Search Talks

The problem of quantum gravity -- i.e. to determine the microscopic structure underlying quantum mechanical theories that reproduce general relativity at long distances -- is a major outstanding problem in modern physics.  Solving the quantum gravitational scattering problem is one sharp way to address this question.  While in principle effective field theory (EFT) provides a systematic framework for solving scattering problems, in quantum gravity the complete answer requires an infinite number of measurements and thereby fails to predict details of the microscopic structure.

I will present two developments that provide new insight into the gravitational scattering problem.   The first is a class of infinite-dimensional symmetries generically found to arise in gauge and gravitational scattering.  The infinite number of constraints implied by the symmetries are equivalent to quantum field theoretic soft theorems, which prescribe the pattern of soft radiation produced during a scattering event.  The second development is a reformulation of the gravitational scattering problem in which Lorentz symmetry is rendered manifest and realized as the action of the global conformal group in two dimensions.  This reformulation, which involves scattering particles of definite boost weight as opposed to energy, offers a new approach precisely because it does not admit the decoupling of low and high-energy physics that underpins the traditional EFT approach.  I will describe new perspectives ensuing from these developments on various properties of the gravitational scattering problem, including collinear limits, infrared divergences and universal behavior associated to black hole formation. 

Many physical states of interest, such as ground states of gapped quantum many-body systems, are expected to obey an area law of entanglement entropy. I will report on a series of recent results that suggest a deep connection between area law and two seemingly unrelated subjects: topological quantum field theory and quantum marginal problem. Recently, we deduced --- only using area law and quantum information-theoretic tools --- the existence of new topological charges and invariants associated with the domain walls between topologically ordered systems in two spatial dimensions. Moreover, the same set of tools were also used in finding a solution to the quantum marginal problem. This is the problem in which one asks whether a set of reduced density matrices on bounded subsystems are compatible with some globally well-defined many-body quantum state. Since this problem was first posed in 1959, a solution that goes beyond the mean-field ansatz has remained elusive until now. These results suggest that area law is not just a qualitative statement about entanglement; it is an important equation that lets us "solve" quantum many-body systems that appear in nature.

Based on arXiv:2008.11793 and arXiv:2010.07424

Axion cosmic strings have for some time been considered a potential source of enhancement of axion dark matter production, and have been the subject of extensive simulations (for references, see out in recent years).  But axion strings are rather peculiar entities.  This talk will explore some aspects of these objects, and suggest that they are not likely to play a distinguished role in early universe cosmology.

The Hubble tension is conventionally viewed as that between the cosmic microwave background (CMB) and the SH0ES measurement. A prominent proposal for a resolution of this discrepancy is to introduce a new component in the early universe, which initially acts as "early dark energy" (EDE), thus decreasing the physical size of the sound horizon imprinted in the CMB and increasing the inferred H_0, bringing it into near agreement with SH0ES. However, this impacts cosmological observables beyond the CMB -- in particular, the large scale structure (LSS) of the universe across a range of redshift. The H_0 tension resolving EDE cosmologies produce scale-dependent changes to the matter power spectrum, including 10% more power at k=1 h/Mpc. Motivated by this, I will present the results of two analyses of LSS constraints on the EDE scenario. Weak lensing and galaxy clustering data (from, e.g., the Dark Energy Survey) significantly constrain the EDE model, and the resulting H_0 is in significant tension with SH0ES. Complementary to this, including data from the Baryon Oscillation Spectroscopic Survey (BOSS), analyzed using the effective field theory (EFT) of LSS,  yields an EDE H_0 value that is in significant (3.6\sigma) tension with SH0ES. These results indicate that current LSS data disfavours the EDE model as a resolution of the Hubble tension, and, more generally, that the EDE model fails to restore cosmological concordance. A sensitivity forecast for EUCLID suggests that future LSS surveys can close the remaining parameter space of the model.

In this talk I will begin by introducing symmetry protected topological (SPT) Floquet systems in 1D. I will describe the topological invariants that characterize these systems,  and highlight their differences from SPT phases arising in static systems.  I will also discuss how the entanglement properties of a many-particle wavefunction depend on these  topological invariants. I will then show that the edge modes encountered in free fermion SPTs are remarkably robust to adding interactions, even in disorder-free systems where generic bulk quantities can heat to infinite temperatures due to the periodic driving. This robustness of the edge modes to heating can be understood in the language of strong modes for free fermion SPTs, and  almost strong modes for interacting SPTs.

I will then outline a tunneling calculation for extracting the long lifetimes of these edge modes by mapping the Heisenberg time-evolution of the edge operator to dynamics of a single particle in Krylov space. 

Detection mechanisms for low mass bosonic dark matter candidates, such the axion or hidden photon, leverage potential interactions with electromagnetic fields, whereby the dark matter (of unknown mass) on rare occasion converts into a single photon. Current dark matter searches operating at microwave frequencies use a resonant cavity to coherently accumulate the field sourced by the dark matter and a near standard quantum limited (SQL) linear amplifier to read out the cavity signal. To further increase sensitivity to the dark matter signal, sub-SQL detection techniques are required. Here we report the development of a novel microwave photon counting technique and a new exclusion limit on hidden photon dark matter. We operate a superconducting qubit to make repeated quantum non-demolition measurements of cavity photons and apply a hidden Markov model analysis to reduce the noise to 15.7 dB below the quantum limit, with overall detector performance limited by a residual background of real photons. With the present device, we perform a hidden photon search and constrain the kinetic mixing angle to  ≤ 1.68×10−15 in a band around 6.011 GHz (24.86 μeV) with an integration time of 8.33 s. This demonstrated noise reduction technique enables future dark matter searches to be sped up by a factor of 1300. By coupling a qubit to an arbitrary quantum sensor, more general sub-SQL metrology is possible with the techniques presented in this work.

The melonic limit of a field theory is a large-N limit in which melonic diagrams dominate, thus differing significantly from the cactus and planar limits of vector and matrix models. It was first discovered in tensor models in zero dimensions, viewed as an approach to quantum gravity, and later in the SYK model. More recently, it has found applications in quantum field theory on a fixed (flat) background as an analytic tool for the study of new fixed points of the renormalization group, i.e. new conformal field theories. In this talk, I will review the main features of the melonic limit, and in view of the recent developments I will revisit an old model by Amit and Roginsky with SO(3) internal symmetry, which is neither a tensor model nor a disordered model like SYK, and yet it has a similar melonic limit. Time permitting, I will also comment on similarities with the fishnet model by Kazakov et al, and on the (in)stability of all such models when complex scaling dimensions appear.

General Relativity remains to this day our best description of gravitational phenomena. Nonetheless, issues such its quantization and cosmological constant problem suggest Einstein’s theory might not be final theory of the gravitational interaction. Motivated by these questions, theorists have proposed a myriad of extensions to General Relativity over the decades. In this seminar, I will focus on theories with extra scalar fields. In particular, I will describe how some of these theories can evade Solar System constraints and yet yield to new effects in the strong-gravity regime of compact objects, i.e. neutron stars and black holes. This is achieved through a process known as spontaneous scalarization, in which a compact object growths 'scalar hair' once certain conditions are met and remains 'bald' otherwise. I will review the basics of this effect and then focus on recent efforts in understanding it for black holes both in isolation and in binaries.

Information theory offers mathematically precise theory of communication and data storage that guided and fueled the information age.  Initially, quantum effects were thought to be an annoying source of noise, but we have since learned that they offer new capabilities and vast opportunities. Quantum information theory seeks to identify, quantify, and ultimately harness these capabilities. A basic resource in this context is a noisy quantum communication channel, and a central goal is to figure out its capacities---what can you do with it?  I’ll highlight the new and fundamentally quantum aspects that arise here, such as the role of entanglement, ways to quantify it, and bizarre new kinds of synergies between resources.  These ideas elucidate the nature of communication in a quantum context, as well as revealing new facets of quantum theory itself.

Have you always wanted to know: What the symmetries of nature are? How black holes process quantum information? What the ultimate UV description of our universe is? Then join me as I continue to develop a new framework to describe scattering:  Celestial Holography.

The Celestial Holography framework applies the holographic principle to spacetimes with vanishing cosmological constant by mapping 4D S-matrix elements to correlators in a 2D conformal field theory. This map possesses a number of surprising features. For example, it emphasizes infinite dimensional symmetry enhancements, which are typically hidden in IR factorization theorems for amplitudes; reorganizes collinear limits as CFT operator product expansions; and mixes UV and IR behavior in a manner that may allow us to make general claims about scattering not obvious from perturbation theory.

Can we show that the UV behavior of amplitudes must be stringy? Can we bootstrap celestial CFTs? Can we unify tools from Loop Quantum Gravity and String Theory?

Maybe, with your help!

I will discuss the conditions satisfied by the 2 → 2 scattering amplitude in unitarity and causal theories, presenting a simple formulation in term of moments of a positive measure. I will identify complete subsets of two-sided constraints, suitable to bound Effective Field Theories (EFTs) at any finite order in the energy expansion. I will also discuss  the importance of IR loop effects on these bounds. A consequence of this study is that EFTs in which the scattering amplitude in some regime grows in energy faster than E6 (e.g. higher spin particles with masses smaller than their inverse size) cannot be UV-completed.