Dark matter remains one of the biggest mysteries in physics and cosmology today. While its gravitational effects are evident, the particle nature and origins of dark matter continue to elude researchers. Now, a new preprint paper proposes an intriguing production mechanism for dark matter dubbed “recycling” .
Overview of the Recycled Dark Matter Mechanism
The recycling mechanism relies on trapping dark sector particles in a false vacuum during a first order phase transition in the early universe. As the universe expands and cools, the false vacuum regions collapse into primordial black holes that eventually evaporate, reproducing the trapped dark particles.
The key aspects of the recycling process are:
- False vacuum trapping: A dark sector scalar field develops a vev, inducing a phase transition. Dark sector particles gain large masses in the true vacuum but remain light in the false vacuum. This results in the particles getting trapped in the false vacuum pockets.
- Black hole formation: The shrinking false vacuum regions compress the trapped particles. Pockets larger than a critical size determined by particle annihilation collapse into primordial black holes.
- Hawking evaporation: The black holes emit Hawking radiation and evaporate before BBN. This releases the trapped dark sector particles, producing ultra-heavy dark matter.
The recycling mechanism relies on two key requirements:
- High scale phase transition > 10^11 GeV to ensure black hole evaporation before BBN
- Multiple dark sector species to prevent annihilation from stopping black hole formation
Details of the Recycling Process
The preprint  utilizes a simple model with a real scalar φ and Dirac fermion χ to illustrate recycling. Key aspects include:
- φ undergoes spontaneous symmetry breaking, acquiring a vev ~10^11-10^15 GeV.
- φ and χ gain masses ~φ’s vev in the true vacuum but remain massless in the false vacuum.
- A portal coupling maintains kinetic equilibrium between the dark and visible sectors.
- Most dark sector particles get trapped in the false vacuum during the phase transition.
- Annihilation into SM particles determines the minimum pocket size that collapses into a PBH.
- All black holes evaporate before BBN, releasing the trapped χ and φ particles.
- φ decays shortly after via the portal coupling, leaving stable χ as the dark matter.
The relic abundance depends on the PBH formation rate, which gets tuned by the phase transition dynamics. The PBH mass distribution can be controlled by the portal coupling.
The recycling mechanism has several novel features:
- Naturally produces ultra-heavy dark matter beyond the unitarity bound
- Achieves correct relic density through exponential suppression of false vacuum pockets
- Results in multiple decoupled species today from one thermalized dark sector
- Gives extended PBH mass functions dependent on IR physics parameters
While recycling offers an intriguing DM production mechanism, many open questions remain for further exploration:
- Alternative dark sector models with multiple stable particles
- Detailed predictions for experimental dark matter searches
- Cosmological signatures like CMB distortions and gravitational waves
- Dynamics of phase transition and PBH formation
- Dependence of PBH properties on model parameters
- Possibility of early matter domination before evaporation
Overall, recycled dark matter links the nature of DM to exciting early universe physics like phase transitions, PBHs, and gravitational waves. As the idea evolves, it may open up new directions for understanding dark matter and the physics of the early universe.