arXiv subjects

Martin White

Publications and source records attributed to Martin White.

At least 379 records · Page 21Linked to original sources

arXiv1996

Cold dark matter models with high baryon content

Recent results have suggested that the density of baryons in the Universe, OmegaB, is much more uncertain than previously thought, and may be significantly higher. We demonstrate that a higher OmegaB increases the viability of critical-density cold dark matter (CDM) models. High baryon fraction offers the twin benefits of boosting the first peak in the microwave anisotropy power spectrum and of suppressing short-scale power in the matter power spectrum. These enable viable CDM models to have a larger Hubble constant than otherwise possible. We carry out a general exploration of high OmegaB CDM models, varying the Hubble constant h and the spectral index n. We confront a variety of observational constraints and discuss specific predictions. Although some observational evidence may favour baryon fractions as high as 20 per cent, we find that values around 10 to 15 per cent provide a reasonable fit to a wide range of data. We suggest that models with OmegaB in this range, with h about 0.5 and n about 0.8, are currently the best critical-density CDM models.

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arXiv1996

The Sachs-Wolfe Effect

We present a pedagogical derivation of the Sachs-Wolfe effect, specifically the factor 1/3 relating the temperature fluctuations to gravitational potentials. The result arises from a cancellation between gravitational redshifts and intrinsic temperature fluctuations which can be derived from a coordinate transformation of the background.

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arXiv1996

The Damping Tail of CMB Anisotropies

By decomposing the damping tail of CMB anisotropies into a series of transfer functions representing individual physical effects, we provide ingredients that will aid in the reconstruction of the cosmological model from small-scale CMB anisotropy data. We accurately calibrate the model-independent effects of diffusion and reionization damping which provide potentially the most robust information on the background cosmology. Removing these effects, we uncover model-dependent processes such as the acoustic peak modulation and gravitational enhancement that can help distinguish between alternate models of structure formation and provide windows into the evolution of fluctuations at various stages in their growth.

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arXiv1996

Distinguishing Causal Seeds from Inflation

Causal seed models, such as cosmological defects, generically predict a distinctly different structure to the CMB power spectrum than inflation, due to the behavior of the perturbations outside the horizon. We provide a general analysis of their causal generation from isocurvature initial conditions by analyzing the role of stress perturbations and conservation laws in the causal evolution. Causal stress perturbations tend to generate an isocurvature pattern of peak heights in the CMB spectrum and shift the first compression, i.e.~main peak, to smaller angular scales than in the inflationary case, unless the pressure and anisotropic stress fluctuations balance in such a way as to reverse the sense of gravitational interactions while also maintaining constant gravitational potentials. Aside from this case, these causal seed models can be cleanly distinguished from inflation by CMB experiments currently underway.

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arXiv1996

A New Test of Inflation

We discuss a new test of inflation from the harmonic pattern of peaks in the cosmic microwave background radiation angular power spectrum. By characterizing the features of alternate models and revealing signatures essentially unique to inflation, we show that inflation could be validated by the next generation of experiments.

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arXiv1996

Acoustic Signatures in the Cosmic Microwave Background

We study the uniqueness and robustness of acoustic signatures in the cosmic microwave background by allowing for the possibility that they are generated by some as yet unknown source of gravitational perturbations. The acoustic {\it pattern} of peak locations and relative heights predicted by the standard inflationary cold dark matter model is essentially unique and its confirmation would have deep implications for the causal structure of the early universe. A generic pattern for isocurvature initial conditions arises due to backreaction effects but is not robust to exotic source behavior inside the horizon. If present, the acoustic pattern contains unambiguous information on the curvature of the universe even in the general case. By classifying the behavior of the unknown source, we determine the minimal observations necessary for robust constraints on the curvature. The diffusion damping scale provides an entirely model independent cornerstone upon which to build such a measurement. The peak spacing, if regular, supplies a precision test.

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arXiv1996

Four-year COBE normalization of inflationary cosmologies

We supply fitting formulae enabling the normalization of slow-roll inflation models to the four-year COBE data. We fully include the effect of the gravitational wave modes, including the predicted relation of the amplitude of these modes to that of the density perturbations. We provide the normalization of the matter power spectrum, which can be directly used for large-scale structure studies. The normalization for tilted spectra is a special case. We also provide fitting functions for the inflationary energy scale of COBE-normalized models and discuss the validity of approximating the spectra by power-laws. In an Appendix, we extend our analysis to include models with a cosmological constant, both with and without gravitational waves.

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arXiv1996

Testing Inflation with Small Scale CMB Anisotropies

We discuss ability of the harmonic pattern of peaks in the CMB angular power spectrum to test inflation. By studying robust features of alternate models, which must all be isocurvature in nature, we reveal signatures unique to inflation. Inflation thus could be validated by the next generation of experiments.

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arXiv1996

Measuring the Curvature of the Universe

We discuss how the curvature of the universe can be robustly measured employing only the gross features of the CMB anisotropy spectrum. Though the position of the first peak is not robust, uncertainties in the model for structure formation can be removed by using the spacing of the acoustic peaks and the location of the damping tail. Combined these provide important consistency tests that can be used to discriminate against a truly exotic model.

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arXiv1996

Cold Dark Matter Models with a Cosmological Constant

We use linear and quasi-linear perturbation theory to analyse cold dark matter models of structure formation in spatially flat models with a cosmological constant. Both a tilted spectrum of density perturbations and a significant gravitational wave contribution to the microwave anisotropy are allowed as possibilities. We provide normalizations of the models to microwave anisotropies, as given by the four-year {\it COBE} observations, and show how all the normalization information for such models, including tilt, can be condensed into a single fitting function which is independent of the value of the Hubble parameter. We then discuss a wide variety of other types of observations. We find that a very wide parameter space is available for these models, provided $Ω_0$ is greater than about 0.3, and that large-scale structure observations show no preference for any particular value of $Ω_0$ in the range 0.3 to 1.

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arXiv1996

Direct Imaging of the CMB from Space

Fundamental information about the Universe is encoded in anisotropies of the Cosmic Microwave Background (CMB) radiation. To make full use of this information, an experiment must image the entire sky with the angular resolution, sensitivity, and spectral coverage necessary to reach the limits set by cosmic variance on angular scales >~10'. Recent progress in detector technology allows this to be achieved by a properly designed space mission that fits well within the scope of NASA's Medium-class Explorer program. An essential component of the mission design is an observing strategy that minimizes systematic effects due to instrumental offset drifts. The detector advances make possible a `spin chopping' approach that has significant technical and scientific advantages over the strategy used by COBE, which reconstructed an image of the sky via inversion of a large matrix of differential measurements. The advantages include increased angular resolution, increased sensitivity, and simplicity of instrumentation and spacecraft operations. For the parameters typical of experiments like the Primordial Structures Investigation (PSI) and the Far InfraRed Explorer (FIRE), we show that the spin-chopping strategy produces images of the sky and power spectra of CMB anisotropies that contain no significant systematic artifacts.

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arXiv1996

The Impact of the CMB on Large-Scale Structure

The COBE detection of microwave anisotropies provides the best way of fixing the amplitude of cosmological fluctuations on the largest scales. We discuss the impact of this new, precise normalization and give fitting formulae for the horizon-crossing amplitude as a function of Ω_0 and n for both open and flat cosmologies. We also discuss the relevant normalization (σ_8) at galaxy-clustering scales. Already it is clear that the inferred σ_8 can be unnaccepatably high for some of the simplest inflationary models, although many minor variants give an adequate fit. Generic topological defect models appear to fare rather badly, and it is unclear whether minor variants or improved calculations will help much. The detection and mapping of structure in the CMB anisotropy spectrum on smaller scales in the near future will enable us to achieve much stronger constraints on models.

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arXiv1996

Cosmic Confusion and Structure Formation

We present a re-analysis of the cosmic confusion hypothesis, elucidating the degree to which ``confusion'' can be expected to hold in a class of flat, adiabatic models. This allows us to devise a simple and accurate fitting function for the height of the first peak in the radiation power spectrum in a wide range of inflationary inspired models. The range preferred by current data is given, together with a discussion of the impact of measurements of the peak height on constraining models of structure formation.

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arXiv1995

Dependence of Inflationary Reconstruction upon Cosmological Parameters

The inflationary potential and its derivatives determine the spectrum of scalar and tensor metric perturbations that arise from quantum fluctuations during inflation. The CBR anisotropy offers a promising means of determining the spectra of metric perturbations and thereby a means of constraining the inflationary potential. The relation between the metric perturbations and CBR anisotropy depends upon cosmological parameters -- most notably the possibility of a cosmological constant. Motivated by some observational evidence for a cosmological constant (large-scale structure, cluster-baryon fraction, measurements of the Hubble constant and age of the Universe) we derive the reconstruction equations and consistency relation to second order in the presence of a cosmological constant. We also clarify previous notation and discuss alternative schemes for reconstruction.

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arXiv1995

The COBE Normalization of CMB Anisotropies

With the advent of the COBE detection of fluctuations in the Cosmic Microwave Background radiation, the study of inhomogeneous cosmology has entered a new phase. It is now possible to accurately normalize fluctuations on the largest observable scales, in the linear regime. In this paper we present a model-independent method of normalizing theories to the full COBE data. This technique allows an extremely wide range of theories to be accurately normalized to COBE in a very simple and fast way. We give the best fitting normalization and relative peak likelihoods for a range of spectral shapes, and discuss the normalization for several popular theories. Additionally we present both Bayesian and frequentist measures of the goodness of fit of a representative range of theories to the COBE data.

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arXiv1995

Why Not Consider Closed Universes?

We consider structure formation and CMB anisotropies in a closed universe, both with and without a cosmological constant. The CMB angular power spectrum and the matter transfer function are presented, along with a discussion of their relative normalization. This represents the first full numerical evolution of density perturbations and anisotropies in a spherical geometry. We extend the likelihood function vs. Omega from the COBE 2-year data to Omega>=1. For large Omega the presence of a very steep rise in the spectrum towards low ell allows us to put an upper limit of Omega<=1.5 (95%CL) for primordial spectra with n<=1. This compares favorably with existing limits on Omega. We show that there are a range of closed models which are consistent with observational constraints while being even older than the currently popular flat models with a cosmological constant. Future constraints from degree scale CMB data may soon probe this region of parameter space. A derivation of the perturbed Einstein, fluid and Boltzmann equations for open and closed geometries is presented in an appendix.

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arXiv1995

Cold Dark Matter Resuscitated?

The Cold Dark Matter (CDM) model has an elegant simplicitly which makes it very predictive, but when its parameters are fixed at their `canonical' values its predictions are in conflict with observational data. There is, however, much leeway in the initial conditions within the CDM framework. We advocate a re-examination of the CDM model, taking into account modest variation of parameters from their canonical values. We find that CDM models with $n=0.8$--0.9 and $h=0.45$--0.50 can fit the available data. Our ``best fit'' CDM model has $n=0.9$, $h=0.45$ and $C_2^{T}/C_2^{S}=0.7$. We discuss the current state of observations which could definitely rule out this model.

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arXiv1995

CMB Anisotropies in the Weak Coupling Limit

We present a new, more powerful and accurate, analytic treatment of cosmic microwave background (CMB) anisotropies in the weakly coupled regime. Three applications are presented: gravitational redshifts in a time dependent potential, the Doppler effect in reionized scenarios, and the Vishniac effect. The Vishniac effect can dominate primary anisotropies at small angles even in late and minimally reionized models in flat dark-matter dominated universes with Harrison-Zel'dovich initial conditions. The techniques developed here refine previous calculations yielding a larger coherence angle for the Vishniac effect and moreover can be applied to non-trivial ionization histories. These analytic expressions may be used to modify results for the standard cold dark matter model to its cosmological constant and reionized extensions without detailed and time consuming recalculation.

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