A Measurement of Large-Scale Peculiar Velocities of Clusters of Galaxies: Results and Cosmological Implications

The Astrophysical Journal (Impact Factor: 5.99). 09/2008; 686(2). DOI: 10.1086/592947
Source: arXiv


Peculiar velocities of clusters of galaxies can be measured by studying the fluctuations in the cosmic microwave background (CMB) generated by the scattering of the microwave photons by the hot X-ray emitting gas inside clusters. While for individual clusters such measurements result in large errors, a large statistical sample of clusters allows one to study cumulative quantities dominated by the overall bulk flow of the sample with the statistical errors integrating down. We present results from such a measurement using the largest all-sky X-ray cluster catalog combined to date and the 3-year WMAP CMB data. We find a strong and coherent bulk flow on scales out to at least > 300 h^{-1} Mpc, the limit of our catalog. This flow is difficult to explain by gravitational evolution within the framework of the concordance LCDM model and may be indicative of the tilt exerted across the entire current horizon by far-away pre-inflationary inhomogeneities. Comment: Ap.J. (Letters), in press. 20 Oct issue (Vol. 686)

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    • "[51]), which is consistent with the expectation that structure formation is the outgrowth of gaussian initial conditions via gravitational instability. Some possible sources of the BF are reviewed by citation [52]; they include attractors [53] [54], super-horizon tilt [41] [55] [56], over-dense regions resulting from bubble collisions [57] or induced by cosmic defects [58]. In recent studies, some new possible sources have been proposed: a combination of over-and under-dense region [40] [59] and void asymmetries in the cosmic web [60]. "
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    ABSTRACT: The bulk flow is a volume average of the peculiar velocities and a useful probe of the mass distribution on large scales. The gravitational instability model views the bulk flow as a potential flow that obeys a Maxwellian Distribution. We use two N-body simulations, the LasDamas Carmen and the Horizon Run, to calculate the bulk flows of various sized volumes in the simulation boxes. Once we have the bulk flow velocities as a function of scale, we investigate the mass and gravitational potential distribution around the volume. We found that matter densities can be asymmetrical and difficult to detect in real surveys, however, the gravitational potential and its gradient may provide better tools to investigate the underlying matter distribution. This study shows that bulk flows are indeed potential flows and thus provides information on the flow sources. We also show that bulk flow magnitudes follow a Maxwellian distribution on scales $>10\ h^{-1}$Mpc.
    Preview · Article · Dec 2015
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    • "It can also be utilized to constrain dark matter-dark energy interaction [63] and modified gravity [64] [65]. Recently, a new regime of the kSZ effect generated by larger scale ( Gpc) inhomogeneities has been realized and applied to probe dark flows [66] [67] [68] [69] [70] [71] [72], test the Copernican principle [73] [74], and constrain the Figure 2. A schematic diagram of the kSZ effect in the collision spacetime. The solid circle (sphere) is the last scattering surface (LSS) seen by us, and the dashed circles (spheres) are the LSS of several representative free electrons, labelled by " e1 " , " e2 " and " e3 " respectively. "
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    ABSTRACT: Perhaps the most controversial idea in modern cosmology is that our observable universe is contained within one bubble among many, all inhabiting the eternally inflating multiverse. One of the few way to test this idea is to look for evidence of the relic inhomogeneities left by the collisions between other bubbles and our own. Such relic inhomogeneities induces a coherent bulk flow over gigaparsec scales. Therefore, bubble collisions leave unique imprints in the cosmic microwave background (CMB) through the kinetic Sunyaev Zel'dovich (kSZ) effect, temperature anisotropies induced by the scattering of photons from coherently moving free electrons in the diffuse intergalactic medium. The kSZ signature produced by bubble collisions has a unique directional dependence and is tightly correlated with the galaxy distribution; it can therefore be distinguished from other contributions to the CMB anisotropies. An important advantage of the kSZ signature is that it peaks on arcminute angular scales, where the limiting factors in making a detection are instrumental noise and foreground subtraction. This is in contrast to the collision signature in the primary CMB, which peaks on angular scales much larger than one degree, and whose detection is therefore limited by cosmic variance. In this paper, we examine the prospects for probing the inhomogeneities left by bubble collisions using the kSZ effect. We provide a forecast for detection using cross-correlations between CMB and galaxy surveys, finding that the detectability using the kSZ effect can be competitive with constraints from CMB temperature and polarization data.
    Preview · Article · Jan 2015 · Journal of Cosmology and Astroparticle Physics
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    • "There are three famous CMB anomalies reported in the WMAP, namely the surprisingly low quadrupole [29] (first seen in COBE), alignment between low multipoles ( " axis of evil " ) [30] [31], and an hemispherical power asymmetry [32] [33] [34]. Based on WMAP data a uniform bulk flow of galaxy clusters is reported ( " dark flow " ) [35] [36]. Other data analyses that indicate a preferred axis include analysis of polarization of electromagnetic radiation propagating over cosmological scales [37] [38], and a " handedness " in the orientation of galaxies [39]. "
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    ABSTRACT: Motivated by the couplings of the dilaton in four-dimensional effective actions, we investigate the cosmological consequences of a scalar field coupled both to matter and a Maxwell-type vector field. The vector field has a background isotropy-violating component. New anisotropic scaling solutions which can be responsible for the matter and dark energy dominated epochs are identified and explored. For a large parameter region the universe expands almost isotropically. Using that the CMB quadrupole is extremely sensitive to shear, we constrain the ratio of the matter coupling to the vector coupling to be less than 10^(-5). Moreover, we identify a large parameter region, corresponding to a strong vector coupling regime, yielding exciting and viable cosmologies close to the LCDM limit.
    Full-text · Article · May 2012 · Journal of High Energy Physics
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