In recent years neutrino oscillations were confirmed experimentaly by the results of the atmospheric neutrino experiments, solar neutrino experiments, among which especially SuperKamiokande and SNO and the terrestrial neutrino oscillation experiments KAMLAND and K2K. Oscillations effects in the early Universe were analyzed and active sterile oscillation solutions for the

atmospheric neutrino anomaly and electron-sterile large mixing angle solution for the solar neutrino problem were excluded from cosmological consideration based on primordial synthesis of He-4.

We have investigated the case of electron-sterile oscillations effective after the electron neutrino freeze-out. We have found that these oscillations can lead to a considerable energy spectrum distortion and depletion of the neutrino, as well as to

a production of a neutrino-antineutrino asymmetry in the resonant oscillations. These kinetic effects of oscillations were shown to have strong influence on the primordial nucleosynthesis.

Numerical analysis of the cosmological production of He-4 in the presence of electron-sterile neutrino oscillations was provided with all known oscillations effects on BBN accounted for. The maximal overproduction of He-4 due to oscillations kinetic effect was shown to correspond to nucleosynthesis with almost 6 additional neutrino types.

Cosmological constraints on oscillation parameters for different cases of initial population of the sterile state and different overproduction of He-4 were calculated. The constraints for initial zero sterile neutrino population exclude LOW sterile

solution to the solar neutrino anomaly in addition to the excluded in previous works LMA sterile solution. Depending on the value of He-4 overroduction considered, the non-zero initially sterile state can lead either to strengthening or to relaxing of the constraints. For an overproduction of 3% the constraints corresponding to non-zero population of the sterile neutrino strengthen with the increase of the degree of population, while for He-4 overproduction of 5% they become weaker. The dotted curves in the following figure illustrate the cosmological constraints corresponding to 3% (blue) and 5% He-4 (red) overproduction and zero-initial population of the sterile neutrino. The solid curves present the change of the constraints when the sterile state was half-full initially.

More detail and illustrations concerning neutrino oscillations effects and constraints are given here and in the publications below.

D. Kirilova, Cosmological Constraints on Neutrino Oscillations -

Relaxed or Strengthened, Proc. Intern. Symposium on Fundamental and

Computational Physics, 26-29 September, Udine, Italy

D.Kirilova, M.Panayotova, More General BBN Constraints on Neutrino

Oscillations, ICTP report IC/IR/2004/13, 2004.

D. Kirilova, Cosmological Constraints on Neutrino Oscillations,

Proc. of Manev International Conference on Historical and Contemporary

Aspects of Astronomy, Theoretical and Gravitational Physics,

20-22 May, 2004, Sofia, Bulgaria

D.Kirilova, M.Panayotova, Cosmological constraints on neutrino

oscillations for initially non-zero sterile state, presented

at 4th Serbian-Bulgarian Astronomical Conference (IV SBGAC),

21-24 April 2004, Belgrade, Serbia

D. Kirilova, Neutrino oscillations and the early Universe

astro-ph/0312569, Central Eur. J. Phys.2: 467-491, 2004. (review).

Kirilova, D.P.: Neutrino oscillations and the early Universe,

Proc. XIII National Conference of Yugoslav Astronomers,

17-20 Oct. 2002, Belgrade, Publ. Astron. Obs. Belgrade, No.75, 2003,

167-178.

Kirilova D., Chizhov M., Neutrino Oscillations in the Early

Universe, Nucl.Phys. B Proc. Suppl. 100, p. 360-362, 2001; hep-ph/0102114.

Kirilova D., Chizhov M., Lepton Asymmetry Effect on Neutrino

Oscillations and Primordial Helium-4, in "Cosmology and Particle

Physics", AIP Conference Proceedings, Melville, New York, 2001,

v.555, p.433-436; eds. J. Garcia-Bellido, R. Durrer, and M. Shaposhnikov;

astro-ph/0101083 (extended version).

Kirilova D., Chizhov M., Cosmological nucleosynthesis and

active-sterile neutrino oscillations with small mass differences:

The resonant case Nucl.Phys.B591, 2000.

Kirilova D., Chizhov M., Neutrino Degeneracy Effect on Neutrino Oscillations

and Primordial Helium Yield, Nucl. Phys. B 534, p. 447-463, 1998.

Kirilova D., Chizhov M., Cosmological Nucleosynthesis and Active-Sterile

Neutrino Oscillations with Small Mass Differences: The Nonresonant Case,

Phys. Rev. D 58, p. 073004-073014, 1998.

Kirilova D., Neutrino Oscillations and Primordial Nucleosynthesis,

JINR E2-88-301, Dubna, p.1-14, 1988.