale, which simultaneously sets the neutrino production and the amplitude of primordial gravitational waves, measured by the tensor-to-scalar ratio r.
Results
The results show that with a heavy neutrino mass around 10^13 GeV and a similar Hubble scale at the end of inflation, the density of produced particles reaches the level needed to generate the observed baryon asymmetry (Y_B ≈ 10^{-10}). The viable parameter space requires a plasma reheating temperature on the order of 10^9 GeV and a kination duration between 6 and 10 e-folds. The predicted tensor-to-scalar indicator r falls in the range from 2×10^{-4} to the current upper limit of 0.034, with over 90% of the allowed region accessible to future cosmic microwave background experiments such as the Simons Observatory. Importantly, the CP violation necessary for the asymmetry automatically satisfies the Davidson-Ibarra bound, confirming the model's consistency with neutrino masses.
Implications
The link between inflation and Majorana neutrinos opens a new window for testing the theory of the early universe. Since both phenomena are governed by a single energy scale, detecting or constraining relic gravitational waves becomes a direct test of the leptogenesis mechanism. This gives inflationary cosmology the missing falsifiability and deepens our understanding of the origin of baryonic matter.
Future development
Future research could aim to include more realistic models of the end of kination and the transition to the radiation era, as well as extending the scenario to other types of seesaw mechanisms. Of particular interest is the connection between CP violation in leptogenesis and CP violation in neutrino oscillations, which would allow predictions to be tested against data from experiments like DUNE and Hyper-Kamiokande. Moreover, the kination phase amplifies gravitational waves at high frequencies, potentially making them detectable by interferometers such as LISA and DECIGO.
Impact
The proposed scenario will impact cosmology, particle physics, and gravitational-wave astronomy, uniting them in the quest for the fundamental question of how matter originated.
Next steps
Next steps include developing explicit microscopic models of inflation with an exit into kination, and a detailed analysis of the flavor structure of neutrinos for testing in oscillation experiments.
Key open problems
The study connects unresolved puzzles: baryogenesis, neutrino masses, and the nature of inflation, and also touches on the quantum birth of matter in an expanding curved spacetime.
🎯 Interestingly, gravitational particle production is a quantum effect where the expansion of spacetime itself pulls particles out of the vacuum. Even in our universe, this could happen if the Hubble scale were comparable to particle masses, but today it's vanishingly small.
Key numbers
- mass of heavy Majorana neutrino: ~10^13 GeV
- inflation scale: ~10^13 GeV
- reheating temperature: ~10^9 GeV
- tensor-to-scalar ratio r: from 2×10^{-4} to 0.034
- observed baryon asymmetry Y_B: 0.879×10^{-10}