Cosmology and Gravitation

The long range nature of gravity complicates the dynamics of self-gravitating many-body systems such as galaxies and dark matter (DM) halos. Relaxation/equilibration of perturbed galaxies and cold dark matter halos is typically a collective, collisionless process. Depending on the perturbation timescale, this process can be impulsive/fast, adiabatic/slow or resonant. First, I shall present a linear perturbative formalism to compute the response (in all three regimes) of disk galaxies to external perturbations such as satellite impacts. I shall elucidate how phase-mixing of the disk response gives rise to phase-space spirals akin to those observed by Gaia in the Milky Way disk, and how these features can be used to constrain the Milky Way’s potential as well as its dynamical history. Next, I shall discuss the secular evolution of a massive perturber due to the back reaction of the near-resonant response of the host galaxy/halo. In this context I shall present two novel techniques to model the secular torque (dynamical friction) experienced by the perturber: 1. a self-consistent, time-dependent, perturbative treatment and 2. a non-perturbative orbit-based framework. These two approaches explain the origin of certain secular phenomena observed in N-body simulations of cored galaxies but unexplained in the standard Chandrasekhar and LBK theories of dynamical friction, namely core-stalling and dynamical buoyancy. I shall briefly discuss some astrophysical implications of these phenomena: potential choking of supermassive black hole mergers in cored galaxies, and the possibility of constraining the inner density profile (core vs cusp) of DM dominated dwarf galaxies and therefore the DM particle nature.

Zoom link:  https://pitp.zoom.us/j/99089663538?pwd=aVVjV2ozMkZRTkE0ZW1Ib0dGUC9tdz09

In this talk, I will present a new tool to constrain low-energy Wilson coefficients in a scalar EFT (scalar for simplicity's sake but the range of applicability is much wider) based on the requirement that such theories should respect causality. Causality will be defined in the sense that no low-energy observer should be able to measure any resolvable time-advance resulting from a scattering event. I will show that these so-called causality bounds are in remarkable agreement with previously derived positivity bounds (where low energy constraints on the 4-point amplitude make use of physical assumptions of the UV completion of the EFT), while being considerably simpler and a better candidate to get cosmological and black hole gravitational bounds.

Zoom link: https://pitp.zoom.us/j/92424925160?pwd=bnRNWE81eEQ4NHY4a28rNGMwTitUdz09

Cosmology and astrophysics with the extragalactic light: background and fluctuations

Abstract: The aggregate light emitted by all extragalactic sources can be measured either as an absolute intensity or through its spatial fluctuations; these are known as line-intensity mapping (LIM) when a particular line transition is targeted. I will discuss how these measurements can be used both to learn about galaxy evolution and to investigate the presence of more speculative sources of radiation, such as decaying dark matter. I will then discuss the prospects of LIM to probe the interstellar and intergalactic medium, as well as structure on large scales. In particular, I will focus on the joint information across different line transitions and across different tracers, such as galaxies and extragalactic CMB foregrounds.

Zoom Link: https://pitp.zoom.us/j/94959896129?pwd=QjhzWS9rczdBNGFndUhhNDZ1K1FLQT09

Can degrees of freedom in the interior of black holes be responsible for the entropy-area law? If yes, what spacetime appears? In this talk, I answer these questions at the semi-classical level. Specifically, a black hole is considered as a bound state consisting of many semi-classical degrees of freedom which exist uniformly inside and have maximum gravity. The distribution of their information determines the interior metric through the semi-classical Einstein equation. Then, the interior is a continuous stacking of AdS_2 times S^2 without horizon or singularity and behaves like a local thermal state. Evaluating the entropy density from thermodynamic relations and integrating it over the interior volume, the area law is obtained with the factor 1/4 for any interior degrees of freedom. Here, the dynamics of gravity plays an essential role in changing the entropy from the volume law to the area law. This should help us clarify the holographic property of black-hole entropy. [arXiv: 2207.14274]

Zoom link: https://pitp.zoom.us/j/99386433635?pwd=VzlLV2U4T1ZOYmRVbG9YVlFIemVVZz09

Weak gravitational lensing by the intervening large-scale structure (LSS) of the Universe is the leading non-linear effect on the anisotropies of the cosmic microwave background (CMB). The integrated line-of-sight gravitational potential that causes the distortion can be reconstructed from the lensed temperature and polarization anisotropies via estimators quadratic in the CMB modes. While previous studies have focused on the lensing power spectrum, upcoming experiments will be sensitive to the bispectrum of the lensing field, sourced by the non-linear evolution of structure. The detection of such a signal would provide additional information on late-time cosmological evolution, complementary to the power spectrum.

Zoom link: https://pitp.zoom.us/j/94880169487?pwd=dzRWcVRwQ2dVdWZ3N2RjOWU2RDUyZz09

In this talk I will share our recent work in developing statistical models based on machine learning methods. In particular, I will discuss posterior sampling in low- and high-dimensional spaces and connect this to two ongoing projects: measuring the small-scale distribution of dark matter and estimating the expansion rate of the Universe. I will discuss how the speed and the accuracy gained by these models are essential for the large volumes of data from the next generation sky surveys. I will finish by mentioning a few other projects and a new initiative for interdisciplinary collaboration in astrophysics and data sciences. 

Zoom link:  https://pitp.zoom.us/j/98316228305?pwd=UWwrZkIwUG1QZFBkYzc1eVdNSW1Ldz09

Mapping the intensity of the 21 cm emission line from neutral hydrogen (HI) is a promising technique for characterizing the 3D matter distribution over large volumes of the Universe and out to high redshifts.  The Canadian Hydrogen Intensity Mapping Experiment (CHIME) is a radio interferometer specifically designed for this purpose.  CHIME recently reported the detection of 21 cm emission from large-scale structure between redshifts 0.8 and 1.4.  This was achieved by stacking maps of the radio sky, constructed from 102 nights of CHIME data, on the angular and spectral locations of galaxies and quasars from the eBOSS clustering catalogs.  In this talk, I will introduce the experiment and provide an overview of the detection.  I will describe key aspects of both the data processing pipeline and the simulation pipeline used to model the stacked signal.  I will discuss the implications of the detection.  Finally, I will evaluate the prospects for using CHIME -- and it's successor, the Canadian Hydrogen Observatory and Radio-transient Detector (CHORD) -- to measure the power spectrum of 21 cm emission, identify the signature of baryon acoustic oscillations, and constrain dark energy.

Zoom link:  https://pitp.zoom.us/j/94362295704?pwd=NnQxa1pteWJVTzVBTVFYUmlsWnlVUT09

The Drake Equation is a thought experiment whose purpose is to understand the ingredients necessary for life and advanced technological civilizations to exist on other worlds in our galaxy.  However, beyond reflecting on life on Earth we have no knowledge of many of these ingredients, such as the number of planets that have life, the number of with intelligent life, the number with advanced civilizations, and the lifetimes of these civilizations. In this talk I will review the Drake Equation and the biases that scientists have traditionally had in discussing this equation and how it has led to the current searches of biological and technological signatures. I will discuss how the Drake Equation looks different if we consider it through the lens of Indigenous methods and sciences and how these methods would lead to a dramatically different view of life in our Galaxy. 

Zoom link: https://pitp.zoom.us/j/95952883179?pwd=a2lzaEc2UWJER2k2VmwzRVgvMVpoQT09

Information about the late-time Universe is imprinted on the small scale CMB as photons travel to us from the surface of last scattering. Several processes are at play and small scale fluctuations are very rich and non-Gaussian in nature. I will review some of the most important effects and I will focus on the Sunyaev-Zel'dovich (SZ) effect and gravitational lensing. I will discuss how a combination of measurements can probe velocity fields at cosmological distances and inform us on cluster energetics. I will also show recent measurements of weak lensing of the CMB and how they can help us interpret intriguing discrepancies in cosmological parameters between the high and low redshift Universe.

Zoom link: https://pitp.zoom.us/j/94451033605?pwd=Tkx4dHZTblMxUFJlZENyblJQVFo2dz09

The huge separation between the Planck scale and typical laboratory scales makes it extremely difficult to detect quantum gravitational effects; however, the situation is in principle much more favourable in cosmology. In particular, the Planck and Hubble scales were only separated by about 5 to 6 orders of magnitude during inflation. This motivates looking for present-day signatures of Planck-scale physics from the early universe. The question, then, is what quantum gravitational effects should we look for, and what are their observational signatures? Here I will discuss predictions for how a generic, quantum gravity-motivated, natural ultraviolet cutoff manifests in primordial power spectra. The cutoff is model-independent, both in the sense that it does not rely on a particular UV completion of quantum gravity, nor does it assume a particular model of inflation. The predicted signature consists of small oscillations that are superimposed on the conventional primordial power spectra, where the template waveform is parameterized by the location of the cutoff between the Planck and Hubble scales. This will allow experiments to place new rigorous bounds on the scale at which quantum gravity effects become important.

Since the 80s, radio sources have been observed to undergo extreme scattering events (ESEs): large, frequency dependent flux modulations due to scattering off the ISM. Recently, the study of these events has undergone a revived interest due to the increase in pulsar timing data, as well as the realization that FRBs will be scattered by the same structures in the ISM. Models of the structures responsible for ESEs range from spherical-cow approximations (e.g. simple Gaussian profiles) to more exotic models (e.g. plasma shells around compact dark matter). Here we present a new model in which ESEs are produced by corrugated sheets in the ISM, which, when projected onto the plane of the sky, generically form A3 cusp catastrophes. We will argue that this model naturally explains several features in scattering data, including observations of PSR 0834+06 and the original Fiedler et al. 0954+658 ESE.  Moreover, this model is consistent with models of pulsar scintillation and does not require exotic physics to explain. We will discuss potential applications to FRB cosmology that arise from the universality of this model.

Zoom link:  https://pitp.zoom.us/j/94260081028?pwd=Vm8zVUd5ak1saWJYZFZ3MWF0L3g2dz09

We examine the validity of the classical approximation of the waterfall phase transition in hybrid inflation from an effective field theory (EFT) point of view. The EFT is constructed by integrating out the waterfall field fluctuations, up to one-loop order in the perturbative expansion. Assuming slow-roll conditions are obeyed, right after the onset of the waterfall phase, we find the backreaction of the waterfall field fluctuations to the evolution of the system can be dominant. In this case the classical approximation is completely spoiled. We derive the necessary constraint that ensures the validity of the EFT.

Zoom Link: https://pitp.zoom.us/j/96209675696?pwd=ZkZ2NGpRVW9vVkdaY1QrWG5GRFltdz09