Scientific Series

In an ordinary quantum field theory, the "split property" implies that the state of the system can be specified independently on a bounded subregion of a Cauchy slice and its complement. This property does not hold for theories of gravity, where observables near the boundary of the Cauchy slice uniquely fix the state on the entire slice. The original formulation of the information paradox explicitly assumed the split property and we follow this assumption to isolate the precise error in Hawking's argument. A similar assumption also underpins the monogamy paradox of Mathur and AMPS. Finally the same assumption is used to support the common idea that the entanglement entropy of the region outside a black hole should follow a Page curve. It is for this reason that computations of the Page curve have been performed only in nonstandard theories of gravity, which include a non-gravitational bath and massive gravitons. The fine-grained entropy at future null infinity does not obey a Page curve for an evaporating black hole in standard theories of gravity but we discuss possibilities for coarse graining that might lead to a Page curve in such cases.

We discuss aspects of the possible transition between small black holes and highly excited fundamental strings. We focus on the connection between black holes and the self gravitating string solution of Horowitz and Polchinski. This solution is interesting because it has non-zero entropy at the classical level and it is natural to suspect that it might be continuously connected to the black hole. Surprisingly, we find a different behavior for heterotic and type II cases. For the type II case we find an obstruction to the idea that the two are connected as classical solutions of string theory, while no such obstruction exists for the heterotic case. We further provide a linear sigma model analysis that suggests a continuous connection for the heterotic case. We also describe a solution generating transformation that produces a charged version of the self gravitating string. This provides a fuzzball-like construction of near extremal configurations carrying fundamental string momentum and winding charges. We provide formulas which are exact in α′ relating the thermodynamic properties of the charged and the uncharged solutions.

Zoom Link: https://pitp.zoom.us/j/96056818482?pwd=cXdxSGRBTEI5N2d6Z21XbTF1Z1lLQT09

Consider a polynomial differential operator in one variable, depending on a small parameter (Planck constant). Under appropriate conditions, the low-energy spectrum admits an asymptotic expansion in hbar.
I will present a way to calculate such a series via a purely "commutative problem", a mixture of variations of Hodge structures and of the Stirling formula. This result came from discussions with A.Soibelman. It seems that we obtain an explanation of an old observation by J.Zinn-Justin of the 

 universal appearance of Bernoulli numbers.

Pulsar magnetospheres admit non-stationary vacuum gaps that are characterized by non-vanishing E•B. These gaps play an important role in plasma production and electromagnetic wave emission and, as I will discuss, are very efficient axion factories. The density of axions produced in a vacuum gap can be several orders of magnitude greater than the ambient dark matter density.  In the strong pulsar magnetic field, a fraction of these axions may convert to photons, giving rise to broadband radio signals. We show that dedicated observations of nearby pulsars with radio telescopes (FAST) and interferometers (SKA) can probe axion-photon couplings that are a few orders of magnitude lower than current astrophysical bounds.

Time at the Planck scale is an unexplored physical regime. It is widely believed that probing Planck time will remain for long an impossible task. Yet, we propose an experiment to test the discreteness of time at the Planck scale and show that it is not far removed from current technological capabilities.

The growing catalog of gravitational wave signals from binary black hole mergers has allowed us to probe the properties of these compact objects more precisely than ever before. In particular, binary black hole spin measurements have the potential to reveal the formation channels of these systems. An accurate and precise measurement of the spins of individual merging black holes is required to model their population-level distributions. In this talk, I will discuss recent work on the measurability of individual black hole spins focusing on those in heavy binaries. I will also present novel spin parameterizations that facilitate the study of specific features of binary black hole evolutionary histories. I will contextualize these individual spin measurements within the observed population of binary black holes and conclude by presenting the latest LIGO-Virgo results for the black hole spin distribution.

When a generic isolated quantum many-body system is driven out of equilibrium, its local properties are eventually described by the thermal ensemble. This picture can be intuitively explained by saying that, in the thermodynamic limit, the system acts as a bath for its own local subsystems. Despite the undeniable success of this paradigm, for interacting systems most of the evidence in support of it comes from numerical computations in relatively small systems, and there are very few exact results. The situation changed recently, with the discovery of certain solvable classes of local quantum circuits, in which finite-time dynamics is accessible and the subsystem-thermalization picture can be verified. After introducing the general picture I will present a recent example (arXiv:2012.12256) of a simple interacting integrable circuit, for which the finite-time dynamics can be exactly described, and the model can be shown to exhibit generic thermalization properties.

Non-gaussianity of primordial density perturbations can be sensitive to very heavy particles at the inflationary Hubble scale (H < 10^(13) GeV). However, the window of observability is often constrained to masses close to H. In this talk, I will discuss a mechanism (dubbed “chemical potential”) for heavy complex scalar fields that can extend this window to masses as large as 60H. The mechanism utilizes the large kinetic energy of the inflaton to enhance particle production, and can impart observable non-gaussianity, f_NL~ O(0.01-10). In the second part of the talk, I will discuss another mechanism where the distinct signature of a heavy field can be imprinted at the level of the power spectrum by violating scale-invariance. This can be achieved through the onset of classical oscillations of the heavy field during inflation, instead of quantum production. We consider the possibility of observing such a signal in the stochastic gravitational wave (GW) background originating from a first-order phase transition in a hidden sector. The signal can be observably large in the GW map while being completely hidden in the standard curvature perturbations such as those of the CMB.

Recent studies revealed that wormhole geometries play a central role in understanding quantum gravity. After disorder-averaging over random couplings, Sachdev-Ye-Kitaev (SYK) model has a collective field description of wormhole saddles. A recent paper by Saad, Shenker, Stanford, and Yao studied the SYK model with fixed couplings and found that the wormhole saddles persist, but that new saddles called “half-wormholes” also appear in the path-integral. 

In this talk, we introduce a “partially disorder-averaged SYK model” and study how these half-wormholes emerge as we gradually fix the coupling constants. This model has a real parameter that smoothly interpolates between the ordinary totally disorder-averaged SYK model and the totally fixed-coupling model. For the large N effective description, in addition to the usual bi-local collective fields, we also introduce a new additional set of local collective fields. These local fields can be understood as the “half” of the bi-local collective fields, and they represent the half-wormholes in the totally fixed-coupling limit. We found that the large N saddles of these local fields vanish in the total-disorder-averaged limit, while they develop non-trivial profiles as we gradually fix the coupling constants. This illuminates how the half-wormhole saddles emerge in the SYK model with fixed couplings.

Primordial gravitational waves have the potential to shed new light on the very early universe. In this talk I will discuss gravitational wave production in a variety of models beyond the simplest, single-field, scenarios and highlight some of their implications for testing inflation with interferometers.