Testing CMB polarization data using position angles

We consider a novel null test for contamination which can be applied to CMB polarization data that involves analysis of the statistics of the polarization position angles. Specifically, we will concentrate on using histograms of the measured position angles to illustrate the idea. Such a test has been used to identify systematics in the NVSS point source catalogue with an amplitude well below the noise level. We explore the statistical properties of polarization angles in CMB maps. If the polarization angle is not correlated between pixels, then the errors follow a simple $\sqrt{N_{pix}}$ law. However this is typically not the case for CMB maps since these have correlations which result in an increase in the variance since the effective number of independent pixels is reduced. Then we illustrate how certain classes of systematic errors can result in very obvious patterns in these histograms, and thus that these errors could possibly be identified using this method. We discuss how this idea might be applied in a realistic context, and make a preliminary analysis of the WMAP7 data, finding evidence of a systematic error in the Q and W band data, consistent with a constant offset in Q and U.Comment: Accepted by MNRA

A Skyrme lattice with hexagonal symmetry

Recently it has been found that the structure of Skyrmions has a close analogy to that of fullerene shells in carbon chemistry. In this letter we show that this analogy continues further, by presenting a Skyrme field that describes a lattice of Skyrmions with hexagonal symmetry. This configuration, a novel `domain wall' in the Skyrme model, has low energy per baryon (about 6% above the Faddeev-Bogomolny bound) and in many ways is analogous to graphite. By comparison to the energy per baryon of other known Skyrmions and also the Skyrme crystal, we discuss the possibility of finding Skyrmion shells of higher charge.Comment: 12 pages, 1 figure. To appear in Phys. Lett.

Constraints on the solid dark universe model

If the dark energy is modelled as a relativistic elastic solid then the standard CDM and $\Lambda$CDM models, as well as lattice configurations of cosmic strings or domain walls, are points in the two-dimensional parameter space $(w,c_{\rm s}^2)$. We present a detailed analysis of the best fitting cosmological parameters in this model using data from a range of observations. We find that the $\chi^2$ is improved by $\sim 10$ by including the two parameters and that the $w=-1$ $\Lambda$CDM model is only the best fit to the data when a large number of different datasets are included. Using CMB observations alone we find that $w=-0.38\pm 0.16$ and with the addition of Large-Scale Structure data $w=-0.62\pm 0.15$ and $\log c_{\rm s}=-0.77\pm 0.28$. We conclude that the models based on topological defects provide a good fit to the current data, although $\Lambda$CDM cannot be ruled out.Comment: 10 page

Evidence for massive neutrinos from CMB and lensing observations

We discuss whether massive neutrinos (either active or sterile) can reconcile some of the tensions within cosmological data that have been brought into focus by the recently released {\it Planck} data. We point out that a discrepancy is present when comparing the primary CMB and lensing measurements both from the CMB and galaxy lensing data using CFHTLenS, similar to that which arises when comparing CMB measurements and SZ cluster counts. A consistent picture emerges and including a prior for the cluster constraints and BAOs we find that: for an active neutrino model with 3 degenerate neutrinos, $\sum m_{\nu}= (0.320 \pm 0.081)\,{\rm eV}$, whereas for a sterile neutrino, in addition to 3 neutrinos with a standard hierarchy and $\sum m_{\nu}= 0.06\,{\rm eV}$, $m_{\nu, \, \rm sterile}^{\rm eff}= (0.450 \pm 0.124)\,{\rm eV}$ and $\Delta N_{\rm eff} = 0.45 \pm 0.23$. In both cases there is a significant detection of modification to the neutrino sector from the standard model and in the case of the sterile neutrino it is possible to reconcile the BAO and local $H_0$ measurements. However, a caveat to our result is some internal tension between the CMB and lensing/cluster observations, and the masses are in excess of those estimated from the shape of the matter power spectrum from galaxy surveys.Comment: 5 pages, 3 figures, clarifications added, comparison with WMAP-9 plus high-l added, version accepted in Physical Review Letter

Approximation of the potential in scalar field dark energy models

We study the nature of potentials in scalar field based models for dark energy - with both canonical and noncanonical kinetic terms. We calculate numerically, and using an analytic approximation around $a\approx 1$, potentials for models with constant equation-of-state parameter, $w_{\phi}$. We find that for a wide range of models with canonical and noncanonical kinetic terms there is a simple approximation for the potential that holds when the scale factor is in the range $0.6\lesssim a\lesssim 1.4$. We discuss how this form of the potential can also be used to represent models with non-constant $w_{\phi}$ and, hence, how it could be used in reconstruction from cosmological data.Comment: 17 pages, 6 figures. Accepted by Phys. Rev.

Optimizing the yield of Sunyaev-Zel'dovich cluster surveys

We consider the optimum depth of a cluster survey selected using the Sunyaev-Zel'dovich effect. By using simple models for the evolution of the cluster mass function and detailed modeling for a variety of observational techniques, we show that the optimum survey yield is achieved when the average size of the clusters selected is close to the size of the telescope beam. For a total power measurement, we compute the optimum noise threshold per beam as a function of the beam size and then discuss how our results can be used in more general situations. As a by-product we gain some insight into what is the most advantageous instrumental set-up. In the case of beam switching observations one is not severely limited if one manages to set the noise threshold close to the point which corresponds to the optimum yield. By defining a particular reference configuration, we show how our results can be applied to interferometer observations. Considering a variety of alternative scenarios, we discuss how robust our conclusions are to modifications in the cluster model and cosmological parameters. The precise optimum is particularly sensitive to the amplitude of fluctuations and the profile of the gas in the cluster.Comment: 16 pages, 18 figure

Multiple-scales analysis of cosmological perturbations in brane-worlds

We present a new approximation method for solving the equations of motion for cosmological tensor perturbations in a Randall-Sundrum brane-world model of the type with one brane in a five-dimensional anti-de Sitter spacetime. This method avoids the problem of coordinate singularities inherent in some methods. At leading order, the zero-mode solution replicates the evolution of perturbations in a four-dimensional Friedmann-Robertson-Walker universe in the absence of any tensor component to the matter perturbation on the brane. At next order, there is a mode-mixing effect, although, importantly, the zero-mode does not source any other modes.Comment: 13 pages, Revte

Effective action approach to cosmological perturbations in dark energy and modified gravity

In light of upcoming observations modelling perturbations in dark energy and modified gravity models has become an important topic of research. We develop an effective action to construct the components of the perturbed dark energy momentum tensor which appears in the perturbed generalized gravitational field equations, {\delta}G_{\mu\nu} = 8{\pi}G{\delta}T_{\mu\nu} + {\delta}U_{\mu\nu} for linearized perturbations. Our method does not require knowledge of the Lagrangian density of the dark sector to be provided, only its field content. The method is based on the fact that it is only necessary to specify the perturbed Lagrangian to quadratic order and couples this with the assumption of global statistical isotropy of spatial sections to show that the model can be specified completely in terms of a finite number of background dependent functions. We present our formalism in a coordinate independent fashion and provide explicit formulae for the perturbed conservation equation and the components of {\delta}U_{\mu\nu} for two explicit generic examples: (i) the dark sector does not contain extra fields, L = L(g_{\mu\nu}) and (ii) the dark sector contains a scalar field and its first derivative L = L(g_{\mu\nu}, {\phi}, \nabla_{\mu}{\phi}). We discuss how the formalism can be applied to modified gravity models containing derivatives of the metric, curvature tensors, higher derivatives of the scalar fields and vector fields.Comment: Version accepted by JCAP. Typos corrected. Covariant decoupling conditions adde

Tight constraints on F- and D-term hybrid inflation scenarios

We use present cosmological data from the cosmic microwave background, large-scale structure and deuterium at high redshifts to constrain supersymmetric F- and D-term hybrid inflation scenarios including possible contributions to the CMB anisotropies from cosmic strings. Using two different realizations of the cosmic string spectrum, we find that the minimal version of the D-term model is ruled out at high significance. F-term models are also in tension with the data. We also discuss possible non-minimal variants of the models.Comment: 4 pages, 2 figure