Directions

Imperial College has its own detailed information on general directions and on getting to the theoretical physics group. The College is located on Prince Consort Road, south of Hyde Park (map). The most convenient access is via tube (South Kensington, Gloucester Road) or buses. The Theoretical Physics group resides on the 5th floor of the Huxley Building. The group also possesses its own description.

Seminars at Imperial College

Found at least 20 result(s)

17.05.2023 (Wednesday)

On Quantum Transitions, Detailed Balance and Nothingness

Regular Seminar Fernando Quevedo (University of Cambridge)

at:
14:00 IC
room B1004
abstract:

We consider vacuum transitions by bubble nucleation among vacua with different values and signs of the cosmological constant Lambda, including both up and down tunnelings. Following the Hamiltonian formalism in four dimensions, we explicitly compute the decay rates for all possible combinations of initial and final values of Lambda and find that up-tunneling may be allowed starting not only from pure dS spacetime but also from pure AdS and Minkowski spacetimes. We trace the difference with the Euclidean approach, for which these transitions are found not to be allowed, to the difference of assigning either vanishing or infinite entropy to both pure AdS and Minkowski. We find that, in all allowed cases, detailed balance is satisfied. Also in the formal limit Lambda -> -infinity, the transition rates for AdS to dS agree with the Hartle-Hawking and Vilenkin amplitudes for the creation of dS from nothing. This is consistent with a proposal of Brown and Dahlen to define 'nothing' as AdS in this limit. We generalise our results to include black hole backgrounds for which transitions are allowed only in certain regimes of the black hole mass M but detailed balance is not satisfied, except for Schwarzschild de Sitter (SdS) to another SdS for which the transition is allowed and detailed balance satisfied. We compute the bubble trajectory after nucleation and find that, contrary to the M = 0 case, the trajectory is not a geodesic for the open universe slicing of dS. We briefly discuss the relevance of our results to the string landscape.

16.05.2023 (Tuesday)

TBA

Regular Seminar Marco Scalisi (Max Planck Institute)

at:
13:15 IC
room Lecture theatre 3, Blackett building
abstract:

10.05.2023 (Wednesday)

Exploring Low-Dimensional Quantum Spectral Curves

Regular Seminar Simon Ekhammar (Uppsala U., Sweden)

at:
14:00 IC
room Lecture Theatre 2, B113
abstract:

The Quantum Spectral Curve (QSC) is a powerful integrability-based formalism capable of computing the non-perturbative spectrum of planar N=4 SYM. The success and utility of QSC motivate trying to extend it beyond N=4 to other instances of the AdS/CFT correspondence where integrability is expected to be present. This has been successfully accomplished for AdS4/CFT3 and a curve has been conjectured for AdS3/CFT2. I will review the basics of the QSC framework in the well-understood AdS5 case and then turn to low-dimensional versions of the QSC. I will discuss the conjectured curve for AdS3 and how it differs from previous iterations of the QSC. Furthermore, I will discuss recent perturbative results with a peculiar structure.

03.05.2023 (Wednesday)

Doubled space and extended supersymmetry

Regular Seminar Ondrej Hulik (Vrije U., Brussels)

at:
15:00 IC
room B1004
abstract:

In this talk I will discuss N=(2,2) susy generalisation of Hull's doubled sigma model. The doubled formulation of the worldsheet provides a description of string theory in which T-duality is promoted to a manifest symmetry. Formulation via N=(2,2) superspace provides a doubled formulation for bi-Hermitian/generalised Kahler target spaces. The theory is described by a single function, a doubled-generalised Kahler potential, supplemented with a manifestly N=(2,2) constraint. If time permits I will ilustrate some of the concepts developed on examples.

02.05.2023 (Tuesday)

Supersymmetric Massive Gravity

Regular Seminar Laura Johnson (Zurich, ETH)

at:
13:15 IC
room H503
abstract:

In this talk, we start by introducing massive spin-2 theories and reviewing some of their key features. Then, using massive spinor helicity variables, we review an on-shell superspace formalism for massive particles in four dimensions. Finally, we apply this formalism to massive spin-2 amplitudes, deriving all spin-2 cubic vertices that are compatible with supersymmetry and exploring the constraints that adding more supersymmetry has on these vertices. Additionally, we discuss how the massive graviton supermultiplets and cubic superamplitudes can be constructed via a double copy of massive Yang—Mills supermultiplets and cubic superamplitudes. We conclude by commenting on possible future directions such as computing the 4-point massive supersymmetric spin-2 amplitudes and the issues that can arise in a massive double copy for higher point amplitudes.

19.04.2023 (Wednesday)

Line defects, rational Q-systems, and higher symmetries in 3d N=4 theories

Regular Seminar Marcus Sperling (Southeast U., Nanjing)

at:
14:00 IC
room H503
abstract:

In this talk, I explore the set of line defects supported by 3D N=4 theories, and their significance in the context of generalised symmetries. I begin by discussing mirror symmetry of line defects using the example of Sp(k) SQCD and its two mirror theories. I then introduce rational Q-systems, a powerful technique borrowed from spin-chains/integrability, for evaluating twisted indices and studying line operator correlation functions. Finally, I highlight the role of line defects in realising mirror symmetry in the presence of non-trivial higher symmetries

29.03.2023 (Wednesday)

Large N factorization and holography

Regular Seminar Alberto Zaffaroni (University of Milano-Bicocca)

at:
14:00 IC
room H503
abstract:

There has been some recent progress in understanding the microscopic derivation of the entropy of supersymmetric AdS black holes using holography and a localisation computation in the dual quantum field theory. In this talk, after a general discussion of the overall picture, I discuss the large N factorization properties of supersymmetric partition functions for CFT with a holographic dual in various dimensions. I show how this factorization is related to a universal gluing formula for the entropy functionals of known AdS black holes in terms of elementary objects called gravitational blocks.

28.03.2023 (Tuesday)

Decoupling limits in Renormalizable Quantum Gravity

Regular Seminar Luca Buoninfante (Nordita)

at:
13:15 IC
room H503
abstract:

A natural way to extend Einstein's General Relativity in the high-energy regime is to introduce higher-order curvature terms in the gravitational Lagrangian. Indeed, by working in the framework of perturbative QFT one can show that quadratic-curvature gravity in four dimensions is strictly renormalizable. The quadratic-curvature terms are multiplied by dimensionless parameters that are related to the masses of the additional gravitational degrees of freedom and to the interaction couplings. In this talk, after having motivated Renormalizable Quantum Gravity, we will study the limits in which those dimensionless parameters tend to zero or to infinity, and show that different types of decoupling can occur. In particular, it will be shown that the presence of a non-zero cosmological constant affects the decoupling in a non-trivial way in the limit where the coefficient of the Weyl-squared term tends to infinity. We will discuss possible physical implications of this mathematical analysis for the high-energy behavior of the spin-2 massive ghost and for the classical limit of the theory. Several concepts that have been developed in the context of massive gravity will naturally emerge in this talk, sometimes with different relevance.

23.03.2023 (Thursday)

Kaluza-Klein Spectrometry for String Theory Compactifications

Regular Seminar Emanuel Malek (Humboldt University of Berlin)

at:
14:00 IC
room SALC 10, Sherfield building
abstract:

I will present a powerful new method that for the first time allows us to compute the Kaluza-Klein spectrum of a large class of string theory compactifications, including those arising in maximal gauged supergravities and beyond. This includes geometries with little to no remaining (super-)symmetries, completely inaccessible by previous methods. I will show how these insights can be used to holographically compute the anomalous dimensions of protected and unprotected operators in strongly-coupled CFTs, as well as to study global properties of their conformal manifolds. I will also show how the method can be used to determine the perturbative stability of non-supersymmetric AdS vacua. We will see the importance of higher Kaluza-Klein modes to the physics of string compactifications, e.g. in realising the compactness of moduli spaces, restoring supersymmetry that is lost in a consistent truncation, and in destabilising vacua that appear to stable in lower-dimensional supergravities.

14.03.2023 (Tuesday)

Non-perturbative strings, asymptotic safety, and the swampland

Regular Seminar Alessia Platania (Perimeter Institute for Theoretical Physics)

at:
13:15 IC
room H503
abstract:

Quantum gravity is undoubtfully one of the most important missing pieces in the understanding of the mathematical structure of our universe. The impossibility of consistently quantizing gravity via perturbative quantum field theory has led to a plethora of different proposals, from asymptotically safe gravity to non-local gravity, loop quantum gravity, and string theory. Different approaches face different problems and have succeeded in different areas. Yet, on the conceptual side, it is not obvious that all these frameworks are inequivalent or unrelated: some theories may be low-energy approximations of others, or could even provide different mathematical descriptions of the same physics. On the technical side, the knowledge gained in an approach could be useful to investigate certain aspects of others. In this spirit, I will review progress in connecting and contrasting two theories: asymptotically safe gravity and string theory. Specifically, I will discuss how to test asymptotic safety using stringy swampland constraints, and how techniques developed in the context of asymptotically safe gravity can be exploited to compute cosmological higher-derivative corrections to all orders in string theory.

07.03.2023 (Tuesday)

Quantum phenomena in the early Universe

Regular Seminar Kimmo Kainulainen (Helsinki Institute of Physics and University of Jyvaskyla)

at:
13:15 IC
room H503
abstract:

Quantum coherence plays essential role in diverse phenomena of relevance in the early universe. Examples include activation of sterile neutrinos, electroweak baryogenesis, resonant leptogenesis and particle production in phase transitions and during the (p)reheating stage after inflation. After a general introduction I will concentrate on the particle production problem. I will show how one can derive coupled, renormalized and tractable quantum kinetic equations for the scalar field 1- and 2-point functions starting from the CTP-formalism, working in the Hartree approximation of the 2PI-action. I will then apply these equations to study particle production and the back-reaction from the non-equilibrium modes on the dynamics of the one-point function. We will see both spinodal and parametric resonances taking place and sometimes overlapping in a novel way, and we follow the process of decoherence and thermalization. Overall, I argue that advanced quantum transport equations are necessary for an accuarate description of many systems of acute interest in cosmology.

13.02.2023 (Monday)

Dissipation of oscillating homogeneous scalar condensates - unusual day and time

Regular Seminar Wenyuan Ai (Kings College London)

at:
12:15 IC
room Blackett 630
abstract:

Scalar condensates are very common objects in cosmology. For example, the inflaton field can be viewed as a scalar condensate before it completely dissipates into ordinary particles during reheating. Axion condensates may have been formed through the vacuum-misalignment mechanism. In this talk, I will discuss the dissipation of oscillating homogeneous scalar backgrounds in flat spacetime and an expanding universe using nonequilibrium quantum field theory. The latter naturally captures the thermal effects and backreaction effects. For quasi-harmonic oscillations, we adopt the multi-scale analysis to obtain analytical approximate expressions for the self-consistent evolution of the scalar condensates in terms of the retarded self-energy and retarded proper four-vertex function, whose imaginary parts characterize different condensate decay channels. At finite temperatures, there are many new condensate decay channels that would be absent at zero temperature. These new channels could play an important role in ensuring a complete dissipation in an expanding universe. The talk is based on the following two papers: JHEP 11 (2021) 160 [arXiv:2108.00254 [hep-ph]]; JHEP 11 (2022) 075 [arXiv:2202.08218 [hep-ph]]

07.02.2023 (Tuesday)

Causal Set Quantum Gravity and the Hard Problem of Consciousness

Regular Seminar Fay Dowker (Imperial College London)

at:
13:15 IC
room H503
abstract:

In this talk I will develop Rafael D. Sorkin’s heuristic that a partially ordered process of the birth of spacetime atoms in causal set quantum gravity can provide an objective physical correlate of our perception of time passing. I will argue that one cannot have an external, fully objective picture of the birth process because the order in which the spacetime atoms are born is a partial order. I propose that live experience in causal set theory is an internal “view” of the objective birth process in which events that are neural correlates of consciousness occur. In causal set theory, what “breathes fire” into a neural correlate of consciousness is that which breathes fire into the whole universe: the unceasing, partially ordered process of the birth of spacetime atoms.

02.02.2023 (Thursday)

Topological recursion, Airy structures, and its generalisations

Regular Seminar Kento Osuga (University of Tokyo)

at:
13:15 IC
room H503
abstract:

Topological recursion has become known as a powerful recursive formalism to compute a variety of invariants in mathematics and physics. The list of applications includes matrix model correlation functions, 2d gravity amplitudes, topological string theory amplitudes, and more. Interestingly, recent study has shown that by introducing the notion of Airy structures, topological recursion can be described in terms of twisted representations of the Virasoro algebra. In this talk, I will first give an introductory overview of topological recursion as well as Airy structures. Then, I will present how one can generalise the current formalism of topological recursion, e.g. by upgrading the Virasoro algebra to the super Virasoro algebra or the q-Virasoro algebra. If time permits, I will also discuss expected applications of such generalisations. This is in part joint with Vincent Bouchard and also in part joint with Nitin Chidambaram. Notice the unusual day!

25.01.2023 (Wednesday)

AdS/CFT at loop order

Regular Seminar Kostas Skenderis (University of Southampton)

at:
14:00 IC
room H503
abstract:

I will discuss how to setup renormalization of bulk loops in AdS and the implications for the AdS/CFT correspondence.

24.01.2023 (Tuesday)

Uncovering the Structure of the epsilon-Expansion

Regular Seminar Andreas Stergiou (King's College London)

at:
13:15 IC
room H503
abstract:

The epsilon-expansion was invented more than 50 years ago and has been used extensively ever since to study aspects of renormalization group flows and critical phenomena. Its most famous applications are found in theories involving scalar fields in (4-epsilon) dimensions. In this talk, we will discuss the structure of the epsilon-expansion and the fixed points that can be obtained within it. We will mostly focus on scalar theories, but we will also discuss theories with fermions as well as line defects. Our motivation is based on the goal of classifying conformal field theories in d=3 dimensions. We will describe recently discovered universal constraints obtained within the framework of the epsilon-expansion and show that a 'heavy handed' quest for fixed points yields a plethora of new ones. These fixed points reveal aspects of the structure of the epsilon-expansion and suggest that a classification of conformal field theories in d=3 is likely to be highly non-trivial.

18.01.2023 (Wednesday)

Celestial chiral algebras, colour-kinematics duality and integrability

Regular Seminar Ricardo Monteiro (Queen Mary University of London)

at:
14:00 IC
room H503
abstract:

We will discuss a connection between OPE algebras appearing in celestial holography for various theories and the `colour-kinematics duality' in the bulk spacetime description of those theories. Both the celestial algebras and the colour-kinematics duality take a particularly simple form for self-dual Yang-Mills and gravity. In particular, we show that the $Lw_{1+\infty}$ celestial algebra recently unveiled in self-dual gravity arises from the soft expansion of an area-preserving diffeomorphism algebra, which plays the role of the kinematic algebra in the colour-kinematics duality. We also present deformations of the celestial algebras resulting from Moyal deformations of the self-dual theories, e.g. the deformation of $Lw_{1+\infty}$ into $LW_{1+\infty}$ in the case of self-dual gravity. In addition, we discuss the relation of these deformations to higher-spin theories of massless particles that can be thought of as extensions of self-dual Yang-Mills and gravity. Finally, we present a proof that tree-level scattering amplitudes in the theories we focus on vanish, signalling their classical integrability, which is an S-matrix version of the Ward conjecture for integrable systems.

17.01.2023 (Tuesday)

Bootstrapping the 6D (2,0) theory with Reinforcement Learning

Regular Seminar Costis Papageorgakis (Queen Mary University of London)

at:
13:15 IC
room H503
abstract:

I will describe a method for approximately solving the crossing equations in a general CFT, using Reinforcement Learning as a stochastic optimiser. I will then present an application of this approach in the context of the 6D (2,0) theory.

12.12.2022 (Monday)

AdS Virasoro-Shapiro from single-valued periods

Regular Seminar Luis Fernando Alday (University of Oxford)

at:
14:00 IC
room B1004
abstract:

We determine the full 1/sqrt(lambda) correction to the flat-space Wilson coefficients which enter the AdS Virasoro-Shapiro amplitude in N=4 SYM theory at strong coupling. The assumption that the Wilson coefficients are in the ring of single-valued multiple zeta values, as expected for closed string amplitudes, is surprisingly powerful and leads to a unique solution to the dispersive sum rules relating Wilson coefficients and OPE data. The corresponding OPE data fully agrees with and extends the results from integrability.

06.12.2022 (Tuesday)

Unitarity and clock dependence in quantum cosmology

Regular Seminar Steffen Gielen (University of Sheffield)

at:
13:15 IC
room H503
abstract:

The problem of time is often discussed as an obstacle in canonical quantisation of gravity: general covariance means there is no preferred time parameter with respect to which evolution could be defined. We can instead characterise dynamics in relational terms by defining one degree of freedom to play the role of an internal clock for the other variables; this leads to a "multiple choice problem". I will review recent results obtained in a quantum cosmological model with three dynamical degrees of freedom: a volume or scale factor variable for the geometry, a massless scalar matter field, and a perfect fluid. Each of these variables can be used as a clock for the other two. We obtain three different theories which, if we require them to have unitary time evolution with respect to the given clock, make very different statements about the fate of the Universe. Only one resolves the classical singularity, and only one leads to a quantum recollapse of the Universe at large volume. Nonclassical behaviour arises whenever a classical solution terminates in finite time so that reflecting boundary conditions are needed to make the theory unitary.