Cosmology and Gravitation

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

I will discuss symmetries that arise at the infrared fixed point of the RG flow of Einstein gravity, including conformal vs. scale invariance in various dimensions, as well as 1-form generalized global symmetries and new anomalies that arise among them.

Zoom link:  https://pitp.zoom.us/j/93217255461?pwd=MXdzWDdVbWZrQzQ5UmVYVkx3US8zZz09

Primordial black holes (PBHs), if they exist, may shed light on long-standing questions on the nature
of dark matter and mechanism driving cosmic inflation. If we associate their origin to the presence of enhanced primordial scalar fluctuations generated during inflation, the underlying dynamics that populates these objects can also provide distinctive sources of gravitational waves (GWs), potentially detectable with current or forthcoming GW experiments. Therefore, their population, along with the associated GW signal offer promising opportunities to shed light on the nature of inflation, by opening up a unique window to its dynamics at scales inaccessible by conventional CMB probes. In this talk, I will review some of the compelling inflationary scenarios able to trigger the formation of such objects in the post-inflationary universe by enhancing the amplitude of the primordial scalar perturbations. In this context, I will discuss single and multi-field realizations with a focus on theoretical aspects of model building, discussing common themes shared among models in conjunction with their distinctive phenomenological implications in the form of a primordial GW background.

Zoom Link: https://pitp.zoom.us/j/98857635213?pwd=UGZoMXY4TnJBSmc1RlUvbGprSWlRQT09

Cosmology presents some puzzling aspects: the cosmological constant, the nature of the big bang, the source of inflation, and perhaps the tension in Hubble constant values. One normally uses standard effective field theory to study cosmology. But we know that such an approach fails in black holes due to the information paradox. We will draw on lessons on black holes to see how quantum gravity effects can modify evolution on macroscopic scales, and that such effects may be important is resolving puzzles in cosmology.

Zoom link:  https://pitp.zoom.us/j/98085421952?pwd=NFBCWXNBNkhrTVpaRkRlOWRtSkczdz09

EMRIs are one of the primary targets of spaceborne gravitational wave (GW) detectors and will be ideal GW sources for testing fundamental laws of gravity. In a generic non-Kerr spacetime,  the EMRI system  is non-integrable due to the lack of the Carter constant. As a result,  chaos along with resonance islands arise in these systems leaving a non-Kerr signature in the EMRI waveform as proposed in many previous studies. In this work, we systematically analyze the dynamics of an EMRI system near orbital resonances and we have derived an effective resonant Hamiltonian that describes the dynamics of the resonant degree of freedom  with the action-angle formalism. We have two major findings: (1) the chaotic orbits in general produce unique commensurate jumps in actions and (2) the EMRI orbits driven by radiation-reaction in general do not cross the resonance islands.

Zoom Link: https://pitp.zoom.us/j/95975225333?pwd=V05NZzQ3cE9neUZpN3RIOCt0UE5mZz09

The astrophysical stochastic gravitational wave background (SGWB) originates from numerous faint sub-threshold  gravitational wave (GW) signals arising from the coalescing binary compact objects. This background is expected to be discovered from the current (or next-generation) network of GW detectors by cross-correlating the signal between multiple pairs of GW detectors. However, detecting this signal is challenging and the correlation is only detectable at low frequencies due to the arrival time delay between different detectors. In this work, we propose a novel technique, Spectrogram Correlated Stacking (or SpeCS), which goes beyond the usual cross-correlation (and to higher frequencies)  by exploiting the higher-order statistics in the time-frequency domain which accounts for the chirping nature of the individual events that comprise SGWB.

We show that SpeCS improves the signal-to-noise for the detection of SGWB by up to an order of magnitude, compared to standard optimal cross-correlation methods which are tuned to measure only the power spectrum of the SGWB signal. SpeCS can probe beyond the power spectrum and its application to the GW data available from the current and next-generation GW detectors would speed up the SGWB discovery. 

based on work with Ramit Dey, Luis Longo, and Suvodip Mukherjee

Zoom link:   https://pitp.zoom.us/j/97091817158?pwd=MHNkdjFQT0plVzJJY2lsOHRxdDdwdz09