Cosmology and Gravitation

In a complete theory of gravity we expect space-time singularities to be regularized by quantum effects. From this point of view, we have two possible regular alternatives to singular black holes (BHs) to describe the ultracompact objects that we saw in our universe: regular black holes and horizonless compact objects. I will talk about the possible structures of these black hole mimickers and the gravitational waves ringdown signal that we expect from their coalescence. In particular I will focus on the deviations in the spectrum of QNMs with respect to singular BHs, their possible detectability and the structure of echoes when backreaction effects are taken into account.

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While the standard $\Lambda$CDM model provides an astonishing fit to cosmological data, there is a 4-6 $\sigma$ tension in the Hubble constant, $H_0$, between the value inferred from early-Universe observables, Planck CMB anisotropy spectra, and the value determined by local measurements from SH0ES collaboration using Type 1a supernovae calibrated with Cepheid variables. As an effort to solve this tension, we develop a method of searching for data-driven solutions to the Hubble tension given cosmological dataset, based on the standard Fisher bias formalism. Taking as proof of principle the case of a time-varying electron mass, and focusing first on Planck CMB data, we demonstrate a modified recombination can solve the Hubble tension and lower $S_8$ to better match weak lensing measurements. Once baryonic acoustic oscillation and uncalibrated supernovae data are included, it is not possible to fully solve the tension with perturbative modifications to recombination. The method we develop in this work can be a useful tool to search for many other possible extensions to the $\Lambda$CDM model, and is expected to inspire a model-building effort from the cosmology and particle physics community.

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In this talk I will show how the gravitational potentials generated by ultralight dark matter halos interact with gravitational waves, resonantly amplifying them. Significant amplifications can be achieved in dense dark matter environments, where the Floquet exponent is increased. The frequency of the amplified gravitational wave is equal to the axion mass when one requires resonance in the first band. For some masses considered, the gravitational wave frequencies fall within the Pulsar Timing Array range, representing an interesting possibility to test ultralight axions as dark matter candidates.

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The fundamental nature of dark matter and dark energy remains unknown and requires new physics to explain. This talk will present a cosmological quest to understand the properties of these two dominant but invisible components of our universe. I will present my work on early and late-universe searches for the identity of dark matter, and discuss cosmological limits on its mass, interactions with Standard Model, and production mechanisms, using the measurements of Big Bang Nucleosynthesis, Cosmic Microwave Background, 21-cm, and cosmic structure. Within the next decade, upcoming surveys will dramatically improve our measurements of these probes, enabling new insights into the invisible universe. If time permits, I will briefly introduce my work on cosmological searches for dark energy, as well as modified gravity.

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The Sunyaev-Zel’dovich Effect—the Doppler boost of low-energy Cosmic Microwave Background photons scattering off free electrons—is an excellent probe of ionized gas residing in distant galaxies. Its two main constituents are the kinematic SZ effect (kSZ), where electrons have a non-zero line-of-sight (LOS) velocity and which probes the electron line-of-sight momentum, and the thermal SZ effect (tSZ), where electrons have high energies due to their temperature, and which probes the electron integrated pressure. These two effects provide complementary information to constrain the thermodynamic profile of gas residing in distant galaxies, which can be further used to understand feedback processes, a necessary ingredient to describe the evolution of the large-scale structure in our Universe. Both tSZ and kSZ can be measured in cross-correlation with large-scale structure. In this talk, I will discuss my past and ongoing measurements of the SZ-galaxy cross-correlation with unWISE galaxies, where to measure the kSZ effect I use the projected-fields estimator. unWISE is a galaxy catalog containing over 500 million galaxies on the full sky and consists of three subsamples of mean redshifts z=0.5, 1.1, 1.5, whose halo occupation distribution I have already constrained. If time permits, I will also present my work on mitigating foregrounds in the SZ cross-correlations, particularly the Cosmic Infrared Background (CIB).

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The current progress in gravitational wave detection opened a new exciting window in cosmology. It is natural to ask ourselves how we can best use this new tool to explore physics beyond the standard model. With this idea in mind, my collaborators and I asked what we could learn from Cosmological Gravitational Wave Backgrounds if they were to be detected to a certain accuracy. By drawing comparison to the cosmic microwave background, we investigate the impact of elastic scattering on any cosmological background. Specifically, we focus on quantifying spectral distortions in the energy density spectrum of CGWB attributed to interaction with beyond-the-standard-model particles. We will also explore the effect that elastic scattering of graviton of Primordial Black holes would have on CGWB in the regime where PBHs account for all the dark matter.

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Zoom link https://pitp.zoom.us/j/98460268383?pwd=RytzWHd5dU1lenRhWG1NYXM3OVJpQT09

In this talk, I will discuss two lines of research revolving around the study of weak gravitational lensing in our Universe. First, I will present a pipeline that has been in development to model the matter power spectrum from large to small scales, focusing on stage-IV photometric galaxy surveys. I will discuss the fundamental ingredients of this pipeline, the consistency checks performed to validate it, and how this new tool fares when performing a full Bayesian parameter estimation analysis with an LSST-like survey. In the second part of this talk, I will present a new and exciting methodology to study weakly-lensed gravitational waves in the wave-optics regime.

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Zoom link https://pitp.zoom.us/j/98447653523?pwd=QjFDdk1LZ25LeDd6Nk5iRCtGbFNYQT09

Future surveys will map the cosmic microwave background (CMB) with unprecedented precision. The high fidelity of the data will present new opportunities to extract deep insights about the history, contents, and evolution of our universe.  However, new tools and techniques will be required to maximize the potential of the forthcoming data.  I will describe the techniques necessary to address the emerging challenges and to harness the exciting opportunities provided by future CMB observations.

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Zoom link https://pitp.zoom.us/j/92999899446?pwd=YTNEUkMrV2RXOWE0Mk11b2tDQ2Q0Zz09

Linear cosmological observables, like the cosmic microwave background (CMB), 
offer a unique avenue to investigate the elusive properties of dark matter 
(DM) through its interactions with Standard Model (SM) particles. The 
precision of contemporary experiments has reached sub-percent-level accuracy 
(<1%), necessitating theoretical formalism of definitive accuracy to extract 
meaningful constraints on DM-SM interactions.
I will highlight theoretical assumptions made ubiquitously in deriving most 
constraints on DM-SM scattering interactions, which could potentially induce 
order ~1 errors--- a large margin of uncertainty for precision-cosmology. I 
will discuss the progress we have made toward finding better alternatives by 
developing the first exact implementations in well-defined, cosmologically 
relevant regions of parameter space to mitigate the errors induced by two 
widely adopted theoretical assumptions.
Our exact results provide points of reference that can help determine the 
regimes where current DM-SM scattering constraints can be trusted, and where 
we need better methods going forward. I will end by motivating a higher-order 
statistical formalism for DM-SM scattering with up to 100x greater 
sensitivity.

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Zoom link https://pitp.zoom.us/j/95952209469?pwd=TERqVlZsMVhvbzJqU1hsalhiUVYxdz09

Deep learning (DL) methods have demonstrated great potential for extracting rich non-linear information from cosmological fields, a challenge that traditional summary statistics struggle to address. Most of these DL methods are discriminative models, i.e., they directly learn the posterior constraints of cosmological parameters. In this talk, I will make the argument that learning the field-level likelihood function using generative modeling approaches such as Normalizing Flows usually leads to more effective extraction of cosmological information. This approach also enables anomaly detection to improve the robustness of the analysis. To scale the modeling to high dimensional data and improve its generalization capabilities, we further incorporate physical inductive biases, such as symmetries and multiscale structure, into the architecture of the normalizing flow models. On mock weak lensing maps, I will show that the model leads to significant improvement in constraining power compared to power spectrum and alternative DL models. I will also show that it is able to detect domain shifts between training simulations and test data, such as noise miscalibration and baryonic effect, which, if left unaddressed, could introduce systematic biases in parameter constraints. Finally, I will talk about our ongoing work on applying this model to the field-level cosmic shear analysis for HSC.

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Zoom link https://pitp.zoom.us/j/97478701784?pwd=UVg4TVp1WFArcXNETG5ITGd0S0NuZz09

Abstract TBA

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Zoom link https://pitp.zoom.us/j/96982969023?pwd=MjgzaU5mZUVuV3pNVG11dHM2WUd4UT09

Major improvements in the theoretical aspects of Cosmology have been possible in recent years due to QFT-inspired methods, such as the effective field theory of large-scale structure (EFTofLSS). In this talk, I will explore further connections between high-energy physics and cosmology. I will present a systematic approach to renormalizing the galaxy bias parameters using path integrals and a finite cutoff scale Λ. I will derive the differential equations of the Wilson-Polchinski renormalization group that describe the evolution of the finite-scale bias parameters with Λ, analogous to the β-function running in QFT. I will then discuss how the RG-flow of EFTofLSS can lead to improvements in the extraction of cosmological parameters and also serve as a tool for sanity checks.

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Zoom link https://pitp.zoom.us/j/93248279658?pwd=Z1M1UnI3eVh3NTZ3NCt4NUtGaW5idz09