arXiv subjects

Andrew R. Liddle

Publications and source records attributed to Andrew R. Liddle.

At least 73 records · Page 4Linked to original sources

arXiv2008

Triple unification of inflation, dark matter, and dark energy using a single field

We construct an explicit scenario whereby the same material driving inflation in the early Universe can comprise dark matter in the present Universe, using a simple quadratic potential. Following inflation and preheating, the density of inflaton/dark matter particles is reduced to the observed level by a period of thermal inflation, of a duration already invoked in the literature for other reasons. Within the context of the string landscape, one can further argue for a non-zero vacuum energy of this field, thus unifying inflation, dark matter and dark energy into a single fundamental field.

astro-ph↗

arXiv2008

The Sunyaev-Zel'dovich temperature of the intracluster medium

The relativistic Sunyaev-Zel'dovich (SZ) effect offers a method, independent of X-ray, for measuring the temperature of the intracluster medium (ICM) in the hottest systems. Here, using N-body/hydrodynamic simulations of three galaxy clusters, we compare the two quantities for a non-radiative ICM, and for one that is subject both to radiative cooling and strong energy feedback from galaxies. Our study has yielded two interesting results. Firstly, in all cases, the SZ temperature is hotter than the X-ray temperature and is within ten per cent of the virial temperature of the cluster. Secondly, the mean SZ temperature is less affected by cooling and feedback than the X-ray temperature. Both these results can be explained by the SZ temperature being less sensitive to the distribution of cool gas associated with cluster substructure. A comparison of the SZ and X-ray temperatures (measured for a sample of hot clusters) would therefore yield interesting constraints on the thermodynamic structure of the intracluster gas.

astro-ph↗

arXiv2007

Stability of multi-field cosmological solutions

We explore the stability properties of multi-field solutions of assisted inflation type, where several fields collectively evolve to the same configuration. In the case of noninteracting fields, we show that the condition for such solutions to be stable is less restrictive than that required for tracking in quintessence models. Our results, which do not rely on the slow-roll approximation, further indicate that to linear order in homogeneous perturbations the fields are in fact unaware of each other's existence. We end by generalizing our results to some cases of interacting fields and to other background solutions and dynamics, including the high-energy braneworld.

astro-ph↗

arXiv2007

When can the Planck satellite measure spectral index running?

We use model selection forecasting to assess the ability of the Planck satellite to make a positive detection of spectral index running. We simulate Planck data for a range of assumed cosmological parameter values, and carry out a three-way Bayesian model comparison of a Harrison-Zel'dovich model, a power-law model, and a model including running. We find that Planck will be able to strongly support running only if its true value satisfies |dn/d ln k| > 0.02.

astro-ph↗

arXiv2007

The XMM Cluster Survey: The Dynamical State of XMMXCS J2215.9-1738 at z=1.457

We present new spectroscopic observations of the most distant X-ray selected galaxy cluster currently known, XMMXCS J2215.9-1738 at z=1.457, obtained with the DEIMOS instrument at the W. M. Keck Observatory, and the FORS2 instrument on the ESO Very Large Telescope. Within the cluster virial radius, as estimated from the cluster X-ray properties, we increase the number of known spectroscopic cluster members to 17 objects, and calculate the line of sight velocity dispersion of the cluster to be 580+/-140 km/s. We find mild evidence that the velocity distribution of galaxies within the virial radius deviates from a single Gaussian. We show that the properties of J2215.9-1738 are inconsistent with self-similar evolution of local X-ray scaling relations, finding that the cluster is underluminous given its X-ray temperature, and that the intracluster medium contains ~2-3 times the kinetic energy per unit mass of the cluster galaxies. These results can perhaps be explained if the cluster is observed in the aftermath of an off-axis merger. Alternatively, heating of the intracluster medium through supernovae and/or Active Galactic Nuclei activity, as is required to explain the observed slope of the local X-ray luminosity-temperature relation, may be responsible.

astro-ph↗

arXiv2007

The evolution of clusters in the CLEF cosmological simulation: X-ray structural and scaling properties

We present results from a study of the X-ray cluster population that forms within the CLEF cosmological hydrodynamics simulation, a large N-body/SPH simulation of the Lambda CDM cosmology with radiative cooling, star formation and feedback. The scaled projected temperature and entropy profiles at z=0 are in good agreement with recent high-quality observations of cool core clusters, suggesting that the simulation grossly follows the processes that structure the intracluster medium (ICM) in these objects. Cool cores are a ubiquitous phenomenon in the simulation at low and high redshift, regardless of a cluster's dynamical state. This is at odds with the observations and so suggests there is still a heating mechanism missing from the simulation. Using a simple, observable measure of the concentration of the ICM, which correlates with the apparent mass deposition rate in the cluster core, we find a large dispersion within regular clusters at low redshift, but this diminishes at higher redshift, where strong "cooling-flow" systems are absent in our simulation. Consequently, our results predict that the normalisation and scatter of the luminosity-temperature relation should decrease with redshift; if such behaviour turns out to be a correct representation of X-ray cluster evolution, it will have significant consequences for the number of clusters found at high redshift in X-ray flux-limited surveys.

astro-ph↗

arXiv2006

Tachyon dark energy models: dynamics and constraints

We explore the dynamics of dark energy models based on a Dirac-Born-Infeld (DBI) tachyonic action, studying a range of potentials. We numerically investigate the existence of tracking behaviour and determine the present-day value of the equation of state parameter and its running, which are compared with observational bounds. We find that tachyon models have quite similar phenomenology to canonical quintessence models. While some potentials can be selected amongst many possibilities and fine-tuned to give viable scenarios, there is no apparent advantage in choosing a DBI scalar field instead of a Klein-Gordon one.

astro-ph↗

arXiv2006

Cosmological model selection

Model selection aims to determine which theoretical models are most plausible given some data, without necessarily asking about the preferred values of the model parameters. A common model selection question is to ask when new data require introduction of an additional parameter, describing a newly-discovered physical effect. We review several model selection statistics, and then focus on use of the Bayesian evidence, which implements the usual Bayesian analysis framework at the level of models rather than parameters. We describe our CosmoNest code, which is the first computationally-efficient implementation of Bayesian model selection in a cosmological context. We apply it to recent WMAP satellite data, examining the need for a perturbation spectral index differing from the scale-invariant (Harrison-Zel'dovich) case.

astro-ph↗

arXiv2006

Nflation: non-gaussianity in the horizon-crossing approximation

We analyze the cosmic non-gaussianity produced in inflation models with multiple uncoupled fields with monomial potentials, such as Nflation. Using the horizon-crossing approximation to compute the non-gaussianity, we show that when each field has the same form of potential, the prediction is independent the number of fields, their initial conditions, and the spectrum of masses/couplings. It depends only on the number of e-foldings after the horizon crossing of observable perturbations. We also provide a further generalization to the case where the fields can have monomial potentials with different powers. Unless the horizon-crossing approximation is substantially violated, the predicted non-gaussianity is too small to ever be observed.

astro-ph↗

arXiv2006

Cosmic reionization constraints on the nature of cosmological perturbations

We study the reionization history of the Universe in cosmological models with non-Gaussian density fluctuations, taking them to have a renormalized $χ^2$ probability distribution function parametrized by the number of degrees of freedom, $ν$. We compute the ionization history using a simple semi-analytical model, considering various possibilities for the astrophysics of reionization. In all our models we require that reionization is completed prior to $z=6$, as required by the measurement of the Gunn--Peterson optical depth from the spectra of high-redshift quasars. We confirm previous results demonstrating that such a non-Gaussian distribution leads to a slower reionization as compared to the Gaussian case. We further show that the recent WMAP three-year measurement of the optical depth due to electron scattering, $τ=0.09 \pm 0.03$, weakly constrains the allowed deviations from Gaussianity on the small scales relevant to reionization if a constant spectral index is assumed. We also confirm the need for a significant suppression of star formation in mini-halos, which increases dramatically as we decrease $ν$.

astro-ph↗

arXiv2006

Model selection as a science driver for dark energy surveys

A key science goal of upcoming dark energy surveys is to seek time evolution of the dark energy. This problem is one of {\em model selection}, where the aim is to differentiate between cosmological models with different numbers of parameters. However, the power of these surveys is traditionally assessed by estimating their ability to constrain parameters, which is a different statistical problem. In this paper we use Bayesian model selection techniques, specifically forecasting of the Bayes factors, to compare the abilities of different proposed surveys in discovering dark energy evolution. We consider six experiments -- supernova luminosity measurements by the Supernova Legacy Survey, SNAP, JEDI, and ALPACA, and baryon acoustic oscillation measurements by WFMOS and JEDI -- and use Bayes factor plots to compare their statistical constraining power. The concept of Bayes factor forecasting has much broader applicability than dark energy surveys.

astro-ph↗

arXiv2006

Cosmic microwave background multipole alignments in slab topologies

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 (\ell = 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.

astro-ph↗

arXiv2006

A Nested Sampling Algorithm for Cosmological Model Selection

The abundance of new cosmological data becoming available means that a wider range of cosmological models are testable than ever before. However, an important distinction must be made between parameter fitting and model selection. While parameter fitting simply determines how well a model fits the data, model selection statistics, such as the Bayesian Evidence, are now necessary to choose between these different models, and in particular to assess the need for new parameters. We implement a new evidence algorithm known as nested sampling, which combines accuracy, generality of application and computational feasibility, and apply it to some cosmological datasets and models. We find that a five-parameter model with Harrison-Zel'dovich initial spectrum is currently preferred.

astro-ph↗

arXiv2005

Flow equations in generalized braneworld scenarios

We discuss the flow equations in the context of general braneworld cosmologies with a modified Friedmann equation, for either an ordinary scalar field or a Dirac-Born-Infeld tachyon as inflaton candidates. The 4D, Randall-Sundrum, and Gauss-Bonnet cases are compared, using the patch formalism which provides a unified description of these models. The inflationary dynamics is described by a tower of flow parameters that can be evolved in time to select a particular subset of points in the space of cosmological observables. We analyze the stability of the fixed points in all the cosmologies (our results in the 4D case already extending those in the literature). Numerical integration of the flow equations shows that the predictions of the Gauss-Bonnet braneworld differ significantly as compared to the Randall-Sundrum and 4D scenarios, whereas tachyon inflation gives tensor perturbations smaller than those in the presence of a normal scalar field. These results are extended to the realization of a noncommutative space-time preserving maximal symmetry. In this case the tensor-to-scalar signal is unchanged, while blue-tilted spectra are favoured.

astro-ph↗

arXiv2005

Direct reconstruction of the quintessence potential

We describe an algorithm which directly determines the quintessence potential from observational data, without using an equation of state parametrisation. The strategy is to numerically determine observational quantities as a function of the expansion coefficients of the quintessence potential, which are then constrained using a likelihood approach. We further impose a model selection criterion, the Bayesian Information Criterion, to determine the appropriate level of the potential expansion. In addition to the potential parameters, the present-day quintessence field velocity is kept as a free parameter. Our investigation contains unusual model types, including a scalar field moving on a flat potential, or in an uphill direction, and is general enough to permit oscillating quintessence field models. We apply our method to the `gold' Type Ia supernovae sample of Riess et al. (2004), confirming the pure cosmological constant model as the best description of current supernovae luminosity-redshift data. Our method is optimal for extracting quintessence parameters from future data.

astro-ph↗

arXiv2005

Structure formation constraints on the Jordan-Brans-Dicke theory

We use cosmic microwave background data from WMAP, ACBAR, VSA and CBI, and galaxy power spectrum data from 2dF, to constrain flat cosmologies based on the Jordan-Brans-Dicke theory, using a Markov Chain Monte Carlo approach. Using a parametrization based on ξ=1/4ω, and performing an exploration in the range \lnξ\in [-9,3], we obtain a 95% marginalized probability bound of \lnξ< -6.2, corresponding to a 95% marginalized probability lower bound on the Brans-Dicke parameter ω>120.

astro-ph↗

arXiv2004

A new calculation of the mass fraction of primordial black holes

We revisit the calculation of the abundance of primordial black holes (PBHs) formed from primordial density perturbations, using a formation criterion derived by Shibata and Sasaki which refers to a metric perturbation variable rather than the usual density contrast. We implement a derivation of the PBH abundance which uses peaks theory, and compare it to the standard calculation based on a Press--Schechter-like approach. We find that the two are in reasonable agreement if the Press--Schechter threshold is in the range $Δ_{\rm th} \simeq 0.3$ to 0.5, but advocate use of the peaks theory expression which is based on a sounder theoretical footing.

astro-ph↗

arXiv2004

How many cosmological parameters?

Constraints on cosmological parameters depend on the set of parameters chosen to define the model which is compared with observational data. I use the Akaike and Bayesian information criteria to carry out cosmological model selection, in order to determine the parameter set providing the preferred fit to the data. Applying the information criteria to the current cosmological data sets indicates, for example, that spatially-flat models are statistically preferred to closed models, and that possible running of the spectral index has lower significance than inferred from its confidence limits. I also discuss some problems of statistical assessment arising from there being a large number of `candidate' cosmological parameters that can be investigated for possible cosmological implications, and argue that 95% confidence is too low a threshold to robustly identify the need for new parameters in model fitting. The best present description of cosmological data uses a scale-invariant (n=1) spectrum of gaussian adiabatic perturbations in a spatially-flat Universe, with the cosmological model requiring only five fundamental parameters to fully specify it.

astro-ph↗