arXiv subjects

T. Buchert

Publications and source records attributed to T. Buchert.

At least 19 recordsLinked to original sources

arXiv2019

4MOST: Project overview and information for the First Call for Proposals

We introduce the 4-metre Multi-Object Spectroscopic Telescope (4MOST), a new high-multiplex, wide-field spectroscopic survey facility under development for the four-metre-class Visible and Infrared Survey Telescope for Astronomy (VISTA) at Paranal. Its key specifications are: a large field of view (FoV) of 4.2 square degrees and a high multiplex capability, with 1624 fibres feeding two low-resolution spectrographs ($R = λ/Δλ\sim 6500$), and 812 fibres transferring light to the high-resolution spectrograph ($R \sim 20\,000$). After a description of the instrument and its expected performance, a short overview is given of its operational scheme and planned 4MOST Consortium science; these aspects are covered in more detail in other articles in this edition of The Messenger. Finally, the processes, schedules, and policies concerning the selection of ESO Community Surveys are presented, commencing with a singular opportunity to submit Letters of Intent for Public Surveys during the first five years of 4MOST operations.

astro-ph.IM↗

arXiv2019

4MOST Consortium Survey 8: Cosmology Redshift Survey (CRS)

The 4MOST Cosmology Redshift Survey (CRS) will perform stringent cosmological tests via spectroscopic clustering measurements that will complement the best lensing, cosmic microwave background and other surveys in the southern hemisphere. The combination of carefully selected samples of bright galaxies, luminous red galaxies, emission-line galaxies and quasars, totalling about 8 million objects over the redshift range $z = 0.15$ to $3.5$, will allow definitive tests of gravitational physics. Many key science questions will be addressed by combining CRS spectra of these targets with data from current or future facilities such as the Large Synoptic Survey Telescope, the Square Kilometre Array and the Euclid mission.

astro-ph.CO↗

arXiv2015

Is there proof that backreaction of inhomogeneities is irrelevant in cosmology?

No. In a number of papers Green and Wald argue that the standard FLRW model approximates our Universe extremely well on all scales, except close to strong field astrophysical objects. In particular, they argue that the effect of inhomogeneities on average properties of the Universe (backreaction) is irrelevant. We show that this latter claim is not valid. Specifically, we demonstrate, referring to their recent review paper, that (i) their two-dimensional example used to illustrate the fitting problem differs from the actual problem in important respects, and it assumes what is to be proven; (ii) the proof of the trace-free property of backreaction is unphysical and the theorem about it fails to be a mathematically general statement; (iii) the scheme that underlies the trace-free theorem does not involve averaging and therefore does not capture crucial non-local effects; (iv) their arguments are to a large extent coordinate-dependent, and (v) many of their criticisms of backreaction frameworks do not apply to the published definitions of these frameworks. It is therefore incorrect to infer that Green and Wald have proven a general result that addresses the essential physical questions of backreaction in cosmology.

gr-qc↗

arXiv2005

The universe seen at different scales

A large-scale smoothed-out model of the universe ignores small-scale inhomogeneities, but the averaged effects of those inhomogeneities may alter both observational and dynamical relations at the larger scale. This article discusses these effects, and comments briefly on the relation to gravitational entropy.

gr-qc↗

arXiv2001

The morphological and dynamical evolution of simulated galaxy clusters

We explore the morphological and dynamical evolution of galaxy clusters in simulations using scalar and vector-valued Minkowski valuations and the concept of fundamental plane relations. In this context, three questions are of fundamental interest: 1. How does the average cluster morphology depend on the cosmological background model? 2. Is it possible to discriminate between different cosmological models using cluster substructure in a statistically significant way? 3. How is the dynamical state of a cluster, especially its distance from a virial equilibrium, correlated to its visual substructure? To answer these questions, we quantify cluster substructure using a set of morphological order parameters constructed on the basis of the Minkowski valuations (MVs). The dynamical state of a cluster is described using global cluster parameters: in certain spaces of such parameters fundamental band-like structures are forming indicating the emergence of a virial equilibrium. We find that the average distances from these fundamental structures are correlated to the average amount of cluster substructure for our cluster samples during the time evolution. Furthermore, significant differences show up between the high- and the low-Omega models. We pay special attention to the redshift evolution of morphological characteristics and find large differences between the cosmological models even for higher redshifts.

astro-ph↗

arXiv2001

Morphological fluctuations of large-scale structure: the PSCz survey

In a follow-up study to a previous analysis of the IRAS 1.2Jy catalogue, we quantify the morphological fluctuations in the PSCz survey. We use a variety of measures, among them the family of scalar Minkowski functionals. We confirm the existence of significant fluctuations that are discernible in volume-limited samples out to 200Mpc/h. In contrast to earlier findings, comparisons with cosmological N-body simulations reveal that the observed fluctuations roughly agree with the cosmic variance found in corresponding mock samples. While two-point measures, e.g. the variance of count-in-cells, fluctuate only mildly, the fluctuations in the morphology on large scales indicate the presence of coherent structures that are at least as large as the sample.

astro-ph↗

arXiv2000

Morphometry of Spatial Patterns

Minkowski functionals constitute a family of order parameters which discriminate spatial patterns according to size, shape and connectivity. Here we point out, that these scalar descriptors can be complemented by vector-valued curvature measures also known as Quermass vectors. Using examples of galaxy clusters, we demonstrate that the Quermass vectors provide additional morphological information on directional features and symmetries displayed by spatial data.

astro-ph↗

arXiv1997

Temporal optimization of Lagrangian perturbation schemes

The Lagrangian perturbation theory on Friedmann-Lemaitre cosmologies is compared with numerical simulations (tree-, adaptive P$^3$M- and PM codes). In previous work we have probed the large-scale performance of the Lagrangian perturbation solutions up to the third order by studying their cross- correlations with N-body simulations for various power spectra (Buchert etal 1994, Melott etal 1995, Weiss etal 1996). Thereby, spatial optimization techniques were applied by (high-frequency-)filtering of the initial power spectra. In this work the novel method of temporal optimization [Shifted-Time- Approximation (STA) and Frozen-Time-Approximation (FTA)] is investigated and used. The method is designed to compensate the native property of Lagrangian perturbation solutions to delay the collapse of structures. The method can be treated analytically. Applying the STA and FTA prescriptions a significant improvement of the performance of Lagrangian perturbation schemes (as measured by cross-correlation statistics) is observed. Using this tool we investigate a local study of special clustering models of dark matter as candidates for typical elements of the large-scale structure in the Universe, and so also focus on the performance of the perturbation solutions on smaller scales at high-spatial resolution. The models analyzed were presented in (Buchert etal 1996) and allow studying typical features of the clustering process in the weakly non-linear regime. The spatial and temporal limits of applicability of the solutions at second and third order are determined and compared with the first-order solution, which is equivalent to the ``Zel'dovich approximation'' (Zel'dovich 1970, 1973) for the type of initial data analyzed.

astro-ph↗

arXiv1996

Minkowski Functionals of Abell/ACO Clusters

We determine the Minkowski functionals for a sample of Abell/ACO clusters, 401 with measured and 16 with estimated redshifts. The four Minkowski functionals (including the void probability function and the mean genus) deliver a global description of the spatial distribution of clusters on scales from $10$ to $60\hMpc$ with a clear geometric interpretation. Comparisons with mock catalogues of N--body simulations using different variants of the CDM model demonstrate the discriminative power of the description. The standard CDM model and the model with tilted perturbation spectrum cannot generate the Minkowski functionals of the cluster data, while a model with a cosmological constant and a model with breaking of the scale invariance of perturbations (BSI) yield compatible results.

astro-ph↗

arXiv1995

Averaging Hypotheses in Newtonian Cosmology

Average properties of general inhomogeneous cosmological models are discussed in the Newtonian framework. It is shown under which circumstances the average flow reduces to a member of the standard Friedmann--Lema\^ıtre cosmologies. Possible choices of global boundary conditions of inhomogeneous cosmologies as well as consequences for the interpretation of cosmological parameters are put into perspective.

astro-ph↗

arXiv1995

Analyzing Galaxy Catalogues with Minkowski Functionals

We discuss the application of Minkowski functionals to galaxy catalogues with emphasis on the boundary corrections and point out the relevance for other statistical methods. The analysis of a cubic subsample of the CfA--1 data serves as an example.

astro-ph↗

arXiv1995

Optimizing Higher-order Lagrangian Perturbation Theory for Cold Dark Matter Models

We report on the performance of Lagrangian perturbation theory up to the second order for the standard cold dark matter (SCDM) and broken scale invariance (BSI) scenarios. We normalize both models to the COBE data, the BSI model serves as an example of models which fit the small-scale power of galaxy surveys. We optimize Lagrangian perturbation solutions by removing small-scale power from the initial data and compare the results with those of numerical simulations. We find an excellent performance of the optimized Lagrangian schemes down to scales around the correlation length or smaller, depending on the statistics used for the comparison. The optimization scheme can be expressed in a way which is independent of the type of fluctuation spectrum and of the size of the simulations.

astro-ph↗

arXiv1995

Minkowski Functionals in Cosmology

Minkowski functionals provide a novel tool to characterize the large-scale galaxy distribution in the Universe. Here we give a brief tutorial on the basic features of these morphological measures and indicate their practical application for simulation data and galaxy redshift catalogues as examples.

astro-ph↗

arXiv1995

Lagrangian Perturbation Approach to the Formation of Large-scale Structure

The present lecture notes address three columns on which the Lagrangian perturbation approach to cosmological dynamics is based: 1. the formulation of a Lagrangian theory of self--gravitating flows in which the dynamics is described in terms of a single field variable; 2. the procedure, how to obtain the dynamics of Eulerian fields from the Lagrangian picture, and 3. a precise definition of a Newtonian cosmology framework in which Lagrangian perturbation solutions can be studied. While the first is a discussion of the basic equations obtained by transforming the Eulerian evolution and field equations to the Lagrangian picture, the second exemplifies how the Lagrangian theory determines the evolution of Eulerian fields including kinematical variables like expansion, vorticity, as well as the shear and tidal tensors. The third column is based on a specification of initial and boundary conditions, and in particular on the identification of the average flow of an inhomogeneous cosmology with a ``Hubble--flow''. Here, we also look at the limits of the Lagrangian perturbation approach as inferred from comparisons with N--body simulations and illustrate some striking properties of the solutions.

astro-ph↗

arXiv1994

Optimized Lagrangian Approximations for Modelling Large--Scale Structure at Non--Linear Stages

Approximations to the exact solutions for gravitational instability in the expanding Universe are extremely useful for understanding the evolution of large--scale structure. We report on a series of tests of Newtonian Lagrangian perturbation schemes using N--body simulations for various power--spectra with scale--independent indices in the range $-3$ to $+1$. The models have been evolved deeply into the non--linear regime of structure formation in order to probe the dynamical and statistical performance of the Lagrangian perturbation schemes (whose first--order solution contains as a subset the celebrated ``Zel'dovich--approximation'', hereafter ZA). These tests reveal properties of the approximations at stages beyond the obvious validity of perturbation theory. Recently, another series of tests of different analytical and semi--numerical approximations for large--scale structure was conducted with the result that ZA displays the best dynamical performance in comparison with the N--body simulations, if the initial data were smoothed before evolving the model, i.e., a truncated form of ZA (TZA). We show in this contribution that the excellent performance of TZA can be further improved by going to second order in the Lagrangian perturbation approach. The truncated second--order Lagrangian scheme provides a useful improvement over TZA especially for negative power indices, which suggests it will be very useful for modelling standard scenarios such as ``Cold--'', ``Hot--'' and ``Mixed--Dark--Matter''.

astro-ph↗

arXiv1994

Cosmogony of Generic Structures

The problem of formation of generic structures in the Universe is addressed, whereby first the kinematics of inertial continua for coherent initial data is considered. The generalization to self--gravitating continua is outlined focused on the classification problem of singularities and metamorphoses arising in the density field. Self--gravity gives rise to an internal hierarchy of structures, and, dropping the assumption of coherence, also to an external hierarchy of structures dependent on the initial power spectrum of fluctuations.

astro-ph↗

arXiv1994

Robust Morphological Measures for Large-Scale Structure

A complete family of statistical descriptors for the morphology of large--scale structure based on Minkowski--Functionals is presented. These robust and significant measures can be used to characterize the local and global morphology of spatial patterns formed by a coverage of point sets which represent galaxy samples. Basic properties of these measures are highlighted and their relation to the `genus statistics' is discussed. Test models like a Poissonian point process and samples generated from a Voronoi--model are put into perspective.

astro-ph↗

arXiv1994

Optimized Lagrangian Approximations for Modelling Large--Scale Structure at Non--Linear Stages

We report on a series of tests of Newtonian Lagrangian perturbation schemes using N--body simulations for various power--spectra with scale--independent indices in the range $-3$ to $+1$. The models have been evolved deeply into the non--linear regime of structure formation in order to probe the dynamical and statistical performance of the Lagrangian perturbation schemes (whose first--order solution contains as a subset the celebrated ``Zel'dovich--approximation'' (hereafter ZA). These tests reveal properties of the approximations at stages beyond the obvious validity of perturbation theory. Recently, another series of tests of different analytical and semi--numerical approximations for large--scale structure was conducted with the result that ZA displays the best dynamical performance in comparison with the N--body simulations, if the initial data were smoothed before evolving the model, i.e., a truncated form of ZA (TZA). We show in this Letter that the excellent performance of TZA can be further improved by going to second order in the Lagrangian perturbation approach. The truncated second--order Lagrangian scheme provides a useful improvement over TZA especially for negative power indices, which suggests it will be very useful for modelling standard scenarios such as ``Cold--'', ``Hot--'' and ``Mixed--Dark--Matter''.

astro-ph↗