Iberian Strings Meetings 

19/01/2021, 11:30 — 12:00 — Online
Pablo Cano, KU Leuven

Novel higher-curvature variations of $R^2$ inflation

Higher-curvature corrections to the gravitational action are a definite prediction of string theory and they may play an important role in the early universe. Indeed, one of the most successful inflationary scenarios is based on the Lagrangian $R+R^2$, but the effect of more general corrections is not yet fully understood. In this talk I will describe extensions of the $R+R^2$ model with a recently identified type of curvature corrections that keep the cosmological equations second-order and which therefore give rise to a well-behaved cosmological evolution. We use holographic methods to constrain the couplings of the new operators and then we derive the predictions for the power spectrum of tensor and scalar primordial fluctuations. The predicted values of the scalar spectral index and the tensor-to-scalar ratio turn out to lie within experimental constraints, but observations in the near future may be able to distinguish the presence of higher-curvature corrections.

See also

GeomR2.pdf

19/01/2021, 11:00 — 11:30 — Online
Matteo Baggioli, Jiao Tong University Shanghai

How small hydrodynamics can go

Numerous experimental and theoretical results in liquids and plasmas suggest the presence of a critical momentum at which the shear diffusion mode collides with a non-hydrodynamic relaxation mode, giving rise to propagating shear waves. This phenomenon, labelled as $k$-gap, could explain the surprising identification of a low frequency elastic behavior in confined liquids. More recently, a formal study of the perturbative hydrodynamic expansion showed that critical points in complex space, such as the aforementioned $k$-gap, determine the radius of convergence of linear hydrodynamics, its regime of applicability. In this talk, we combine the two new concepts and we study the radius of convergence of linear hydrodynamics in real liquids by using several data from simulations and experiments. We generically show that the radius of convergence increases with temperature and it surprisingly decreases with the interactions coupling. More importantly, we find that such radius is universally set by the characteristic interatomic distance of the liquid, which provides a natural microscopic bound. We finally compare our results with those from holographic theories.

See also

talk_baggioli.pdf

19/01/2021, 10:00 — 10:30 — Online
David Mateos, University of Barcelona

Strong-coupling dynamics and entanglement in de Sitter space

We use holography to study the dynamics of a strongly-coupled gauge theory in four-dimensional de Sitter space with Hubble rate $H$. The gauge theory is non-conformal with a characteristic mass scale $M$. We solve Einstein’s equations numerically and determine the time evolution of homogeneous gauge theory states. If their initial energy density is high compared with $H^4$ then the early-time evolution is well described by viscous hydrodynamics with a non-zero bulk viscosity. At late times the dynamics is always far from equilibrium. The asymptotic late-time state preserves the full de Sitter symmetry group and its dual geometry is a domain-wall in $AdS_5$. The approach to this state is characterised by an emergent relation of the form $P = w E$ that is different from the equilibrium equation of state in flat space. The constant $w$ does not depend on the initial conditions but only on $H/M$ and is negative if the ratio $H/M$ is close to unity. The event and the apparent horizons of the late-time solution do not coincide with one another, reflecting its non-equilibrium nature. In between them lies an “entanglement horizon” that cannot be penetrated by extremal surfaces anchored at the boundary, which we use to compute the entanglement entropy of boundary regions. If the entangling region equals the observable universe then the extremal surface coincides with a bulk cosmological horizon that just touches the event horizon, while for larger regions the extremal surface probes behind the event horizon.

See also

Iberian Strings 2021-Lisbon_dmateos.pdf

19/01/2021, 09:00 — 10:00 — Online
Malcom Perry, DAMTP, Cambridge

Soft Charges, Soft Hair and Black Hole Entropy

Review lecture on Asymptotic symmetries in gravity.

I will describe the concept of soft charge emerging from the nature of the BMS group. An explicit construction of the supertranslation charges and their integrability will be presented together with arguments as to why this leads to conserved charges. I will extend this to other asymptotic charges. I will then explain how to construct soft black hole hair and show how it can be used to determine the Hawking black hole entropy through holography. I will conclude by looking at the implications for the information paradox.

See also

talk_malcolm_perry.pdf

19/01/2017, 17:30 — 18:00 — Abreu Faro Amphitheatre
José Edelstein, University of Santiago de Compostela

Causality in $3D$ massive gravity theories

We study the constraints coming from local causality requirement in various $2+1$ dimensional dynamical theories of gravity. We show that causality and unitarity are compatible with each other and they both require the Newton constant to be negative. This is in contrast with what happens in the higher dimensional case. We discuss the reasons behind this result.

19/01/2017, 17:00 — 17:30 — Abreu Faro Amphitheatre
Olivera Miskovic, PUCV Valparaiso

Conformal mass in Lovelock AdS gravity

Conformal mass in AdS gravity encodes the physical information about conserved charges of a system in the electric part of the Weyl tensor. Based on the Noether theorem, we show that the conformal mass can be defined for any Lovelock AdS gravity, provided the AdS vacuum is a non-degenerate solution of the field equations. We find an explicit expression for the conformal mass in this case.

See also

Conformal Love_Lisbon_olivera.pdf

19/01/2017, 16:00 — 16:30 — Abreu Faro Amphitheatre
Benedikt Richter, LMU & Instituto Superior Técnico Lisbon

Classical Holographic Codes

We introduce classical holographic codes. These can be understood as concatenated probabilistic codes and can be represented as networks uniformly covering hyperbolic space. In particular, classical holographic codes can be interpreted as maps from bulk degrees of freedom to boundary degrees of freedom. Interestingly, they are shown to exhibit features similar to those expected from the AdS/CFT correspondence. Among these are a version of the Ryu-Takayanagi formula and intriguing properties regarding bulk reconstruction and boundary representations of bulk operations. We discuss the relation of our findings with expectations from AdS/CFT and, in particular, with recent results from quantum error correction.

See also

CHC_talk_ibstrings_Benedikt_Richter.pdf

19/01/2017, 15:30 — 16:00 — Abreu Faro Amphitheatre
Javier Martín García, Autónoma University of Madrid

Holographic Complexity

Twenty years after the Maldacena model for holography, new entries for the AdS/CFT dictionary are still being created. After a brief review of holographic entanglement entropy and its interpretation in terms of tensor networks, I will introduce some recent proposals about the bulk dual to the quantum computational complexity. With these tools, we compute the complexity of a peculiar degenerate system, i.e. near-extremal hyperbolic black holes, and study its anomalous behavior.

See also

Javier_Martin-Garcia_Lisboa.pdf

19/01/2017, 14:30 — 15:30 — Abreu Faro Amphitheatre
José Barbon, CSIC & Autónoma University of Madrid

Chaos and complexity in black holes

Recently, a flux of ideas from the theory of quantum chaos and quantum complexity has influenced discussions of black hole dynamics in the framework of holography. In this talk I will attempt a review of these developments.

See also

Barbon-Lisboa.pdf

19/01/2017, 12:00 — 12:30 — Abreu Faro Amphitheatre
Oscar Lasso Andino, Autónoma University of Madrid

On $SO(3)$-gauged maximal $d=8$ supergravities

I will show how the tensor hierarchy of generic, bosonic, $8$-dimensional field theories is built. Studying the form of the most general 8-dimensional bosonic theory with Abelian gauge symmetries only and no massive deformations we determined the tensors that occur in the Chern-Simons terms of the (electric and magnetic) field strengths and the action for the electric fields. Having constructed the most general Abelian theory we study the most general gaugings of its global symmetries and the possible massive deformations using the embedding tensor formalism, constructing the complete tensor hierarchy using the Bianchi identities. We find the explicit form of all the field strengths of the gauged theory up to the $6$-forms and also the equations of motion. We find that some equations of motion are not simply the Bianchi identities of the dual fields, but combinations of them. With the results at hand I will show how to construct explicitly a $1$-parameter family of $SO(3)$-gauged maximal $d=8$ supergravities that interpolates continuously between the theory constructed by Salam and Sezgin by Scherk-Schwarz compactification of $d=11$ supergravity and the theory constructed in arXiv:hep-th/0012032 by dimensional reduction of the so called massive $11$-dimensional supergravity proposed by Meessen and Ortín earlier.

The talk is based on the articles: arXiv:hep-th/1605.05882v1, arXiv:hep-th/1605.09629v2.

See also

Oscar Lasso Andino - On SO(3) families of gauged supergravities in 8-dimensions.pdf

19/01/2017, 11:30 — 12:00 — Abreu Faro Amphitheatre
Javier Tarrio, Université Libre de Bruxelles

Holography with a Landau pole

Holography for UV-incomplete gauge theories is important but poorly understood. A paradigmatic example is d=4, N=4 super Yang-Mills coupled to $N_f$ quark flavors, which possesses a Landau pole at a UV scale $ΛLP$. The dual gravity solution exhibits a UV singularity at a finite proper distance along the holographic direction. Despite this, holographic renormalization can be fully implemented via analytic continuation to an AdS solution. The presence of a UV cut-off manifests itself in several interesting ways. At energies $E \ll ΛLP$ no pathologies appear, as expected from effective field theory. In contrast, at scales $E \lesssim ΛLP$ the gravitational potential becomes repulsive, and at temperatures $T \lesssim ΛLP$ the specific heat becomes negative.

See also

Tarrio.pdf

19/01/2017, 11:00 — 11:30 — Abreu Faro Amphitheatre
David Rodríguez, University of Oviedo

Breaking the sound barrier in AdS/CFT

It has been conjectured that the speed of sound in holographic models with UV fixed points has an upper bound set by the value of the quantity in conformal field theory. If true, this would set stringent constraints for the presence of strongly coupled quark matter in the cores of physical neutron stars, as the existence of two-solar-mass stars appears to demand a very stiff Equation of State. In this article, we present a family of counter examples to the speed of sound conjecture, consisting of strongly coupled theories at finite density. The theories we consider include N = 4 super Yang-Mills at finite R-charge density and non-zero gaugino masses, while the holographic duals are Einstein-Maxwell theories with a minimally coupled scalar in a charged black hole geometry. We show that even for a small breaking of conformal invariance, the speed of sound approaches the conformal value from above at large chemical potentials.

See also

Breaking the sound barrier from holography_david_rodriguez.pdf

19/01/2017, 10:00 — 10:30 — Abreu Faro Amphitheatre
Amadeo Jimenez, Friedrich-Schiller-University Jena

Surface States in Holographic Weyl Semimetals

Weyl semimetals (WSMs) are novel gapless topological states of matter with electronic low-energy excitations behaving as left- and right-handed Weyl fermions. Remarkably this is the first known experimental realization of Weyl fermions. As in topological insulators, the existence of surface states is guaranteed by topology. Moreover, it has been shown that the surface states of a WSM form so-called Fermi arcs connecting the projections of the Weyl nodes onto the surface Brillouin zone. I will discuss these states at strong coupling within the so called Holographic Weyl Semimetal.

See also

AMADEO.pdf

19/01/2017, 09:00 — 10:00 — Abreu Faro Amphitheatre
Koenraad Schalm, University of Leiden

Applied string theory: Understanding strange metals in the lab with virtual black holes

AdS/CFT has given us an unprecedented new holographic window in strongly coupled physics. In particular the existence of charged black holes in AdS predicts the existence of novel quantum critical fixed points distinct from the conventional theory of critical phenomena. The distinct features of these novel quantum critical points show a remarkable resemblance with the profoundly mysterious behavior of exotic strange metal states of quantum matter, e.g. in high Tc superconductors. Recent experiments strongly indicate that this resemblance is more than superficial. This has put us at the cusp of a new era in theoretical physics: we will present the case that current experiments can and will test a holographic gravity model as the theory of the strange metal state.

See also

Schalm-Lisbon-Jan2017.pdf

18/01/2017, 13:00 — 13:30 — Abreu Faro Amphitheatre
John Huerta, Instituto Superior Técnico Lisbon

M-theory from the superpoint

One mysterious facet of M-theory is how a 10-dimensional string theory can "grow an extra dimension" to become 11-dimensional M-theory. Physically, the process is understood via brane condensation. Mathematically, Fiorenza, Sati, and Schreiber have proposed that brane condensation coincides with extending superspacetime, viewed as a Lie superalgebra, by the cocycle in Lie algebra cohomology which encodes the brane's WZW term. The resulting extension can be regarded as an "extended superspacetime'' where still other super p-branes may live, whose condensates yield further extensions, and so on. In this way, all the super p-branes of string theory and M-theory fit into a hierarchy called "the brane bouquet". In this talk, we show how the brane bouquet grows out of the simplest kind of supermanifold, the superpoint.

This is joint work with Urs Schreiber.

See also

superpoint_talk_huerta.pdf

18/01/2017, 12:30 — 13:00 — Abreu Faro Amphitheatre
Marián Lledo, University of Valencia

Superfields and superschemes

We exploit the examples of constrained superfields that appear in some supergravity cosmological models to put forward an interpretation of superfields that makes use of basic algebraic geometry tools as schemes or its super version, superschemes, perhaps not well known in the physics literature. In this way, non regular superschemes are shown to appear naturaly from physical constraints which challenge the standard interpretations.

See also

superschemestalk.pdf

18/01/2017, 12:00 — 12:30 — Abreu Faro Amphitheatre
Madalena Lemos, DESY Hamburg

Bootstrapping $N=2$ SCFTs

We discuss the bootstrap program applied to four-dimensional $N=2$ superconformal field theories, with special focus on analytical results. As a particular case we also consider the recently found $N=3$ SCFTs in four dimensions. We constrain the allowed space of SCFTs by making use of the existence of a protected subsector captured by a two-dimensional chiral algebra. Finally we discuss implications for the numerical bootstrap program.

See also

bootstrap_lemos.pdf

18/01/2017, 11:30 — 12:00 — Abreu Faro Amphitheatre
Silvia Nagy, Instituto Superior Técnico Lisbon

BPS black holes and the double copy

We show how one can construct the N=2 matter coupled supergravity theory as a product of N=2 SYM with a bosonic theory. We use the gauge-gravity dictionary obtained to build multi-centered BPS black hole solutions from SYM BPS solutions.

See also

s_nagy.pdf

18/01/2017, 10:00 — 11:00 — Abreu Faro Amphitheatre
Alessandro Vichi, EPFL Lausanne

Conformal bootstrap: non-perturbative QFT's under siege

Originally formulated in the 70's, the conformal bootstrap is the ambitious idea that one can use internal consistency conditions to carve out, and eventually solve, the space of conformal field theories. In this talk I will review recent developments in the field which have boosted this program to a new level.

I will present a method to extract quantitative informations in strongly-interacting theories, such as 3D Ising, $O(N)$ vector model and even systems without a Lagrangian formulation. I will explain how these techniques have led to the world record determination of several critical exponents.

See also

Alessandro_Vichi.pdf

18/01/2017, 09:00 — 10:00 — Abreu Faro Amphitheatre
Ricardo Monteiro, CERN Geneva

Gravity as a double copy of gauge theory

I will give an overview of the relations expressing gravity as a "double copy" of gauge theory. These relations appeared first in string theory, and have been used to compute scattering amplitudes in theories of (super)gravity. At the level of perturbative amplitudes, I will review current research topics such as the colour-kinematics duality in gauge theory and the new formalism of the scattering equations. I will then discuss the extension of the double-copy procedure to classical solutions in general relativity, including our best known black hole spacetimes.

See also

talk._monteiro.pdf