Cosmology and Gravitation

With much of the cosmological information in the primary CMB having already been mined, the next decade of CMB observations will revolve around the secondary CMB lensing effect, which will touch nearly all aspects of observation in some way. At the same time, the increasingly low noise levels of these future observations will render existing "quadratic estimator" methods for analyzing CMB lensing obsolete. This leaves us in an exciting place where new methods need to be developed to fully take advantage of the upcoming generation of CMB data just on our doorstep. I will describe my work developing such new lensing analysis tool, made possible by Bayesian methods, modern statistical techniques, and borrowing ideas from machine learning. I will present the recent first-ever application of such methods to data (from the South Pole Telescope; https://arxiv.org/abs/2012.01709) and discuss prospects for this analysis in the future with regards to not just lensing but also primordial B modes, reionization, and extragalactic foreground fields.

The primordial non-Gaussianity (PNG) is a key feature to screen various inflationary models and it is one of the main targets in the next generation galaxy surveys. In particular, the local-type of PNG makes the galaxy bias scale-dependent in large-scales, which is known as the scale-dependent bias, allowing to constrain the local-type PNG from galaxy surveys. In this talk, I will present the galaxy shape correlation, called the ``intrinsic alignment'', can explore the angular-dependent PNG. Using N-body simulations, we show the angular-dependent PNG induces the scale-dependent bias in the intrinsic alignment, just as the angular-independent PNG leads to the scale-dependent bias in the galaxy number density correlation. By combining photometric and spectroscopic surveys to measure the intrinsic alignment, future galaxy surveys are potentially capable of constraining the angular-dependent PNG better than CMB experiments.

The discovery of astrophysical gravitational waves has opened a new avenue to explore the cosmos using transients. I will discuss a few new frontiers in the field of physical cosmology and fundamental physics that can be explored using gravitational waves from the current generation gravitational wave detectors such as LIGO/Virgo, and in the future from gravitational wave detectors such as LISA, Einstein Telescope, and Cosmic Explorer. I will elucidate the existence of synergies between electromagnetic probes and gravitational wave probes and their importance in understanding the standard model of cosmology and unveiling new physics.

Tensions between measurements in the early and the late universe could be the first hint of new physics beyond the cosmological standard model. In particular, the clustering of large scale structure and the current value of the Hubble parameter show intriguing discrepancies between measurements in the early and late universe. In this talk, I review the most common ways of easing these two tensions and focus specifically on parameter extensions and various models of dark matter, such as warm dark matter, cannibalistic dark matter, dark matter interactions, and dark radiation. To constrain these models a variety of cosmological probes, both current and future, is required at different scales and redshifts.

The Dark Energy Survey (DES) is a photometric galaxy survey which, using measurements of distortions to galaxy shapes from weak gravitational lensing and other observables, we can use to test the validity of our standard cosmological model, LambdaCDM. As an example of this, I will motivate and discuss a recent analysis of the DES Year 1 data (described in https://arxiv.org/abs/2010.05924) in which we use a "growth-geometry split" parameterization to check the consistency of constraints from structure growth and expansion history. I will also highlight some of the ongoing work on the DES Year 3 analysis, as well as challenges we will face as we subject LambdaCDM to increasingly precise tests with future cosmological experiments.

Torsion is a popular ingredient in gravity, yet fraught with quantum and classical pathologies. I develop a novel torsion theory, consistent with power-counting and unitarity. The Friedmann equations emerge (with dark energy and radiation), as do pp waves and the Schwarzschild vacuum, all without an Einstein-Hilbert term. I show that cosmology sees torsion as a non-canonical scalar, revealing a rich phenomenology of conformal or waterfall inflatons, and cuscutons. I finally argue that future work will be driven less by toy-models, and more by computer surveys. I advocate Hamiltonian and effective field theory approaches to non-Riemannian geometry in general, relevant to ultraviolet completion and modified gravity alike. Such methods should be oriented towards the ultimate test of gravity: observed cosmological structure.

In the first part of the talk I will review some recent progress in large-scale structure theory and show how it can be used to measure cosmological parameters from current and future redshift surveys. Then I will discuss some ongoing challenges in the modeling of galaxy clsutering data and covariance matrices. Finally, I will present a systematic calculation of the probability distribution function for the dark matter density field and discuss its potential as a cosmological probe.

Gravitational waves (GWs) have already proved immensely powerful for constraining cosmological extensions of GR, both from data-driven and theoretical perspectives. However, GWs really come into their own when used in combination with complementary electromagnetic data. I’ll start by reviewing some of the bounds on extended gravity theories from GW detections to date. I'll introduce the formalism, the phenomenology, and the astrophysical pitfalls of these tests. Finally, we'll explore the impact of future experiments like LISA and accompanying galaxy surveys on the remaining parameter space of modified gravity theories.

In order to infer cosmological parameters from galaxy survey data, we typically use summary statistics such as the power spectrum and we need an accurate estimate of their covariance matrix. The traditional process of obtaining the covariance involves simulating thousands of mocks. I will present an analytic approach for the covariance matrix which is more than four orders of magnitude faster than mocks and show its validation with an analysis of the BOSS DR12 data. Furthermore, our analytic approach is free of sampling noise which makes it useful for upcoming surveys like DESI and Euclid. Towards the end, I will change gears and talk about some recent work on the assembly bias of neutral hydrogen.
 

The standard model of cosmology is built upon on a series of propositions on how the early, intermediate, and late epochs of the Universe behave. In particular, it predicts that dark energy and dark matter currently pervades the cosmos. Understanding the properties of the dark sector is plausibly the biggest challenge in theoretical physics. There is, however, a broad assumption in cosmology that the Universe on its earlier stages is fully understood and that discrepancies between the standard model of cosmology and current data are suggestive of distinct dark energy properties. Uncertainties on the validity of this hypothesis are not usually taken into account when forecasting survey capabilities, even though our investigations might be obfuscated if the intermediate and early Universe did behave abnormally. In this colloquium, I propose a program to investigate dark energy and earlier aspects of our Universe simultaneously, through space missions in the 2020s in combination with ground-based observatories. This program will help guide the strategy for the future LSST and WFIRST supernovae and weak lensing surveys. My investigations on how properties of the early and intermediate Universe affect inferences on dark energy (and vice-versa) will also support community understanding of how future missions can be employed to test some of the core hypotheses of the standard model of cosmology.

The new gravitational-wave signal GW190521in LIGO and Virgo marks the first observational detection of the elusive intermediate-mass black holes. The detection also confirms there exist a new class of black holes in the mass gap predicted by the pair-instability supernovae theory. In this talk, I will discuss the process that went behind inferring the astrophysical properties of this historic discovery. I would briefly address the alternative scenarios we looked into for a possible exotic origin of this signal, including any violation of General Relativity. For the upcoming ESA/NASA space mission LISA, I would highlight how this discovery opens a unique epoch of multi-band, multi-messenger astronomy.

The outskirts of accreting dark matter haloes exhibit a sudden drop in density delimiting the virialized region. After briefly describing the physics shaping this feature and how it is measured, I will discuss its applications. I will examine its connection to structure formation and how it can constrain the screening mechanisms of beyond-GR models of gravity.