Cosmology

The deep learning architecture associated with ChatGPT and related generative AI products is known as transformers. Initially applied to Natural Language Processing, transformers and the self-attention mechanism they exploit have gained widespread interest across the natural sciences. We will present the mathematics underlying the attention mechanism and describe the basic transformer architecture. We will then describe applications to time series and imaging data in astronomy and discuss possible foundation models.

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Zoom link https://pitp.zoom.us/j/91226066758?pwd=TWZ5RVliMjVKYXdLcHdya09lNWZhQT09

Black hole thermodynamics plays a central role in the theoretical landscape, acting as both synthesizer and sieve for concepts in field theory, quantum gravity, information theory, and more. While its formulation and applications are well understood in asymptotically flat and anti-de Sitter spaces, significant difficulties arise when generalizing to cosmological settings. In this talk, I will discuss how the thermodynamic properties of black holes can be understood in such scenarios through the Euclidean path integral. I will also discuss black holes in a more generalized, semi-classical setting, and suggest a consistent framework for describing a wide variety of dynamical black hole models and ultracompact objects, with consequences for horizon formation, back-reaction, and the growth of astrophysical black holes.

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Zoom link https://pitp.zoom.us/j/97647270696?pwd=bXhFYWYyYUlrTEUyVXhtOUNDakhhQT09

I will present work on probing the large scale structure of the universe using CMB lensing from the upcoming Data Release 6 of the Atacama Cosmology Telescope (ACT) and cross-correlations with galaxies from the unWISE galaxy catalog. My talk will focus on how our highly competitive constraints from CMB lensing and CMB lensing cross-correlations can provide insight into the widely discussed “S8/sigma8 tension”. For this purpose I will briefly introduce the high fidelity CMB lensing reconstruction obtained by the ACT Collaboration and results from the analysis of the lensing auto-correlation. I will discuss new results from the cross-correlation between ACT CMB lensing and unWISE galaxies, highlighting improvements to the analysis pipeline compared to previous work on the cross-correlation between Planck CMB lensing and unWISE by some of my collaborators (Krolewski et al. 2021). I will also show a reanalysis of Planck CMB lensing x unWISE and a joined analysis of the ACT and Planck CMB lensing cross-correlations with unWISE.

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Zoom link https://pitp.zoom.us/j/99192611116?pwd=TU9iMjhrejVESjNRdi92M0ZXN2ZEQT09

Low mass galaxies challenge our picture of galaxy formation and are an intriguing laboratory for the study of star formation, feedback and dark matter physics. I will present results from high resolution, cosmological simulations that contain many (isolated) dwarf galaxies [the MARVEL dwarfs] as well as satellite dwarf galaxies [the DC Justice League]. Together, they create the largest collection of high-resolution simulated dwarf galaxies to date and the first flagship suite to resolve ultra-faint dwarf galaxies in multiple environments. This sample spans a wide range of physical (stellar and halo mass), and evolutionary properties (merger history). I will present results and predictions constraining star formation, feedback and dark matter physics soon testable by telescopes like JWST, Rubin's LSST and the Roman Space Telescope. Finally, I will present new work on measuring galaxy shapes and the diversity of rotation curves in the dwarf galaxy mass regime which may be used to distinguish dark matter model.

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Zoom link: https://pitp.zoom.us/j/99038411436?pwd=OFd1SEdUUXJkd0NLeWtrTUxGR0FCUT09

Standard model of cosmology is based on the cosmological principle. The cosmological principle states that the Universe is statistically homogeneous and isotropic on large scales. Is this hypothesis supported by the observations ? After a historical survey of the field, I shall use the high redshift data from radio galaxies and quasars to show that the early Universe does not seem to be isotropic and the rest frame of cosmic microwave background radiation does not coincide with the rest frame of distant sources. I shall also demonstrate that the cosmological principle is violated at a statistical significance of over 5-sigma.

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Zoom link: https://pitp.zoom.us/j/94290315794?pwd=STFpalNNb1NNUHNPNWcvZlkreWNpZz09

The onset of the formation of structure in the early universe was marked by the monolithic collapse of smooth peaks in the initial density field. This process creates prompt rho ~ r^-1.5 density cusps of dark matter, which persist largely unaltered through the subsequent growth of dark matter halos around them. Consequently, in the standard collisionless dark matter paradigm, these prompt cusps are expected to be enormously abundant, and one resides at the center of every halo and subhalo. Prompt cusps present new opportunities to test the nature of dark matter. In annihilating dark matter models, the abundance of these features and the high density inside them greatly influence the intensity and morphology of the annihilation signal. For example, if the Galactic Center gamma-ray excess is due to annihilating dark matter, then a matching signal from unresolved prompt cusps should be detectable elsewhere. Moreover, the properties of prompt cusps are closely linked to details of the primordial density field. In warm dark matter models, prompt cusps are expected to be large enough to influence stellar motions within galaxies at detectable levels.

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Zoom link: https://pitp.zoom.us/j/98307421845?pwd=V3BqZmtyQ09XcjBwNEltTzFPTHJPUT09

 I will present a galaxy formation model within the Lambda Cold Dark Matter (LCDM) framework that is calibrated on the results of galaxy formation simulations and some of the empirical properties of nearby dwarf galaxies. I will then use the model to interpret a number of ostensible challenges to the LCDM framework, such as the "too-big-too-fail problem", "central density problem" and the "planes of satellites" problem and will argue that none of these pose a serious challenge to LCDM, as the corresponding observations can be largely understood within the current galaxy formation modeling. I will also show that the same galaxy formation model can explain the abundance of UV-bright galaxies at z>5 measured by the Hubble Space Telescope and James Webb Space Telescope recently, if the expected increase of burstiness of star formation in galaxies towards early epochs is taken into account.

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Zoom link https://pitp.zoom.us/j/91798519705?pwd=Nk9rM0tFSXcrWDhLdXFhVmJWbGgvUT09

While General Relativity has withstood tests on solar system scales, progress in observational cosmology now enables tests on the largest scales. I will present results from our tests of gravity using Dark Energy Survey Year 3 weak lensing and clustering data in addition to a variety of complementary data. One outcome of this analysis was the necessity to further explore lensing consistency so I will present preliminary results of such tests using the latest Cosmic Microwave Background (CMB) lensing data. These analyses are setting the scene for future tests of fundamental physics with CMB and galaxy surveys: I will show expected results and challenges from the Rubin Observatory. I will finally argue for the use of machine learning for theory exploration, to better organize our efforts within the future experimental landscape. As PI is a leading center in outreach, I will end my talk by sharing my experience with science content creation on various social media platforms.

Zoom link: https://pitp.zoom.us/j/96929387143?pwd=WW1CZElpMkN2U1RQUjB3VERxRVQ5dz09

I will introduce the stability problem for spacetimes from the initial value formulation perspective in general relativity. After introducing some notions on how to quantitatively characterize (in)stability, I will present a result for a class of spacetimes called gravitational solitons which exhibit slower decay compared to black holes. This is joint work with Hari Kunduri (McMaster University).

Zoom Link: TBD

The universe has turned out to be simpler than expected on both very small and very large scales. We propose a minimal, highly predictive framework connecting particle physics to cosmology. Instead of introducing an ``attractor” phase such as inflation we extrapolate the observed universe all the way back to the initial singularity where we impose a CPT symmetric boundary condition via a generalization of the method of images. If the hot plasma in the early universe is perfectly conformal, so is the singularity. The cosmos may then be analytically extended to a ``mirror image” universe prior to the bang. Using this new boundary condition we calculate the gravitational entropy for cosmologies with radiation, matter, Lambda and space curvature, finding it favours spatially flat, homogeneous and isotropic universes with a small positive cosmological constant in accord with observation. To maintain conformal symmetry, we include unusual Dim-0 (dimension zero) fields. They improve the SM’s coupling to gravity, cancelling the vacuum energy and two local “Weyl” anomalies, without introducing additional propagating modes. They also cancel pathologies introduced into the graviton propagator by loops of SM particles. Cancellation requires precisely 3 generations of SM fermions, each with a RH neutrino. It also requires a composite Higgs, presumably built with the Dim-0 fields. One of the RH neutrinos, if stable, is a viable candidate for the dark matter which will be tested soon. The Dim-0 fields source scale-invariant curvature perturbations in the early universe. Subject to two simple but crucial theoretical assumptions, the amplitude and spectral tilt match the observations with remarkable accuracy. (See arXiv:2302.00344 and references therein).

Zoom Link: https://pitp.zoom.us/j/95784600151?pwd=dkppa2s3ZDM4NG5yb0ZVV2w5SXErdz09