arXiv subjects

J. Richard Bond

Publications and source records attributed to J. Richard Bond.

174 records · Page 10Linked to original sources

arXiv1997

Forecasting Cosmic Parameter Errors from Microwave Background Anisotropy Experiments

Accurate measurements of the cosmic microwave background (CMB) anisotropies with an angular resolution of a few arcminutes can be used to determine fundamental cosmological parameters such as the densities of baryons, cold and hot dark matter, and certain combinations of the cosmological constant and the curvature of the Universe to percent-level precision. Assuming the true theory is a variant of inflationary cold dark matter cosmologies, we calculate the accuracy with which cosmological parameters can be determined by the next generation of CMB satellites, MAP and Planck. We pay special attention to: (a) the accuracy of the computed derivatives of the CMB power spectrum C_L; (b) the number and choices of parameters; (c) the inclusion of prior knowledge of the values of various parameters.

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arXiv1997

The Lyman Alpha Forest within The Cosmic Web

Observations indicate galaxies are distributed in a filament-dominated web-like structure; classic examples are the Coma and Perseus-Pisces superclusters. Numerical experiments at high and low redshift of viable structure formation theories also show filament-dominance; in particular, the gasdynamical simulations of Lyman alpha clouds at redshifts 2-6 that we concentrate on here. We understand why this is so in terms of rare events (peak patches) in the medium and the web pattern of filaments that bridge the gaps between the peaks along directions defined by their (oriented) tidal fields. We present an overview of these ideas and their practical application in crafting high resolution well-designed simulations. We show the utility of this by taking a highly filamentary subvolume found in a galactic-scale simulation, compressing its important large scale features onto a handful of numbers defining galactic-scale peak-patch constraints, which are then used to construct constrained initial conditions for a higher resolution simulation appropriate for study of the Lyman alpha forest.

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arXiv1997

Constraints on Compact Hyperbolic Spaces from COBE

The (large angle) COBE DMR data can be used to probe the global topology of our universe on scales comparable to and just beyond the present ``horizon''. For compact topologies, the two main effects on the CMB are: [1] the breaking of statistical isotropy in characteristic patterns determined by the photon geodesic structure of the manifold and [2] an infrared cutoff in the power spectrum of perturbations imposed by the finite spatial extent. To make a detailed confrontation of these effects with the COBE maps requires the computation of the pixel-pixel temperature correlation function for each topology and for each orientation of it relative to the sky. We present a general technique using the method of images for doing this in compact hyperbolic (CH) topologies which does not require spatial eigenmode decomposition. We demonstrate that strong constraints on compactness follow from [2] and that these limits can be improved by exploiting the details of the geodesic structure for each individual topology ([1]), as we show for the flat 3-torus and selected CH models.

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arXiv1997

CMB Power Spectrum Estimation

We explore power spectrum estimation in the context of a Gaussian approximation to the likelihood function. Using the Saskatoon data, we estimate the power averaged through a set of ten filters designed to make the errors on the power estimates uncorrelated. We also present an improvement to using the window function, $W_l$, for calculating bandpower estimates.

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arXiv1996

SPH P3MG Simulations of the Lyman-alpha Forest

Our understanding of the Lyman-$α$ forest has received a great boost with the advent of the Keck Telescope and large 3D hydrodynamical simulations. We present new simulations using the SPH technique with a P^3MG (Particle-Particle Particle-MultiGrid) non-periodic gravity solver. Our method employs a high resolution (1 kpc) inner volume, essential for capturing the complex gas physics, a larger low resolution volume, essential for correct larger scale tidal fields and a self-consistently applied, uniform tidal field to model the influence of ultra long waves. We include a photoionizing UV flux and relevant atomic cooling processes. We use constrained field realisations to probe a selection of environments and construct a statistical sample representative of the wider universe. We generate artificial Lyman-$α$ spectra and fit Voigt profiles. We examine the importance of (1) the photoionizing flux level and history, (2) tidal environment and (3) differing cosmologies, including CDM and CDM+{$Λ$}. With an appropriate choice for the UV flux, we find that the data is fit quite well if the rms density contrast is $\sim$ 1 at $z\sim 3$ on galaxy scales.

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arXiv1996

The Theoretical Agenda in CMB Research

The terrain that theorists cover in this CMB golden age is described. We ponder early universe physics in quest of the fluctuation generator. We extoll the virtues of inflation and defects. We transport fields, matter and radiation into the linear (primary anisotropies) and nonlinear (secondary anisotropies) regimes. We validate our linear codes to deliver accurate predictions for experimentalists to shoot at. We struggle at the computing edge to push our nonlinear simulations from only illustrative to fully predictive. We are now phenomenologists, optimizing statistical techniques for extracting truths and their errors from current and future experiments. We begin to clean foregrounds. We join CMB experimental teams. We combine the CMB with large scale structure, galaxy and other cosmological observations in search of current concordance. The brave use all topical data. Others carefully craft their prior probabilities to downweight data sets. We are always unbiased. We declare theories sick, dead, ugly. Sometimes we cure them, resurrect them, rarely beautify them. Our goal is to understand how all cosmic structure we see arose and what the Universe is made of, and to use this to discover the laws of ultrahigh energy physics. Theorists are humble, without hubris.

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arXiv1996

Cosmic Parameter Estimation Combining Sub-Degree CMB Experiments with COBE

We describe the Bayesian-based signal-to-noise eigenmode method for cosmological parameter estimation, show how it can be used to optimally compress large CMB anisotropy data sets to manageable sizes, and apply it to the DMR 4-year, South Pole and Saskatchewan data, individually and in combination. A simple prior probability method is used to include large scale structure observations. Estimates of the Hubble parameter, vacuum energy density, baryon fraction and primordial spectral tilt derived from the combined data are given.

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arXiv1995

How Filaments are Woven into the Cosmic Web

Observations indicate galaxies are distributed in a filament-dominated web-like structure. Numerical experiments at high and low redshift of viable structure formation theories also show filament-dominance. We present a simple quantitative explanation of why this is so, showing that the final-state web is actually present in embryonic form in the overdensity pattern of the initial fluctuations, with nonlinear dynamics just sharpening the image. The web is largely defined by the position and primordial tidal fields of rare events in the medium, with the strongest filaments between nearby clusters whose tidal tensors are nearly aligned. Applications of the cosmic web theory to observations include probing cluster-cluster bridges by weak gravitational lensing, X-rays, and the Sunyaev-Zeldovich effect and probing high redshift galaxy-galaxy bridges by low column density Lyman alpha absorption lines.

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arXiv1994

Signal-to-Noise Eigenmode Analysis of the Two-Year COBE Maps

To test a theory of cosmic microwave background fluctuations, it is natural to expand an anisotropy map in an uncorrelated basis of linear combinations of pixel amplitudes --- statistically-independent for both the noise and the signal. These $S/N$-eigenmodes are indispensible for rapid Bayesian analyses of anisotropy experiments, applied here to the recently-released two-year COBE {\it dmr} maps and the {\it firs} map. A 2-parameter model with an overall band-power and a spectral tilt $ν_{ΔT}$ describes well inflation-based theories. The band-powers for {\it all} the {\it dmr} $53,90,31$ $a$+$b$ GHz and {\it firs} 170 GHz maps agree, $\{(1.1\pm 0.1)\times 10^{-5}\}^{1/2}$, and are largely independent of tilt and degree of (sharp) $S/N$-filtering. Further, after optimal $S/N$-filtering, the {\it dmr} maps reveal the same tilt-independent large scale features and correlation function. The unfiltered {\it dmr} $53$ $a$+$b$ index $ν_{ΔT}+1$ is $1.4\pm 0.4$; increasing the $S/N$-filtering gives a broad region at (1.0--1.2)$\pm$0.5, a jump to (1.4--1.6)$\pm$0.5, then a drop to 0.8, the higher values clearly seen to be driven by $S/N$-power spectrum data points that do not fit single-tilt models. These indices are nicely compatible with inflation values ($\sim$0.8--1.2), but not overwhelmingly so.

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arXiv1994

Testing Inflation with the Cosmic Background Radiation

In inflation cosmologies, cosmic structure develops through the gravitational instability of the inevitable quantum noise in primordial scalar fields. I show how the acceleration of the universe defines the shape of the primordial spectrum of gravitational metric and scalar field fluctuations. I assess how we can determine the shape and overall amplitude over the five decades or so of spatial wavelengths we can probe, and use current data ... to show how far we are in this program. Broad-band power amplitudes are given for CMB anisotropy detections up to spring 1994 ... I show that COBE band-powers found with full Bayesian analysis of the $53,90,31$ $a$+$b$ GHz first year DMR (and FIRS) maps are in good agreement, and are essentially independent of spectral slope and degree of (sharp) signal-to-noise filtering. Further, after (smooth) optimal signal-to-noise filtering (\ie Weiner-filtering), the different DMR maps reveal the same large scale features and correlation functions with little dependence upon slope. However, the most probable slope depends upon how the maps are filtered: with no filtering whatsoever, the slope is high, but the power is not described by a single-slope law; as filtering is increased, the index moves nearer to inflation predictions. in {\it Relativistic Cosmology}, ed. M. Sasaki, Academic Press (1994) Proc. 8th Nishinomiya-Yukawa Memorial Symposium, Japan, October 1993

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arXiv1994

How Well Can Cosmological Parameters Be Estimated from CMB Observations?

The CMB anisotropy depends sensitively upon the slope and amplitude of primordial density and gravitational wave fluctuations, the baryon density, the Hubble constant, the cosmological constant, the ionization history, {\it etc.} We report on recent work showing how well multi-scale measurements of the anisotropy power spectrum can resolve these factors. We identify a hypersurface in cosmic parameter space that can be accurately localized by observations, but along which the likelihood will hardly vary. Other cosmic observations will be needed to break this degeneracy.

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arXiv1994

CMB Broad-Band Power Spectrum Estimation

The natural outcome of theoretical calculations of microwave background anisotropy is the angular power spectrum ${\cal C}_\ell$ as a function of multipole number $\ell$. Experimental ${\cal C}_\ell$'s are needed for direct comparison. Estimation procedures using statistics linear in the pixel amplitudes as well as the conventional but less useful quadratic combinations are described. For most current experiments, a single broad-band power amplitude is all that one can get with accuracy. Results are given for the Capri-meeting detections. Mapping experiments, sensitive to many base-lines, can also give spectral ``colour'' information, either with a series of contiguous narrow-band powers or as parameterized by a local ``colour'' index $n_{ΔT}$ (scale invariant is -2, white noise is 0). Bayesian analyses of the full first year DMR and FIRS maps give very similar band-powers (\eg $Q_{rms,PS}=17.9 \pm 2.9 μK$ {\it c.f.} $18.6 \pm 4.7 μK$ for $n_{ΔT}=-2$) and colour indices (with 1 and 2 sigma error bars) $n_{ΔT}+3=2.0^{+0.4;+0.7}_{-0.4;-1.0}$ and $ 1.8^{+0.6;+0.9}_{-0.8;-1.3}$ ({\it c.f.} the value 1.15 for a ``standard'' scale invariant CDM model). The 53 and 90 GHz DMR maps, as well as the FIRS map, have residual short-distance noise which steepens $n_{ΔT}$. Allowing the pixel error bars to increase absorbs much of the residual, but further exploration is needed to see if a second residual evident in the data which is responsible for the high $n_{ΔT}$ is from systematic errors or is physical.

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