zobov (ZOnes Bordering On Voidness) is an algorithm that finds density depressions in a set of points, without any free parameters, or assumptions about shape. It uses the Voronoi tessellation to ...estimate densities, which it uses to find both voids and subvoids. It also measures probabilities that each void or subvoid arises from Poisson fluctuations. This paper describes the zobov algorithm, and the results from its application to the dark matter particles in a region of the Millennium simulation. Additionally, the paper points out an interesting high-density peak in the probability distribution of dark matter particle densities.
We examine the Lagrangian divergence of the displacement field, arguably a more natural object than the density in a Lagrangian description of a cosmological large-scale structure. This quantity, ...which we denote ψ, quantifies the stretching and distortion of the initially homogeneous lattice of dark-matter particles in the universe. ψ encodes similar information as the density, but the correspondence has subtleties. It corresponds better to the log-density A than the overdensity δ. A Gaussian distribution in ψ produces a distribution in A with slight skewness; in δ, we find that in many cases the skewness is further increased by 3.
A local spherical-collapse-based (SC) fit found by Bernardeau gives a formula for ψ's particle-by-particle behaviour that works quite well, better than applying the Lagrangian perturbation theory (LPT) at first or second (2LPT) order. In 2LPT, there is a roughly parabolic relation between initial and final ψ that can give overdensities in deep voids, so low-redshift, high-resolution 2LPT realizations should be used with caution. The SC fit excels at predicting ψ until streams cross; then, for particles forming haloes, ψ plummets as in a waterfall to −3. This gives a new method for producing N-particle realizations. Compared to LPT realizations, such SC realizations give reduced stream-crossing, and better visual and 1-point-probability density function (PDF) correspondence to the results of full gravity. LPT, on the other hand, predicts large-scale flows and the large-scale power-spectrum amplitude better, unless an empirical correction is added to the SC formula.
ABSTRACT
In a cold dark matter universe, cosmological structure formation proceeds in rough analogy to origami folding. Dark matter occupies a three‐dimensional ‘sheet’ of free‐fall observers, ...non‐intersecting in six‐dimensional velocity–position phase space. At early times, the sheet was flat like an origami sheet, i.e. velocities were essentially zero, but as time passes, the sheet folds up to form cosmic structure. This paper further illustrates this analogy, and clarifies a Lagrangian definition of caustics and streams: caustics are two‐dimensional surfaces in this initial sheet along which it folds, tessellating Lagrangian space into a set of three‐dimensional regions, i.e. streams. The main scientific result of this paper is that streams may be coloured by only two colours, with no two neighbouring streams (i.e. streams on either side of a caustic surface) coloured the same. The two colours correspond to positive and negative parities of local Lagrangian volumes. This is a severe restriction on the connectivity and therefore arrangement of streams in Lagrangian space, since arbitrarily many colours can be necessary to colour a general arrangement of three‐dimensional regions. This stream two‐colourability has consequences from graph theory, which we explain. Then, using N‐body simulations, we test how these caustics correspond in Lagrangian space to the boundaries of haloes, filaments and walls. We also test how well outer caustics correspond to a Zel'dovich‐approximation prediction.
We measure hot and cold spots on the microwave background associated with supercluster and supervoid structures identified in the Sloan Digital Sky Survey luminous red galaxy catalog. The structures ...give a compelling visual imprint, with a mean temperature deviation of 9.6 +/- 2.2 muK, i.e., above 4 capital sigma . We interpret this as a detection of the late-time integrated Sachs-Wolfe (ISW) effect, in which cosmic acceleration from dark energy causes gravitational potentials to decay, heating or cooling photons passing through density crests or troughs. In a flat universe, the linear ISW effect is a direct signal of dark energy.
We discuss an idealized model of halo formation, in which a collapsing halo node is tetrahedral, with a filament extruding from each of its four faces, and with a wall connecting each pair of ...filaments. In the model, filaments generally spin when they form, and the halo spins if and only if there is some rotation in filaments. This is the simplest possible fully three-dimensional halo collapse in the ‘origami approximation’, in which voids are irrotational, and the dark-matter sheet out of which dark-matter structures form is allowed to fold in position–velocity phase space, but not stretch (i.e. it cannot vary in density along a stream). Up to an overall scaling, the four filament directions, and only three other quantities, such as filament spins, suffice to determine all of the collapse's properties: the shape, mass, and spin of the halo; the densities per unit length and spins of all filaments; and masses per unit area of the walls. If the filaments are arranged regular-tetrahedrally, filament properties obey simple laws, reminiscent of angular-momentum conservation. The model may be most useful in understanding spin correlations between neighbouring galaxies joined by filaments; these correlations would give intrinsic alignments between galaxies, essential to understand for accurate cosmological weak-lensing measurements.
It was recently shown that applying a Gaussianizing transform, such as a logarithm, to the nonlinear matter density field extends the range of useful applicability of the power spectrum by a factor ...of a few smaller. Such a transform dramatically reduces nonlinearities in both the covariance and the shape of the power spectrum. Here, analyzing Coyote Universe real-space dark-matter density fields, we investigate the consequences of these transforms for cosmological parameter estimation. The power spectrum of the log-density provides the tightest cosmological parameter error bars (marginalized or not), giving a factor of 2-3 improvement over the conventional power spectrum in all five parameters tested. For the tilt, ns , the improvement reaches a factor of five. Similar constraints are achieved if the log-density power spectrum and conventional power spectrum are analyzed together. Rank-order Gaussianization seems just as useful as a log transform to constrain ns , but not other parameters. Dividing the overdensity by its dispersion in few-Mpc cells, while it diagonalizes the covariance matrix, does not seem to help with parameter constraints. We also provide a code that emulates these power spectra over a range of concordance cosmological models.
Here we present a simple, parameter-free, non-perturbative algorithm that gives low-redshift cosmological particle realizations accurate to few-Megaparsec scales, called muscle (MUltiscale ...Spherical-ColLapse Evolution). It has virtually the same cost as producing N-body-simulation initial conditions, since it works with the ‘stretch’ parameter ψ, the Lagrangian divergence of the displacement field. It promises to be useful in quickly producing mock catalogues, and to simplify computationally intensive reconstructions of galaxy surveys. muscle applies a spherical-collapse prescription on multiple Gaussian-smoothed scales. It achieves higher accuracy than perturbative schemes (Zel'dovich and second-order Lagrangian perturbation theory – 2LPT), and, by including the void-in-cloud process (voids in large-scale collapsing regions), solves problems with a single-scale spherical-collapse scheme. Slight further improvement is possible by mixing in the 2LPT estimate on large scales. Additionally, we show the behaviour of ψ for different morphologies (voids, walls, filaments, and haloes). A python code to produce these realizations is available at http://skysrv.pha.jhu.edu/∼neyrinck/muscle.html.
We present a cosmic void catalog using the large-scale structure galaxy catalog from the Baryon Oscillation Spectroscopic Survey (BOSS). This galaxy catalog is part of the Sloan Digital Sky Survey ...(SDSS) Data Release 12 and is the final catalog of SDSS-III. We take into account the survey boundaries, masks, and angular and radial selection functions, and apply the ZOBOV void finding algorithm to the Galaxy catalog. We identify a total of 10,643 voids. After making quality cuts to ensure that the voids represent real underdense regions, we obtain 1,228 voids with effective radii spanning the range 20-100 and with central densities that are, on average, 30% of the mean sample density. We release versions of the catalogs both with and without quality cuts. We discuss the basic statistics of voids, such as their size and redshift distributions, and measure the radial density profile of the voids via a stacking technique. In addition, we construct mock void catalogs from 1000 mock galaxy catalogs, and find that the properties of BOSS voids are in good agreement with those in the mock catalogs. We compare the stellar mass distribution of galaxies living inside and outside of the voids, and find no large difference. These BOSS and mock void catalogs are useful for a number of cosmological and galaxy environment studies.
Intergalactic filaments spin Xia, Qianli; Neyrinck, Mark C; Cai, Yan-Chuan ...
Monthly notices of the Royal Astronomical Society,
09/2021, Letnik:
506, Številka:
1
Journal Article
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ABSTRACT
Matter in the Universe is arranged in a cosmic web, with a filament of matter typically connecting each neighbouring galaxy pair, separated by tens of millions of light-years. A quadrupolar ...pattern of the spin field around filaments is known to influence the spins of galaxies and haloes near them, but it remains unknown whether filaments themselves spin. Here, we measure dark matter velocities around filaments in cosmological simulations, finding that matter generally rotates around them, much faster than around a randomly located axis. It also exhibits some coherence along the filament. The net rotational component is comparable to, and often dominant over, the known quadrupolar flow. The evidence of net rotations revises previous emphasis on a quadrupolar spin field around filaments. The full picture of rotation in the cosmic web is more complicated and multiscale than a network of spinning filamentary rods, but we argue that filament rotation is substantial enough to be an essential part of the picture. It is likely that the longest coherently rotating objects in the Universe are filaments. Also, we speculate that this rotation could provide a mechanism to generate or amplify intergalactic magnetic fields in filaments.
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