Institute of Cosmology and Gravitation, University of Portsmouth

About: Institute of Cosmology and Gravitation, University of Portsmouth is a based out in . It is known for research contribution in the topics: Galaxy & Redshift. The organization has 297 authors who have published 1207 publications receiving 76919 citations.

Neutrino masses from clustering of red and blue galaxies: a test of astrophysical uncertainties

Abstract: Combining measurements of the galaxy power spectrum and the cosmic microwave background (CMB) is a powerful means of constraining the summed mass of neutrino species , but is subject to systematic uncertainties due to non-linear structure formation, redshift-space distortions and galaxy bias. We empirically test the robustness of neutrino mass results to these effects by separately analysing power spectra of red and blue galaxies from the Sloan Digital Sky Survey (SDSS-II) Data Release 7 (DR7), combined with the CMB 5-yr Wilkinson Microwave Anisotropy Probe (WMAP5) data. We consider fitting for a range of maximum wavenumber k using 12 different galaxy bias models. For example, using a new model based on perturbation theory and including redshift-space distortions, the all-galaxy power spectrum combined with WMAP5 for a wavenumber range of k < 0.2 h Mpc−1 yields 95 per cent confidence limit eV. The red and blue galaxy power spectra give 0.41 and 0.63 eV, respectively, for this model. Using mock catalogues, we find the expected difference in these limits assuming a true neutrino mass of zero is 0.10 ± 0.14 eV. Thus, the difference of 0.22 eV between upper limits on neutrino mass for red and blue galaxies is approximately 1σ from the expected value. We find similar results for the other models and k ranges tested. This indicates good agreement for current data but hints at possible issues for next-generation surveys. Being able to perform such systematic tests is advantageous, and future surveys would benefit by including broad galaxy populations and luminosities that enable such a decomposition.

Relativistic initial conditions for N-body simulations

Abstract: Initial conditions for (Newtonian) cosmological N-body simulations are usually set by re-scaling the present-day power spectrum obtained from linear (relativistic) Boltzmann codes to the desired initial redshift of the simulation. This back-scaling method can account for the effect of inhomogeneous residual thermal radiation at early times, which is absent in the Newtonian simulations. We analyse this procedure from a fully relativistic perspective, employing the recently-proposed Newtonian motion gauge framework. We find that N-body simulations for ΛCDM cosmology starting from back-scaled initial conditions can be self-consistently embedded in a relativistic space-time with first-order metric potentials calculated using a linear Boltzmann code. This space-time coincides with a simple ``N-body gauge'' for z < 50 for all observable modes. Care must be taken, however, when simulating non-standard cosmologies. As an example, we analyse the back-scaling method in a cosmology with decaying dark matter, and show that metric perturbations become large at early times in the back-scaling approach, indicating a breakdown of the perturbative description. We suggest a suitable ``forwards approach" for such cases.

Observational signatures of the theories beyond Horndeski

Abstract: In the approach of the effective field theory of modified gravity, we derive the equations of motion for linear perturbations in the presence of a barotropic perfect fluid on the flat isotropic cosmological background. In a simple version of Gleyzes-Langlois-Piazza-Vernizzi (GLPV) theories, which is the minimum extension of Horndeski theories, we show that a slight deviation of the tensor propagation speed squared ct2 from 1 generally leads to the large modification to the propagation speed squared cs2 of a scalar degree of freedom O. This problem persists whenever the kinetic energy ρX of the field phi is much smaller than the background energy density ρm, which is the case for most of dark energy models in the asymptotic past. Since the scaling solution characterized by the constant ratio ρX/ρm is one way out for avoiding such a problem, we study the evolution of perturbations for a scaling dark energy model in the framework of GLPV theories in the Jordan frame. Provided the oscillating mode of scalar perturbations is fine-tuned so that it is initially suppressed, the anisotropic parameter η=−Φ/Ψ between the two gravitational potentials Ψ and Φ significantly deviates from 1 for ct2 away from 1. For other general initial conditions, the deviation of ct2 from 1 gives rise to the large oscillation of Ψ with the frequency related to cs2. In both cases, the model can leave distinct imprints for the observations of CMB and weak lensing

Spherical collapse in modified gravity with the Birkhoff theorem

Abstract: We study structure formation in a phenomenological model of modified gravity which interpolates between Acold dark mtter (ACDM) and Dvali-Gabadadze-Porrati (DGP) gravity. In our model, the Friedmann equation assumes the form H 2 (a) = (1 - Ω m )H α (a) + Ω m /a 3 with the interpolation parameter a. Generalization of spherical collapse by using the Birkhoff theorem along with the modified growth equation shows that the overdensity for spherical collapse δ c in these models is significantly lowered compared to ACDM, leading to enhanced number densities of massive clusters and enhanced cluster merging rates. We find that δ c ( z ) is well fitted by a function of the form δ c (z) = a - b exp(-cz). We examine the sensitivity of Planck's and SPT's Sunyaev-Zel'dovich survey to constrain the modified gravity parametrization, and find that these experiments can easily distinguish between models with a cosmological constant and modified gravity by including prior constraints from cosmic microwave background (CMB) temperature and polarization anisotropies. Applying a Fisher matrix formalism yields an expected accuracy of Δα ≃ 0.04 (SPT),..., 0.07(Planck) for a combined measurement of the cluster redshift distribution and the CMB temperature and polarization power spectrum, assuming ACDM as the fiducial cosmology.

The Vainshtein Mechanism in the Cosmic Web

Abstract: We investigate the dependence of the Vainshtein screening mechanism on the cosmic web morphology of both dark matter particles and halos as determined by ORIGAMI. Unlike chameleon and symmetron screening, which come into effect in regions of high density, Vainshtein screening instead depends on the dimensionality of the system, and screened bodies can still feel external fields. ORIGAMI is well-suited to this problem because it defines morphologies according to the dimensionality of the collapsing structure and does not depend on a smoothing scale or density threshold parameter. We find that halo particles are screened while filament, wall, and void particles are unscreened, and this is independent of the particle density. However, after separating halos according to their large scale morphological environment, we find no difference in the screening properties of halos in filaments versus halos in clusters. We find that the fifth force enhancement of dark matter particles in halos is greatest well outside the virial radius. We confirm the theoretical expectation that even if the internal field is suppressed by the Vainshtein mechanism, the object still feels the fifth force generated by the external fields, by measuring peculiar velocities and velocity dispersions of halos. Finally, we investigate the morphology and gravity model dependence of halo spins, concentrations, and shapes.