The Random Universe

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Abstract: The three primary CMB observables: temperature fluctuations T and polarization fluctuations E and B (and their cross-correlations) are powerful probes of cosmology and reionization physics. There is a fourth observable, though: the fluctuations in the optical depth. While somewhat futuristic, it may offer additional constraints on reionization and cosmology. Using the most advanced numerical simulations of cosmic reionization to date, I explore its potential reach as a novel probe of early structure formation.

Abstract:I present a Gaussianity test for Planck polarization data by analyzing the statistics of singular (unpolarized) points in the polarization field and show clear evidence of non-Gaussianity in both E and B modes at almost all angular scales. We created publicly available software that can perform such a test on any polarization map in HEALPix format with full or incomplete sky coverage. I also propose an alternative way to measure the CMB anisotropy and polarization at low multipoles l=1,2,3 using the anisotropic SZ effect.

Abstract:The anthropic principle in physics states that our existence implies certain constraints on the natural conditions under which we evolved. In statistics, a corresponding anthropic principle can be used to infer properties of the models we should fit to data. For example, experiments are typically aimed to have a precision sufficient to estimate effects of interest but without overkill; it is rare to have an estimate that is 10 standard errors from zero, We demonstrate through several examples in social and medical sciences how the anthropic principle, combined with Bayesian inference, can be used to improve statistical practice.

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Abstract: In 2002, Andrew wrote a now-classic paper that connected black-hole demographics to pulsar timing arrays and in so doing articulated clearly the science case for PTA searches for gravitational waves. I will review this work briefly and then discuss more recent work, initiated by a 2018 paper with Andrew, on a harmonic approach to thinking about PTA observables.

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Abstract: Peculiar velocities have long been recognized as a powerful source of information about the growth of structure in the universe. Andrew’s early work with Pedro Ferreira, Roman Juskiewicz, Marc Davis, and Hume Feldman originally got me interested in mean pairwise velocities. But the limitations of galaxy distance estimates long precluded velocities from being a reliable and competitive cosmological probe. This began to change with ACT’s first detection of the kinematic Sunyaev-Zeldovich effect in 2012. The coming years hold the prospect of mapping the three-dimensional velocity field using the kSZ and transverse lensing effects by cross-correlations of large galaxy surveys like LSST and high-resolution microwave background maps from ACT and Simons Observatory.

Abstract: The remarkable success of recent cosmology in pinpointing a concordance phenomenological model from the observations of the perturbed universe relies on several foundational assumptions — homogeneity, isotropy of the universe, and nearly scale-invariant, nearly Gaussian, tiny, seed primordial fluctuations in the cosmic spacetime fabric that are expected to inherit its basic symmetries. Increasingly precise cosmological observations, such as the exquisite measurements of the CMB sky from the ESA Planck space mission, have opened up the possibility of robust, independent tests of these assumptions. I will present the results of my research program, which focuses on searching for subtle patterns that could expose richness beyond the rather simplistic cosmological model and early universe paradigm.

Abstract: I will review the current status of using cosmic clocks and chronometers to pin down the cosmological model reflecting on Andrew’s early insights on how and why we should use timing besides yard stick to unveil the nature of the cosmos.

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Abstract: Bayesian model selection has become a cornerstone of modern cosmological inference. After briefly reviewing the underlying ideas, I will trace the method's journey through cosmology over the past three decades. I begin with its first application to the field in Andrew Jaffe's 1996 study of the Hubble constant and the odds on rival cosmological models, in which the evidence for a handful of low-dimensional models was obtained essentially by hand, using analytic approximations and direct numerical integration. I then contrast this with a recent analysis of the dark energy equation of state, in which the evidence plays a central role in obtaining a free-form "flexknot" reconstruction of w(z), computed with GPU-accelerated nested sampling over a variable-dimension parameter space, and is further used to ask whether the data favour dynamical dark energy or whether the apparent signal is instead an observational systematic. Throughout, the same evidence and its automatic Occam's razor remain the guiding principle.

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Abstract: If dark matter is composed of particles lighter than about 10 eV, wave phenomena become important. We will discuss both the astrophysical implications in the ultra-light regime, and experimental implications for axion detection in a broad range of masses.

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Abstract: years. In this approach we measure two-point correlation functions on and between two sets of fields: galaxy density fields, and the gravitational lensing fields. This method has proved more effective in constraining the late time matter power spectrum than either statistic alone. Separately, there has been a significant push towards studying higher order statistics of weak lensing fields. A very broad range of such statistics has been explored, from three-point functions to morphological statistics and machine learning-driven approaches. In this talk I will discuss work with Mehraveh Nikjoo, Ben Moews, and Ottavia Truttero that unifies these two approaches and explores higher-order statistics on combined lensing and clustering fields. As a first go we have been exploring weak lensing by extended ridge structures in galaxy density, which have been shown to trace underlying cosmic web filaments. I’ll also talk about life as one of Andrew’s early PhD students back in the mid 2000s!

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Abstract:Non-Gaussianity is a powerful clue to the physics of the early Universe, but its traces must be searched for in the complex late-time Universe. I will take an excursion through recent and ongoing work by my research group using large-scale structure to look for these traces: from the bispectrum to the mass function, from assembly bias to simulations, and finally to rare objects such as galaxy clusters. By the end I hope I will convince you that Andrew had the right idea in looking at the CMB: the late-time Universe does not give up its secrets quite so gracefully.