Scientific Series

The fact that the Einstein-Hilbert action, by itself, does not lead to a well-posed variational principle has become textbook knowledge. It can be made well-posed by the addition of suitable boundary terms. There are many boundary terms available in the literature, of which the most famous and most widely used is the Gibbons-Hawking-York (GHY) boundary term. The GHY term is ostensibly defined only for a non-null boundary. There have been very few efforts in the literature to extend its definition to null boundaries. The speaker will present his group's work towards finding a boundary term to render the Einstein-Hilbert action well-posed in the presence of null boundaries. He shall discuss a proposed boundary term, associated boundary conditions, outstanding issues and possible applications.

Gravitational waves, as predicted by Einstein one hundred years ago, have been detected by the advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) in September last year. This finding marks the beginning of gravitational-wave astronomy. From now on, we are able to probe our universe using both long-range forces in nature. 

In the foreseeable future, the gravitational wave astronomy will be exponentially growing, with more detectors targeting gravitational waves at different frequencies and more events detected every week. This will provide us great opportunities but also impose challenges as well. In the first part of my talk, I will discuss the physics of gravitational wave detectors, emphasizing its basic principles and dominant noise sources, as well as ongoing efforts to improve detector sensitivity to fulfill the growing need. In the second half of my talk, I will discuss the astrophysics of gravitational-wave sources - especially compact sources such as black holes and neutron stars. I shall explain the spacetime dynamics, gravitational-wave emission during compact binary mergers and possible electromagnetic radiation from these compact objects. In the next few years with new gravitational-wave observations and possible electromagnetic counterpart measurements, we may dramatically improve our understandings on the astrophysics of these sources.

I will present recent result on constructing effective quantum gravity theory as a locally covariant QFT. The approach I advocate uses the BV formalism for dealing with the gauge freedom and Epstein-Glaser renormalization to control the UV divergences. I will show how gauge invariant observables that satisfy a weak notion of locality can be constructed in this framework and I will sketch the argument for perturbative background independence. Recently these ideas were applied to models relevant in cosmology.

I will discuss recent work on big crunch singularities produced in asymptotic AdS cosmologies using gauge/gravity duality. The dual description consists of a constant mass deformation of ABJM theory on de Sitter space and is well-defined and stable for small deformations.

  There is a critical deformation where the theory becomes unstable at weak and at strong coupling. I will discuss a field theory diagnostic of this instability as well as boundary two-point correlators calculated via the geodesic approximation. Near the critical deformation a second saddle point contribution enters, in which the spacelike geodesics probe the high curvature region near the singularity. Its contribution strongly enhances the long-distance correlations. This has a natural interpretation in the weakly coupled boundary theory where the critical point corresponds to a massless limit.

I will discuss phenomena associated with particle production and field excitation during inflation. In the first part of the talk I will present several
signatures that can originate from the coupling of an axion inflaton to gauge fields. In the second part I will test the robustness of the standard
implications associated with the detection of a gravity wave (GW) signal at CMB scales, by discussing what conditions can allow a visible
sourced GW background that exceeds the vacuum one (without simultaneously overproduce scalar perturbations), and by providing a concrete
example. In the final part I will review the phenomenology of trapped inflation, which is an interesting and minimal mechanism where particle production
provides the main friction for the motion of the inflaton.

I will demonstrate that SL(2,Z) duality is a property of all low-energy effective Abelian theories with electric or magnetic charges  The duality will be verified at one loop by comparing the amplitudes in the case of an electron and the dyon that is its SL(2,Z) image, and I will show that it can be extended order by order in perturbation theory.  I will discuss how the duality generically breaks down at high energies, and show how the results apply to the Seiberg-Witten theory.

In this talk I discuss the effects of nonlinear backreaction of small scale density inhomogeneities in general relativistic cosmology. It has been proposed that in an inhomogeneous universe, nonlinear terms in the Einstein equation could, if properly averaged and taken into account, affect the large scale Friedmannian evolution of the universe. In particular, it was hoped that these terms might mimic a cosmological constant and eliminate the need for dark energy. After reviewing some of these approaches, and some of their flaws, I will describe a perturbative framework (developed with R. Wald) designed to properly take into account these effects. In our framework, we assume that the spacetime metric is "close"---within 1 part in 10^4, except near strong field objects---to a background metric of FLRW symmetry, but we do not assume that the background metric satisfies the Friedmann equation. We also do not require that spacetime derivatives of the metric be close to derivatives of the background metric. This allows for significant deviations in geodesics, and very large curvature inhomogeneities. A priori, this framework also allows for significant backreaction, which would take the form of new effective matter sources in the Friedmann equation. Nevertheless, we prove that if the matter stress-energy tensor satisfies the weak energy condition, then large matter inhomogeneities on small scales cannot produce significant backreaction effects on large scales, and in particular cannot account for dark energy. As I will also review here, with a suitable ‘dictionary,’ Newtonian cosmologies provide excellent approximations to cosmological solutions to Einsteinʼs equation (with dust and a cosmological constant) on all scales. Our results thereby provide strong justification for the mathematical consistency and validity of the LCDM model within the context of general relativistic cosmology. While our rigorous framework makes use of 1-parameter families and weak limits, in this talk I will provide a simple heuristic discussion that places emphasis on the manner in which "averaging" is done, and the fact that one is solving the Einstein equation.

In any quantum field theory, the energy flux at a point of spacetime can be negative.  This would produce a repulsive gravitational field causing nearby light rays to defocus.  This in turn threatens to produce a variety of exotic phenomena including traversable wormholes, warp drives, time machines, and evasion of singularity theorems.  I will describe a new "quantum focusing conjecture" that prevents such pathologies.  In the flat spacetime limit it reduces to a novel lower bound on the energy density, which can be proven for several classes of field theories.

In this talk, I discuss several application of semileptonic B-meson form factors.  Topics include the determination of $|V_{ub}|$ and $|V_{cb}|$, hints for new physics in semitauonic decays, and Standard-Model predictions for flavor-changing-neutral-current processes: $B\to P\nu\bar{\nu}$ and $B\to P\ell^+\ell^-$, where $P$ denotes a pion or kaon.
I will also cover some details of the underlying lattice-QCD calculations at a nontechnical level.