Showing papers by "Institute of Cosmology and Gravitation, University of Portsmouth published in 2006"

Inflation dynamics and reheating

Abstract: The theory of inflation with single and multiple fields is reviewed paying particular attention to the dynamics of adiabatic and entropy/isocurvature perturbations which provide the primary means of testing inflationary models. The theory and phenomenology of reheating and preheating after inflation is reviewed providing a unified discussion of both the gravitational and nongravitational features of multifield inflation. In addition inflation in theories with extra dimensions and models such as the curvaton scenario and modulated reheating which provide alternative ways of generating large-scale density perturbations are covered. Finally interesting observational implications are discussed that can result from adiabatic-isocurvature correlations and non-Gaussianity.

Galaxy bimodality versus stellar mass and environment

Abstract: We analyse a z < 0.1 galaxy sample from the Sloan Digital Sky Survey focusing on the variation in the galaxy colour bimodality with stellar mass M and projected neighbour density Σ, and on measurements of the galaxy stellar mass functions. The characteristic mass increases with environmental density from about 10 10. 6 to 10 10.9 M ⊙ (Kroupa initial mass function, H 0 = 70) for Σ in the range 0.1-10 Mpc -2 . The galaxy population naturally divides into a red and blue sequence with the locus of the sequences in colour-mass and colour-concentration indices not varying strongly with environment. The fraction of galaxies on the red sequence is determined in bins of 0.2 in log Σ and log M (12 x 13 bins). The red fraction f r generally increases continuously in both Σ and M such that there is a unified relation: f, = F(Σ, M). Two simple functions are proposed which provide good fits to the data. These data are compared with analogous quantities in semi-analytical models based on the Millennium N-body simulation: the Bower et al. and Croton et al. models that incorporate active galactic nucleus feedback. Both models predict a strong dependence of the red fraction on stellar mass and environment that is qualitatively similar to the observations. However, a quantitative comparison shows that the Bower et al. model is a significantly better match; this appears to be due to the different treatment of feedback in central galaxies.

Non-Gaussianity of the primordial perturbation in the curvaton model

Abstract: We use the {delta}N formalism to investigate the non-Gaussianity of the primordial curvature perturbation in the curvaton scenario for the origin of structure. We numerically calculate the full probability distribution function allowing for the noninstantaneous decay of the curvaton and compare this with analytic results derived in the sudden-decay approximation. We also present results for the leading-order contribution to the primordial bispectrum and trispectrum. In the sudden-decay approximation we derive a fully nonlinear expression relating the primordial perturbation to the initial curvaton perturbation. As an example of how non-Gaussianity provides additional constraints on model parameters, we show how the primordial bispectrum on cosmic microwave background scales can be used to constrain variance on much smaller scales in the curvaton field. Our analytical and numerical results allow for multiple tests of primordial non-Gaussianity, and thus they can offer consistency tests of the curvaton scenario.

The SCUBA Half-Degree Extragalactic Survey – II. Submillimetre maps, catalogue and number counts

Abstract: We present maps, source catalogue and number counts of the largest, most complete and unbiased extragalactic submillimetre survey: the 850-μm SCUBA Half-Degree Extragalactic Survey (SHADES). Using the Submillimetre Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope (JCMT), SHADES mapped two separate regions of sky: the Subaru/XMM–Newton Deep Field (SXDF) and the Lockman Hole East (LH). Encompassing 93 per cent of the overall acquired data (i.e. data taken up to 2004 February 1), these SCUBA maps cover 720 arcmin2 with a rms noise level of about 2 mJy and have uncovered >100 submillimetre galaxies. In order to ensure the utmost robustness of the resulting source catalogue, data reduction was independently carried out by four subgroups within the SHADES team, providing an unprecedented degree of reliability with respect to other SCUBA catalogues available from the literature. Individual source lists from the four groups were combined to produce a robust 120-object SHADES catalogue; an invaluable resource for follow-up campaigns aiming to study the properties of a complete and consistent sample of submillimetre galaxies. For the first time, we present deboosted flux densities for each submillimetre galaxy found in a large survey. Extensive simulations and tests were performed separately by each group in order to confirm the robustness of the source candidates and to evaluate the effects of false detections, completeness and flux density boosting. Corrections for these effects were then applied to the data to derive the submillimetre galaxy source counts. SHADES has a high enough number of detected sources that meaningful differential counts can be estimated, unlike most submillimetre surveys which have to consider integral counts. We present differential and integral source number counts and find that the differential counts are better fit with a broken power law or a Schechter function than with a single power law; the SHADES data alone significantly show that a break is required at several mJy, although the precise position of the break is not well constrained. We also find that a 850-μm survey complete down to 2 mJy would resolve 20–30 per cent of the far-infrared background into point sources.

More on ghosts in the Dvali-Gabadaze-Porrati model

Abstract: It is shown by an explicit calculation that the excitations about the self-accelerating cosmological solution of the Dvali-Gabadaze-Porrati model contain a ghost mode. This raises serious doubts about viability of this solution. Our analysis reveals the similarity between the quadratic theory for the perturbations around the self-accelerating universe and an Abelian gauge model with two St\"uckelberg fields.

The XMM Cluster Survey: A Massive Galaxy Cluster at z = 1.45

Abstract: We report the discovery of XMMXCS J2215.9-1738, a massive galaxy cluster at z=1.45, which was found in the XMM Cluster Survey. The cluster candidate was initially identified as an extended X-ray source in archival XMM data. Optical spectroscopy shows that six galaxies within a ~60" diameter region lie at z=1.45+/-0.01. Model fits to the X-ray spectra of the extended emission yield kT=7.4+2.7-1.8 keV (90% confidence); if there is an undetected central X-ray point source, then kT=6.5+2.6-1.8 keV. The bolometric X-ray luminosity is LX=4.4+0.8-0.6C 1044 ergs s-1 over a 2 Mpc radial region. The measured TX, which is the highest for any known cluster at z>1, suggests that this cluster is relatively massive for such a high redshift. The redshift of XMMXCS J2215.9-1738 is the highest currently known for a spectroscopically confirmed cluster of galaxies.

The 2dF-SDSS LRG and QSO (2SLAQ) luminous red galaxy survey

Abstract: We present a spectroscopic survey of almost 15 000 candidate intermediate-redshift luminous red galaxies (LRGs) brighter than i = 19.8, observed with 2dF on the Anglo-Australian Telescope. The targets were selected photometrically from the Sloan Digital Sky Survey (SDSS) and lie along two narrow equatorial strips covering 180 deg 2 . Reliable redshifts were obtained for 92 per cent of the targets and the selection is very efficient: over 90 per cent have 0.45 < z < 0.8. More than 80 per cent of the ∼11 000 red galaxies have pure absorption-line spectra consistent with a passively evolving old stellar population. The redshift, photometric and spatial distributions of the LRGs are described. The 2SLAQ data will be released publicly from mid-2006, providing a powerful resource for observational cosmology and the study of galaxy evolution.

Observational constraints on self-accelerating cosmology

Abstract: The Dvali-Gabadadze-Porrati (DGP) braneworld model provides a simple alternative to the standard lambda cold dark matter cosmology, with the same number of parameters. There is no dark energy---the late universe self-accelerates due to an infrared modification of gravity. We compute the joint constraints on the DGP model from supernovae, the cosmic microwave background shift parameter, and the baryon oscillation peak in the Sloan Digital Sky Survey luminous red galaxy sample. Flat DGP models are within the joint 2 sigma contour, but the lambda cold dark matter model provides a significantly better fit to the data. These tests are based on the background dynamics of the DGP model, and we comment on further tests that involve structure formation.

Curvature and isocurvature perturbations from two-field inflation in a slow-roll expansion

Abstract: We calculate the power spectra of primordial curvature and isocurvature perturbations from a general two-field inflation model at next-to-leading order correction in a slow-roll expansion. In particular we calculate the spectral indices to second order in slow-roll parameters. We show that the cross correlation of the curvature and isocurvature perturbations at the time of Hubble exit during inflation is nonzero at first order in slow-roll parameters. We apply our results to different classes of inflation, including inflaton and curvaton scenarios. The spectrum of primordial gravitational waves, curvature and isocurvature perturbations obey generalized consistency relations in two-field inflation models. We give the first two consistency relations in an infinite hierarchy.

A Measurement of the Quadrupole Power Spectrum in the Clustering of the 2dF QSO Survey

Abstract: We report on a measurement of the quadrupole power spectrum in the two degree field (2dF) QSO redshift (2QZ) survey. The analysis used an algorithm parallel to that for estimating the standard monopole power spectrum without first requiring computation of the correlation function or the anisotropic power spectrum. The error on the quadrupole spectrum was rather large, but the best-fit value of the bias parameter from the quadrupole spectrum is consistent with that from previous investigations of the 2dF data.

Cosmological dynamics and dark energy with a nonlinear equation of state: a quadratic model

Abstract: We investigate the general relativistic dynamics of Robertson-Walker models with a nonlinear equation of state (EoS), focusing on the quadratic case $P={P}_{o}+\ensuremath{\alpha}\ensuremath{\rho}+\ensuremath{\beta}{\ensuremath{\rho}}^{2}$. This may be taken to represent theTaylor expansion of any arbitrary barotropic EoS, $P(\ensuremath{\rho})$. With the right combination of ${P}_{o}$, $\ensuremath{\alpha}$ and $\ensuremath{\beta}$, it serves as a simple phenomenological model for dark energy, or even unified dark matter. Indeed we show that this simple model for the EoS can produce a large variety of qualitatively different dynamical behaviors that we classify using dynamical systems theory. An almost universal feature is that accelerated expansion phases are mostly natural for these nonlinear EoS's. These are often asymptotically de Sitter thanks to the appearance of an effective cosmological constant. Other interesting possibilities that arise from the quadratic EoS are closed models that can oscillate with no singularity, models that bounce between infinite contraction/expansion and models which evolve from a phantom phase, asymptotically approaching a de Sitter phase instead of evolving to a ``big rip''. In a second paper we investigate the effects of the quadratic EoS in inhomogeneous and anisotropic models, focusing, in particular, on singularities.

Crossing the phantom divide without phantom matter

Abstract: A class of braneworld models can lead to phantom-like acceleration of the late universe, but without the need for any phantom matter. In the simplest models, the universe contains only cold dark matter and a cosmological constant. We generalize these models by introducing a quintessence field. The new feature in our models is that quintessence leads to a crossing of the phantom divide, w = −1. This is a purely gravitational effect, and there is no phantom instability. Furthermore, the Hubble parameter is always decreasing, and there is no big rip singularity in the future.

Observational constraints on phantomlike braneworld cosmologies

Abstract: We investigate a simple braneworld model in which the universe contains only cold dark matter and a cosmological constant, but the effective dark energy is phantomlike because of extra-dimensional gravity effects. Modified gravity screens the cosmological constant {lambda}, allowing for a larger {lambda}. In practice, observations do not favor any significant screening. We use supernova data, the cosmic microwave background shift parameter, and the baryon oscillation peak in the galaxy distribution to constrain the model. We find the mean value of {omega}{sub m} with 68% confidence limits, and an upper limit on {omega}{sub {lambda}} at the 68% confidence level. The best-fit model is very close to a standard LCDM model, but the LCDM model provides a better fit since it has one less parameter.

Universal fitting formulae for baryon oscillation surveys

Abstract: The next generation of galaxy surveys will attempt to measure the baryon oscillations in the clustering power spectrum with high accuracy. These oscillations encode a preferred scale which may be used as a standard ruler to constrain cosmological parameters and dark energy models. In this paper we present simple analytical fitting formulae for the accuracy with which the preferred scale may be determined in the tangential and radial directions by future spectroscopic and photometric galaxy redshift surveys. We express these accuracies as a function of survey parameters such as the central redshift, volume, galaxy number density and (where applicable) photometric redshift error. These fitting formulae should greatly increase the efficiency of optimizing future surveys, which requires analysis of a potentially vast number of survey configurations and cosmological models. The formulae are calibrated using a grid of Monte Carlo simulations, which are analysed by dividing out the overall shape of the power spectrum before fitting a simple decaying sinusoid to the oscillations. The fitting formulae reproduce the simulation results with a fractional scatter of 7 per cent (10 per cent) in the tangential (radial) directions over a wide range of input parameters. We also indicate how sparse-sampling strategies may enhance the effective survey area if the sampling scale is much smaller than the projected baryon oscillation scale.

Cosmological dynamics and dark energy with a quadratic equation of state: Anisotropic models, large-scale perturbations, and cosmological singularities

Abstract: In standard general-relativistic cosmology, for fluids with a linear equation of state (EoS) $P=w\ensuremath{\rho}$ or scalar fields, the high isotropy of the universe requires special initial conditions: singularities are velocity dominated and anisotropic in general In brane world effective 4-dimensional cosmological models an effective term, quadratic in the energy density, appears in the evolution equations, which has been shown to be responsible for the suppression of anisotropy and inhomogeneities at the singularity under reasonable assumptions Thus in the brane world isotropy is generically built in, and singularities are matter dominated There is no reason why the effective EoS of matter should be linear at the highest energies, and an effective nonlinear EoS may describe dark energy or unified dark matter (Paper I [K Ananda and M Bruni, preceding article, Phys Rev D 74, 023523 (2006)]) In view of this, here we investigate the effects of a quadratic EoS in homogenous and inhomogeneous anisotropic cosmological models in general relativity, in order to understand if in this standard context the quadratic EoS can isotropize the universe at early times With respect to Paper I [K Ananda and M Bruni, preceding article, Phys Rev D 74, 023523 (2006)], here we use the simplified EoS $P=\ensuremath{\alpha}\ensuremath{\rho}+{\ensuremath{\rho}}^{2}/{\ensuremath{\rho}}_{c}$, which still allows for an effective cosmological constant and phantom behavior, and is general enough to analyze the dynamics at high energies We first study homogenous and anisotropic Bianchi I and V models, focusing on singularities Using dynamical systems methods, we find the fixed points of the system and study their stability We find that models with standard nonphantom behavior are in general asymptotic in the past to an isotropic fixed point IS, ie in these models even an arbitrarily large anisotropy is suppressed in the past: the singularity is matter dominated Using covariant and gauge-invariant variables, we then study linear anisotropic and inhomogeneous perturbations about the homogenous and isotropic spatially flat models with a quadratic EoS We find that, in the large-scale limit, all perturbations decay asymptotically in the past, indicating that the isotropic fixed point IS is the general asymptotic past attractor for nonphantom inhomogeneous models with a quadratic EoS

Likelihood techniques for the combined analysis of CMB temperature and polarization power spectra

Abstract: We consider the shape of the likelihood and posterior surfaces to be used when fitting cosmological models to cosmic microwave background (CMB) temperature and polarization power spectra measured from experiments. In the limit of an all-sky survey with Gaussian-distributed pixel noise we show that the true combined likelihood of the four CMB power spectra (TT, TE, EE and BB) has a Wishart distribution and we discuss the properties of this function. We compare various fits to the posterior surface of the Cl values, both in the case of a single auto-power spectrum and for a combination of temperature and polarization data. In the latter case, it is important that the fits can accurately match the Wishart distribution in the limit of near full-sky coverage. Simple extensions of auto-power spectrum fits to include polarization data generally fail to match correlations between the different power spectra in this limit. Directly fitting pixel values on large scales, as undertaken by the Wilkinson Microwave Anisotropy Probe team in their analysis of the 3-yr data, avoids the complications of characterizing the shape of the posterior for the power spectra. Finally, we demonstrate the importance of the likelihood distribution on analytic marginalization, and provide a formula for analytic marginalization over a calibration error given an all-sky survey.

The 2dF-SDSS LRG and QSO Survey: the star formation histories of luminous red galaxies

Abstract: We present a detailed investigation into the recent star formation histories of 5697 luminous red galaxies (LRGs) based on the Hδ (4101 A), and [O II] (3727 A) lines and the D4000 index. LRGs are luminous (L > 3L ∗ ) galaxies which have been selected to have photometric properties consistent with an old, passively evolving stellar population. For this study, we utilize LRGs from the recently completed 2dF-SDSS LRG and QSO Survey (2SLAQ). Equivalent widths of the Hδ and [O II] lines are measured and used to define three spectral types, those with only strong Hδ absorption (k+a), those with strong [O II] in emission (em) and those with both (em+a). All other LRGs are considered to have passive star formation histories. The vast majority of LRGs are found to be passive (∼80 per cent); however, significant numbers of k+a (2.7 per cent), em+a (1.2 per cent) and em LRGs (8.6 per cent) are identified. An investigation into the redshift dependence of the fractions is also performed. A sample of SDSS MAIN galaxies with colours and luminosities consistent with the 2SLAQ LRGs is selected to provide a low-redshift comparison. While the em and em+a fractions are consistent with the low-redshift SDSS sample, the fraction of k+a LRGs is found to increase significantly with redshift. This result is interpreted as an indication of an increasing amount of recent star formation activity in LRGs with redshift. By considering the expected lifetime of the k+a phase, the number of LRGs which will undergo a k+a phase can be estimated. A crude comparison of this estimate with the predictions from semi-analytic models of galaxy formation shows that the predicted

Examining the effect of the map‐making algorithm on observed power asymmetry in WMAP data

Abstract: We analyze first-year data of WMAP to determine the significance of asymmetry in summed power between arbitrarily defined opposite hemispheres. We perform this analysis on maps that we create ourselves from the time-ordered data, using software developed independently of the WMAP team. We find that over the multipole range l = [2, 64], the significance of asymmetry is ~10-4, a value insensitive to both frequency and power spectrum. We determine the smallest multipole ranges exhibiting significant asymmetry and find 12, including l = [2, 3] and [6, 7], for which the significance → 0. Examination of the 12 ranges indicates both an improbable association between the direction of maximum significance and the ecliptic plane (significance ~0.01) and that contours of least significance follow great circles inclined relative to the ecliptic at the largest scales. The great circle for l = [2, 3] passes over previously reported preferred axes and is insensitive to frequency, while the great circle for l = [6, 7] is aligned with the ecliptic poles. We examine how changing map-making parameters, e.g., foreground masking, affects asymmetry. Only one change appreciably reduces asymmetry: asymmetry at large scales (l ≤ 7) is rendered insignificant if the magnitude of the WMAP dipole vector (368.11 km s-1) is increased by ≈1-3 σ (≈2-6 km s-1). While confirmation of this result requires the recalibration of the time-ordered data, such a systematic change would be consistent with observations of frequency-independent asymmetry. We conclude that the use of an incorrect dipole vector, in combination with a systematic or foreground process associated with the ecliptic, may help to explain the observed power asymmetry.

Black hole production in tachyonic preheating

Abstract: We present fully non-linear simulations of a self-interacting scalar field in the early universe undergoing tachyonic preheating. We find that density perturbations on subhorizon scales which are amplified by tachyonic instability maintain long range correlations even during the succeeding parametric resonance, in contrast to the predictions from the standard models of preheating dominated by parametric resonance. As a result the final spectrum exhibits memory and is not universal in shape. We find that throughout the subsequent era of parametric resonance the equation of state of the universe is almost dust-like; hence the Jeans wavelength is much smaller than the horizon scale. If our 2D simulations are accurate reflections of the situation in 3D, then there are wide regions of parameter space ruled out by overproduction of black holes. It is likely however that realistic parameter values, consistent with COBE/WMAP normalization, are safely outside this black hole overproduction region.

Dynamics and Non-Gaussianity in the Weak-Dissipative Warm Inflation Scenario

Abstract: We calculate the general solutions for a warm inflationary scenario with weak dissipation, reviewing the dissipative dynamics of the two-fluid system, and calculate the bispectrum of the gravitational field fluctuations generated in the case where dissipation of the vacuum potential during inflation is the mechanism for structure formation, but is the subdominant effect in the dynamics of the scalar field during slow-roll. The bispectrum is nonzero because of the self-interaction of the scalar field. We compare the predictions with both those of standard, or ``supercooled``, inflationary models, and warm inflation models with strong dissipation and consider the detectability of these levels of non-Gaussianity in the bispectrum of the cosmic microwave background. We find that the levels of non-Gaussianity for warm and supercooled inflation are an order of magnitude different.

Cosmic microwave background multipole alignments in slab topologies

Abstract: Several analyses of the microwave sky maps from the Wilkinson Microwave Anisotropy Probe (WMAP) have drawn attention to alignments amongst the low-order multipoles. Amongst the various possible explanations, an effect of cosmic topology has been invoked by several authors. We focus on an alignment of the first four multipoles (l = 2 to 5) found by Land and Magueijo (2005), and investigate the distribution of their alignment statistic for a set of simulated cosmic microwave background maps for cosmologies with slab-like topology. We find that this topology does offer a modest increase in the probability of the observed value, but that even for the smallest topology considered the probability of the observed value remains below one percent.

Chaplygin gas in light of recent integrated Sachs–Wolfe effect data

Abstract: We investigate the possibility of constraining Chaplygin dark energy models with current integrated Sachs–Wolfe effect data. In the case of a flat universe we found that generalized Chaplygin gas models must have an energy density such that Ωc > 0.55 and an equation of state w 0.55 and w < −0.65 at 95% c.l. Better measurements of the CMB–LSS correlation will be possible with the next generation of deep redshift surveys. This will provide independent and complementary constraints on unified dark energy models such as the Chaplygin gas.

Gravitational waves and cosmological braneworlds: A characteristic evolution scheme

Abstract: Motivated by the problem of the evolution of bulk gravitational waves in Randall-Sundrum cosmology, we develop a characteristic numerical scheme to solve $1+1$ dimensional wave equations in the presence of a moving timelike boundary. The scheme exhibits quadratic convergence, is capable of handling arbitrary brane trajectories, and is easily extendible to non-AdS bulk geometries. We use our method to contrast two different prescriptions for the bulk fluctuation initial conditions found in the literature; namely, those of Hiramatsu et al. and Ichiki and Nakamura. We find that, if the initial data surface is set far enough in the past, the late-time waveform on the brane is insensitive to the choice between the two possibilities; and we present numeric and analytic evidence that this phenomenon generalizes to more generic initial data. Observationally, the main consequence of this work is to reaffirm previous claims that the stochastic gravitational wave spectrum is predominantly flat ${\ensuremath{\Omega}}_{\mathrm{GW}}\ensuremath{\propto}{f}^{0}$, in contradiction with naive predictions from the effective 4-dimensional theory. Furthermore, this flat spectrum result is predicted to be robust against uncertainties in (or modifications of) the bulk initial data, provided that the energy scale of brane inflation is high enough.

Gravastars supported by nonlinear electrodynamics

Abstract: Gravastar models have recently been proposed as an alternative to black holes, mainly to avoid the problematic issues associated with event horizons and singularities. In this work, a wide variety of gravastar models within the context of nonlinear electrodynamics are constructed. Using the $F$ representation, specific forms of Lagrangians are considered describing magnetic gravastars, which may be interpreted as self-gravitating magnetic monopoles with charge $g$. Using the dual $P$ formulation of nonlinear electrodynamics, electric gravastar models are constructed by considering specific structural functions, and the characteristics and physical properties of the solutions are further explored. These interior nonlinear electrodynamic geometries are matched to an exterior Schwarzschild spacetime at a junction interface.

The luminosities, sizes and velocity dispersions of Brightest Cluster Galaxies: Implications for formation history

Abstract: The size-luminosity relation of early-type Brightest Cluster Galaxies (BCGs), R_e ~ L^0.88, is steeper than that for the bulk of the early-type galaxy population, for which R_e ~ L^0.68. In addition, although BCGs are hardly offset from the Fundamental Plane defined by the bulk of the early-type population, they show considerably smaller scatter. The larger than expected sizes of BCGs, and the increased homogeneity, are qualitatively consistent with models which seek to explain the colors of the most massive galaxies by invoking dry dissipationless mergers, since dissipation tends to reduce the sizes of galaxies, and wet mergers which result in star formation would tend to increase the scatter in luminosity at fixed size and velocity dispersion. Furthermore, BCGs define the same g-r color-magnitude relation as the bulk of the early-type population. If BCGs formed from dry mergers, then BCG progenitors must have been red for their magnitudes, suggesting that they hosted older stellar populations than typical for their luminosities. Our findings have two other consequences. First, the R_e-L relation of the early-type galaxy population as a whole (i.e., normal plus BCG) exhibits some curvature. Some of this curvature must be a consequence of the fact that an increasing fraction of the most luminous galaxies are BCGs. The second consequence is suggested by the fact that, despite following a steeper size-luminosity relation, BCGs tend to define a tight relation between dynamical mass R_e sigma^2/G and luminosity. As consequence, we find that BCGs define a shallower sigma-L relation than the bulk of the early-type galaxy population.

Cosmology with inhomogeneous magnetic fields

Abstract: We review spacetime dynamics in the presence of large-scale electromagnetic fields and then consider the effects of the magnetic component on perturbations to a spatially homogeneous and isotropic universe. Using covariant techniques, we refine and extend earlier work and provide the magnetohydrodynamic equations that describe inhomogeneous magnetic cosmologies in full general relativity. Specialising this system to perturbed Friedmann-Robertson-Walker models, we examine the effects of the field on the expansion dynamics and on the growth of density inhomogeneities, including non-adiabatic modes. We look at scalar perturbations and obtain analytic solutions for their linear evolution in the radiation, dust and inflationary eras. In the dust case we also calculate the magnetic analogue of the Jeans length. We then consider the evolution of vector perturbations and find that the magnetic presence generally reduces the decay rate of these distortions. Finally, we examine the implications of magnetic fields for the evolution of cosmological gravitational waves.

Bayesian photometric redshifts for weak-lensing applications

Abstract: The next generation of weak gravitational lensing surveys are capable of generating good measurements of cosmological parameters, provided that, amongst other requirements, adequate redshift information is available for the background galaxies that are measured. It is frequently assumed that photometric redshift techniques provide the means to achieve this. Here, we compare Bayesian and frequentist approaches to photometric redshift estimation, particularly at faint magnitudes. We identify and discuss the biases that are inherent in the various methods, and describe an optimum Bayesian method for extracting redshift distributions from photometric data.

Numerical study of curvature perturbations in a brane-world inflation at high-energies

Abstract: We study the evolution of scalar curvature perturbations in a brane-world inflation model in a 5D anti-de Sitter space–time. The inflaton perturbations are confined to a 4D brane but they are coupled to the 5D bulk metric perturbations. We numerically solve full coupled equations for the inflaton perturbations and the 5D metric perturbations using the Hawkins–Lidsey inflationary model. At an initial time, we assume that the bulk is unperturbed. We find that the inflaton perturbations at high energies are strongly coupled to the bulk metric perturbations even on subhorizon scales, leading to the suppression of the amplitude of the comoving curvature perturbations at a horizon crossing. This indicates that the linear perturbations of the inflaton field do not obey the usual 4D Klein–Gordon equation due to the coupling to 5D gravitational fields on small scales, and it is necessary to quantize the coupled brane–bulk system in a consistent way in order to calculate the spectrum of the scalar perturbations in a brane-world inflation.

Crossing the phantom divide without phantom matter

Abstract: A class of braneworld models can lead to phantom-like acceleration of the late universe, but without the need for any phantom matter. In the simplest models, the universe contains only cold dark matter and a cosmological constant. We generalize these models by introducing a quintessence field. The new feature in our models is that quintessence leads to a crossing of the phantom divide, $w=-1$. This is a purely gravitational effect, and there is no phantom instability. Furthermore, the Hubble parameter is always decreasing, and there is no big rip singularity in the future.