Why is there more matter than antimatter?
The observable universe contains essentially no primordial antimatter: for every ~10^9 photons there is roughly one baryon, and zero antibaryons. Sakharov showed this requires baryon-number violation, C/CP violation and departure from equilibrium — but the measured CP violation in the Standard Model is far too small.
Why it is hard
The asymmetry was fixed in the first microseconds, at energies unreachable by colliders and erasable by later cosmology. Electroweak baryogenesis needs a first-order phase transition the 125 GeV Higgs doesn't provide; leptogenesis is elegant but its key signatures (heavy right-handed neutrinos, lepton-number violation) may lie beyond direct reach.
Current evidence
No primordial antimatter domains are seen in cosmic rays or via annihilation signatures at galaxy-cluster scales. Neutrino oscillations prove lepton-sector mixing, keeping leptogenesis viable; neutrinoless double-beta decay experiments are searching for the required lepton-number violation.
Possible approaches
Neutrinoless double-beta decay ton-scale experiments, precision CP-violation measurements at DUNE/Hyper-K, gravitational-wave signatures of a first-order electroweak transition, and new searches for electric dipole moments.
Related topics
Sources
- Planck and the cosmic microwave background — European Space Agency
- Research Highlights — IceCube Neutrino Observatory
- Hubble Big Bang — NASA Science
- COBE Science — CMB Blackbody Spectrum — NASA Science