The Lemaître-Tolman-Bondi solution has received much attention as a possible alternative to Dark Energy, as it is able to account for the apparent acceleration of the Universe without any exotic matter content. However, in order to make rigorous comparisons between these models and cosmological observations, such as the integrated Sachs-Wolfe effect, baryon acoustic oscillations and the observed matter power spectrum, it is absolutely necessary to have a proper understanding of the linear perturbation theory about them. Here we present this theory in a fully general, and gauge-invariant form. It is shown that scalar, vector and tensor perturbations interact, and that the natural gauge invariant variables in Lemaître-Tolman-Bondi cosmology do not correspond straightforwardly to the usual Bardeen variables, in the limit of spatial homogeneity. We therefore construct new variables that reduce to pure scalar, vector and tensor modes in this limit.

Dark energy observations may be explained within general relativity using an inhomogeneous Hubble‐scale depression in the matter density and accompanying curvature, which evolves naturally out of an Einstein–de Sitter (EdS) model. We present a simple parametrization of a void which can reproduce concordance model distances to arbitrary accuracy, but can parametrize away from this to give a smooth density profile everywhere. We show how the Hubble constant is not just a nuisance parameter in inhomogeneous models because it affects the shape of the distance–redshift relation. Independent Hubble‐rate data from age estimates can, in principle, serve to break the degeneracy between concordance and void models, but the data are not yet able to achieve this. Using the latest Constitution supernova data set, we show that robust limits can be placed on the size of a void which is roughly independent of its shape. However, the sharpness of the profile at the origin cannot be well constrained due to supernova being dominated by peculiar velocities in the local universe. We illustrate our results using some recently proposed diagnostics for the Friedmann models.

We present the confusion-limited 1.28 GHz MeerKAT DEEP2 image covering one q » ¢ 68 FWHM b Unified Astronomy Thesaurus concepts: Radio telescopes (1360); Galaxy counts (588); Star formation (1569)

We have simulated the formation of a galaxy cluster in a ΛCDM universe using twelve different codes modeling only gravity and non-radiative hydrodynamics (ART, AREPO, HYDRA and 9 incarnations of GADGET). This range of codes includes particle based, moving and fixed mesh codes as well as both Eulerian and Lagrangian fluid schemes. The various GAD-GET implementations span traditional and advanced smoothed-particle hydrodynamics (SPH) schemes. The goal of this comparison is to assess the reliability of cosmological hydrodynamical simulations of clusters in the simplest astrophysically relevant case, that in which the gas is assumed to be non-radiative. We compare images of the cluster at z = 0, global properties such as mass, and radial profiles of various dynamical and thermodynamical quantities. The underlying gravitational framework can be aligned very accurately for all the codes allowing a detailed investigation of the differences that develop due to the various gas physics implementations employed. As expected, the mesh-based codes ART and AREPO form extended entropy cores in the gas with rising central gas temperatures. Those codes employing traditional SPH schemes show falling entropy profiles all the way into the very centre with correspondingly rising density profiles and central temperature inversions. We show that methods with arXiv:1503.06065v1 [astro-ph.CO]

Building on the initial results of the nIFTy simulated galaxy cluster comparison, we compare and contrast the impact of baryonic physics with a single massive galaxy cluster, run with 11 state-of-the-art codes, spanning adaptive mesh, moving mesh, classic and modern SPH approaches. For each code represented we have a dark matter only (DM) and non-radiative (NR) version of the cluster, as well as a full physics (FP) version for a subset of the codes. We compare both radial mass and kinematic profiles, as well as global measures of the cluster (e.g. concentration, spin, shape), in the NR and FP runs with that in the DM runs. Our analysis reveals good consistency ( < ≈ 20%) between global properties of the cluster predicted by different codes when integrated quantities are measured within the virial radius R 200 . However, we see larger differences for quantities within R 2500 , especially in the FP runs. The radial profiles reveal a diversity, especially in the cluster centre, between the NR runs, which can be understood straightforwardly from the division of codes into classic SPH and non-classic SPH (including the modern SPH, adaptive and moving mesh codes); and between the FP runs, which can also be understood broadly from the division of codes into those that include AGN feedback and those that do not. The variation with respect to the median is much larger in the FP runs with different baryonic physics prescriptions than in the NR runs with different hydrodynamics solvers.