TESTING THE GRAVITATIONAL-INSTABILITY HYPOTHESIS

We challenge a widely accepted assumption of observational cosmology: that successful reconstruction of observed galaxy density fields from measured galaxy velocity fields (or vice versa), using the methods of gravitational instability theory, implies that the observed large-scal e structures and large-scale flows were produced by the action of grav ity. This assumption is false, in that there exist nongravitational th eories that pass the reconstruction tests and gravitational theories w ith certain forms of biased galaxy formation that fail them. Gravitati onal instability theory predicts specific correlations between large-s cale velocity and mass density fields, but the same correlations arise in any model where (a) structures in the galaxy distribution grow fro m homogeneous initial conditions in a way that satisfies the continuit y equation, and (b) the present-day velocity field is irrotational and proportional to the time-averaged velocity field. We demonstrate thes e assertions using analytical arguments and N-body simulations. If lar ge-scale structure is formed by gravitational instability, then the ra tio of the galaxy density contrast to the divergence of the velocity f ield yields an estimate of the density parameter OMEGA (or, more gener ally, an estimate of beta = OMEGA0.6/b, where b is an assumed constant of proportionality between galaxy and mass density fluctuations). In nongravitational scenarios, the values of OMEGA or beta estimated in t his way may fail to represent the true cosmological values. However, e ven if nongravitational forces initiate and shape the growth of struct ure, gravitationally induced accelerations can dominate the velocity f ield at late times, long after the action of any nongravitational impu lses. The estimated beta approaches the true value in such cases, and in our numerical simulations the estimated beta values are reasonably accurate for both gravitational and nongravitational models. Reconstru ction tests that show correlations between galaxy density and velocity fields can rule out some physically interesting models of large-scale structure. In particular, successful reconstructions constrain the na ture of any bias between the galaxy and mass distributions, since proc esses that modulate the efficiency of galaxy formation on large scales in a way that violates the continuity equation also produce a mismatc h between the observed galaxy density and the density inferred from th e peculiar velocity field. We obtain successful reconstructions for a gravitational model with peaks biasing, but we also show examples of g ravitational and nongravitational models that fail reconstruction test s because of more complicated modulations of galaxy formation.