Large-scale Peculiar Velocity Research Articles

Cosmic strings: Large-scale mass fluctuations and velocity fields

In this paper we calculate the large scale mass fluctuations and peculiar velocity fields in the cosmic string scenario in different dark matter-dominated universes. By using Turok et al.'s string density fluctuation spectra before gravitational growth and the usual linear perturbation theory, we get the mean square root mass fluctuation 〈( δM M ) 2(M)〉 1 2 and obtain the first calculation of large-scale peculiar velocity fields in the cosmic string picture. If we assume galaxies trace the mass distribution, normalization of δM/ M at scale 8 Mpch −1 requires Gμ > 10 −5, which is too large to be allowed by Big Bang nucleosynthesis theory and the observed background radiation anisotropy, where μ is the mass per unit string length. Pancaking at z = 3 in neutrino-dominated universes requires even larger Gμ. The peculiar velocity decreases more slowly with increasing distance than in usual cosmological models, but this is still not enough to explain Collins et al.'s observed data. Our results indicate that cosmic string density fluctuations alone cannot form such large scale structures as observed.

Cosmic string induced peculiar velocities

We calculate analytically the typical peculiar velocities on scales from 10h^-1^ to 60h^-1^ Mpc with cosmic string loops as the dominant source of primordial gravitational perturbations. From this we calculate the joint probability of finding steaming velocities of at least 600 km s^-1^ on large scales and local peculiar velocities of less than 800 km s^-1^. We consider a range in the parameters of βGμ appropriate for both hot (HDM) and cold (CDM) dark matter scenarios. An {OMEGA} = 1 CDM universe is assumed with the loops randomly placed on a smooth background. It is shown how the effects can be estimated of loops breaking up and being born with a spectrum of sizes. It is found that to obtain large-scale streaming velocities of at least 600 km s^-1^ it is necessary that either a large value for βGμ or the effect of loop fissioning and production details be considerable. For CDM with Gμ = 10^-6^, β = 9, the maximum joint probability is 7% and requires the parent size spectrum to be 18 times larger than the evolved daughter spectrum. The string induced velocity spectrum is relatively flat out to scales of 2t_eq_ /a_eq_ and then drops off quickly. The flatness is a signature of string models of galaxy formation. With HDM a larger value of βGμ is necessary for galaxy formation since accretion on small scales starts later. Hence, with HDM the peculiar velocity spectrum will be larger on large scales and the flat region will extend further and lie lower relative to larger scales. A consequence of these effects is that the joint probability can be as much as 5 times larger than in the CDM scenario. If large-scale peculiar velocities greater than 600 km s^-1^ are real, then it is concluded that strings plus CDM have difficulties. The advantages of strings plus HDM in this regard will be explored in greater detail in a later paper.

Formation and Evolution of Galaxies

At the beginning of this review period a number of arguments were put forward against the neutrino model which became popular in 1980-1983: too high a rate of the structure evolution at the non-linear stage and the same difficulty in the galaxy formation. As a consequence, many other schemes of the structure origin have been elaborated: models with “cold” particles, with unstable missing mass, etc. In these models the missing mass is in the form of weakly interacting particles (axion, photino, gravitino, etc.), or of usual particles (e.g., neutrino) but with properties that are out of the ordinary (e.g. instability). However, the standard neutrino model cannot yet be regarded as rejected, the more so in view of the recent data on the large-scale peculiar velocities.

Double inflation - A possible resolution of the large-scale structure problem

A model is presented for the large-scale structure of the universe in which two successive inflationary phases resulted in large small-scale and small large-scale density fluctuations. This bimodal density fluctuation spectrum in an Omega = 1 universe dominated by hot dark matter leads to large-scale structure of the galaxy distribution that is consistent with recent observational results. In particular, large, nearly empty voids and significant large-scale peculiar velocity fields are produced over scales of about 100 Mpc, while the small-scale structure over less than about 10 Mpc resembles that in a low-density universe, as observed. Detailed analytical calculations and numerical simulations are given of the spatial and velocity correlations. 44 references.

The large-scale peculiar velocity field in flat models of the universe

The inflationary universe scenario predicts a flat universe and both adiabatic and isocurvature primordial density perturbations with the Zel'dovich spectrum. The two simplest realizations, models dominated by hot or cold dark matter, seem to be in conflict with observations. Flat models with two components of mass density, where one of the components of mass density is smoothly distributed, are examined, and the large-scale peculiar velocity field for these models is computed. For the smooth component the authors consider relativistic particles, a relic cosmological term, and light strings. At present the observational situation is unsettled, but, in principle, the large-scale peculiar velocity field is a very powerful discriminator between these different models.

Large-scale peculiar velocities from cosmic strings

Recent observations1–3 indicate that large regions of the Universe 60 h−1 Mpc in radius (where h is the Hubble constant in units of 100 km s−1 Mpc−1) may be streaming coherently with velocities up to 1,000 km s−1 with respect to the rest frame determined by the cosmic microwave background. In this letter the peculiar velocities which arise in the cosmic string theory of galaxy formation are estimated using numerical simulations. In contrast to models of formation of structure based on primordial linear energy density fluctuations with random phases, the distribution of velocities is highly non-gaussian on scales smaller than 50 h−1 Mpc. The most likely velocities are approximately scale independent on scales between 5 and 50 h−1 Mpc.

Microwave background anisotropies, large-scale peculiar velocity fields, and clustering evolution in a warm-hot dark matter cosmological model

We study the linear evolution of adiabatic density fluctuations in a model with dark matter made of two kinds of collisionless particles, both relativistic at their decoupling. We obtain the mass variance deltaM/M, the rms large-scale peculiar velocity fields, and the microwave radiation SSA. Agreement with observations is found for a primeval spectral index n = 1, assuming deltaM/M = 10/sup -14.5/ M/sub sun/ today, with nonlinear effects starting at zroughly-equal6. To lower such redshift, either nroughly-equal0.5 or a biased galaxy formation or both are required. In the latter case we expect an evolution of clustering which might be observationally tested.

Scale-invariant density perturbations, anisotropy of the cosmic microwave background, and large-scale peculiar velocity field

The large-scale peculiar velocity field and the large- and intermediate-angular scale anisotropy of the cosmic microwave background are studied in inflationary cosmological models of critical density and containing primordial scale-invariant adiabatic density perturbations. Comparison with recent observations by de Vaucouleurs and Peters Provides tentative support for a cold dark matter scenario in which the dark matter is not appreciably less clustered than the luminous galaxy distribution.