Statistical characteristics of observed Ly-$\alpha$ forest and the shape   of initial power spectrum
  Properties of $\sim$ 5000 observed Ly-$\alpha$ absorbers are investigated using the model of formation and evolution of DM structure elements based on the Zel'dovich theory. This model is generally consistent with simulations of absorbers formation, accurately describes the Large Scale Structure observed in the galaxy distribution at small redshifts and emphasizes the generic similarity of the LSS and absorbers.   The simple physical model of absorbers asserts that they are composed of DM and gaseous matter and it allows us to estimate the column density and overdensity of DM and gaseous components and the entropy of the gas trapped within the DM potential wells. The parameters of DM component are found to be consistent with theoretical expectations for the Gaussian initial perturbations with the WDM--like power spectrum. We demonstrate the influence of the main physical factors responsible for the absorbers evolution.   The analysis of redshift distribution of absorbers confirms the self consistence of the assumed physical model and allows to estimate the shape of the initial power spectrum at small scales what in turn restricts the mass of dominant fraction of DM particles to $M_{DM}\approx 0.6 - 2$ keV. Our results verify the redshift variations of intensity of the UV background by about $4 - 6$ times and indicate that, perhaps, the available observations underestimate the intensity of this background. 
M. Demia\'nski, A. Doroshkevich & V.Turchaninov