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Probing Fundamental Physics with the Large-Scale Structure of the Universe

Final Report Summary - FUNDPHYSICSANDLSS (Probing Fundamental Physics with the Large-Scale Structure of the Universe)

Over the past decade, cosmology, the study of the origin and evolution of the Universe and structure within it, has evolved into a precision science. While we have learned much about the evolution and history of the Universe, many of the most pressing questions remain unanswered. Some of these questions, which have significant implications for our basic understanding of nature, are:
- Why is the Universe accelerating ?
- What is dark matter ?
- Does our current theory of gravity - General Relativity - correctly describe the Universe as a whole ?
- How did the Universe begin, and what is the origin of structure in the Universe ?

The overarching goal of the project is to make progress on these fronts through measurements of the large-scale structure of the Universe. In particular, the research attempts to a) probe our theory of gravity, and b) unravel the nature and origin of the initial seed fluctuations out of which all structure in the Universe formed. Beyond these two science goals, the proposed research includes c) new ways of measuring weak gravitational lensing, which is the most promising tool to study the "dark" ingredients of the cosmos that do not interact with light (such as dark matter and neutrinos). The project has pursued this research program on all three fronts of numerical simulations, analytical models, and observational constraints. Significant progress has been achieved over the course of the project.

On research objective a), Schmidt has organized a workshop at MPA bringing together the leading experts in modified gravity simulations. A code comparison project was initiated at this workshop, with the aim of providing cross checks and quantifying the performance of these highly sophisticated codes. We found excellent agreement between the different codes for each modified gravity model. This is a significant advance beyond the state of the art in the field. Further, tight Solar System constraints on so-called disformal gravity models were obtained, which either rule out this entire class of models, or require the introduction of screening mechanisms. Second, an end-to-end validation of the galaxy clustering analysis pipeline used to infer cosmological constraints from the Baryon Oscillations Spectroscopic Survey DR12 galaxy sample was performed. The validation was successful, allowing the authors to place the first fully simulation-calibrated constraints on modified gravity from galaxy clustering.

On objective b), Schmidt and colleagues derived the general perturbative relation between matter and galaxies (biasing) in the presence of primordial non-Gaussianity. Further, galaxy shape correlations (alignments) were identified as another probe of inflation through large-scale structure that is complementary to galaxy clustering. Further research focused on the precise characterization of primordial non-Gaussianity generated in standard single-field slow-roll scenarios, and on any possible contamination by late-time nonlinear gravitational evolution. These works were based on the Conformal Fermi Coordinate (CFC) approach previously developed by Dai, Pajer, and Schmidt. One of the key results is that no contamination of primordial non-Gaussianity occurs within the CFC frame.

On objective c), Schmidt was actively involved in efforts of comparing different magnification measurements applied to the COSMOS data set, and comparing all of them with shear measurements. He participated in several workshops on weak lensing magnification at Universitat Autonoma de Barcelona, and the International Space Science Institute (ISSI) in Bern, which focused on weak lensing magnification. Since then, Schmidt has focused on modeling a significant physical contaminant of weak lensing measurements, namely intrinsic shape and size correlations (also known as intrinsic alignments). These need to be understood in order to properly interpret current and future galaxy shape (shear) and size (magnification) correlations. This work is still ongoing, and expected to be published in the near future.