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)

12.07.2022 (Tuesday)

Emergent Cosmology From Matrix Theory

Regular Seminar Samuel Laliberte (McGill University)

at:
14:00 IC
room H503
abstract:

Matrix theory is a proposed non-perturbative definition of superstring theory in which space is emergent. Recently, it was shown that a 4-dimensional expanding universe can emerge in the IKKT matrix model, with another 6 spatial dimensions stabilized at the string scale. This scenario was also explored in the BFSS model, in which case the emerging phase yields a scale-invariant spectrum of scalar and tensor perturbation. In this talk, we will discuss recent progress in understanding these results. More precisely, we will discuss a possible way of obtaining the metric out of the matrices in the IKKT model, and ways to probe symmetry breaking in the BFSS model.

05.07.2022 (Tuesday)

The Higgs/Amplitude Mode from Holography

Regular Seminar Aristomenis Donos (Durham university)

at:
14:00 IC
room H503
abstract:

Second order thermal phase transitions are driven my an order parameter which comes with an amplitude. Fluctuations of this amplitude lead to a slowly decaying mode whose gap closes to zero at the critical point. I will use holographic techniques to discuss in detail how this gapped mode determines the linear response of scalar operators close to the phase transition and give a geometric expression for the dissipative coefficient that fixes the relevant Green’s functions.

28.06.2022 (Tuesday)

Extended operators in CFTs, twists of 4d N=2 theories and extended VOAs.

Regular Seminar Matteo Lotito (University of Massachusetts Amherst)

at:
14:00 IC
room B1004
abstract:

In recent years, it has been understood how local operators do not paint the entire picture of a quantum field theory, but we need to introduce extended operators to understand finer details about these theories. Motivated by this, we investigate a subset of these extended operators in particular in the context of 4d N=2 superconformal field theories. I will start by introducing the setup that we use to describe the possible configurations of these extended operators. I will also compare and contrast the operators that appear in our framework with the more familiar (Wilson and 't Hooft) line operators. Time permitting, I will then introduce twists of these theories by a choice of an appropriate nilpotent supercharge. The restriction to (extended) operators living in the cohomology of this supercharge gives rise to interesting algebraic structures, that are analogous to, or rather an extension of, the 2d vertex operator algebras that have now become familiar familiar in the context of 4d N=2 theories.

14.06.2022 (Tuesday)

CosmoLattice - The Art of Simulating the Early Universe

Regular Seminar Daniel Figueroa (IFIC (CSIC/UV), Valencia, Spain)

at:
14:00 IC
room B1004
abstract:

In this talk we will introduce CosmoLattice, a modern code for simulating the non-linear dynamics of interactive scalar-gauge theories in an expanding universe. As a demonstration of its power we will solve three very different problems of early Universe cosmology: i) the generation and use of gravitational waves as a probe of particle couplings, ii) the dynamics of non-minimally coupled scalar fields in the Jordan frame, and iii) the non-linear dynamics of helical gauge field production during the last efoldings of axion-inflation

31.05.2022 (Tuesday)

Testing massive fields during inflation with the cosmic microwave background

Regular Seminar Matteo Braglia (IFT/UAM)

at:
14:00 IC
room B1004
abstract:

Massive fields oscillating around their minima produce specific oscillatory patterns in the density perturbations that record the time dependence of the scale factor in the primordial Universe (hence the name 'Primordial Standard Clocks'). In this talk, I present recent developments on the model building of inflationary classical primordial standard clocks (CPSC) and emphasize their observable consequences. I then present constraints on these models from the latest Planck temperature and polarization data and show how CPSC can address anomalies at different multipoles. Finally, I discuss the prospects for detecting CPSCs with upcoming CMB missions and to distinguish them from other types of features.

24.05.2022 (Tuesday)

Criticality in Eternal Inflation

Regular Seminar Justin Khoury (University of Pennsylvania)

at:
16:00 IC
room .
abstract:

The string landscape, together with the mechanism of eternal inflation for populating vacua, leads to the seemingly inescapable conclusion that we are part of a vast multiverse. As an inhabitant of the multiverse, how should we reason probabilistically about the expected physical properties of our observable universe? Probabilities in eternal inflation are usually defined in terms of frequencies, but this approach has failed to yield a unique answer. In this talk, I will present a different approach to the problem, based on Bayesian reasoning. By adopting the least informative priors on the model parameters, we will be led to well-defined, time-reparametrization invariant (and non-anthropic) probabilities for occupying different vacua. Remarkably, these probabilities favor vacua whose surrounding landscape topography is that of a deep valley or funnel, akin to folding funnels of naturally-occurring proteins. Furthermore, by modeling the landscape of vacua as a random network, I will show that our probabilities favor regions that are close to the directed percolation phase transition. As usual, the predictive power of criticality lies in scale invariant observables characterized by critical exponents. As an example, I will show that the probability distribution for the cosmological constant is power-law, with a particular critical exponent, and favors a naturally small and positive vacuum energy. Tantalizingly, this hints at a deep connection between non-equilibrium critical phenomena on the landscape and the near-criticality of our universe.

17.05.2022 (Tuesday)

dS vs Quintessence: who sinks in the swampland?

Regular Seminar Michele Cicoli (Bologna University)

at:
14:00 IC
room B1004
abstract:

I will discuss different theoretical and phenomenological challenges of quintessence model building in string theory in order to be able to give an informed answer to the question in the title.

26.04.2022 (Tuesday)

Constraining the fundamental theory: cosmological observations and theoretical consistency.

Regular Seminar Anna Tokareva (Imperial College London)

at:
14:00 IC
room 503 Huxley
abstract:

I can formulate the purpose of my research as an attempt to look to the quantum gravity through both the theoretical and observational windows. My previous research was mainly dedicated to obtaining both observational predictions and theoretical constraints for the models of inflation which can be stated as 'minimal'. They contain only the fields which are proved to exist: Higgs field and gravity. As the results of this work, we proved the self-consistency and investigated the reheating mechanism in a model describing inflation driven by the interplay of Higgs and gravity. My recent research is more focused on the theoretical window to the nature of quantum gravity based on the scattering amplitudes and dispersive relations. The scattering amplitudes through the graviton exchange contain the IR singularities in forward limit. The divergences at $t\rightarrow 0$ can be cancelled in the dispersive relation only if the UV limit of the amplitude is tuned in a specific way which establishes the non-trivial connection between UV and IR forms of the amplitude. We show that this connection can be expressed in terms of the Laplace transform and it can give an information about the UV amplitude in the limit $t \log{s}\rightarrow 0$. We discuss the implications of this approach for QED with gravity.

05.04.2022 (Tuesday)

Gravitational-wave cosmology with dark sirens: state of the art and perspectives for 3G detectors

Regular Seminar Michele Mancarella (Geneva University)

at:
14:00 IC
room 539 Blackett
abstract:

Gravitational-wave (GW) cosmology provides a new way to measure the expansion history of the Universe and test General Relativity (GR) at cosmological scales in the tensor sector, based on the fact that GWs are direct distance tracers. Obtaining the redshift (whose knowledge is essential to test cosmology) is instead the challenge of GW cosmology. In absence of a direct electromagnetic counterpart to the GW event, the source goes under the name of ``dark siren'' and statistical techniques are used. This talk aims at giving an overview of the state-of-the-art for these techniques as well as discussing perspectives for the future. After introducing GW cosmology and statistical methods, I will present the latest measurements of the Hubble parameter and of the phenomenon of ``modified GW propagation'' (that takes place whenever GR is modified at cosmological scales), obtained from the latest Gravitational Wave Transient Catalog 3 with new, independent, open-source codes. In particular, the two techniques applied to real data so far consist in using the statistical correlation with galaxy catalogues and information from the mass distribution of Binary Black Holes. I will discuss methodological aspects, relevant sources of systematics, the interplay with population studies, current challenges and possible ways forward. I will finally present some not-yet-applied ideas for statistical dark siren techniques, in particular for third generation (3G) ground-based GW detectors.

29.03.2022 (Tuesday)

Multi-messenger cosmology: cross-correlating Stochastic Gravitational Wave Backgrounds and Cosmic Microwave Background

Regular Seminar Angelo Ricciardon (Padova University)

at:
14:00 IC
room .
abstract:

Gravitational Waves (GWs) have become one of the most powerful tools to explore our universe from its early epoch until nowadays, thanks to their freely propagating nature. After the GW detections from resolved sources by the LIGO/Virgo collaboration the next target of present and future ground and space-based interferometers is the detection of the stochastic background of GW (SGWB), both astrophysical or cosmological. General Relativity provides us with an extremely powerful and for free tool to extract astrophysical and cosmological information from the SGWB: the cross-correlation with other cosmological tracers, since their anisotropies share a common origin and the same perturbed geodesics. In this talk I will present some recent results about the study of the cross-correlation of the cosmological and astrophysical SGWBs with Cosmic Microwave Background (CMB) anisotropies, showing that future GW detectors, such as LISA or BBO, have the ability to measure such cross-correlation signals. I will also present a new tool in this context which can be used to reconstruct the expected SGWB maps starting from high resolution real Planck CMB maps.

08.03.2022 (Tuesday)

Quantum gravity S-matrix Bootstrap

Regular Seminar Andrea Guerrieri (Tel-Aviv University)

at:
14:00 IC
room Online
abstract:

In this talk I will review some of the recent developments in the S-matrix Bootstrap focussing on applications to effective field theories. As an example, I will apply the bootstrap methods to the supergravity effective field theory in ten dimensions. I will show the improved numerical bootstrap bounds on the first correction to the universal graviton scattering and compare the result with the String Theory predictions. In the last part, I will comment on some new numerical ideas that will boost the explorations in different dimensions and for higher dimensional operators.

01.03.2022 (Tuesday)

Classical general relativity from the double copy and the kinematic algebra of Yang-Mills theory

Regular Seminar Gabriele Travaglini (QMUL)

at:
14:00 IC
room .
abstract:

Scattering amplitudes of elementary particles exhibit a fascinating simplicity, which is entirely obscured in textbook Feynman-diagram computations. While these quantities find their primary application to collider physics, describing the dynamics of the tiniest particles in the universe, they also characterise the interactions among some of its heaviest objects, such as black holes. Violent collisions among black holes occur where tremendous amounts of energy are emitted, in the form of gravitational waves. 100 years after having been predicted by Einstein, their extraordinary direct detection in 2015 opened a fascinating window of observation of our universe at extreme energies never probed before, and it is now crucial to develop novel efficient methods for highly needed high-precision predictions. Thanks to their inherent simplicity, amplitudes are ideally suited to this task. I will begin by reviewing the computation of a very familiar quantity Newton's potential, from scattering amplitudes and unitarity. I will then explain how to compute directly observable quantities such as the scattering angle for light or for gravitons passing by a heavy mass such as a black hole. These computations are further simplified thanks to a remarkable, yet still mysterious connection between scattering amplitudes of gluons (in Yang-Mills theory) and those of gravitons (in Einstein's General relativity), known as the "double copy", whereby the latter amplitudes can be expressed, schematically, as sums of squares of the former -- a property that cannot be possibly guessed by simply staring at the Lagrangians of the two theories. I will conclude by discussing the prospects of performing computations in Einstein gravity to higher orders in Newton's constant using a new, gauge-invariant version of the double copy, and as an example I will briefly discuss the computation of the scattering angle for classical black hole scattering to third post-Minkowskian order (or O(G^3) in Newton's constant G).

09.02.2022 (Wednesday)

DFT and alpha'-corrections

Regular Seminar Linus Wulff (Masaryk University)

at:
14:00 IC
room Online
abstract:

I will argue that Double Field Theory (DFT) can describe some, but not all, alpha'-corrections to the tree-level string effective action. In particular, I will discuss how the first and second alpha'-corrections to the bosonic and heterotic string can be derived from DFT.

08.02.2022 (Tuesday)

Reviving chaotic inflation with fermion production: a supergravity model

Regular Seminar Lorenzo Sorbo (University of Massachusetts Amherst)

at:
14:00 IC
room Online
abstract:

Processes of particle production during inflation can increase the amplitude of the scalar metric perturbations. We show that such a mechanism can naturally arise in supergravity models where an axion-like field drives large field inflation. In this class of models one generally expects instanton-like corrections to the superpotential. We show, by deriving the equations of motion in models of supergravity with a stabilizer, that such corrections generate an interaction between the inflaton and its superpartner. This inflaton-inflatino interaction term is rapidly oscillating, and can lead to copious production of inflatinos during inflation, filling the Fermi sphere up to momenta much larger than the Hubble parameter. In their turn, those fermions source inflaton fluctuations, increasing their amplitude, and effectively lowering the tensor-to-scalar ratio for the model. This allows, in particular, to bring the model where the inflaton potential is quadratic (plus negligibly small instanton corrections) to agree with all existing observations.

01.02.2022 (Tuesday)

Small Cosmological Constants in String Theory

Regular Seminar Liam McAllister (Cornell University)

at:
14:30 IC
room Remote
abstract:

We construct vacua of string theory in which all moduli are stabilized and the magnitude of the cosmological constant is exponentially small. The vacua are supersymmetric AdS4 solutions in flux compactifications of type IIB string theory on orientifolds of Calabi-Yau hypersurfaces. I will explain the advances in computing topological data in Calabi-Yau compactifications that led to these solutions, then speculate about implications for the cosmological constant problem. The vacuum energy is small because we ensure the exact cancellation of all perturbative contributions, through an explicit choice of integer parameters determined by the topology and quantized fluxes. The nonperturbative contributions that remain are exponential in these integers. Finding cosmological constants of small magnitude in this landscape is exponentially easier than in Bousso-Polchinski landscapes. Extending this approach to positive cosmological constants in realistic universes is a difficult open problem.

26.01.2022 (Wednesday)

4d N=2 supergravity observables from Nekrasov-like partition functions

Regular Seminar Kiril Hristov (Sofia University)

at:
14:00 IC
room Online
abstract:

We reinterpret the OSV formula for the on-shell action/entropy function of asymptotically flat BPS black holes as a fixed point formula that is formally equivalent to a recent gluing proposal for asymptotically AdS4 black holes. This prompts a conjecture that the complete perturbative answer for the most general gravitational building block of 4d N=2 supergravity at a single fixed point takes the form of a Nekrasov-like partition function with equivariant parameters related to the higher-derivative expansion of the prepotential. In turn this leads to a simple localization-like proposal for a set of supersymmetric partition functions in (UV completed) 4d N=2 supergravity theories. The conjecture is shown to be in agreement with a number of available results for different BPS backgrounds with both Minkowski and AdS asymptotics. In particular, it follows that the OSV formula comes from the unrefined limit of the general expression including only the so-called W tower of higher derivatives, while the on-shell action of pure (Euclidean) AdS4 with round S3 boundary comes from the NS limit that includes only the T tower.

25.01.2022 (Tuesday)

Is the Hubble constant a constant?

Regular Seminar Eoin Colgain (Sogang University)

at:
14:00 IC
room Online
abstract:

In recent years Adam Riess' SH0ES collaboration has made it fashionable to question Lambda-CDM through a series of steadily more precise local determinations of the Hubble constant, the latest of which currently stands at H0 = 73 ± 1 km/s/Mpc. On the other hand, questioning the FLRW paradigm is still taboo. However, if there is a 5 sigma discrepancy with Planck, then a good explanation is required. In the talk, I will explain why H0 should be bounded above by H0 ~ 71 km/s/Mpc in any FLRW cosmology, before presenting some observations that appear to challenge the working FLRW assumption that the Universe is isotropic and homogeneous. Time permitting, I will spell out the implications of a higher local H0 for dark energy models.

19.01.2022 (Wednesday)

The limitations of the holographic idea

Regular Seminar Samir Mathur (Ohio State University)

at:
14:00 IC
room Online
abstract:

The black hole information paradox has been tightened to a precise contradiction by the small corrections theorem. Resolving the puzzle thus needs an order unity correction to semiclassical dynamics at the horizon. Remarkably, in string theory we find that microstates of black holes are `fuzzballs' with no horizon, which resolves the paradox. An alternative to the fuzzball paradigm is has been sought through a `wormhole paradigm' where the horizon would be continue to be described by semiclassical physics on a code subspace of the full quantum degrees of freedom. This wormhole paradigm can however can be ruled out by an extension of the small corrections theorem. We argue that the notions of ER=EPR etc underlying the wormhole paradigm are incorrect, and that the error arises from using the eternal spacetime geometry which is itself inconsistent with the requirements of unitarity.

03.01.2022 (Monday)

String (In)Stability Issues with Broken Supersymmetry

Regular Seminar Augusto Sagnotti (Scuola Normale Superiore and INFN)

at:
14:00 IC
room B1004
abstract:

I shall review some key features of the three ten-dimensional string models with broken supersymmetry that are free of tachyonic modes. Their leading back-reactions are runaway “tadpole potentials”, which have important effects on their ambient spacetimes. When these are explored in detail within the low-energy effective theory, some surprising features emerge: ·Tadpole potentials can drive interesting spontaneous compactifications; ·In Cosmology, they can lead to the peculiar “climbing scenario” for fast-to-slow-roll transitions. Puzzling instabilities typically accompany broken supersymmetry in String Theory. However, they are absent in the former setting, and point to a mere breakdown of isotropy in the latter, which resonates with the very emergence of compact dimensions. I shall address these issues, trying to emphasize potential lessons and some key open questions.

08.12.2021 (Wednesday)

TBA

Regular Seminar Andreas Schachner ()

at:
14:00 IC
room H503
abstract: