10 research outputs found
Model selection applied to reconstruction of the Primordial Power Spectrum
The preferred shape for the primordial spectrum of curvature perturbations is
determined by performing a Bayesian model selection analysis of cosmological
observations. We first reconstruct the spectrum modelled as piecewise linear in
\log k between nodes in k-space whose amplitudes and positions are allowed to
vary. The number of nodes together with their positions are chosen by the
Bayesian evidence, so that we can both determine the complexity supported by
the data and locate any features present in the spectrum. In addition to the
node-based reconstruction, we consider a set of parameterised models for the
primordial spectrum: the standard power-law parameterisation, the spectrum
produced from the Lasenby & Doran (LD) model and a simple variant
parameterisation. By comparing the Bayesian evidence for different classes of
spectra, we find the power-law parameterisation is significantly disfavoured by
current cosmological observations, which show a preference for the LD model.Comment: Minor changes to match version accepted by JCA
Cosmological Applications of a Wavelet Analysis on the Sphere
The cosmic microwave background (CMB) is a relic radiation of the Big Bang and as such it contains a wealth of cosmological information. Statistical analyses of the CMB, in conjunction with other cosmological observables, represent some of the most powerful techniques available to cosmologists for placing strong constraints on the cosmological parameters that describe the origin, content and evolution of the Universe. The last decade has witnessed the introduction of wavelet analyses in cosmology and, in particular, their application to the CMB. We review here spherical wavelet analyses of the CMB that test the standard cosmological concordance model. The assumption that the temperature anisotropies of the CMB are a realisation of a statistically isotropic Gaussian random field on the sphere is questioned. Deviations from both statistical isotropy and Gaussianity are detected in the reviewed works, suggesting more exotic cosmological models may be required to explain our Universe. We also review spherical wavelet analyses that independently provide evidence for dark energy, an exotic component of our Universe of which we know very little currently. The effectiveness of accounting correctly for the geometry of the sphere in the wavelet analysis of full-sky CMB data is demonstrated by the highly significant detections of physical processes and effects that are made in these reviewed work
Reconstruction of the Dark Energy equation of state
One of the main challenges of modern cosmology is to investigate the nature
of dark energy in our Universe. The properties of such a component are normally
summarised as a perfect fluid with a (potentially) time-dependent
equation-of-state parameter . We investigate the evolution of this
parameter with redshift by performing a Bayesian analysis of current
cosmological observations. We model the temporal evolution as piecewise linear
in redshift between `nodes', whose -values and redshifts are allowed to
vary. The optimal number of nodes is chosen by the Bayesian evidence. In this
way, we can both determine the complexity supported by current data and locate
any features present in . We compare this node-based reconstruction with
some previously well-studied parameterisations: the Chevallier-Polarski-Linder
(CPL), the Jassal-Bagla-Padmanabhan (JBP) and the Felice-Nesseris-Tsujikawa
(FNT). By comparing the Bayesian evidence for all of these models we find an
indication towards possible time-dependence in the dark energy
equation-of-state. It is also worth noting that the CPL and JBP models are
strongly disfavoured, whilst the FNT is just significantly disfavoured, when
compared to a simple cosmological constant . We find that our node-based
reconstruction model is slightly disfavoured with respect to the CDM
model.Comment: 17 pages, 5 figures, minor correction
Observational constraints on conformal time symmetry, missing matter and double dark energy
The current concordance model of cosmology is dominated by two mysterious
ingredients: dark matter and dark energy. In this paper, we explore the
possibility that, in fact, there exist two dark-energy components: the
cosmological constant , with equation-of-state parameter
, and a `missing matter' component with , which we
introduce here to allow the evolution of the universal scale factor as a
function of conformal time to exhibit a symmetry that relates the big bang to
the future conformal singularity, such as in Penrose's conformal cyclic
cosmology. Using recent cosmological observations, we constrain the present-day
energy density of missing matter to be . This is
consistent with the standard CDM model, but constraints on the energy
densities of all the components are considerably broadened by the introduction
of missing matter; significant relative probability exists even for
, and so the presence of a missing matter component
cannot be ruled out. As a result, a Bayesian model selection analysis only
slightly disfavours its introduction by 1.1 log-units of evidence. Foregoing
our symmetry requirement on the conformal time evolution of the universe, we
extend our analysis by allowing to be a free parameter. For this more
generic `double dark energy' model, we find and
, which is again consistent with the standard
CDM model, although once more the posterior distributions are
sufficiently broad that the existence of a second dark-energy component cannot
be ruled out. The model including the second dark energy component also has an
equivalent Bayesian evidence to CDM, within the estimation error, and
is indistinguishable according to the Jeffreys guideline.Comment: Revised version emphasising a different version of the underlying
symmetry, as published in JCA
"Author! Author!" : Shakespeare and biography
Original article can be found at: http://www.informaworld.com/smpp/title~content=t714579626~db=all Copyright Informa / Taylor & Francis Group. DOI: 10.1080/17450910902764454Since 1996, not a year has passed without the publication of at least one Shakespeare biography. Yet for many years the place of the author in the practice of understanding literary works has been problematized, and even on occasions eliminated. Criticism reads the “works”, and may or may not refer to an author whose “life” contributed to their meaning. Biography seeks the author in the works, the personality that precedes the works and gives them their characteristic shape and meaning. But the form of literary biography addresses the unusual kind of “life” that puts itself into “works”, and this is particularly challenging where the “works” predominate massively over the salient facts of the “life”. This essay surveys the current terrain of Shakespeare biography, and considers the key questions raised by the medium: can we know anything of Shakespeare's “personality” from the facts of his life and the survival of his works? What is the status of the kind of speculation that inevitably plays a part in biographical reconstruction? Are biographers in the end telling us as much about themselves as they tell us about Shakespeare?Peer reviewe
A search for primordial anisotropies in the cosmic microwave background radiation: first observations at 13.5 GHz with the Cosmic Anisotropy Telescope
We report the first observations of the cosmic microwave background radiation (CMBR) at 13.5 GHz made with the Cosmic Anisotropy Telescope (CAT) near Cambridge. After removal of foreground radio sources brighter than 10 mJy, using the Ryle Telescope (RT), the map shows evidence for residual structure on a scale of approximately 1/2 deg with an rms variation of 18 mJy. We intend to use multifrequency measurements to distinguish between Galactic and CMBR components