University of Crete HEP Seminars
Small Cosmological Constant in String Theory
| Speaker: | Liam McAllister |
| Institution: | Cornell U. |
| Time: | Tuesday 13 February 2024, 17:15 |
| Venue: | Webinar |
| Abstract: | Why is the vacuum energy in our Universe exponentially small in natural units? Motivated by this difficult problem, we ask a related but cleaner question: do there exist controlled anti-de Sitter solutions of string theory in which the internal space is small but the four-dimensional spacetime is exponentially large compared to the string length? We give an affirmative answer, by explicit construction, in Calabi-Yau compactifications of type IIB string theory. In this talk I will begin with a general overview of the problem of finding vacua in string theory, explain the physical mechanism at work in our solutions, and comment on the prospects for de Sitter solutions along similar lines. |
| Recording: | The recorded talk can be found here. |
Logarithmic corrections in AdS/CFT
| Speaker: | Marina David |
| Institution: | KU Leuven |
| Time: | Tuesday 20 February 2024, 13:15 |
| Venue: | Webinar |
| Abstract: | It is well known that in the semi-classical limit, the entropy of black holes is universally given by the Bekenstein-Hawking formula. There are in fact corrections to this formula arising from higher derivative terms in the gravitational path integral or quantum effects to due matter fields propagating on a fixed gravitational background. Evaluating such corrections is a challenging endeavor, but there is one term, of logarithmic form, that is more accessible. In this talk, I will discuss how logarithmic corrections in four-dimensional AdS gravity theories can be extracted via the heat kernel and the differences between the logarithmic term in asymptotically flat and AdS spacetimes. I will show that our results match the one-loop computations from holographic field theories when it is known, and explain how the logarithmic correction produces constraints on effective field theories coupled to gravity . |
| Recording: | The recorded talk can be found here. |
Tearing down spacetime with quantum disentanglement
| Speaker: | Roberto Emparan |
| Institution: | Barcelona U. and ICREA |
| Time: | Tuesday 27 February 2024, 13:15 |
| Venue: | Webinar |
| Abstract: | If spacetime is holographically built up from the quantum entanglement of microscopic degrees of freedom, it should also be possible to split it apart by disentangling these same degrees of freedom. However, studying this phenomenon with holographic methods reveals a puzzle: the disentangled state appears to keep a large entanglement entropy. I will review this problem and then explain how to resolve it. Interestingly, the solution involves bulk quantum effects of a kind brought to bear on another long-standing enigma in black hole thermodynamics, namely, the entropy of near-extremal Reissner-Nordstrom black holes. . |
| Recording: | The recorded talk can be found here. |
Chaotic and Thermal Aspects in the String S-Matrix
| Speaker: | Diptarka Das |
| Institution: | Indian Inst. Tec. |
| Time: | Tuesday 5 March 2024, 13:15 |
| Venue: | Webinar |
| Abstract: | We compute tree level scattering amplitudes involving more than one highly excited states in bosonic string theory. We use these amplitudes to understand chaotic and thermal aspects of the excited string states lending support to the Susskind-Horowitz- Polchinski correspondence principle. The unaveraged amplitudes exhibit chaos in the resonance distribution as a function of kinematic parameters, which can be described by random matrix theory. Upon coarse-graining these amplitudes exponentiate, and give certain thermal indications. . |
| Recording: | The recorded talk can be found here. |
Gravity, Horizons and Open EFTs
| Speaker: | Clifford P. Burgess |
| Institution: | Perimeter and McMaster U. |
| Time: | Tuesday 12 March 2024, 13:15 |
| Venue: | Webinar |
| Abstract: | Wilsonian effective theories exploit hierarchies of scale to simplify the description of low-energy behaviour and play as central a role for gravity as for the rest of physics. They are useful both when hierarchies of scale are explicit in a gravitating system and more generally for understanding precisely what controls the size of quantum corrections in gravitational systems. But effective descriptions are also relevant for open systems (e.g. fluid mechanics as a long-distance description of statistical systems) for which the `integrating out' of unobserved low-energy degrees of freedom complicate a straightforward application of Wilsonian methods. Observations performed only on one side of an apparent horizon provide examples where open system descriptions also arise in gravitational physics. This chapter describes some early adaptations of Open Effective Theories (i.e. techniques for exploiting hierarchies of scale in open systems) in gravitational settings. Besides allowing the description of new types of phenomena (such as decoherence) these techniques also have an additional benefit: they sometimes can be used to resum perturbative expansions at late times and thereby to obtain controlled predictions in a regime where perturbative predictions otherwise generically fail. |
| Recording: | The recorded talk can be found here. |
A bootstrap study of RG flows in AdS_2
| Speaker: | Edoardo Lauria |
| Institution: | LPENS |
| Time: | Tuesday 19 March 2024, 13:15 |
| Venue: | Webinar |
| Abstract: | For QFTs in AdS the boundary correlation functions remain conformal even if the bulk theory has a scale. This allows one to constrain RG flows with numerical conformal bootstrap methods. In this talk, I will discuss how to apply this idea to study RG flows between two-dimensional CFTs, focusing on deformations of the tricritical and ordinary Ising model. I will present non-perturbative constraints for the boundary correlation functions of these flows and compare them with conformal perturbation theory in the vicinity of the fixed points. I will also discuss a completely general constraint on the sign of the TTbar deformation in two dimensions, and how it emerges from the numerical conformal bootstrap . |
| Recording: | The recorded talk can be found here. |
Fermion-monopole scattering and generalized symmetries
| Speaker: | Marieke van Beest |
| Institution: | Stony Brook U. |
| Time: | Tuesday 26 March 2024, 16:15 |
| Venue: | Webinar |
| Abstract: | I will discuss the problem of electrically charged, massless fermions scattering off magnetic monopoles. The interpretation of the outgoing states has long been a puzzle, as they can carry fractional quantum numbers. I will argue that such outgoing particles live in the twisted sector of a 3-dimensional topological surface, which ends on the monopole. This leads to new predictions for the outgoing state in QED as well as the Standard Model. Moreover, the surface is often non-invertible, and as such the outgoing radiation not only carries unconventional flavor quantum numbers, but is often trailed by a topological field theory.. |
| Recording: | The recorded talk can be found here. |
Weak chaos and mixed dynamics in the string S-matrix
| Speaker: | Mihailo Cubrovic |
| Institution: | Belgrade U. |
| Time: | Tuesday 9 April 2024, 13:15 |
| Venue: | Webinar |
| Abstract: | We investigate chaotic dynamics in tree-level S-matrices describing the scattering of tachyons, photons and gravitons on highly excited open and closed bosonic strings, motivated by the string/black hole complementarity. The eigenphase spacing distribution and other indicators of quantum chaotic scattering suggest that the dynamics is only weakly chaotic, consisting of both regular/Poisson and chaotic/Wigner-Dyson processes. Only for special values of momenta and (for photon scattering) scattering angles do we find strong chaos of random matrix type. These special values correspond to a crossover between two regimes of scattering, dominated by short versus long partitions of the total occupation number of the highly excited string; they also maximize the information entropy of the S-matrix. The lack of strong chaos suggests that perturbative dynamics of highly excited strings can never describe the universal properties and maximal chaos of black hole horizons. The recorded talk can be found here. |
Holographic Weyl Anomalies for 4d Defects in 6d SCFTs
| Speaker: | Brandon Robinson |
| Institution: | INFN, Milan Bicocca |
| Time: | Tuesday 16 April 2024, 13:15 |
| Venue: | Webinar |
| Abstract: | In this note, we study 1/4- and 1/2-BPS co-dimension two superconformal defects in the 6d N = (2,0) A_{N−1} SCFT at large N using their holographic descriptions as solutions of 11d supergravity. In this regime, we are able to compute the defect contribution to the sphere entanglement entropy and the change in the stress-energy tensor one-point function due to the presence of the defect using holography. From these quantities, we are then able to unambiguously compute the values for two of the twenty-nine total Weyl anomaly coefficients that characterize 4d conformal defects in six and higher dimensions.We are able to demonstrate the consistency of the supergravity description of the defect theories with the average null energy condition on the field theory side. For each class of defects that we consider, we also show that the A-type Weyl anomaly coefficient is non-negative. Lastly, we uncover and resolve a discrepancy between the on-shell action of the 7d 1/4-BPS domain wall solutions and that of their 11d uplift. . |
| Recording: | The recorded talk can be found here. |
Algebraically special solutions and accelerating black hole thermodynamics
| Speaker: | Ioannis Papadimitriou |
| Institution: | Athens U. |
| Time: | Tuesday 25 April 2024, 14:15 |
| Venue: | Webinar |
| Abstract: | Algebraically special solutions constitute a broad class of generally time dependent but analytically known solutions of Einstein's equations. The physics they describe, however, remains relatively less understood, mainly due to the boundary conditions they satisfy at asymptotic infinity. In this talk I will review algebraically special solutions with a negative cosmological constant in four dimensions and will present accelerating black holes as a particular example. I will then discuss aspects the variational problem for accelerating AdS black holes and its relation to thermodynamics. The recorded talk can be found here. |
Applied SUSY QFT
| Speaker: | Giulio Bonelli |
| Institution: | SISSA, Trieste |
| Time: | Tuesday 30 April 2024, 14:15 |
| Venue: | Webinar |
| Abstract: | The study of Black-Hole perturbation theory is a classical problem in General Relativity and crucial to study gravitational waves. Due to the high order of symmetry of the BH gravitational field and the consequent separation of variables, the first order problem reduces to the study of linear ordinary second order differential equations. The resulting ODEs are of Fuchsian type and therefore, as already observed long ago by A.M.Polyakov, can be solved exactly in terms of classical - regular or irregular - Virasoro conformal blocks. By making use of the specific explicit expressions of the latter implied by the AGT dual perspective (susy gauge theory) on the conformal field theory, it is possible to explicitly solve the connection problem of the resulting (confluent)Heun equation and give novel exact and explicit formulas for the grey body factor, quasi-normal modes and Love numbers of diverse black holes. This will be explicitly applied to 4D Kerr and Schwarzschild-(A)de Sitter BHs and BH Compton scattering amplitudesThe recorded talk can be found here. |
Logarithmic Corrections to Kerr Thermodynamics
| Speaker: | Daniel Kapec |
| Institution: | Harvard U. |
| Time: | Tuesday 14 May 2024, 16:15 |
| Venue: | Webinar |
| Abstract: | To an outside observer, a black hole appears to be an ordinary quantum mechanical system with finite entropy and highly chaotic internal dynamics. Nevertheless, the low-temperature thermodynamics of the Kerr black hole presents several puzzles. For instance, the leading order semiclassical approximation to the black hole density of states predicts a surprisingly large ground state degeneracy, while poorly understood quantum corrections are known to become increasingly important at low temperatures. I will review the modern picture of black holes as quantum systems and then discuss a recent result on the leading correction to the low-temperature thermodynamics of the Kerr black hole that resolves many of the old puzzles. The recorded talk can be found here |