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A. N. Lasenby

Publications and source records attributed to A. N. Lasenby.

At least 109 records · Page 6Linked to original sources
arXiv1999

Bayesian `Hyper-Parameters' Approach to Joint Estimation: The Hubble Constant from CMB Measurements

Recently several studies have jointly analysed data from different cosmological probes with the motivation of estimating cosmological parameters. Here we generalise this procedure to take into account the relative weights of various probes. This is done by including in the joint χ^2 function a set of `Hyper-Parameters', which are dealt with using Bayesian considerations. The resulting algorithm (in the case of uniform priors on the log of the Hyper-Parameters) is very simple: instead of minimising \sum χ_j^2 (where χ_j^2 is per data set j) we propose to minimise \sum N_j \ln (χ_j^2) (where N_j is the number of data points per data set j). We illustrate the method by estimating the Hubble constant H_0 from different sets of recent CMB experiments (including Saskatoon, Python V, MSAM1, TOCO and Boomerang).

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arXiv1999

Microwave background anisotropies due to non-linear structures in open and Lambda universes

A new method arising from a gauge-theoretic approach to general relativity is applied to the formation of clusters in an expanding universe. The three cosmological models (Omega_0=1, Omega_Lambda=0), (Omega_0=0.3, Omega_Lambda=0.7), (Omega_0=0.3, Omega_Lambda=0) are considered, which extends our previous application (Lasenby et al. 1999 -"http://xxx.soton.ac.uk/abs/astro-ph/9810123"-, Dabrowski et al. 1999 -"http://xxx.soton.ac.uk/abs/astro-ph/9810149"-). A simple initial velocity and density perturbation of finite extent is imposed at the epoch z=1000 and we investigate the subsequent evolution of the density and velocity fields for clusters observed at redshifts z=1, z=2 and z=3. Photon geodesics and redshifts are also calculated so that the Cosmic Microwave Background (CMB) anisotropies due to collapsing clusters can be estimated. We find that the central CMB temperature decrement is slightly stronger and extends to larger angular scales in the non-zero Omega_Lambda case. This effect is strongly enhanced in the open case. Gravitational lensing effects are also considered and we apply our model to the reported microwave decrement observed towards the quasar pair PC 1643+4631 A & B.

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arXiv1999

Detection of Cosmic Microwave Background Structure in a Second Field with the Cosmic Anisotropy Telescope

We describe observations at frequencies near 15 GHz of the second 2x2 degree field imaged with the Cambridge Cosmic Anisotropy Telescope (CAT). After the removal of discrete radio sources, structure is detected in the images on characteristic scales of about half a degree, corresponding to spherical harmonic multipoles in the approximate range l= 330--680. A Bayesian analysis confirms that the signal arises predominantly from the cosmic microwave background (CMB) radiation for multipoles in the lower half of this range; the average broad-band power in a bin with centroid l=422 (theta = 51') is estimated to be Delta_T/T=2.1^{+0.4}_{-0.5} x 10^{-5}. For multipoles centred on l=615 (theta =35'), we find contamination from Galactic emission is significant, and constrain the CMB contribution to the measured power in this bin to be Delta_T/T <2.0 x 10^{-5} (1-sigma upper limit). These new results are consistent with the first detection made by CAT in a completely different area of sky. Together with data from other experiments, this new CAT detection adds weight to earlier evidence from CAT for a downturn in the CMB power spectrum on scales smaller than 1 degree. Improved limits on the values of H_0 and Omega are determined using the new CAT data.

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arXiv1999

Cosmological parameters from cluster abundances, CMB and IRAS

We combine information on cosmological parameters from cluster abundances, CMB primordial anisotropies and the IRAS 1.2 Jy galaxy redshift survey. We take as free parameters the present values of the total matter density of the universe, Omega_m, the Hubble parameter, h, sigma_8, and the IRAS biasing factor, b_IRAS. We assume that the universe is spatially flat, with a cosmological constant, and that structure formed from adiabatic initial fluctuations with a Harrison-Zel'dovich power spectrum. The nucleosynthesis value Omega_b=0.019/h^2 is adopted. We use the full three- and four- dimensional likelihood functions for each data set and marginalise these to two- and one- dimensional distributions in a Bayesian way, integrating over the other parameters. It is shown that the three data sets are in excellent agreement, with a best fit point of Omega_m= 1-Omega_Lambda=0.36, h=0.54, sigma_8=0.74, and b_IRAS=1.08. This point is within one sigma of the minimum for each data set alone. Pairs of these data sets have their degeneracies in sufficiently different directions that using only two data sets at a time is sufficient to place good constraints on the cosmological parameters. We show that the results from each of the three possible pairings of the data are also in good agreement. Finally, we combine all three data sets to obtain marginalised 68 per cent confidence intervals of 0.30<Omega_m<0.43, 0.48<h<0.59, 0.69<sigma_8<0.79, and 1.01<b_IRAS<1.16. For the best fit parameters the CMB quadrupole is Q_rms-ps=18.0 uK, Gamma=0.15, Omega_b=0.066 and the age of the universe is 16.7 Gyr.

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arXiv1999

Cosmic Microwave background map making with data between 10 and 90GHz

We use data from the Tenerife 10, 15 and 33 GHz beamswitching experiments along with the COBE 53 and 90 GHz data to separate the cosmic microwave background (CMB) signal from the Galactic signal and create two maps at high Galactic latitude. The new multi-MEM technique is used to obtain the best reconstruction of the two channels. The two maps are presented and known features are identified within each. We find that the Galactic contribution to both the 15 and 33 GHz Tenerife data is small enough to be ignored when compared to the errors in the data and the magnitude of the CMB signal.

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arXiv1999

Combining Supernovae and LSS Information with the CMB

Observations of the Cosmic Microwave Background (CMB), large scale structure (LSS) and standard candles such as Type 1a Supernovae (SN) each place different constraints on the values of cosmological parameters. We assume an inflationary Cold Dark Matter model with a cosmological constant, in which the initial density perturbations in the universe are adiabatic. We discuss the parameter degeneracies inherent in interpreting CMB or SN data, and derive their orthogonal nature. We then present our preliminary results of combining CMB and SN likelihood functions. The results of combining the CMB and IRAS 1.2 Jy survey information are given, with marginalised confidence regions in the H_0, Omega_m, b_IRAS and Q_rms-ps directions assuming n=1, Omega_Lambda+Omega_m=1 and Omega_b h^2=0.024. Finally we combine all three likelihood functions and find that the three data sets are consistent and suitably orthogonal, leading to tight constraints on H_0, Omega_m, b_IRAS and Q_rms-ps, given our assumptions.

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arXiv1998

Separation of foregrounds from cosmic microwave background observations with the MAP satellite

Simulated observations of a $10\dg \times 10\dg$ field by the Microwave Anisotropy Probe (MAP) are analysed in order to separate cosmic microwave background (CMB) emission from foreground contaminants and instrumental noise and thereby determine how accurately the CMB emission can be recovered. The simulations include emission from the CMB, the kinetic and thermal Sunyaev-Zel'dovich (SZ) effects from galaxy clusters, as well as Galactic dust, free-free and synchrotron. We find that, even in the presence of these contaminating foregrounds, the CMB map is reconstructed with an rms accuracy of about 20 $μ$K per 12.6 arcmin pixel, which represents a substantial improvement as compared to the individual temperature sensitivities of the raw data channels. We also find, for the single $10\dg \times 10\dg$ field, that the CMB power spectrum is accurately recovered for $\ell \la 600$.

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arXiv1998

The effect of point sources on satellite observations of the cosmic microwave background

We study the effect of extragalactic point sources on satellite observations of the cosmic microwave background (CMB). In order to separate the contributions due to different foreground components, a maximum-entropy method is applied to simulated observations by the Planck Surveyor satellite. In addition to point sources, the simulations include emission from the CMB and the kinetic and thermal Sunyaev-Zel'dovich (SZ) effects from galaxy clusters, as well as Galactic dust, free-free and synchrotron emission. We find that the main input components are faithfully recovered and, in particular, that the quality of the CMB reconstruction is only slightly reduced by the presence of point sources. In addition, we find that it is possible to recover accurate point source catalogues at each of the Planck Surveyor observing frequencies.

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arXiv1998

Observations of the Cosmic Microwave Background and Implications for Cosmology and Large Scale Structure

Observations of the Cosmic Microwave Background (CMB) are discussed, with particular emphasis on current ground-based experiments and on future satellite, balloon and interferometer experiments. Observational techniques and the effects of contaminating foregrounds are highlighted. Recent CMB data is used with large scale structure (LSS) data to constrain cosmological parameters and the complementary nature of CMB, LSS and supernova distance data is emphasized.

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arXiv1998

Review of Ground-Based CMB Experiments

We present in this paper a brief review of Ground-Based Cosmic Microwave Background (CMB) experiments. We first recall the main experimental problems and adopted solutions. We then review the Tenerife experiments, giving an update together with some new results. Then results and problems from other experiments are highlighted including IAC-Bartol, Python, Saskatoon, Mobile Anisotropy Telecope (MAT) and the Owen Valley Radio Observatory (OVRO) experiments. We next move on to the future ground experiments, in particular new interferometers such as the Very Small Array (VSA), the Cosmic Background Imager (CBI) and the Degree Angular Scale Interferometer (DASI/VCA). To finish, very recent work is presented on joint likelihood analysis for estimation of cosmological parameters where both CMB results and Large Scale Structure (LSS) surveys are considered.

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arXiv1998

Microwave background anisotropies and non-linear structures II. Numerical computations

A new method for modelling spherically symmetric inhomogeneities is applied to the formation of clusters in an expanding Universe. We impose simple initial velocity and density perturbations of finite extent and we investigate the subsequent evolution of the density field. Photon paths are also calculated, allowing a detailed consideration of gravitational lensing effects and microwave background anisotropies induced by the cluster. We apply the method to modelling high-redshift clusters and, in particular, we consider the reported microwave decrement observed towards the quasar pair PC1643+4631 A&B. We also consider the effect on the primordial microwave background power spectrum due to gravitational lensing by a population of massive high-redshift clusters.

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arXiv1998

Microwave Background Anisotropies and Nonlinear Structures I. Improved Theoretical Models

A new method is proposed for modelling spherically symmetric inhomogeneities in the Universe. The inhomogeneities have finite size and are compensated, so they do not exert any measurable gravitational force beyond their boundary. The region exterior to the perturbation is represented by a Friedmann-Robertson-Walker (FRW) Universe, which we use to study the anisotropy in the cosmic microwave background (CMB) induced by the cluster. All calculations are performed in a single, global coordinate system, with nonlinear gravitational effects fully incorporated. An advantage of the gauge choices employed here is that the resultant equations are essentially Newtonian in form. Examination of the problem of specifying initial data shows that the new model presented here has many advantages over `Swiss cheese' and other models. Numerical implementation of the equations derived here is described in a subsequent paper.

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arXiv1998

Joint estimation of cosmological parameters from CMB and IRAS data

Observations of large scale structure (LSS) and the Cosmic Microwave Background (CMB) each place separate constraints on the values of cosmological parameters. We calculate a joint likelihood based on various CMB experiments and the IRAS 1.2Jy galaxy redshift survey and use this to find an overall optimum with respect to the free parameters. Our formulation self-consistently takes account of the underlying mass distribution, which affects both the CMB potential fluctuations and the IRAS redshift distortion. This not only allows more accurate parameter estimation, but also removes the parameter degeneracy which handicaps calculations based on either approach alone. The family of Cold Dark Matter (CDM) models analysed corresponds to a spatially-flat universe with an initially scale-invariant spectrum and a cosmological constant. Free parameters in the joint model are the mass density due to all matter (Omega_m), Hubble's parameter (h = H_0 / 100km/s Mpc^-1), the quadrupole normalisation of the CMB power spectrum (Q) in uK, and the IRAS light-to-mass bias (b_i). Throughout the analysis, the baryonic density (Omega_b) is required is to satisfy the nucleosynthesis constraint Omega_b h^2 = 0.024. Results from the two data sets show good agreement, and the joint optimum lies at Omega_m=0.39, h=0.53, Q=16.96uK, and b_i=1.21. The 68 per cent confidence intervals are: 0.29<Omega_m<0.53, 0.39<h<0.58, 15.34<Q<17.60, and 0.98<b_i<1.56. For the above parameters the normalisation and shape of the mass power-spectrum are sigma_8=0.67 and Gamma=0.15, and the age of the Universe is 16.5Gyr.

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arXiv1998

A maximum-entropy method for reconstructing the projected mass distribution of gravitational lenses

The maximum-entropy method is applied to the problem of reconstructing the projected mass density of a galaxy cluster using its gravitational lensing effects on background galaxies. We demonstrate the method by reconstructing the mass distribution in a model cluster using simulated shear and magnification data to which Gaussian noise is added. The mass distribution is reconstructed directly and the inversion is regularised using the entropic prior for this positive additive distribution. For realistic noise levels, we find that the method faithfully reproduces the main features of the cluster mass distribution not only within the observed field but also slightly beyond it. We estimate the uncertainties on the reconstruction by calculating an analytic approximation to the covariance matrix of the reconstruction values of each pixel. This result is compared with error estimates derived from Monte-Carlo simulations for different noise realisations and found to be in good agreement.

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arXiv1998

A covariant and gauge-invariant analysis of cosmic microwave background anisotropies from scalar perturbations

We present a new, fully covariant and manifestly gauge-invariant expression for the temperature anisotropy in the cosmic microwave background radiation resulting from scalar perturbations. We pay particular attention to gauge issues such as the definition of the temperature perturbation and the placing of the last scattering surface. In the instantaneous recombination approximation, the expression may be integrated up to a Rees-Sciama term for arbitrary matter descriptions in flat, open and closed universes. We discuss the interpretation of our result in the baryon-dominated limit using numerical solutions for conditions on the last scattering surface, and confirm that for adiabatic perturbations the dominant contribution to the anisotropy on intermediate scales (the location of the Doppler peaks) may be understood in terms of the spatial inhomogeneity of the radiation temperature in the baryon rest frame. Finally, we show how this term enters the usual Sachs-Wolfe type calculations (it is rarely seen in such analyses) when subtle gauge effects at the last scattering surface are treated correctly.

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arXiv1998

10 GHz Tenerife CMB observations at 8 degree resolution and their analysis using a new maximum entropy method

The complete set of data from the Tenerife 10 GHz (8 degree FWHM) twin-horn, drift scan experiment is described. These data are affected by both long-term atmospheric baseline drifts and short term noise. A new maximum entropy procedure, utilising the time invariance and spatial continuity of the astronomical signal, is used to achieve a clean separation of these effects from the astronomical signal, and to deconvolve the effects of the beam-switching. We use a fully positive/negative algorithm to produce two-dimensional maps of the intrinsic sky fluctuations. Known discrete sources and Galactic features are identified in the deconvolved map. The data from the 10 GHz experiment, after baseline subtraction with MEM, is then analysed using conventional techniques and new constraints on Galactic emission are made.

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arXiv1997

Recent developments in the calculation of CMB anisotropies

We discuss three recent developments in the calculation of CMB anisotropies. First, we consider relativistic corrections to the Sunyaev-Zel'dovich effect. By extending the Kompaneets equation to the relativistic domain we are able to derive simple analytic forms for the SZ spectral distortion. Second, we discuss covariant kinetic theory methods for the gauge-invariant calculation of CMB anisotropies. We present a covariant version of the Boltzmann equation which we use to derive covariant and gauge-invariant equations describing a CDM model, which are independent of background curvature and type of perturbation (scalar, vector or tensor). By integrating the covariant Boltzmann equation along the line of sight, and using the instantaneous recombination approximation, we obtain an expression for the large-scale anisotropy which agrees with an expression derived recently by Dunsby. Third, we discuss recent accurate calculations of the CMB anisotropy in global defect models of cosmic structure formation. In such models the effects of vector modes are significant and when combined with the effects of decoherence lead to a suppression of the Doppler peaks. The result is that global defect models may already be at variance with the growing volume of CMB and large scale structure data.

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