The effects of collective neutrino-plasma interactions on the linear wave spectrum supported by a magnetized electron-positron plasma in the presence of a neutrino-antineutrino medium are investigated. When a pair-symmetric background neutrino-plasma medium is perturbed by space-charge waves (electrostatic waves associated with electron-positron charge separation), our analysis shows that the neutrino and antineutrino fluids also separate and the pair symmetry of the background medium is broken. The cosmological implications of this pair-symmetry breaking mechanism are briefly discussed.PACS numbers: 13.10.+q, 52.30.-q, 98.80.Cq
3/2MeV rad/sec) is larger than the neutrino-matter collision rate c/ℓ ν by a factor ω p ℓ ν /c ≃ 1.3 × 10 21 T −7/2 MeV , which is larger than unity when the neutrino-matter temperatures are smaller than 10 6 MeV. In addition, the ratio of the plasma frequency to the photon-matter collision rate (c/ℓ γ ≃ 4.8 × 10 17 T 3 MeV sec −1 ) is expressed as ω p ℓ γ /c ≃ 560 T −3/2 MeV , which exceeds unity for temperatures less than 70 MeV. Hence, the short-time evolution of the primordial neutrino-plasma medium (in the temperature range 1 MeV < T < 10 MeV) appears to be dominated by collective (collisionless) effects.