Search Talks

In this talk, we explore the possibility of gravitational wave production due to ultra-relativistic bubble wall collisions. This occurs due to a process of post-inflationary vacuum decay that takes place via quantum tunnelling within a warped throat (of Randall-Sundrum type). We emphasise the differences between vacuum decay via quantum tunnelling, and a thermal first order phase transition, and how potential gravitational wave signals from both processes differ. We explore a specific example in the context of type IIB string theory, although we argue that our conclusions are more generally applicable to theories with hidden sectors featuring metastable vacua. Many such transitions could have occurred in the post-inflationary Universe, as a large number of throats with exponentially different IR scales can be present in the string landscape, potentially leading to several signals of widely different frequencies – a soundscape connected to the landscape of vacua. Future detectors like LISA will have the required sensitivity to detect these potential signals.

I will discuss the potential for the LISA space-based interferometer to detect the stochastic Gravitational Wave (GW) background produced from different mechanisms during inflation. In particular, I will present the GW contributions from particle production during inflation, inflationary spectator fields with varying speed of sound, effective field theories of inflation with specific patterns of symmetry breaking and models leading to the formation of primordial black holes. I will show that LISA is able to probe these inflationary scenarios beyond the irreducible vacuum tensor modes expected from any inflationary background.

Quantum phase transitions arise at zero temperature when ground state energy meets non-analyticity upon tuning a non-thermal parameter.

Physical properties around quantum critical points (QCPs) are of extensive current interests because the fierce competition between critical quantum and thermal fluctuations near the QCPs can strongly affect dynamics and thermodynamics, leading to unconventional physics.

Despite of its great interests, however, it remains challenge to analytically understand such real frequency dynamics in strongly-correlated quantum systems, even for the dynamics in the quantum systems of one spatial dimension.

In the talk, motivated by recent experimental progresses, we shall take the challenge via analytically exploring finite temperature dynamics at the QCP of 3(space)+1(time) quantum φ^4 model, and zero-temperature spin dynamics in the quantum critical regime of XXZ Heisenberg model.

During the exciting excursion, sophisticated but powerful theoretical tools were introduced, and a bunch of new physics beyond conventional pictures were discovered. At the end we shall also discuss the implication of our results for experiments in relevant materials and suggest proper future experimental setups to test our theoretical findings.

By applying loop quantum gravity techniques to 2+1 gravity with a positive cosmological constant Λ, we show how the local gauge symmetry of the theory encoded in the constraint algebra acquires the quantum group structure of SOq(4). By means of an Inonu-Wigner contraction of the quantum group bi-algebra we obtain the kappa-Poincaré algebra of the flat quantum space-time symmetries.

The anti-de Sitter (AdS) space is of great interest in contemporary
theoretical physics due to the AdS/CFT correspondence. However, the
question of stability of AdS space is unanswered till now. After
giving the motivation for studies of asymptotically AdS spaces, I will
review dynamics of such spacetimes in the context of AdS instability
problem. This survey will include: evidence for instability of AdS
space, existence and properties of time-periodic solutions, and
finally an application of analytical technique called multiscale or
resonant approximation approach. If time permits, I will comment on
other asymptotically AdS solutions. Along with the results, I will
highlight some details of methods relevant to the topic.

How does thermalization in quantum systems work? Naively, the unitary time evolution prevents thermalization, but one can easily show that in general quantum systems thermalize when brought into contact with a thermal bath. In noninteracting systems, the approach to the thermal value can be either ballistic or diffusive depending on particle statistics and bath temperature.

However, many systems cannot be thermalized when placed in a bath: glasses.

I will discuss a disorderfree model of an organic electronic glass that is formed through rapid supercooling. Geometric frustration and long-range interactions cause the Arrhenius-type freezing.

Quenched disorder can also lead to glassiness, a phenomenon known as many-body localization. In this case, thermalization is prevented by the existence of extensively many local integrals of motion. I will show how to compute these integrals of motion and their properties.

I will argue that the standard model contains a rather strong hint that -- instead of being simply an ordinary continuous 4D manifold -- spacetime is actually the product of a 4D manifold and a certain discrete/finite 6D space (i.e. there are 6D discrete/finite "extra dimensions").  I will introduce this idea and the evidence for it in simple way, and then discuss various outstanding puzzles and future directions.