Oliver F. Piattella, Davi C. Rodrigues, Júlio C. Fabris, José A. de Freitas Pacheco
Solutions of Vlasov-Einstein equation for collisionless particles (representing dark matter) are discussed in the context of a flat Friedmann universe. It is shown that, after decoupling from the primordial plasma, the dark matter phase-space density indicator Q remains constant during the expansion of the universe, prior to structure formation. This well known result is valid for non-relativistic particles and is not "observer dependent" as in solutions derived from the Vlasov-Poisson system. In the linear regime, the inclusion of velocity dispersion effects permits to define the physical Jeans length for collisionless matter as a function of the primordial phase-space density indicator Q, i.e., lambda_J = (5pi/G)^(1/2) Q^(-1/3) rho^(-1/6). The comoving Jeans wavenumber at matter-radiation equality is smaller by a factor of 2-3 than the comoving wavenumber due to free streaming, contributing to the cut-off of the density fluctuation power spectrum at the lowest scales. The physical differences between these two scales are discussed. For dark matter particles of mass equal to 200 GeV, the derived Jeans mass is 4.3 x 10^(-6) solar masses.
View original:
http://arxiv.org/abs/1306.3578
No comments:
Post a Comment