Scientific Series

Quantum theory can be understood as a theory of information processing in the circuit framework for operational probabilistic theories. This approach presupposes a definite casual structure as well as a preferred time direction. But in general relativity, the causal structure of space-time is dynamical and not predefined, which indicates that a quantum theory that could incorporate gravity requires a more general operational paradigm. In this talk, I will describe recent progress in this direction. First, I will show how relaxing the assumption that local operations take place in a global causal structure leads to a generalized framework that unifies all signaling and non-signaling quantum correlations in space-time via an extension of the density matrix called the process matrix. This framework also contains a new kind of correlations incompatible with any definite causal structure, which violate causal inequalities, the general theory of which I am going to present. I will then present an extension of the process matrix framework, in which no predefined causal structure is assumed even locally. This is based on a more general, time-neutral notion of operation, which leads to new insights into the problem of time-reversal symmetry in quantum mechanics, the meaning of causality, and the fact that we remember the past but not the future. In the resultant generalized formulation of quantum theory, operations are associated with regions that can be connected in networks with no directionality assumed for the connections. The theory is compatible with timelike loops and other acausal structures.

I will discuss a class of non-compact solutions to the Strominger-Hull system, the first order system of equations for preserving N=1 supersymmetry in heterotic compactifications to four dimensions. The solutions consists of the conifold and its Z2 orbifold with Abelian gauge fields and non-zero three-form flux. The heterotic Bianchi Identity is solved in a large charge limit of the gauge fields, where it is shown that the topological term p1(TX) can be consistently neglected. At large distances, these solutions are locally Ricci-flat. For a given flux, the family of solutions has three real parameters, the size of the pair of two spheres in the IR and the dilaton zero mode. There exists an explicit analytic solution for the decoupled near horizon region where for a given flux, the size of the cycles is frozen and the only parameter is the dilaton zero mode. This near horizon region also has an exactly solvable worldsheet CFT. When one of the two cycles has vanishing size the near horizon region disappears, but a solution on the unorbifolded resolved conifold still exists.

Asymptotically AdS spacetimes with reflecting boundary conditions represent a natural setting for studying superradiant instabilities of rotating or charged black holes. In the first part of this talk, I prove that all asymptotically AdS black holes with ergoregions in dimension d ≥ 4 are linearly unstable to gravitational perturbations. This proof uses the canonical energy method of Hollands and Wald in a WKB limit. In the second part of the talk, I consider a charged Reissner-Nordstrom-AdS black hole---which is superradiantly unstable to charged scalar field perturbations at the linear level---and study the full *nonlinear* evolution of the instability. In this special case, the instability occurs even for spherically symmetric perturbations, which simplifies the analysis and allows for the use of numerical general relativity simulations. Our results show that nonlinear backreaction causes the black hole to lose charge and mass to the scalar field as the instability proceeds. Eventually, higher scalar field harmonics become nonsuperradiant, and they are reabsorbed into the black hole. The final state is described by a “hairy” black hole, surrounded by a scalar condensate in the fundamental (lowest) mode. I discuss implications of this work on the original problem of the rotating black hole superradiant instability.

The kinetic Sunyaev-Zel'dovich effect is a direct probe of the distribution and velocity of electrons on cosmological scales.  Recent progress in Cosmic Microwave Background observations allow statistical detections of this subtle effect originating from a number of different tracers populations.  In my talk, I will review the observational status, highlight the consequences for astrophysics and cosmology and discuss future directions.

This is a joint work with A.Kuznetsov and L.Rybnikov.

We study a moduli problem on a nodal curve of arithmetic genus 1, whose solution is an open subscheme in the zastava space for projective line. This moduli space is equipped with a natural Poisson structure, and we compute it in a natural coordinate system. We compare this Poisson structure with the trigonometric Poisson structure on the transversal slices in an affine flag variety.

We conjecture that certain generalized minors give rise to a cluster structure on the trigonometric zastava.

The structure of entanglement can yield new physical insights into strongly interacting quantum critical states. I’ll describe key properties of the entanglement entropy of conformal field theories (CFTs) in 2+1d. In particular, we’ll see that sharp corners in the entangling surface contribute a regulator-independent function that depends non-trivially on the corner angle. I’ll argue that in the smooth limit this function yields the 2-point function of the stress tensor. This sheds light on recent cutting edge simulations of the quantum critical Ising, XY and Heisenberg models. I’ll also present a new lower bound for this function. I will then generalize to Rényi entropies, which yields a simple procedure to extract the thermal entropy using corner entanglement of the groundstate alone. Connections will be made to CFTs in 1+1d and 3+1d, as well as to Lifshitz theories.

I distinguish two types of reduction within the context of quantum-classical relations, which I designate “formal” and “empirical”. Formal reduction holds or fails to hold solely by virtue of the mathematical relationship between two theories; it is therefore a two-place, a priori relation between theories. Empirical reduction requires one theory to encompass the range of physical behaviors that are well-modeled in another theory; in a certain sense, it is a three-place, a posteriori relation connecting the theories and the domain of physical reality that both serve to describe. Focusing on the relationship between classical and quantum mechanics, I argue that while certain formal results concerning singular ℏ→0 limits have been taken to preclude the possibility of reduction between these theories, such results at most provide support for the claim that singular limits block reduction in the formal sense; little if any reason has been given for thinking that they block reduction in the empirical sense. I then briefly outline a strategy for empirical reduction that is suggested by work on decoherence theory, arguing that this sort of account remains a fully viable route to the empirical reduction of classical to quantum mechanics and is unaffected by such singular limits.

In effective field theory, causality fixes the signs of certain interactions.  I will describe how these Lorentzian constraints are encoded in the Euclidean theory, and use the conformal bootstrap to derive analogous causality constraints in CFT. Applied to spinning fields, these constraints include (some of) the Hofman-Maldacena bounds derived from conformal collider physics. I will also discuss applications to holographic theories.

We propose the discovery of the electroweak monopole as the final test of the standard model. Unlike the Dirac's monopole in electrodynamics which is optional, the electroweak monopole must exist within the framework of the standard model because the $U(1)_{em}$ becomes non-trivial. We estimate the mass of the monopole to be around 4 to 7 TeV, and expect the production rate to be relatively large, $(1/\alpha_{em})^2$ times bigger than the WW production rate. This implies that the MoEDAL detector at LHC could have a real chance to detect it.  

The global redshifted 21-cm radiation background is expected to be a powerful probe of the re-heating and re-ionization of the intergalactic medium. However, its measurement is technically challenging: one must extract the small, frequency-dependent signal from under much brighter and spectrally smooth foregrounds. Traditional approaches to study the global signal have used single-antenna systems, where one must calibrate out frequency-dependent structure in the overall system gain, as well as remove the noise bias from auto-correlating a single amplifier output. I will review these approaches, and critically examine several recent proposals to measure the global background using interferometric setups. In particular, using very general principles, I will show that the latters' sensitivity is directly related to two characteristics: the cross-talk between the readout channels (i.e. the signal picked up at one antenna when the other one is driven) and the correlated noise due to thermal fluctuations of lossy elements (e.g. absorbers or the ground) radiating into both channels. I will also briefly discuss the implications and future prospects for interferometric methods.

If the dark matter is made up of a bosonic particle, it can be ultralight, with a mass potentially much below 1 eV. Well-known DM candidates of this type include pseudoscalars like the QCD axion, and vectors such as hidden photons kinetically mixed with the Standard Model. Moduli, even-parity scalars with nonderivative couplings to the SM, can also be light dark matter. I will show that they cause tiny fractional oscillations of SM parameters, such as the electron mass and the fine-structure constant, in turn modulating length and time scales of atoms. Rods and clocks, used in gedanken experiments in relativity, have since transformed into actual precision instruments. The size of acoustic resonators and the frequency of optical clocks can now be measured to 1 part in 10^22 and 10^18, respectively, and thus constitute sensitive probes of moduli. 

In this talk, I will give an overview of the parameter space of modulus dark matter, and discuss the sensitivity of the proposed experiments compared to existing constraints from fifth-force tests.