Cosmology and Gravitation

Pulsar timing arrays (PTAs) consist of a set of regularly monitored millisecond pulsars with extremely stable rotational periods. The arrival time of pulses can be altered by the passage of gravitational waves (GWs) between them and the Earth, thus serving as a galaxy-wide GW detector. Evidence for the first detection of low-frequency (~nHz) gravitational waves has recently been reported across multiple PTA collaborations, opening a new observational window into the Universe. Although the origin of the GW signal is yet to be determined, the dominant sources are expected to be inpiralling supermassive black holes (SMBHs). I will discuss a recent work in which we compare the GW detections by PTAs with the expected signal implied by existing electromagnetic observations in a simple but robust manner. This study suggests that the currently measured GW amplitude is larger than expected by a significant amount. I will then show that additional information regarding the typical number of sources contributing to the background can already be inferred from current PTA data.

Recent advances in General Relativity point toward unanticipated, and dynamic, relations between ultracompact objects and the universe they inhabit. The possibility for strongly gravitating systems, like astrophysical black holes (BHs) and their embedding cosmology, to directly interact has been dubbed "cosmological coupling." We focus on recent results from the DOE Stage IV Dark Energy (DE) Spectroscopic Instrument (DESI), which strongly suggest that DE is dynamical. Using typical empirical models for the cosmic star-formation rate density as a proxy to BH production, we show that the DESI-inferred time-evolution of DE is consistent with cosmologically coupled stellar-collapse BHs as the source of DE. The predicted cosmological expansion rate today, H_0 = 69.94 +/- 0.81 km/Mpc/s, is in excellent agreement with H_0 = 69.58 +/- 1.58 km/Mpc/s recently reported by the Chicago-Carnegie Hubble Program using Cepheid, Tip of the Red Giant Branch, and J-Region Asymptotic Giant Branch stellar distance-ladder calibrations. With DESI Redshift Space Distortions and Year 3 datasets on the horizon, we highlight exciting prospects for further observational confrontation in the near term.

In this seminar, I will discuss recent progress towards developing robust methods to constrain PNG in the non-linear regime based on the LSS consistency relations — non-perturbative statements about the structure of LSS correlation functions derived from symmetries of the LSS equations of motion. Specifically, I will present non-perturbative models for the squeezed matter bispectrum and collapsed matter trispectrum in the presence of local PNG, as well as in the presence of a more general “Cosmological Collider” signal sourced by inflationary massive particle exchange. Using N-body simulations with modified initial conditions, I will demonstrate that these models yield unbiased constraints on the amplitude of PNG deep into the non-linear regime (k~2 h/Mpc at z=0). Finally, I will discuss how these non-perturbative methods can provide insight into the scale-dependent bias signature associated with the Cosmological Collider scenario.

I will discuss recent progress on the initial conditions for inflation and how this can potentially enhance gravitational wave non-Gaussianity (NG). In particular, there is a significant additional NG contribution in the flattened configuration, offering a straightforward way to boost parity-violating NG from Chern-Simons gravity, which typically suffers from graviton ghost-production. Furthermore, I will explore how this shape – at the level of the trispectrum – can enable direct measurement of NG in the stochastic gravitational wave background (SGWB) using pulsar timing arrays (PTA). This is particularly intriguing as it allows us to investigate NG at scales complementary to those observed in the CMB, which could help us distinguish between various early universe models and physical mechanisms.

In this talk, I will present a published and an ongoing work in the direction of emergent gravity. The first is what I dubbed as the generalized Unruh effect, a mapping from arbitrary states in the Fock space of positive Minkowski (Kruskal) modes to Rindler (Schwarzschild) modes obtained by Bogoliubov transformation. The special case of vacuum state -- thermal bath mapping has been well known in the textbooks, the original Unruh effect. I will discuss the interesting physical implications of the generalized Unruh effect on the black hole information paradox. In the second part, I will give a novel conjecture of the dark energy when considering the spacetime as a 4d volume-conserved fluid. Einstein's equation with an always-positive metric term (Lambda, but not necessarily constant) can be interpreted as the differential version of this assumed conservation of 4d volume at linear order. The rich phenomenology of this theory will be discussed, including its solution to the anthropic problem of our currently dark-energy-dominated universe, and high-redshift over-evolved astrophysical objects that have been recently popping up in the JWST survey.

Present-day galaxies still contain a multitude of clues about their formation histories. But which properties should we best look at and what do they actually reveal about the past? Using cosmological simulations, I will show what insights about a galaxy's formation and merger history can be gained from a diverse set of measurements. For this, I will show how the overall galaxy shapes and tidal features in the outskirts are related to the inner kinematics. These results give us indications about the correlation with the recent merger histories of the galaxies. Finally, I will discuss how early gas-rich mergers can be revealed through the ages of globular cluster populations.

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In this talk, I will present a novel framework for modeling the weak lensing source galaxy redshift distribution and galaxy intrinsic alignment via a shared luminosity function. In the context of LSST Year 1 and Year 10 cosmic shear analysis, I demonstrate the significant impact of the luminosity function on source galaxy redshift distributions and intrinsic alignment contamination. I establish the influence of Schechter luminosity function parameters on the redshift distribution of a magnitude-limited sample and show the effects of marginalizing over these parameters in intrinsic alignment modeling. I forecast how this joint modeling approach affects cosmological parameter constraints, finding that it yields constraints comparable to standard analyses while mitigating potential biases from incorrectly fixed luminosity function parameters. I highlight the specific impact of the luminosity function shape on the cosmic shear data vector and discuss the method's potential for modeling generic selection functions and extending to a 3x2pt analysis with galaxy bias incorporation. While focused on LSST cosmic shear, this framework is broadly applicable to weak lensing surveys.

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4 months after it opened, the Tacoma Narrows bridge, one of the world's
longest suspension bridges at the time, collapsed spectacularly in the first
storm that hit it. Though it was built to exceed all of the standrds at the
time, something  clearly went wrong. The failure was filmed from almost the
beginning to the end (about 1 hour), and that film has been shown to almost
all first year physics or engineering classes as an example of resonance, that
explanation is clearly nonsense. What happened? Why did it collapse. The
explanation is closely linked to, for example, the reason that clarinets or
flutes, or even violins, make their music. With Daniel Green (whom you may
remember from his time at Toronto) we were able to show in detail what
happened.

One big question in the physics of large-scale structures is how we can trace back the evolution of cosmic structures and reconstruct the initial density field. The universe we observe today is dotted with galaxy clusters separated by vast voids, in sharp contrast to its initial state, which was nearly uniform with only minor density fluctuations. The evolution from this early uniformity to today's complex structure of galaxies is a profound transformation, with many intermediate processes still unexplained. This talk focuses on this transformation, aiming to reconstruct both the initial density and the displacement fields of galaxies observed in spectroscopic surveys. I will discuss new reconstruction algorithms that depend weakly, if at all, on a cosmological model. These algorithms have paved the way for new astrophysical achievements in several directions, such as: Baryon Acoustic Oscillations (BAO) analysis, morphological analysis of proto-halos, and kinematic Sunyaev-Zel'dovich (kSZ) effect.

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The growth of cosmic structures provides a bridge between observations and theories. In the first part of this talk, I will review the sigma8 tension between early- and late-time measurements of cosmic structure growth, and its physical implications. Are we seeing a chink in the standard cosmological model’s armor? To unveil the origin of this tension (and possibly new physics in the late Universe), galaxy clustering and spectroscopic surveys will play an indispensable role. In the second part of the talk, I will introduce a novel framework to extract cosmological information from millions of galaxies in spectroscopic surveys, directly at the field level—i.e. the three-dimensional map level with millions of grid points. The goal is to achieve a percent-level constraint on sigma8 and growth of structure with stage-IV surveys like DESI. I will further discuss recent progress towards this goal, highlighting latest breakthroughs, results in a community data challenge, and remaining challenges.

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Elucidating the nature of dark matter, dark energy, and dynamics of the early universe stand as major challenges of modern cosmology and particle physics. I will present a new program of addressing these challenges with galaxy surveys. This program builds on theoretical particle physics tools and allows for sub-percent precision analytic understanding of galaxy clustering on large scales. I will share some results of this program that include new measurements of fundamental cosmological parameters and constraints on new physics beyond the standard models of particles and cosmology.

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Neutrinos and other light relic particles leave a number of imprints in the cosmic microwave background anisotropies and on maps of the large-scale structure of our Universe. These imprints can not only demonstrate the presence of these particles and constrain their masses, but can provide insight into their nature via signatures of interactions or other behavior that changes during the history of the Universe. I will describe this physics, present constraints on non-standard neutrino self-interactions and other forms of dark radiation from cosmic datasets. I will also discuss prospects for detecting neutrino mass and highlight how relic neutrinos force us to adopt new technology when modeling structures in the Universe today.

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