17 research outputs found
Rendering dark energy void
Includes abstract.Includes bibliographical references (leaves 114-120).The current model of cosmology, the Friedman-Lemaitre-Robertson-Valker model, assumes that the universe is approximately homogeneous and isotropic on very large scales. Further assuming flatness and dark energy in the form of Einstein's cosmological constant A then implies that the latter contributes roughly 73% of the total energy of the universe, cold dark matter (CD'I) 23SiC, and baryons, the matter we are made, only 4%
On the evolution of large-scale structure in a cosmic void
Includes bibliographical references.Future large-scale structure surveys are expected to pin-down the properties of dark energy significantly more by mapping the cosmic web to unprecedented precision. To take advantage of such state-of-the-art technologies, the evermore accurate modelling of structure formation is absolutely necessary. While relativistic linear and non-relativistic (Newtonian) non-linear effects have been well established (although improvements are still being made), a fairly unexplored area is the impact of relativistic, non-linear effects on structure formation. As an attempt in this direction, we consider linear perturbations of a Lemaître-Tolman-Bondi (LTB) spacetime. LTB models are spherically symmetric but inhomogeneous exact dust solutions to the Einstein field equations. They are known to accommodate most observations of the background universe without dark energy. In this work we present a new numerical code to solve the set of coupled partial differential equations that describe the evolution of the (polar) perturbations, test it in the case of a Hubble-scale LTB void, and demonstrate its excellent stability and convergence. We then explore the solutions for a variety of generic initial conditions. The variable that closely resembles the Newtonian potential is shown to excite propagating (tensor) as well as rotational (vector) modes at the percent-level. Comparing our results to that which ignores the full coupling, we estimate percent-level corrections to the amplitude of the galaxy correlation function when only the scalar degrees of freedom are included. In addition, we showed that the anisotropic correlation function can nevertheless be used as a test of the Copernican Principle. Note that our code has applications to other scenarios as well in which spherical symmetry is a good approximation, such as the lensing of gravitational waves by intervening halos/voids
Galaxy correlations and the BAO in a void universe: structure formation as a test of the Copernican Principle
A suggested solution to the dark energy problem is the void model, where accelerated expansion is
replaced by Hubble-scale inhomogeneity. In these models, density perturbations grow on a radially
inhomogeneous background. This large scale inhomogeneity distorts the spherical Baryon Acoustic
Oscillation feature into an ellipsoid which implies that the bump in the galaxy correlation function
occurs at different scales in the radial and transverse correlation functions. We compute these for the
first time, under the approximation that curvature gradients do not couple the scalar modes to vector
and tensor modes. The radial and transverse correlation functions are very different from those of
the concordance model, even when the models have the same average BAO scale. This implies that
if void models are fine-tuned to satisfy average BAO data, there is enough extra information in the
correlation functions to distinguish a void model from the concordance model. We expect these new
features to remain when the full perturbation equations are solved, which means that the radial and
transverse galaxy correlation functions can be used as a powerful test of the Copernican Principle.Web of Scienc
Galaxy correlations and the BAO in a void universe: structure formation as a test of the Copernican Principle
A suggested solution to the dark energy problem is the void model, where
accelerated expansion is replaced by Hubble-scale inhomogeneity. In these
models, density perturbations grow on a radially inhomogeneous background. This
large scale inhomogeneity distorts the spherical Baryon Acoustic Oscillation
feature into an ellipsoid which implies that the bump in the galaxy correlation
function occurs at different scales in the radial and transverse correlation
functions. We compute these for the first time, under the approximation that
curvature gradients do not couple the scalar modes to vector and tensor modes.
The radial and transverse correlation functions are very different from those
of the concordance model, even when the models have the same average BAO scale.
This implies that if void models are fine-tuned to satisfy average BAO data,
there is enough extra information in the correlation functions to distinguish a
void model from the concordance model. We expect these new features to remain
when the full perturbation equations are solved, which means that the radial
and transverse galaxy correlation functions can be used as a powerful test of
the Copernican Principle.Comment: 12 pages, 8 figures, matches published versio
nIFTy galaxy cluster simulations – II. Radiative models
We have simulated the formation of a massive galaxy cluster (M = 1.110) in a CDM universe using
10 different codes (RAMSES, 2 incarnations of AREPO and 7 of GADGET), modeling
hydrodynamics with full radiative subgrid physics. These codes include
Smoothed-Particle Hydrodynamics (SPH), spanning traditional and advanced SPH
schemes, adaptive mesh and moving mesh codes. Our goal is to study the
consistency between simulated clusters modeled with different radiative
physical implementations - such as cooling, star formation and AGN feedback. We
compare images of the cluster at , global properties such as mass, and
radial profiles of various dynamical and thermodynamical quantities. We find
that, with respect to non-radiative simulations, dark matter is more centrally
concentrated, the extent not simply depending on the presence/absence of AGN
feedback. The scatter in global quantities is substantially higher than for
non-radiative runs. Intriguingly, adding radiative physics seems to have washed
away the marked code-based differences present in the entropy profile seen for
non-radiative simulations in Sembolini et al. (2015): radiative physics +
classic SPH can produce entropy cores. Furthermore, the inclusion/absence of
AGN feedback is not the dividing line -as in the case of describing the stellar
content- for whether a code produces an unrealistic temperature inversion and a
falling central entropy profile. However, AGN feedback does strongly affect the
overall stellar distribution, limiting the effect of overcooling and reducing
sensibly the stellar fraction.Comment: 20 pages, 13 figures, submitted to MNRA
nIFTy galaxy cluster simulations - IV. Quantifying the influence of baryons on halo properties
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 smoothed
particle hydrodynamics (SPH) approaches. For each code represented we have a dark-matteronly
(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 per cent) between global
properties of the cluster predicted by different codes when integrated quantities are measured
within the virial radius R200. However, we see larger differences for quantities within R2500,
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 active galactic nucleus 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
Rendering Dark Energy Void
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 parameterization of a void which can reproduce concordance
model distances to arbitrary accuracy, but can parameterize 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 is not yet able to achieve this. Using the latest
Constitution supernova dataset 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.Comment: 12 pages, 14 figures, typos corrected, matches the version published
in MNRAS. DOI adde
nIFTy galaxy cluster simulations – I. Dark matter and non-radiative models
We have simulated the formation of a galaxy cluster in a Ʌ cold dark matter universe using 13 different codes modelling only gravity and non-radiative hydrodynamics (RAMSES, ART, AREPO, HYDRA and nine 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 GADGET implementations span classic and modern 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 RAMSES, ART and AREPO form extended entropy cores in the gas with rising central gas temperatures. Those codes employing classic 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 modern SPH schemes that allow entropy mixing span the range between these two extremes and the latest SPH variants produce gas entropy profiles that are essentially indistinguishable from those obtained with grid-based methods