Constraints on the Models for Structure Formation from the Abundance of   Damped Lyman Alpha Systems
  Models for structure formation attempt to predict the power spectrum of density perturbations in the present universe from the initial power spectrum and the nature of dark matter. Observational constraints on the power spectrum at different scales in the present epoch can, therefore, be used to eliminate (or choose between) different theoretical models. Such a comparison is fairly easy at large scales (at which linear theory is valid), and one can use observations like the MBR anisotropy, large scale steaming motions etc to constrain the models. But to discriminate between the models effectivley, it is necessay to constrain the power spectrum at small scales. The most reliable constraints on the power spectra at small scales come from the predicted abundance of bound systems which can be estimated reasonably accurately using Press-Schecter (or similar) methods$^1$. In the past, this method has been used in conjunction with the quasar abundance$^{2-4}$ and cluster abundance$^{5-7}$. We show here that the abundance of damped lyman alpha systems (DLAS, hereafter), provides a far stronger constraint on the models for structure formation. Models with a mixture of hot and cold dark matter $^{8-11}$ (which are consistent with large scale observations) are strongly ruled out by the DLAS constraints while models with cosmological constant $^{12}$ are marginally inconsistent. It is also possible to combine the constraints from the abundance of clusters, DLAS and QSO's to obtain model-independent bounds on the power spectrum at the nonlinear scales. These bounds are to be respected by any viable model for structure formation. 
K.Subramanian and T.Padmanabhan