Peculiar Velocity and Gravity as Cosmological Probes

Abstract

We summarize the theoretical implications of recent observations of peculiar velocities, represented as (1) empirical models, such as the bulk flow or the Great Attractor infall model and (2) velocity correlation tensor, derived directly from the “raw data”. Our tests have varying power to constrain theories for the growth of cosmic structure. Each of these tests has its own drawbacks and technical limitations. However, one result seems to be particularly robust: the predictions of the canonical biased cold dark matter (CDM) scenario fail all tests by a large margin. A change of the interpretation or a drastic revision of the existing fits to the observational data would be required to save this theory. The isocurvature baryon-dominated models, recently proposed by Peebles (hereafter PIB) fare much better and naturally lead to large scale flows, although some of these models may actually have too much large scale power.

We also consider the magnitude and direction of the observed dipole anisotropy of the galaxy distribution, derived from the IRAS catalogue. In contrast to the peculiar velocity data, constraints on cosmological models, based on this statistic (and the associated estimate of the local peculiar gravity) appear to be rather weak and cannot distinguish between models with a wide variation of large scale power, such as CDM and PIB. Estimates of the density parameter, Ω, based on the IRAS data may be misleading as one cannot be assured that there is indeed neglible power on scales beyond the sample depth.