University of Crete HEP Seminars
Amplitudes for Hawking Radiation
| Speaker: | Donal O'Connell |
| Institution: | Edinburgh U. |
| Time: | 27 January, 13:15 |
| Venue: | Webinar |
| Abstract: | In recent years, ideas from quantum field theory, including effective field theory and the on-shell methods of scattering amplitude, have been successfully applied to classical gravitational-wave physics. In this talk, I will show that these ideas extend to the semiclassical case of Hawking radiation. Along the way, I will discuss the relationship between Bogoliubov coefficients and generalised amplitudes. |
| Recording: | The recorded talk can be found here. |
Emergent Viscous Hydrodynamics From a Single Quantum Particle
| Speaker: | Jorge Noronha |
| Institution: | Illinois U. |
| Time: | 10 February, 18:15 |
| Venue: | Webinar |
| Abstract: | We present a concrete example showing how quantum decoherence in open quantum systems gives rise to hydrodynamic behavior at late times and long wavelengths. Focusing on a single non-relativistic quantum particle linearly coupled to a thermal bath of harmonic oscillators a la Caldeira and Leggett, we demonstrate that position-space decoherence renders the reduced density matrix nearly diagonal. Expanding around this diagonal limit, we derive a moment hierarchy akin to the BBGKY hierarchy which, when truncated at second order, yields transient viscous hydrodynamic equations similar to those used in heavy-ion collisions, with shear and bulk viscosities determined by decoherence and the coupling to the environment. In the asymptotic limit, these equations reduce to the Navier-Stokes equations, show here to emerge from decoherence. This suggests that even single-particle dynamics (coupled to a bath) can exhibit collective behavior, offering insight into the success of hydrodynamics in exotic systems, such as high-energy proton-proton collisions. |
| Recording: | The recorded talk can be found here. |
Near-extremal hydrodynamics and the holographic product formula
| Speaker: | Edwan Preau |
| Institution: | Edinburgh U. |
| Time: | 17 February, 13:15 |
| Venue: | Webinar |
| Abstract: | I will discuss the recently introduced holographic product formula in the context of near-extremal hydrodynamics, which corresponds to the regime of energy ω, momentum k and temperature T much smaller than a hard scale μ. I will explain how this formula determines the general form taken by holographic spectral functions in this regime. This form simplifies in the extremal limit T ≪ ω, k ≪ μ, for which the low-temperature gapless modes and the IR conformal behavior factorize. I will then present some examples with different types of gapless modes and IR CFTs, including new numerical results for low temperature quasi-normal modes. |
| Recording: | The recorded talk can be found here. |
For inflation to gravitational waves and black holes and back
| Speaker: | Aris Katsis |
| Institution: | CCTP |
| Time: | 23 February, 15:15 |
| Venue: | Second floor seminar room |
| Abstract: | We interpret cosmological perturbations as resulting from the evolution of primordial, quantum fluctuations of the inflaton field. Then working within the framework of cosmic inflation we can extract information about the early universe by tracing back the evolution of late-time snapshots, like the observed CMB sky, the galaxy surveys for large-scale structure. The possibility of primordial black hole abundances and primordial gravitational waves have only reinforced this line of thought. Dramatic changes in the power spectrum of fluctuations linked to such phenomena can be attributed to exotic features of the background inflaton potentials over which the fluctuations evolve. I will present inflationary potentials that result into a bump at the small scales of the scalar power spectrum and hence an expected increase in the power spectrum of the gravitational waves, and discuss the intuition behind them based on [JCAP 06 (2025) 010] and [2602.04055]. |
Quantum Mechanics of the Hartle-Hawking State
| Speaker: | Jordan Cotler |
| Institution: | Harvard U. |
| Time: | 24 February, 17:15 |
| Venue: | Webinar |
| Abstract: | The no-boundary state in de Sitter offers a compelling framework for specifying the initial conditions of our universe. Many (but famously not all) features of the state agree beautifully with empirical observations and have served as a foundation for initial conditions in inflationary cosmology. We explore its relation to entropy puzzles in de Sitter spacetimes. We then compute the norm of the no-boundary state and show that the contributions from geometries with spherical spatial slices, including the Hartle-Hawking geometry and fluctuations around it, vanish to all loops in perturbation theory. We then discuss the resulting implications for theoretical cosmology. |
| Recording: | The recorded talk can be found here. |
A curious set of self-dualities in scalar field theory
| Speaker: | Paul Romatschke |
| Institution: | TU Wien |
| Time: | 3 March, 13:15 |
| Venue: | Second floor seminar room |
| Abstract: |
In this talk, I consider one-component scalar field theory with quartic self-interaction in two regimes: a broken phase with non-vanishing VEV |
The baryon junction
| Speaker: | Zohar Komargodski |
| Institution: | Simons Center, NY |
| Time: | 3 March, 17:15 |
| Venue: | Webinar |
| Abstract: | Using the flux tube effective field theory, we quantize the baryon junction. We discuss the mass of the baryon junction and some theoretical constraints on it. We discuss some recent measurements of the baryon junction mass. We show that the baryon junction mass is also related to the interaction strength between closed strings. |
| Recording: | The recorded talk can be found here. |
The dark matter problem as a topsy-turvy version of the strong CP problem
| Speaker: | Tom Melia |
| Institution: | IPMU Tokyo |
| Time: | 10 March, 13:15 |
| Venue: | Webinar |
| Abstract: | The vacuum states of QCD generically break CP symmetry, and are parameterised by the ‘QCD theta’ variable. The strong CP problem arises because experiment shows that the amount of QCD CP violation must be exceedingly small, at odds with our generic expectation. This problem has a dynamical solution – the QCD axion – which zeroes any observable effects of CP violation through a relaxation mechanism. The vacuum states of electromagnetism and gravity generically break Lorentz symmetry, and are parameterised by a ‘shadow’ charge and matter density. Similar to the case of theta-vacua, we should expect that we live in a generic state of these gauge theories, and would thus observe ‘shadow’ fluids in the universe. Because these fluids can behave like cold dark matter, we could view dark matter’s existence as a confirmation of our expectations about the vacuum of EM and GR. On the other hand, there is a well-known relaxation mechanism – inflation – that, if realised in our universe, would dynamically ruin this solution to the DM problem by diluting the shadow fluids away. Thus one is led to a topsy-turvy version of the strong CP problem – one has to address how not to dynamically relax a generic vacuum state. Along the way, I will point out that the Hamiltonian formalism makes clear that 1) symmetry-based approaches to the strong CP problem do not solve it, and 2) choosing generic vacuum states in GR (that do not satisfy the Wheeler-DeWitt equation) offers an extremely simple solution to the ‘problem of time evolution’. |
| Recording: | The recorded talk can be found here. |
Defect RG and Defect EE
| Speaker: | Christopher Rosen |
| Institution: | CCTP |
| Time: | 12 March, 15:15 |
| Venue: | Second floor seminar room |
| Abstract: | Over the last two decades, the framework of quantum information theory has provided a powerful perspective on the physics of quantum field theories. In particular, quantum entanglement has emerged as an efficient means for organizing our understanding of the behaviour of quantum field theories at different length scales. An important application of this perspective is the unification of proofs of the celebrated c/F/a theorems, and (in some cases) their extensions to systems with reduced spacetime symmetries. One interesting example of a system with reduced spacetime symmetries is a "defect quantum field theory", in which the presence of a defect breaks the ambient theory's Poincare invariance at least in part. Comparatively little is known about the behavior of defect quantum field theories under renormalization group flow, and it is natural to wonder if the information theoretic perspective can be exploited to better understand these systems as well. I will pursue this line of thought in several examples of defect conformal field theories which are particularly ammenable to calculation. In each case, the sphere entanglement entropy can be calculated explicitly and is shown to take a simple form in terms of "defect data" inherent to the defect. Arguing for the existence of an RG flow between particular examples of such defect theories, I will comment on the behaviour of the universal part of this entanglement entropy along the putative flow. |
Frontiers in hadron structure from lattice QCD
| Speaker: | Dimitra Pefkou |
| Institution: | UC Berkeley & LBNL Berkeley |
| Time: | 17 March, 18:15 |
| Venue: | Webinar |
| Abstract: | The structure of the proton and other hadrons can be expressed in various ways, e.g. by considering non-local correlations between its constituents, or the distribution of fundamental properties like charge and energy inside it, which are encoded in its parton distribution functions (PDFs) and its form factors respectively. Fully characterizing hadron structure is important not only to properly describe how it emerges from the degrees of freedom of quantum chromodynamics (QCD), but also because it is input and sometimes a major source of uncertainty in collider searches for new physics. Lattice QCD is the only approach to theoretically predict these quantities from first principles. I will present recent lattice studies that either aim to increase precision of well-studied hadron properties like the nucleon axial charge and PDFs via improved techniques, or to provide the first glimpse into new ways to probe hadron structure, e.g. via the gravitational form factors. |
| Recording: | The recorded talk can be found here. |
Ordered phases of matter at arbitrarily high temperature
| Speaker: | Andrew Lucas |
| Institution: | Colorado U. |
| Time: | 24 March, 13:15 |
| Venue: | Webinar |
| Abstract: | Usually in statistical physics, high temperature phases are disordered and featureless while low temperature phases may be ordered. For example, solid ice becomes liquid water upon heating; the solid is an ordered phase that spontaneously breaks translational and rotational symmetries, while the liquid is a disordered phase which restores these symmetries in a typical state. In fact, mathematicians have proved that many classes of common models in statistical physics, such as interacting spins in lattice models, always enter a disordered phase at sufficiently high temperature. On the other hand, counterexamples to the intuition that heating destroys order have also been known for a long time: for example, helium-3 under very high pressure can go from a liquid to a solid after heating it at very low temperature. As I will explain, any such transition from disorder to order upon heating is necessarily characterized by “entropic order”, where ordering one degree of freedom enables many more fluctuations in another, causing entropy (not energy) to stabilize an ordered phase. I will then show how to build an entropically-ordered phase of matter at arbitrarily high temperature, with a few simple and illustrative examples of lattice gases that heat into solids (and remain solids forever upon further heating!). I will also explain how entropic order underlies the recent demonstration that 2+1d conformal field theories that spontaneously break Z2 symmetry at finite temperature, exist, and how to use these field theoretic ideas to build a toy model of high-temperature superconductivity, stabilized by entropic order. |
| Recording: | The recorded talk can be found here. |
Near extremal Black Holes, extended hydrodynamics and quantum gravitational effects
| Speaker: | Elias Kiritsis |
| Institution: | CCTP |
| Time: | 26 March, 15:15 |
| Venue: | Second floor seminar room |
| Abstract: | Charged asymptotically AdS black holes are known to be dual to strongly coupled QFTs at finite density. In the near extremal limit, where temperature is much smaller that the charge density , they are known to develop an IR scaling symmetry encoded in the near horizon AdS_2 geometry. It has also been argued that the low energy dynamics associated with such black holes, which is hydrodynamics, is valid at much higher energies and momenta than what is conventionally assumed. We show concrete evidence of this by comparing exact charge and energy correlators to various improved hydrodynamic approximations that resum the effect of non-trivial AdS_2 poles. When the transverse space to the AdS_2 geometry has finite volume, there is a non-zero energy scale E_\text{gap}, scaling as 1/charge for large charge, below which quantum-gravitational corrections due to the fluctuations of the nearly-gapless Schwarzian modes become important. Such corrections to the retarded Green's function are calculated at different relative values of \omega, T, and E_\text{gap}. The \omega \to 0 limit is used to define the shear viscosity \eta. As the temperature is lowered below the charge density, quantum corrections are found to increase the value of \eta with respect to its famous semiclassical result and the shear viscosity diverges as \sqrt{E_\text{gap}/T} at T \ll E_\text{gap}, in accord with corresponding results for the absorption cross section. The quantum result for the ratio \eta/s, where s is the entropy density, dips below the semiclassical limit of 1/4\pi when E_\text{gap} \ll T \ll \mu,then turns back to increase towards lower temperatures, and finally diverges at temperatures much below E_\text{gap}. Comments on the similarity of near-extremal black holes and glasses as well as their effect on the Aretakis instability will be advanced. |
Observers, path integrals and a consistent evaluation of the Hartle-Hawking proposal
| Speaker: | Adam Levine |
| Institution: | MIT |
| Time: | 31 March, 18:15 |
| Venue: | Webinar |
| Abstract: | In this talk, we will discuss recent work attempting to develop a framework for describing non-perturbative gravitational physics relative to an observer. We apply our proposal to an observer that lives in a closed universe and one that falls behind a black hole horizon. We find that the Hilbert space that describes the experience of the observer is much larger than the Hilbert space in the absence of an observer. In the case of closed universes, the Hilbert space is not one-dimensional, as calculations in the absence of the observer suggest. Rather, its dimension scales exponentially with G^{-1}_N. We will then discuss these ideas in the context of cosmology and their applications to the study of the Hartle-Hawking no-boundary state. Based upon 2501.02632 and 2602.02682. |
| Recording: | The recorded talk can be found here. |
Covariant holographic c-functions revisited
| Speaker: | Jose Manuel Penin |
| Institution: | CCTP |
| Time: | 2 April, 15:15 |
| Venue: | Second floor seminar room |
| Abstract: | We propose a covariant holographic c-function defined directly in ten dimensions, constructed from the extrinsic curvature of codimension-two slices of the Einstein-frame metric. The definition requires no special coordinate choice, factorizable warp Ansatz, or consistent truncation, and reduces to known holographic c-functions in the regimes where their assumptions apply. We evaluate the covariant expression in a range of top-down string backgrounds, including flows across dimensions, confining geometries, and examples where the ten-dimensional geometry cannot be written in a standard factorizable form. In all cases the behavior is as expected: the function monotonically interpolates between AdS fixed points when present and decreases towards zero in confining infrared regions. We also highlight open conceptual issues, including the absence of a bulk-action derivation and of a known field-theory counterpart, and briefly comment on possible links to entanglement-entropy c-functions. |
Analyticity of the Black Hole S-Matrix
| Speaker: | Miguel Correia |
| Institution: | Mc Gill U. |
| Time: | 21 April, 18:15 |
| Venue: | Webinar |
| Abstract: | I will derive the analytic structure of the S-matrix for classical wave scattering on a Schwarzschild black hole in 4-dimensions. The argument relies on the analytic continuation of the gravitational potential, with the singularity behind the horizon playing a crucial role. We observe the emergence of Stokes phenomena leading to a surprising branch cut in the upper half frequency plane, seemingly in tension with causality. I will discuss how this apparent paradox is solved and mention some future bootstrap applications of these results. |
| Recording: | The recorded talk can be found here. |
CFTs are lazy
| Speaker: | Vasilis Niarchos |
| Institution: | CCTP |
| Time: | 23 April, 15:15 |
| Venue: | Second floor seminar room |
| Abstract: | I will describe how to compute correlation functions in arbitrary CFTs in any spacetime dimension in a few seconds from almost nothing and how this reorganizes the whole conformal bootstrap program. |
A Rosetta Stone for Wilson Line Defects
| Speaker: | Julius Julius |
| Institution: | ENS Paris |
| Time: | 28 April, 13:15 |
| Venue: | Webinar |
| Abstract: | I will discuss the construction of a map between weak (gauge) and strong (string) coupling degrees of freedom for the supersymmetric Wilson line-defect in the planar N=4 Super-Yang-Mills theory. By analysing the Partition Functions at zero and infinite coupling, I will propose a map from degrees of freedom capturing single- and singlet two-particle states at zero coupling to infinite coupling. This map predicts that the dimension of states in these particular sectors doubles as it goes from zero to infinite coupling. This prediction is tested against the non-perturbative spectrum of insertions on the Wilson line obtained using integrability. I will end with a discussion with some possible conclusions that could be drawn from this exercise. |
| Recording: | The recorded talk can be found here. |
Einstein's gravity with boundaries
| Speaker: | Toby Wiseman |
| Institution: | Imperial Coll. |
| Time: | 5 May, 13:15 |
| Venue: | Webinar |
| Abstract: | We will review some properties of boundaries to spacetime both in a Riemannian and Lorentzian setting for Einstein’s gravity. We will outline some surprising new results about what boundary conditions allow a good behaviour (well-posedness) for the bulk theory. |
| Recording: | The recorded talk can be found here. |
Fortuitous states for 4d black holes
| Speaker: | Connor Behan |
| Institution: | Perimeter Institute & ICTP SAIFR Sao Paulo |
| Time: | 12 May, 17:15 |
| Venue: | Webinar |
| Abstract: | The AdS/CFT correspondence makes it possible in principle to study explicit black hole microstates using field theory techniques. In practice, this is still difficult because the field theory is only dual to gravity when it is strongly coupled. A more modest goal is to focus on supersymmetric black holes which have the minimal energy allowed by their other charges. The superconformal index suggests that many of these are protected from quantum corrections and can therefore be constructed at weak coupling. I will explain how one can identify candidates for black holes by searching the Hilbert space of ABJM theory. On general grounds, these should depend sensitively on trace relations at finite N. This principle, known as fortuity, was first established in a similar study of maximally supersymmetric Yang-Mills theory. I will show that similarities between the two theories extend to the quantitative level thereby allowing additional progress to be made without a brute force search. |
| Recording: | The recorded talk can be found here. |
Conformal Turaev-Viro Theory and 3D Gravity
| Speaker: | Thomas Hartman |
| Institution: | Cornell U. |
| Time: | 19 May, 18:15 |
| Venue: | Webinar |
| Abstract: | I will discuss recent work on the path integral of pure 3D gravity on a finite region of spacetime, constructed by gluing together hyperbolic tetrahedra. In the dual CFT, this amplitude calculates statistics of black hole matrix elements. The exact path integral is computed by Conformal Turaev-Viro theory, a topological theory based on triangulations, which is related to Virasoro TQFT by a modular S-transform. |
| Recording: | The recorded talk can be found here. |