Estimating cosmological bulk flows:cmb vs. luminosity function methods

Abstract

In the framework of the standard cosmological model, a leading role is played by the gravitational instability theory. Matter density perturbations, in the early epoch of our Universe, have been evolved under the effect of their own gravity, substantially decoupling themselves from the general expansion of the Universe as a whole, giving rise to the cosmic structures that we can see today. Because of the external gravitational forces acting on them, our own Galaxy and its neighbors, i.e. the Local Group, have an induced peculiar velocity with respect to the CMB rest frame. Amplitude and direction of the Local Group’s velocity are well known quantities from the CMB sky observations. This motion, in fact, induces a dipole in the temperature of the CMB sky, TCMB = 3:355 0:008 mK, from which a velocity of the Local Group of vLG = 627 22 km/s have been obtained, toward direction (l; b) = (276 3 ; 30 3 ). One of the most studied subjects of the observational cosmology is the convergence depth of this motion. As come out from theory, if we are able to estimate the coherent motion of a region large enough, we should measure a null value of the velocity amplitude of such region as a whole with respect to the CMB frame. So that, this volume should contains all the mass sources responsable for the Local Group motion. We refer to this coherent motion caused by gravity as bulk flow. The Cosmological Principle garantees large scale homogeneity and isotropy and thus implied that, on large enough scales, the bulk flow should vanish. When that bulk flow is extended to region too large to be justified by the gravitational instability model, it is termed dark flow. So far, many studies have been performed to estimate its amplitude, direction and convergence depth (the scale at which the bulk flow is consistent with zero) based on different techniques. The one widely used relies on the measurement of the galaxy peculiar velocities inferred from distance indicators. These type of studies have a long history and yet there is still no consensus on what is the convergence scale of the bulk flow in our local Universe. The most recent results still show a significant desagrement. Feldman et:al: in 2010 found a bulk flow of 416 78 km/s on scale of 100h􀀀1 Mpc, too large to be accounted for in the standard CDM cosmological model. On the contrary Nusser & Davis in 2011, using a different technique to analyze a trimmed version of the same dataset on the same scale, found that the bulk flow has amplitude of 257 44 km/s in agreement with model’s prediction. This outstanding dichotomy has, however, been succeeded by an even more surprising result. From the analysis of the CMB temperature fluctuations maps Kashlinsky et:al: detected a dipole-like anisotropy that they attributed to the presence of a large bulk flow of 600 􀀀 1000 km/s on the gigantic scale of 400 700 h􀀀1Mpc. This result 3 is in strong conflict with CDM prediction and, perhaps, with the gravitational instability pictures and the Cosmological Priciple itself. Addictional methods to measure the bulk flow indipendently on peculiar velocity or CMB technique maps have been recently proposed by Nusser and Davis in 2011. It allows to estimate bulk flows from the apparent brightening / dimming of galaxies derived from determining luminosities from redshifts rather than from distances , and tentatively applied to the available datasets. The scope of this Thesis is twofold. The first one is to test the validity of the method proposed by Kashlinsky et:al: using a set of mock CMB maps obtained from sophisticated hydrodynamical simulations. Appling the Kashlinsky’s same data-analysis procedure to a set of controlled experiment allows to estimate random errors and, which is more relevant, assess possible systematic effects. The second one is to implement the luminosity function method of Nusser and Davis and appling it to a set of mock redshift catalogs mimicking the spectroscopic SDSS sample, to check whether it might be used to detect, and which what significance, a dark flow with the same amplitude and scale of that claimed by Kashlinsky et:al:.