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.

Simulating the quartic Galileon gravity model on adaptively refined meshes

Abstract: We develop a numerical algorithm to solve the high-order nonlinear derivative-coupling equation associated with the quartic Galileon model, and implement it in a modified version of the RAMSES N-body code to study the effect of the Galileon field on the large-scale matter clustering. The algorithm is tested for several matter field configurations with different symmetries, and works very well. This enables us to perform the first simulations for a quartic Galileon model which provides a good fit to the cosmic microwave background (CMB) anisotropy, supernovae and baryonic acoustic oscillations (BAO) data. Our result shows that the Vainshtein mechanism in this model is very efficient in suppressing the spatial variations of the scalar field. However, the time variation of the effective Newtonian constant caused by the curvature coupling of the Galileon field cannot be suppressed by the Vainshtein mechanism. This leads to a significant weakening of the strength of gravity in high-density regions at late times, and therefore a weaker matter clustering on small scales. We also find that without the Vainshtein mechanism the model would have behaved in a completely different way, which shows the crucial role played by nonlinearities in modified gravity theories and the importance of performing self-consistent N-body simulations for these theories.

Finding high-redshift strong lenses in DES using convolutional neural networks

Abstract: Author(s): Jacobs, C; Collett, T; Glazebrook, K; McCarthy, C; Qin, AK; Abbott, TMC; Abdalla, FB; Annis, J; Avila, S; Bechtol, K; Bertin, E; Brooks, D; Buckley-Geer, E; Burke, DL; Carnero Rosell, A; Carrasco Kind, M; Carretero, J; Da Costa, LN; Davis, C; De Vicente, J; Desai, S; Diehl, HT; Doel, P; Eifler, TF; Flaugher, B; Frieman, J; Garcia-Bellido, J; Gaztanaga, E; Gerdes, DW; Goldstein, DA; Gruen, D; Gruendl, RA; Gschwend, J; Gutierrez, G; Hartley, WG; Hollowood, DL; Honscheid, K; Hoyle, B; James, DJ; Kuehn, K; Kuropatkin, N; Lahav, O; Li, TS; Lima, M; Lin, H; Maia, MAG; Martini, P; Miller, CJ; Miquel, R; Nord, B; Plazas, AA; Sanchez, E; Scarpine, V; Schubnell, M; Serrano, S; Sevilla-Noarbe, I; Smith, M; Soares-Santos, M; Sobreira, F; Suchyta, E; Swanson, MEC; Tarle, G; Vikram, V; Walker, AR; Zhang, Y; Zuntz, J | Abstract: We search Dark Energy Survey (DES) Year 3 imaging data for galaxy-galaxy strong gravitational lenses using convolutional neural networks. We generate 250 000 simulated lenses at redshifts g 0.8 from which we create a data set for training the neural networks with realistic seeing, sky and shot noise. Using the simulations as a guide, we build a catalogue of 1.1 million DES sources with 1.8 l g − i l 5, 0.6 l g − r l 3, r mag g 19, g mag g 20, and i mag g 18.2. We train two ensembles of neural networks on training sets consisting of simulated lenses, simulated non-lenses, and real sources. We use the neural networks to score images of each of the sources in our catalogue with a value from 0 to 1, and select those with scores greater than a chosen threshold for visual inspection, resulting in a candidate set of 7301 galaxies. During visual inspection, we rate 84 as 'probably' or 'definitely' lenses. Four of these are previously known lenses or lens candidates. We inspect a further 9428 candidates with a different score threshold, and identify four new candidates. We present 84 new strong lens candidates, selected after a few hours of visual inspection by astronomers. This catalogue contains a comparable number of high-redshift lenses to that predicted by simulations. Based on simulations, we estimate our sample to contain most discoverable lenses in this imaging and at this redshift range.

Cosmological phase space analysis of the F(X)-V(ϕ) scalar field and bouncing solutions

Abstract: We analyze the dynamical system defined by a universe filled with a barotropic fluid plus a scalar field with modified kinetic term of the form $\mathcal{L}=F(X)\ensuremath{-}V(\ensuremath{\phi})$. After a suitable choice of variables that allows us to study the phase space of the system, we obtain the critical points and their stability. We find that some of them reduce to the ones defined for the canonical case when $F(X)=X$. We also study the field energy conditions to have a nonsingular bounce.

Cosmological constraints combining H(z), CMB shift and SNIa observational data

Abstract: Recently H(z) data obtained from differential ages of galaxies have been proposed as a new geometrical probe of dark energy. In this paper we use those data, combined with other background tests (CMB shift and SNIa data), to constrain a set of general relativistic dark energy models together with some other models motivated by extra dimensions. Our analysis rests mostly on Bayesian statistics, and we conclude that LCDM is at least substantially favoured, and that extradimensional models are less favoured than general relativistic ones.

Post-Planck constraints on interacting vacuum energy

Abstract: We present improved constraints on an interacting vacuum model using updated astronomical observations including the first data release from Planck. We consider a model with one dimensionless parameter, $\ensuremath{\alpha}$, describing the interaction between dark matter and vacuum energy (with fixed equation of state $w=\ensuremath{-}1$). The background dynamics correspond to a generalized Chaplygin gas cosmology, but the perturbations have a zero sound speed. The tension between the value of the Hubble constant, ${H}_{0}$, determined by Planck data plus WMAP polarization ($\mathrm{Planck}+\mathrm{WP}$) and that determined by the Hubble Space Telescope (HST) can be alleviated by energy transfer from dark matter to vacuum ($\ensuremath{\alpha}g0$). A positive $\ensuremath{\alpha}$ increases the allowed values of ${H}_{0}$ due to parameter degeneracy within the model using only cosmic microwave background data. Combining with additional data sets of including supernova type Ia (SN Ia) and baryon acoustic oscillation (BAO), we can significantly tighten the bounds on $\ensuremath{\alpha}$. Redshift-space distortions (RSD), which constrain the linear growth of structure, provide the tightest constraints on vacuum interaction when combined with Planck+WP, and prefer energy transfer from vacuum to dark matter ($\ensuremath{\alpha}l0$) which suppresses the growth of structure. Using the combined data sets of $\mathrm{Planck}+\mathrm{WP}+\mathrm{Union}2.1+\mathrm{BAO}+\mathrm{RSD}$, we obtain the constraint on $\ensuremath{\alpha}$ to be $\ensuremath{-}0.083l\ensuremath{\alpha}l\ensuremath{-}0.006$ (95% C.L.), allowing low ${H}_{0}$ consistent with the measurement from 6dF Galaxy survey. This interacting vacuum model can alleviate the tension between RSD and $\mathrm{Planck}+\mathrm{WP}$ in the $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ model for $\ensuremath{\alpha}l0$, or between HST measurements of ${H}_{0}$ and Planck+WP for $\ensuremath{\alpha}g0$, but not both at the same time.