Impacts of Dark Stars on Reionization and Signatures in the Cosmic Microwave Background

We perform a detailed and systematic investigation of the possible impacts of dark stars upon the reionization history of the Universe, and its signatures in the cosmic microwave background (CMB). We compute hydrogen reionization histories, CMB optical depths and anisotropy power spectra for a range of stellar populations including dark stars. If dark stars capture large amounts of dark matter via nuclear scattering, reionization can be substantially delayed, leading to decreases in the integrated optical depth to last scattering and large-scale power in the EE polarization power spectrum. Using the integrated optical depth observed by WMAP7, in our canonical reionization model we rule out the section of parameter space where dark stars with high scattering-induced capture rates tie up more than ~90% of all the first star-forming baryons, and live for over ~250 Myr. When nuclear scattering delivers only moderate amounts of dark matter, reionization can instead be sped up slightly, modestly increasing the CMB optical depth. If dark stars do not obtain any dark matter via nuclear scattering, effects upon reionization and the CMB are negligible. The effects of dark stars upon reionization and its CMB markers can be largely mimicked or compensated for by changes in the existing parameters of reionization models, making dark stars difficult to disentangle from astrophysical uncertainties, but also widening the range of standard parameters in reionization models that can be made consistent with observations.Comment: Small changes in response to referee's comments; matches version accepted for publication in ApJ. 14 pages, 9 figure

Milky Way Satellites Shining Bright in Gravitational Waves

The population of Milky Way satellite galaxies is of great interest for cosmology, fundamental physics, and astrophysics. They represent the faint end of the galaxy luminosity function, are the most dark-matter dominated objects in the local Universe, and contain the oldest and most metal-poor stellar populations. Recent surveys have revealed around 60 satellites, but this could represent less than half of the total. Characterization of these systems remains a challenge due to their low luminosity. We consider the gravitational wave observatory LISA as a potential tool for studying these satellites through observations of their short-period double white dwarf populations. LISA will observe the entire sky without selection effects due to dust extinction, complementing optical surveys, and could potentially discover massive satellites hidden behind the disk of the galaxy.Comment: 7 pages, 2 figure

Large-scale secondary polarization of the cosmic microwave background

We present two examples of secondary effects on the large-scale polarization of the cosmic microwave background (CMB). We begin with a review of the standard model of cosmology, as well as the predictions of bulk flows by the linear theory of structure growth and the mathematical formalism of polarization. Following this review, we explore the impact of the epoch of reionization on the large-scale polarization of the CMB, paying particular attention to the effects of an alternate reionization history containing dark stars. Subsequently, we derive the CMB polarization signature of large-scale bulk flows and examine its potential for detection by future experiments.Nous présentons deux exemples d'effets secondaires sur la polarisation du fond diffus cosmologique (CMB) à grande échelle. Nous débutons avec une revue du modèle standard de la cosmologie, une prédiction des mouvements d'ensemble des amas de galaxies par la théorie de la croissance linéaire des structures ainsi qu'une présentation du formalisme mathématique de la polarisation. Suivant cette revue, nous étudions l'impact de l'époque de réionisation sur la polarisation à grande échelle du CMB, en se concentrant sur les effets d'un modèle alternatif de réionisation incluant des étoiles sombres. Ensuite, nous dérivons la signature en polarisation dans le CMB engendrée par un mouvement d'ensemble à grande échelle et examinons la mesurabilité de ce signal par des expériences futures

Characterizing the nanohertz gravitational wave background produced by a cosmological population of binary supermassive black holes

The subject of this thesis is modelling the gravitational wave signal produced by binary supermassive black holes in the nHz frequency band. We begin with a review of large scale structure and galaxies as well as supermassive black holes and their origins, relation to their host galaxies, and potential to form binaries, followed by a review of gravitational wave formalism and its application to binary supermassive black holes and the pulsar timing arrays which could detect such a signal.The review finished, we develop population synthesis models in to make statistical predictions about the total population of binary supermassive black holes expected to be emitting gravitational waves at z ≲ 4. The population synthesis models are based on large N-body simulations and empirical scaling relations between dark matter halos, galaxy properties, and supermassive black holes. We use this simulated population to make predictions on the spectrum of the low-frequency stochastic gravitational wave background and discuss the variance due to Poisson noise in the population, astrophysical and cosmological uncertainties, and frequency resolution. Subsequently, we present a framework for treating the angular information in the pulsar timing array response to a general gravitational wave background. We show that the angular power spectrum of the all-sky maps of gravitational redshift induced by the gravitational waves is mathematically equivalent to the standard Hellings & Downs curve analysis, but has its own advantages. We use this equivalence to examine the expected variance on the Hellings & Downs curve for the cases of individual strong sources of gravitational waves and for a stochastic gravitational wave background. Finally, we discuss the sensitivity of pulsar timing arrays to the angular power spectrum.Cette thèse a pour but de créer un modèle d'ondes gravitationnelles (avec une fréquence de nanohertz) produites par des systèmes binaires de trous noirs supermassifs. Nous commençons avec une revue des structures à grande échelle et des galaxies et poursuivons avec une description des trous noirs supermassifs et de leurs origines, de leurs liens avec les galaxies qui les contiennent et de leurs capacités à former des systèmes binaires. Cette première partie et suivie d'une revue des fondements ma-thématiques des ondes gravitationnelles et de leur application au cas particulier des ondes produites par les systèmes binaires de trous noirs supermassifs et aux réseaux de pulsars pouvant les détecter. Après la revue, nous développons un modèle qui décrit la population prédite de trous noirs supermassifs binaires produisant des ondes gravitationnelles à z ?? ≲ 4. Ce modèle est basé sur de grandes simulations de N corps et sur les liens empiriques reliant les halos de matière sombre, les propriétés des galaxies et les trous noirs supermassifs. En utilisant cette population simulée, nous prédisons la forme du spectre d'ondes gravitationnelles à basse fréquence et nous discutons la variance causée par le bruit de grenaille dans la population, par l'incertitude astrophysique et cosmologique et par la résolution des fréquences.Ensuite, nous présentons un cadre mathématique pour le traitement de l'information angulaire dans la réponse d'un réseau de pulsars à un fond général d'ondes gravitationnelles. Nous démontrons que le spectre de puissance angulaire d'une carte du décalage vers le rouge gravitationnel est l'équivalent mathématique de l'analyse standard utilisant la courbe de Hellings & Downs. Cependant, l'analyse du spectre de puissance angulaire possède ses propres avantages. Nous utilisons cette équivalence pour examiner la variance attendue de la courbe de Hellings & Downs dans le cas d'une seule source d'ondes gravitationnelles et dans le cas d'un fond stochastique d'ondes gravitationnelles. Pour finir, nous discutons la sensibilité d'un réseau de pulsars au spectre de puissance angulaire

Bayesian parameter estimation of stellar-mass black-hole binaries with LISA

We present a Bayesian parameter-estimation pipeline to measure the properties of inspiralling stellar-mass black hole binaries with LISA. Our strategy (i) is based on the coherent analysis of the three noise-orthogonal LISA data streams, (ii) employs accurate and computationally efficient post-Newtonian waveforms accounting for both spin-precession and orbital eccentricity, and (iii) relies on a nested sampling algorithm for the computation of model evidences and posterior probability density functions of the full 17 parameters describing a binary. We demonstrate the performance of this approach by analyzing the LISA Data Challenge (LDC-1) dataset, consisting of 66 quasi-circular, spin-aligned binaries with signal-to-noise ratios ranging from 3 to 14 and times to merger ranging from 3000 to 2 years. We recover 22 binaries with signal-to-noise ratio higher than 8. Their chirp masses are typically measured to better than $0.02 M_\odot$ at $90\%$ confidence, while the sky-location accuracy ranges from 1 to 100 square degrees. The mass ratio and the spin parameters can only be constrained for sources that merge during the mission lifetime. In addition, we report on the successful recovery of an eccentric, spin-precessing source at signal-to-noise ratio 15 for which we can measure an eccentricity of $3\times 10^{-3}$.Comment: 15 pages, 9 figures, 2 tables (accepted to Physical Review D on 19 July 2021

Comparison of statistical sampling methods with ScannerBit, the GAMBIT scanning module

We introduce ScannerBit, the statistics and sampling module of the public, open-source global fitting framework GAMBIT. ScannerBit provides a standardised interface to different sampling algorithms, enabling the use and comparison of multiple computational methods for inferring profile likelihoods, Bayesian posteriors, and other statistical quantities. The current version offers random, grid, raster, nested sampling, differential evolution, Markov Chain Monte Carlo (MCMC) and ensemble Monte Carlo samplers. We also announce the release of a new standalone differential evolution sampler, Diver, and describe its design, usage and interface to ScannerBit. We subject Diver and three other samplers (the nested sampler MultiNest, the MCMC GreAT, and the native ScannerBit implementation of the ensemble Monte Carlo algorithm T-Walk) to a battery of statistical tests. For this we use a realistic physical likelihood function, based on the scalar singlet model of dark matter. We examine the performance of each sampler as a function of its adjustable settings, and the dimensionality of the sampling problem. We evaluate performance on four metrics: optimality of the best fit found, completeness in exploring the best-fit region, number of likelihood evaluations, and total runtime. For Bayesian posterior estimation at high resolution, T-Walk provides the most accurate and timely mapping of the full parameter space. For profile likelihood analysis in less than about ten dimensions, we find that Diver and MultiNest score similarly in terms of best fit and speed, outperforming GreAT and T-Walk; in ten or more dimensions, Diver substantially outperforms the other three samplers on all metrics