Cosmology and Gravitation

21-cm emission from neutral hydrogen atoms provides a unique window into galaxy formation and cosmology in the first billion years of cosmic history. As the first galaxies form after the Big Bang, they generate intense ultraviolet and X-ray radiation fields, which ionize and heat their otherwise neutral surroundings. The resulting modulations in the brightness of 21-cm emission relative to the background can be detected in principle by a single, well-calibrated dipole receiver, as features in the sky-averaged radio spectrum below ~200 MHz. Spatial fluctuations in the 21-cm background are expected also, and can in principle be detected statistically with the current generation of interferometers. In just the last few years, enormous progress has been made on both fronts. The sky-averaged 78 MHz feature reported by the EDGES collaboration in 2018 caused a flurry of activity, largely aimed at explaining its anomalous amplitude. Since then, the MWA, LOFAR, and HERA have all reported upper limits on the 21-cm power spectrum. In this talk, I will focus in particular on the first limits from HERA -- the most stringent limits reported to date -- and describe their implications for galaxy formation and cosmology. I will also discuss the ongoing EDGES controversy, and how JWST and SPHEREx can provide independent tests of astrophysical scenarios that produce EDGES-like 21-cm absorption troughs at frequencies below ~100 MHz.

In the fuzzy dark matter model, dark matter consists of “axion-like” ultra-light scalar particles of mass around 10⁻²² eV. This candidate behaves similarly to cold dark matter on large scales, but exhibits different properties on smaller (galactic) scales due to macroscopic wave effects arising from the extremely light particles’ large de Broglie wavelengths. It has both particle physics motivations and a rich astrophysical phenomenology, giving rise to notable differences in the structures on highly non-linear scales due to the manifestation of wave effects, which can impact a number of contentious small-scale tensions in the standard cosmological model, ΛCDM. Some of the unique features include transient wave interference patterns and granules, the presence flat-density cores (solitons) at the centers of dark matter halos, and the formation of quantized vortices. I will present large numerical simulations of cosmic structure formation with this dark matter model – including the full non-linear wave dynamics – using a pseudo-spectral method to numerically solve the Schrödinger–Poisson equations, and the significant computational challenges associated with these equations. I will discuss several observables, such as the evolution of the matter power spectrum, the fuzzy dark matter halo mass function, dark matter halo density profiles, and the question of a fuzzy dark matter core–halo mass relation, using results obtained from these simulations, and contrast them with corresponding results for the cold dark matter model.

Most of the efforts in searching for dark energy (DE) have focused on its gravitational signatures, and in particular on constraining its equation of state. However, there is a lot to be learned about DE by getting off the beaten track. I will first focus on non-gravitational interactions of DE with visible matter, leading to the possibility of "direct detection of dark energy" (analogous to direct detection of dark matter): I will argue that such interactions can and potentially may already have been detected in experiments such as XENON1T, while discussing complementary cosmological and astrophysical signatures. I will then discuss early- and late-time consistency tests of LCDM, and how these may shed light on (early and late) DE in relation to the Hubble tension. I will present two such tests based on the early ISW effect and the ages of the oldest astrophysical objects in the Universe.

Complex and poorly understood astrophysics impacts our ability to constrain cosmological and astrophysical models from the large scale structure. Two major sources of systematic errors are galaxy biasing (non-linear mapping between dark matter and galaxies) and baryonic feedback (impact of supernovae or AGN on LSS). In the first part of my talk, I will describe a hybrid perturbation theory model of galaxy biasing and show its validation at sub-percent accuracy. I will then describe the cosmological constraints obtained using this model on the measurements from the first three years of observations of the Dark Energy Survey (DES). In the second part of my talk, I will describe tomographic measurements and analysis of the cross-correlations between thermal Sunyaev-Zel'dovich (tSZ) effect and gravitational lensing. Using data from ACT, Planck, and DES, we obtain the highest significance (20 sigma) measurements to-date and use them to constrain models for the pressure profiles of halos across a wide range of halo mass and redshift. We find evidence for reduced pressure in low mass halos, consistent with predictions for the effects of increased feedback from AGN. Lastly, we also comment on application of this study to the sigma8 tension and hydrostatic mass bias as inferred from the cluster count analysis.

In this talk I will give an overview of recent and ongoing work regarding rotating black holes in dynamical Chern-Simons (dCS) gravity. dCS gravity is a well motivated modified theory of gravity which has been extensively studied in gravitational and cosmological contexts. I will first discuss unique geometric structures, `the Chern-Simons caps,' which slowly rotating black holes in dCS gravity were recently found to possess. Motivated by the dCS caps, I will then discuss superradiance in the context of slowly rotating dCS black holes and show that there are corrections to the usual solution for a Kerr black hole. Lastly, I will comment on the observable implications for these corrections and point towards avenues for future work.

Zoom Link: https://pitp.zoom.us/j/95228483630?pwd=dWk1c3p5dUU3RXJrNEhIT2M3Tk1Kdz09

Patterns and complex textures are ubiquitous in astronomical data but challenging to quantify. I will present a new powerful statistic called the “scattering transform”. It borrows ideas from convolutional neural nets (CNNs) while retaining the advantages of traditional statistics. As an example, I will show its application to weak lensing cosmology, where it outperforms classic statistics and is on a par with CNNs. I will also show interesting visual interpretations of the scattering statistics and possible extensions of this “mathematical neural network” idea. I argue that the scattering transform provides a powerful new approach in cosmology and beyond.

Related papers:

https://arxiv.org/abs/2112.01288

https://arxiv.org/abs/2103.09247

https://arxiv.org/abs/2006.08561

Zoom Link: https://pitp.zoom.us/j/91612161747?pwd=bnQrVmo4ZjBjaUdQMDBNZGhFS2NPQT09

Over the next ten years, the Vera C. Rubin Observatory (VRO) will observe ∼10 million active galactic nuclei (AGN) with a regular and high cadence. During this time, the intensities of most of these AGN will vary stochastically. Moreover, these fluctuations may also be connected to the high-energy astrophysical neutrino (HEAN) flux observed by IceCube. In this talk, I explore the prospects to quantify these fluctuations with VRO-measurements of AGN light curves and also evaluate the capacity of VRO, in tandem with various current and upcoming neutrino telescopes, to establish AGN as HEAN emitters.  I find that AGN variability measurements will be so precise as to allow the AGN to be separated into up to ∼ 10 different correlation-timescale bins. I also show that if the correlation time varies as some power of the luminosity, the normalization and power-law index of that relation will be determined to O(10^{−4}%).  Finally, I find that it may be possible to detect AGN contributions at the ~ 3\sigma level to the HEAN flux even if these AGN contribute only ~10% of the HEAN flux.
 

Quantum fluctuations in inflation provide the seeds for the large scale distribution of matter today. According to the standard paradigm, these fluctuations induce density perturbations that are adiabatic and Gaussian distributed. In this limit, all the information is contained within the two-point correlation function, or equivalently, the power spectrum. Today, the distribution of matter is far from Gaussian, with structures forming across a vast range of scales. Despite this, almost all analyses of observational data are performed using two-point functions. This begs the question: what information lies in higher-point statistics? 

In this seminar, I will present a pedagogical overview of the non-Gaussian correlation functions, and demonstrate how they can be used both to sharpen constraints on known physical parameters, and to provide stringent tests of new physics occurring in the early Universe. One of the major barriers to constraining cosmology from the higher-point functions is computational: measuring the statistics with conventional techniques is infeasible for current and future datasets. I will discuss new methods capable of reducing the computational cost by orders of magnitude, and show how this facilitates a number of exciting new tests of the cosmological model.

Self-gravitating quantum matter may exist in a wide range of cosmological and astrophysical settings: from the very early universe through to present-day boson stars. Such quantum matter arises in UltraLight Dark Matter (ULDM): an exciting axion-like particle candidate which keeps the successes of CDM on large scales but alleviates tensions on small scales. This small scale behavior is due to characteristic cores in ULDM called solitons, which also correspond to the ground state of the self-gravitating quantum system governing ULDM. We calculate the full spectrum of eigenstates and decompose simulations of ULDM into these states, allowing us to precisely track the evolution of the tell-tale soliton cores and the surrounding halo “skirt”. Using this formalism, we investigate formation of halos through binary soliton collisions and the dependence of the final halo product on initial parameters. We further link characteristic ULDM halo behavior—such as the soliton “breathing mode” and random walk of the center of mass—to the presence of certain modes. Finally, we comment on the relationship between eigenenergies and oscillatory timescales present in the system, as well as future directions for understanding ULDM through the language of its eigenstates.

There exist various scenarios for the very early universe that could potentially be the explanation for the observed properties of the cosmic microwave background. The current paradigm -- inflationary cosmology -- has rightfully received much attention, but it is not the only theoretically viable explanation. Indeed, several alternative scenarios exist, for example a contracting universe prior to a bounce or a slowly expanding emerging universe. It thus bares the question: how can we discriminate between the various theories, both from a theoretical and an observational point of view? A few pathways to answering this question are discussed in this talk.

Perturbative QFT calculations in de Sitter are plagued by a variety of divergences. One particular kind, the secular growth terms, cause the naive perturbation expansion to break down at late times. Such contributions often arise from loop integrals, which are notoriously hard to compute in dS. We discuss an approach to evaluate such loop integrals, for a scalar field theory in a fixed de Sitter background. Our method is based on the Mellin-Barnes representation of correlation functions, which enables us to regulate divergences for scalars of any mass while preserving the symmetries of dS. The resulting expressions have a similar structure as a standard dimreg answer in flat space QFT. These features of the regulator are illustrated with two examples. Along the way, we illuminate the physical origin of these divergences and their interpretation within the framework of the dynamical renormalization group. Our calculations naturally reveal additional infrared divergences for massless scalar fields in de Sitter, that are not present in the massive case. Such loop corrections can be incorporated as systematic improvements to the Stochastic Inflation framework, allowing for a more precise description of the IR dynamics of massless fields in de Sitter.