"Quantum Gravity and All of That" seminar series is aimed at bringing together experts, postdocs and students working on the following topics:
Quantum gravity and strings, including the early Universe scenarios
Quantum gravity and black holes
Information paradox
Holography and AdS/CFT, including holography and complexity, holography and neutron star physics
Quantum information, including the information theory
and related subjects, including gravitational waves from compact objects and phase transitions.
The seminars are expected to have about 1 hour for the main talk and 30+ minutes for discussions. We pay a special account for open discussions and exchange of opinions as the main driving force in our research community. The principal schedule is one seminar per two weeks on Thursdays at 16:00 CET (or CEST) time. This may be occasionally adjusted to account special time requests of speakers.
The seminars and afterwards discussions will be zoom-based. You have to register following the Registration link above to be included in the mailing list. Talk proposals and other inquires can be submitted through the seminar e-mail (see below).
"Quantum Gravity and All of That" seminar series is aimed at bringing together experts, postdocs and students working on the following topics:
Quantum gravity and strings, including the early Universe scenarios
Quantum gravity and black holes
Information paradox
Holography and AdS/CFT, including holography and complexity, holography and neutron star physics
Quantum information, including the information theory
and related subjects, including gravitational waves from compact objects and phase transitions.
The seminars are expected to have about 1 hour for the main talk and 30+ minutes for discussions. We pay a special account for open discussions and exchange of opinions as the main driving force in our research community. The principal schedule is one seminar per two weeks on Thursdays at 16:00 CET (or CEST) time. This may be occasionally adjusted to account special time requests of speakers.
The seminars and afterwards discussions will be zoom-based. You have to register following the Registration link above to be included in the mailing list. Talk proposals and other inquires can be submitted through the seminar e-mail (see below).
Abstract:
In the first part of my talk, Starobinsky inflation is reviewed and extended in the framework of the modified F(R) gravity in order to accommodate production of primordial black holes (PBH) at smaller scales. Those black holes may form dark matter (DM) in the current Universe. The density and frequency of the PBH-induced gravitational waves (GW) are derived and compared to the sensitivity curves of the future space-based gravitational interferometers (LISA, DECIGO, TianQin, Taiji), which makes it possible to probe our models. In the second part of my talk, an upgrade of our models
to supergravity is introduced, and a possible connection to the MSSM and SM of elementary particles is outlined.
Thursday, April 3, 2025, 15:00 CEST (06:00 Los Angeles, 09:00 New York, 16:00 Moscow, 21:00 Shanghai, 22:00 Tokyo)
Title: Bootstrap Program for Holographic Defects
Speaker: Xinan Zhou (KITS & ShanghaiTech U.)
Abstract:
I will give an overview of the recent progress in developing analytic bootstrap techniques for studying holographic defects. These holographic defects include a wide array of interesting systems ranging from Wilson-'t Hooft loops, surface defects to theories on real project space and giant gravitons. I will focus on defect two-point functions and introduce general techniques to efficiently compute them at tree and loop levels in AdS, as well as their stringy corrections.
Thursday, April 15, 2021, 17:00 CEST (08:00 Los Angeles, 11:00 New York, 18:00 Moscow, 00:00 Tokyo)
Title: Horizons at a quantised black hole
Speaker: Gerard 't Hooft (Utrecht University)
Abstract: Black holes more than an order of magnitude heavier than the
Planck mass can be described in detail by using only Quantum Field Theory on a
curved background, complemented with a unitarity condition for matter passing
a horizon. This condition appears to be unique, leading to a very special doctrine,
giving new hopes for finding a systematic treatment of quantum gravity.
Thursday, April 29, 2021, 17:00 CEST (08:00 Los Angeles, 11:00 New York, 18:00 Moscow, 00:00 Tokyo)
Title: Simplicity in the Black Hole Interior
Speaker: Netta Engelhardt (Massachusetts Institute of Technology)
Abstract:
It has often been speculated that the black hole interior in holography has high computational complexity in the dual CFT. I will argue that operators in the black hole interior are simple whenever they lie outside of the outermost (quantum) extremal surface. That is, that non-minimal quantum extremal surfaces are the only source of high complexity in AdS/CFT. This is the converse of the so-called Python's lunch conjecture, which proposes that operators with support behind the outermost quantum extremal surface - e.g. Hawking partners - are highly complex.
Thursday, May 13, 2021, 17:00 CEST (08:00 Los Angeles, 11:00 New York, 18:00 Moscow, 00:00 Tokyo)
Title: Why Resolving the Singularity Problem of General Relativity
is Not Just a Quantum Gravity Issue
Speaker: Roger Penrose (University of Oxford)
Abstract:
Over many years, there has been much impressive theorizing about how quantum mechanics might modify the space-time structure of general relativity at extremely tiny scales. A good deal of this work has been aimed at resolving the space-time singularity problem. However, there are strong reasons for believing that this cannot provide an overall resolution of that problem, the big bang having a totally different structure from the expected singularities in black holes.
Conformal geometry provides a very different outlook, revealing issues deeply related to the 2nd law of thermodynamics and leading to an exotic cosmological picture (conformal cyclic cosmology, or CCC), with various predictions, some now confirmed in observations of previously unexpected effects—most particularly those referred to as “Hawking points”.
Thursday, May 27, 2021, 17:00 CEST (08:00 Los Angeles, 11:00 New York, 18:00 Moscow, 00:00 Tokyo)
Title: Finding Pythons in Unexpected Places
Speaker: Geoffrey Penington (University of California, Berkeley; Princeton, Institute for Advanced Study)
Abstract:
We argue that novel (highly nonclassical) quantum extremal surfaces play a crucial role in reconstructing the black hole interior even for isolated, single-sided, non-evaporating black holes (i.e. with no auxiliary reservoir). Specifically, any code subspace where interior outgoing modes can be excited will have a quantum extremal surface in its maximally mixed state. We argue that as a result, reconstruction of interior outgoing modes is always exponentially complex. Our construction provides evidence in favor of a strong Python's lunch proposal: that nonminimal quantum extremal surfaces are the exclusive source of exponential complexity in the holographic dictionary. We also comment on the relevance of these quantum extremal surfaces to the geometrization of state dependence in the typicality arguments for firewalls.
Thursday, June 10, 2021, 17:00 CEST (08:00 Los Angeles, 11:00 New York, 18:00 Moscow, 00:00 Tokyo)
Title: Bulk observables in Jackiw-Teitelboim gravity
Speaker: Thomas Mertens (Ghent University)
Abstract:
Using a definition of bulk diff-invariant observables, we go into the bulk of 2d Jackiw-Teitelboim gravity. By mapping the computation to a Schwarzian path integral, we study exact bulk correlation functions and discuss their physical implications. We describe how the black hole thermal atmosphere gets modified by quantum gravitational corrections. Finally, we will discuss how higher topological effects further modify the spectral density and detector response in the Unruh heat bath. Based on arXiv:1902.11194, 1903.10485, 2005.13058 and work in progress.
Thursday, June 17, 2021, 17:00 CEST (08:00 Los Angeles, 11:00 New York, 18:00 Moscow, 00:00 Tokyo)
Title: Contrasting the fuzzball and wormhole paradigms
Speaker: Samir Mathur (Ohio State U.)
Abstract:
We summarize the information paradox, and explain how the fuzzball paradigm resolves this paradox in string theory. We then use the small corrections theorem to show that if one looks for an alternative to fuzzballs, then the quantum gravity theory will have to possess nonlocalities. We then consider the nonlocalities postulated in the wormhole paradigm, and the difficulties created by such postulates.
Thursday, July 8, 2021, 17:00 CEST (08:00 Los Angeles, 11:00 New York, 18:00 Moscow, 00:00 Tokyo)
Title: Effective Methods For Component Field Transparency For Arbitrary Superfields
Speaker: Sylvester James Gates (Brown University)
Abstract:
In 1974 Salam and Strathdee introduced methods based superspace
which facilitated the construction of arbitrary representations of
spacetime supersymmetry. While this approach allows for the derivation
of component field results for low dimensions, it becomes remarkably
opaque for higher dimensional superfields. To overcome this disability,
recently there has been introduced new methods based on 'adynkra'
networks, Dynkin Label/Young Tableaux, Plethysm, and IT algorithms.
Details of this approach are illustrated in the case of ten and eleven
dimensional theories.
Thursday, September 2, 2021, 17:00 CEST (08:00 Los Angeles, 11:00 New York, 18:00 Moscow, 00:00 Tokyo)
Title: Black holes and information — an update
Speaker: Steve Giddings (University of California, Santa Barbara)
Abstract:
For perspective on the information problem, one can state a “Black hole theorem,” and examine its possible
loopholes, and what they tell us about the structure of quantum gravity.
Different proposals can be characterized by how they might evade this theorem;
for example an interesting question is how replica wormholes do so. Key questions are whether black holes
can be thought of as subsystems in quantum gravity, and whether they have additional interactions with their
environments not predicted by local quantum field theory.
Thursday, September 16, 2021, 17:00 CEST (08:00 Los Angeles, 11:00 New York, 18:00 Moscow, 00:00 Tokyo)
Title: Towards demystification of black holes
Speaker: Gia Dvali (Munich, Max Planck Institute & Munich U., ASC)
Abstract:
For a long time, certain well-known properties of Black holes, such as the area form of the entropy
as well as a long time-scale of information retrieval, have been considered to be mysteries.
We explain that black holes turn out not to be exceptional in this respect. Instead, the above
seemingly-mysterious properties follow from certain universal bounds on information storage and retrieval.
They are imposed by unitarity and are fully non-perturbative. The objects that saturate these consistency
bounds exhibit the properties identical to black holes. A long list of examples includes saturated solitons, instantons
and baryons in renormalizable field theories. Perhaps the most striking example is the color glass condensate in
ordinary QCD. We predict new observable phenomena for such objects, including black holes. We focus on
the phenomenon of the ``memory burden", the essence of which is that the information
pattern stored by the system back-reacts and resists decay. For black holes, this predicts a dramatic
deviation from usually assumed evolution, and has potentially important implications for primordial black
holes as dark matter candidates.
Thursday, October 28, 2021, 17:00 CEST (08:00 Los Angeles, 11:00 New York, 18:00 Moscow, 00:00 Tokyo)
Title: Nonsingular Cosmologies and Black Holes with S-Branes
Speaker: Robert Brandenberger (McGill University)
Abstract:
S-Branes are objects motivated by superstring theory
which appear in a low energy effective action for our four
dimensional space-time. I will demonstrate how such branes
can be used to obtain nonsingular cosmologies (specifically,
a nonsingular realization of the Ekpyrotic scenario) and
nonsingular black holes.
Thursday, November 04, 2021, 17:00 CET (09:00 Los Angeles, 12:00 New York, 19:00 Moscow, 01:00 (next day) Tokyo)
Title: From Newtonian to ultra-relativistic gravitational scattering at 3rd post-Minkowskian order
Speaker: Gabriele Veneziano (CERN, Collège de France)
Abstract:
Using the eikonal approximation for quantum scattering I will argue that the non-relativistic (Newtonian) and
ultra-relativistic (massless) limits of classical gravitational scattering are smoothly connected at 3rd
post-Minkowskian (3PM) order provided radiation reaction effects are properly taken into account.
This is how a two-year-old puzzle raised by the first 3PM result by Bern et al. was solved about a year ago.
Extending the connection to the next PM order looks both important and non-trivial.
Thursday, November 18, 2021, 17:00 CET (08:00 Los Angeles, 11:00 New York, 19:00 Moscow, 01:00 (next day) Tokyo)
Title: Discrete gravity
Speaker: Slava Mukhanov (Munich U., ASC)
Abstract:
We assume that the points in volumes smaller than an elementary volume (which may have a Planck size) are
indistinguishable in any physical experiment. This naturally leads to a picture of a discrete space with
a finite number of degrees of freedom per elementary volume. In such discrete spaces, each elementary cell
is completely characterized by displacement operators connecting a cell to the neighboring cells and by
the spin connection. We define the torsion and curvature of the discrete spaces and show that in
the limiting case of vanishing elementary volume the standard results for the continuous curved differentiable
manifolds are completely reproduced.
Thursday, November 25, 2021, 17:00 CET (08:00 Los Angeles, 11:00 New York, 19:00 Moscow, 01:00 (next day) Tokyo)
Title: Revisiting Coleman-de Luccia transitions in the AdS regime using holography
Speaker: Elias Kiritsis (Université de Paris; University of Crete)
Abstract:
Coleman-de Luccia processes for AdS to AdS decays in Einstein-scalar theories are studied. Such tunnelling
processes are interpreted as vev-driven holographic RG flows of a quantum field theory on de Sitter space-time.
These flows do not exist for generic scalar potentials, which is the holographic formulation of the fact that
gravity can act to stabilise false AdS vacua. The existence of Coleman-de Luccia tunnelling solutions in
a potential with a false AdS vacuum is found to be tied to the existence of exotic RG flows in the same potential.
Such flows are solutions where the flow skips possible fixed points or reverses direction in the coupling.
This connection is employed to construct explicit potentials that admit Coleman-de Luccia instantons in AdS and
to study the associated tunnelling solutions. Thin-walled instantons are observed to correspond to dual field
theories with a parametrically large value of the dimension Δ for the operator dual to the scalar field,
casting doubt on the attainability of this regime in holography. From the boundary perspective, maximally
symmetric instantons describe the probability of symmetry breaking of the dual QFT in de Sitter. It is argued
that, even when such instantons exist, they do not imply an instability of the same theory on flat space or
on R×S3.
Thursday, December 2, 2021, 17:00 CET (08:00 Los Angeles, 11:00 New York, 19:00 Moscow, 01:00 (next day) Tokyo)
Title: Planckian and Trans-Planckian physics in black holes
and quantum space-time
Speaker: Norma Sanchez (CNRS OP-SU Paris & Chalonge-de Vega School)
Abstract:
In the absence of a complete quantum theory of gravity, we start from quantum physics and its foundational
milestone: The universal classical-quantum (or wave-particle) duality, which we extend to gravity and
the trans-Planck domain. As a consequence, classical and semiclassical gravity, quantum planckian and
trans-planckian domains are covered, and the usual quantum domaine as well. A new quantum space-time region
emerges. Quantum discrete space-time levels and mass levels are derived, covering from the large
astrophysical/cosmological objects to quantum elementary particles, crossing the Planck scale and beyond it.
Both, black holes and de Sitter cosmology are discussed.
Thursday, December 16, 2021, 17:00 CET (08:00 Los Angeles, 11:00 New York, 19:00 Moscow, 01:00 (next day) Tokyo)
Title: Quantum field theories without infinities and naturalness
Speaker: Mikhail Shaposhnikov (EPFL)
Abstract:
The standard way to do computations in Quantum Field Theory (QFT) is plagued by infinities and fine-tunings
leading to the conception of "naturalness", which requires the cancellation of quadratic divergences by
new particles with masses right above the Fermi scale. At the same time, the ultimate outcome of any QFT
(the Standard Model in particular) is the prediction of all kinds of finite particle cross-sections in terms of
a few finite input parameters (such as the mass of an electron and fine-structure constant in quantum
electrodynamics). In this talk, I will describe how to relate the parameters of the theory to observables
without running into divergences in Feynman diagrams. The existence of such a technique suggests that the
"hierarchy problem" is not really physical, but rather an artifact of the conventional procedure of
renormalisation of QFTs.
Thursday, January 13, 2022, 17:00 CET (08:00 Los Angeles, 11:00 New York, 19:00 Moscow, 01:00 (next day) Tokyo)
Title: Inflation after Planck and BICEP
Speaker: Andrei Linde (Stanford University)
Abstract:
I will discuss a broad class of inflationary models, cosmological attractors, which can describe all presently
available inflation-related observational data using a single parameter. These models generalize the Starobinsky
model and Higgs inflation, but they are sufficiently flexible to account for an arbitrary amount of inflationary
gravitational waves.
Thursday, January 20, 2022, 17:00 CET (08:00 Los Angeles, 11:00 New York, 19:00 Moscow, 01:00 (next day) Tokyo)
Title: Gravity and the Crossed Product
Speaker: Edward Witten (Princeton, Inst. Advanced Study)
Abstract:
Recently Leutheusser and Liu [1,2] identified an emergent algebra of Type III1 in the operator
algebra of N=4 super Yang-Mills theory for large N. Here we describe some 1/N corrections
to this picture and show that the emergent Type III1 algebra becomes an algebra of
Type II∞.
The Type II∞ algebra is the crossed product of the Type III1 algebra by its modular
automorphism group. In the context of the emergent Type II∞ algebra, the entropy of
a black hole state is well-defined up to an additive constant, independent of the state.
This is somewhat analogous to entropy in classical physics.
Thursday, January 27, 2022, 17:00 CET (08:00 Los Angeles, 11:00 New York, 19:00 Moscow, 01:00 (next day) Tokyo)
Title: Sequestered inflation in M-theory and string theory
Speaker: Renata Kallosh (Stanford University)
Abstract:
We build supergravity models which sequester phenomenology of inflation from the Planck scale physics.
The construction involves the nilpotent chiral multiplet representing non-linearly realized supersymmetry and
an anti-D3 brane. We use novel type flux superpotentials originating from M-theory and type IIB string theory and
the associated Goldstone multiplets. Our construction explains the origin of inflationary plateau potentials
favored by the current BICEP/Keck and Planck data and provides the B-mode targets for future CMB experiments.
Thursday, February 10, 2022, 16:00 CET (07:00 Los Angeles, 10:00 New York, 18:00 Moscow, 00:00 (next day) Tokyo)
Title: Failure of the split property in gravity and the information paradox
Speaker: Suvrat Raju (ICTS, Bangalore)
Abstract:
In an ordinary quantum field theory, the "split property" implies that the state of a system can be specified
independently on a bounded subregion of a Cauchy slice and its complement. This property does not hold
for theories of gravity. It can be shown in specific examples that observables near the boundary of a Cauchy
slice uniquely fix the state on the entire slice. The original formulation of the information paradox explicitly
assumed the split property and we follow this assumption to isolate the precise error in Hawking's argument.
A similar assumption also underpins the monogamy paradox of Mathur and AMPS. Finally the same assumption is used
to support the common idea that the entanglement entropy of the region outside a black hole should follow
a Page curve. It is for this reason that recent computations of the Page curve have been performed only
in nonstandard theories of gravity, which include a nongravitational bath and massive gravitons. We discuss
possibilities for coarse graining that might lead to a Page curve in standard theories of gravity.
Thursday, February 24, 2022, 17:00 CET (08:00 Los Angeles, 11:00 New York, 19:00 Moscow, 01:00 (next day) Tokyo)
Title: The end of the evaporation
Speaker: Carlo Rovelli (Aix Marseille University, Perimeter Institute)
Abstract:
Sidestepping the question of what precisely happens at the end of the evaporation in addressing the black hole
information issue is misleading, because this question bears on assumptions mistakenly taken for granted.
I distinguish the three independent regions where quantum gravity becomes relevant in the evolution of a black
hole spacetime, and summarise the multiple indications that loop quantum gravity gives about each of them.
They are consistent with unitarity, no information loss, and no Page curve. The black hole horizon is a trapping
horizon but not an event horizon; the von Neumann entropy is larger than the thermodynamic entropy governing
the thermodynamics of the black hole before the end of the evaporation. The black hole interior tunnels into
a white hole geometry, bounded by a long living anti trapped horizon. I show why earlier arguments dismissing
the possibility of this scenario do not apply.
Thursday, March 3, 2022, 17:00 CET (08:00 Los Angeles, 11:00 New York, 19:00 Moscow, 01:00 (next day) Tokyo)
Title: Applications of black hole scattering
Speaker: Nava Gaddam (Utrecht U.)
Abstract:
In this talk, I will describe scattering in the presence of black holes. I will try to convince you that
i) there is a new regime in the phase space of quantum gravity where scattering amplitudes can reliably be calculated in the low energy theory,
ii) Hawking's free field theory breaks down after Page time (implying information retrieval in local effective field theory), and that
iii) there is an emergent soft limit near the horizon, resulting in new soft graviton theorems in the presence of black holes.
Finally, I will conclude with some more applications.
Thursday, March 10, 2022, 17:00 CET (08:00 Los Angeles, 11:00 New York, 19:00 Moscow, 01:00 (next day) Tokyo)
Title: Gravity, Geometry and the Quantum
Speaker: Abhay Ashtekar (Penn State U.)
Abstract:
I will begin with a broad perspective on the the conceptual problems of unifying the principles of general relativity and quantum mechanics, and then introduce loop quantum gravity, emphasizing the interplay between gravity, geometry and the quantum that lies at its foundation. This approach has resolved some of the long standing fundamental issues, such as: (i) What is the quantum nature of space-time geometry? (ii) Are singularities of classical general relativity naturally resolved and, if so, what is the consequent physics beyond Einstein? and (iii) How does one formulate and address physical questions when there is no classical spacetime in the background? In the second part of the talk, I will discuss some recent advances in quantum cosmology to illustrate that the approach has matured sufficiently to commence the creation of 2-way bridges between fundamental theory and observations. In contrast to, say, string theory where the focus appears to have shifted to creating a powerful toolkit for various branches of physics and mathematics, in loop loop quantum gravity, the focus continues to be on questions rooted in quantum gravity, proper. The talk will not assume prior knowledge of loop quantum gravity.
Thursday, March 24, 2022, 17:00 CET (09:00 Los Angeles, 12:00 New York, 19:00 Moscow, 01:00 (next day) Tokyo)
Title: Large and small speed of light expansion of general relativity
Speaker: Niels Obers (Niels Bohr Institute and Nordita)
Abstract:
The quest for a consistent theory of quantum gravity is one of the most important outstanding problems in
theoretical physics. In the landscape of physical theories, quantum gravity sits at the corner where all the
physical constants (speed of light, Newton’s and Planck’s constant) are finite. A region that is often
overlooked is the nonrelativistic gravity regime. Contrary to common lore, it has become clear that the theory
of nonrelativistic gravity is much richer than was so far appreciated, containing much more than just Newtonian
gravity. I will review recent developments in the large speed of light expansion of GR and its coupling to
matter, and present its geometric formulation in terms of a novel version of Newton-Cartan geometry. I will also
discuss recent insights into the complementary case when the speed of light goes to zero, which is known as the
Carroll or ultra-local expansion of gravity, in which case the underlying geometric structure is Carroll
geometry. Finally, I will briefly comment on approaches towards embedding such limits in string theory and
holography.
Thursday, April 7, 2022, 17:00 CEST (08:00 Los Angeles, 11:00 New York, 18:00 Moscow, 00:00 (next day) Tokyo)
Title: Quantum chaos and unitary black hole evaporation
Speaker: Larus Thorlacius (Iceland U.)
Abstract:
We revisit the black hole information problem in the context of the AdS/CFT correspondence. The formation and
evaporation of small AdS black holes is governed by unitary time evolution on the dual gauge theory side but how
does this translate to the gravitational side? We argue that the eigenstate thermalisation hypothesis can
explain the validity of semiclassical gravity for local bulk observables. Small AdS black holes correspond
to states with finite energy width in the holographic dual, and observables that are smooth functions on
the classical phase space will self-average over a large number of energy eigenstates, exponential in the
Bekenstein-Hawking entropy, giving expectation values that are consistent with semiclassical gravity up to small
corrections. On the other hand, the semiclassical bulk description breaks down at leading order for transition
amplitudes which probe the unitary evolution of the theory.
Thursday, April 21, 2022, 17:00 CEST (08:00 Los Angeles, 11:00 New York, 18:00 Moscow, 00:00 (next day) Tokyo)
Title: Summing Large Logarithms from Loops of Inflationary Gravitons
Speaker: Richard Woodard (Florida U.)
Abstract:
Quantum gravitational corrections on flat space background do not affect particle kinematics at all, and only make fractional changes of order G/r^2 to long range forces. The situation during inflation is very different because (1) the Hubble parameter H allows fractional corrections of the form G H^2 and (2) the continuous production of inflationary gravitons introduces a secular element. As a result, corrections to both particle kinematics and long range forces typically grow like logarithms of the scale factor and/or the spatial separation. If inflation persists long enough, this growth must eventually cause perturbation theory to break down, begging the question of what happens next. I report on recent progress in summing the very similar large logarithms which occur in nonlinear sigma models by combining a variant of Starobinsky's stochastic formalism with a variant of the renormalization group. I discuss how this technique can be generalized to quantum gravity.
This talk is based on arXiv:2110.08715
Thursday, May 5, 2022, 17:00 CEST (08:00 Los Angeles, 11:00 New York, 18:00 Moscow, 00:00 (next day) Tokyo)
Title: Quantum Hair and Black Hole Information
Speaker: Stephen Hsu (Michigan State U.)
Abstract:
I discuss recent results concerning the quantum state of the gravitational field of a compact matter source such
as a black hole. These results demonstrate the existence of quantum hair, violating the classical No Hair
Theorems. I then discuss how this quantum hair affects Hawking radiation, allowing unitary evaporation of black
holes. Small corrections to leading order Hawking radiation amplitudes, with weak dependence on the external
graviton state, are sufficient to produce a pure final radiation state. The radiation state violates
the factorization assumption made in standard formulations of the information paradox. These conclusions are
consequences of long wavelength properties of quantum gravity: no special assumptions are made concerning short
distance (Planckian) physics.
Thursday, May 19, 2022, 17:00 CEST (08:00 Los Angeles, 11:00 New York, 18:00 Moscow, 00:00 (next day) Tokyo)
Title: Localizing Gravity and Supergravity on Braneworlds
Speaker: Kellogg Stelle (Imperial College)
Abstract:
Supergravity brane solutions provide a variety of contexts for effectively obtaining lower-dimensional physics
within a higher-dimensional theory, but with noncompact transverse spaces instead of a usual Kaluza-Klein
compactification. Two such contexts will be discussed. One is a promotion of the static "vacuum" brane solution
to a full braneworld supergravity theory for the degree of supersymmetry possessed by the static brane.
This allows for a "consistent truncation" brane supergravity ansatz in which the vacuum brane serves as
a standard "skeleton" for embedding in the higher dimensional theory. The second is a more complete localization
which exists in situations where the transverse wave equation spectrum starts with a normalizable zero mode
state separated by a mass gap from the continuum expected in a noncompact transverse space. An example based on
a transverse Eguchi-Hanson space will be discussed.
Thursday, June 2, 2022, 17:00 CEST (08:00 Los Angeles, 11:00 New York, 18:00 Moscow, 00:00 (next day) Tokyo)
Title: Gauge Independent Effective Field Equations
Speaker: Shun Pei Miao (National Cheng Kung University)
Abstract:
I describe a technique for removing gauge dependence from graviton loop corrections to the effective field equations. I present explicit results on flat space background for a massless, minimally coupled scalar and for electromagnetism. I then describe how the procedure generalizes to de Sitter background.
This talk is based on arXiv:1708.06239.
Thursday, June 16, 2022, 17:00 CEST (08:00 Los Angeles, 11:00 New York, 18:00 Moscow, 00:00 (next day) Tokyo)
Title: Prediction of the tensor-to-scalar ratio in primordial cosmology from quantum gravity with
purely virtual particle
Speaker: Damiano Anselmi (Pisa Univ.)
Abstract:
I discuss the quantization of purely virtual particles in quantum gravity and its applications to primordial
cosmology. The inflation mechanism is unique and coincides with the Starobinsky one. However, the Weyl squared
term included in the action, which must be understood as purely virtual, changes the final predictions.
In particular, the tensor-to-scalar ratio r lies within the range 0.0004 < r < 0.0035. Tilts and higher-order
corrections can be calculated by viewing inflation as a certain RG flow. The spectra are understood as
correlation functions that obey Callan-Symanzik equations. They can be expressed as expansions in powers of
a running "fine structure" constant of inflation.
Thursday, June 30, 2022, 17:00 CEST (08:00 Los Angeles, 11:00 New York, 18:00 Moscow, 00:00 (next day) Tokyo)
Title: Ultra-Planckian scattering from a QFT for gravity
Speaker: Bob Holdom (Toronto University)
Abstract:
Quantum quadratic gravity (QQG) is a proposal for a UV complete quantum field theory of gravity (Stelle 1977).
Its perturbative expansion displays unitarity without positivity. We calculate ultra-Planckian scattering
amplitudes and cross sections and show that this lack of positivity is associated with cancellations that give
good high energy behaviour. Ultra-Planckian scattering may then have features in common with the QCD description
of high energy scattering.
Thursday, July 14, 2022, 17:00 CEST (08:00 Los Angeles, 11:00 New York, 18:00 Moscow, 00:00 (next day) Tokyo)
Title: Asymptotic Safety and Friends
Speaker: Frank Saueressig (Radboud University)
Abstract:
We give a pedagogical introduction to the gravitational asymptotic
safety program and the functional renormalization group which
constitutes the main computational tool for exploring this scenario
before highlighting some recent developments. A specific focus of the
exposition will be on highlighting similarities and differences to other
approaches to quantum gravity including non-local constructions.
Thursday, September 15, 2022, 17:00 CEST (08:00 Los Angeles, 11:00 New York, 18:00 Moscow, 00:00 (next day) Tokyo)
Title: Quantum Conformal Gravity
Speaker: Philip Mannheim (University of Connecticut)
Abstract:
Conformal symmetry is a natural symmetry in physics since it is the full symmetry of the light cone. If all particles are to get their masses by symmetry breaking then conformal symmetry is the symmetry of the unbroken Lagrangian. Like Yang-Mills theories conformal symmetry has a local extension, namely conformal gravity, a pure metric-based candidate alternative to the non-conformal invariant standard Newton-Einstein theory of gravity. With its dimensionless coupling constant quantum conformal gravity is power counting renormalizable. Since its equations of motion are fourth-order derivative equations conformal gravity has long been thought to possess unacceptable ghost states of negative norm that would violate unitarity. However, on constructing the quantum Hilbert space Bender and Mannheim found that this not to be the case. Conformal gravity is thus offered as a completely consistent and unitary quantum theory of gravity, one that requires neither the extra dimensions nor the supersymmetry of string theory. As formulated via local conformal invariance there is no intrinsic classical gravity, with gravity instead being intrinsically quantum-mechanical, with the observed classical gravity being output rather than input. The contribution of the graviton loops of conformal gravity enables conformal gravity to solve the cosmological constant problem. Like Yang-Mills the potential of conformal gravity contains both a Newtonian term and a linear potential. Together with a quadratic potential that the theory also contains conformal gravity is able to explain the systematics of galactic rotation curves without any need for galactic dark matter. Since all mass is to be dynamical there cannot be a fundamental double-well Higgs potential in the theory. Instead, the Higgs boson is generated dynamically, with the hierarchy problem then being solved.
Thursday, September 29, 2022, 17:00 CEST (08:00 Los Angeles, 11:00 New York, 18:00 Moscow, 00:00 (next day) Tokyo)
Title: Quadratic Gravity in 2022
Speaker: Alberto Salvio (University Tor Vergata)
Abstract:
An extension of general relativity (GR) obtained by adding local quadratic terms to the action will be considered. Such theory can be a viable UV completion of GR. The additional terms soften gravity above a certain energy scale and render gravity renormalizable. The presence of 4 derivatives implies via the Ostrogradsky theorem that the classical Hamiltonian is unbounded from below. Nevertheless, I will argue that the relevant solutions are not unstable, but metastable: when the energies are much below a threshold (that is high enough to describe the whole cosmology) runaways are avoided. Remarkably, the chaotic inflation theory of initial conditions ensures that such bound is satisfied and testable implications for the early universe will be discussed. I will also argue that the basic unitarity condition is satisfied when the theory is correctly formulated at the quantum level. Moreover, thanks to the UV softening of gravity, sufficiently light objects must be horizonless and I will discuss explicit analytic examples of horizonless ultracompact objects, which have interesting physical implications.
Thursday, October 13, 2022, 17:00 CEST (08:00 Los Angeles, 11:00 New York, 18:00 Moscow, 00:00 (next day) Tokyo)
Title: Effective quantum gravity, cosmological constant and the Standard Model of particle physics
Speaker: Ilya Shapiro (Juiz de Fora)
Abstract:
The cosmological constant problem (CCP) and the formulation of
consistent quantum gravity are on the shortlist of the most
important unsolved fundamental problems of physics. In the case
of CCP the problem is to explain the extremely precise (55 orders
in the Standard Model) fine-tuning between the independent vacuum
part and the induced one, which is a function of symmetry breaking
in the models of particle physics. The situation with CCP is so
difficult that it makes sense to give up on attempting its
solution and to accept the need for fine-tuning between the
vacuum and induced counterparts of the observed energy density
of the vacuum. In this case, we meet the challenging situation
with the renormalization group running of the vacuum or induced
summands of the cosmological constant at low energies.
Assuming the effective approach to quantum gravity and the
Vilkovisky-DeWitt scheme of unique effective action, one can
derive the exact, well-defined, renormalization group running of
the vacuum cosmological constant. It turns out that, owing to the
mentioned fine-tuning with the induced part, this running imposes
severe restrictions on the possible extensions of the Minimal
Standard Model of particle physics, concerning the magnitude of
the vacuum expectation value of the corresponding Higgs fields.
Observation: The talk is based on the collaboration with
B. Giacchini and T.P. Netto.
Thursday, October 27, 2022, 17:00 CEST (08:00 Los Angeles, 11:00 New York, 18:00 Moscow, 00:00 (next day) Tokyo)
Title: Simple Models for Quadratic Gravity
Speaker: John Donoghue (Amherst)
Abstract:
Quadratic gravity is in some ways an attractive option for quantum gravity, but it also is problematic because of the presence of the unstable ghost. Here I focus on this aspect by using some theories which are simpler than the full gravitational theory, but which share the same feature. These provide interesting QFTs on their own. Besides topics such as the classical limit, positivity and causality, I will argue that the running of couplings in theories like quadratic gravity needs to be recalculated. These models are amenable to lattice methods, which may ultimately provide non-perturbative insights.
Thursday, November 10, 2022, 17:00 CET (08:00 Los Angeles, 11:00 New York, 19:00 Moscow, 01:00 (next day) Tokyo)
Title: The continuum limit of spin foams — is it GR?
Speaker: Bianca Dittrich (Perimeter)
Abstract:
I will present first results on the continuum limit of spin foams, which allow for a new phenomenological approach, based on continuum quantum field theory.
Spin foams are discretized path integrals for quantum gravity based on a rigorous definition of quantum geometry. This does however lead to very complicated amplitudes, making e.g. the extraction of a continuum limit difficult. Thus, a long-standing open question was whether spin foams do describe gravity in their semi-classical and continuum limit.
The situation has changed with the recent introduction of effective spin foams, which on the one hand are much more amenable to numerical simulations, but also offer a much more transparent encoding of the dynamics. This has allowed first systematics results on the continuum limit, revealing that this limit is described by an area metric (instead of a length metric) dynamics. The space of area metric configurations represents an extension of the length metric space. This extension results from the quantum geometric input and is controlled by the Barbero-Immirzi parameter and the Planck lenghts. Integrating out these additional quantum degrees of freedom we obtain an effective action of the length metric, given by the Einstein-Hilbert term and a Weyl-squared term.
This results can be confirmed by an independent construction, that works entirely in the continuum, and is based on a modification of the Plebanski formulation of Gravity.
Thursday, November 24, 2022, 17:00 CET (08:00 Los Angeles, 11:00 New York, 19:00 Moscow, 01:00 (next day) Tokyo)
Title: Black holes and cosmology in a limiting curvature gravity theory
Speaker: Valeri Frolov (Alberta Univ.)
Abstract:
I shall discuss a recently proposed limiting curvature theory of gravity and its application to the problem of singularities in cosmology and inside black holes. In this theory the growth of the curvature is suppressed by specially chosen inequality constraints included in the gravity action. In this model the Universe has a bounce instead of the initial Big Bang singularity. We also consider a case of a spherically symmetric four-dimensional black hole and demonstrate that imposed curvature constraints modify a solution in the black hole interior. Instead of forming the curvature singularity the modified metric describes a space which is exponentially expanding in one direction and oscillating in the other two directions. The spacetime is complete and its polynomial curvature invariants are uniformly bounded.
Thursday, December 8, 2022, 17:00 CET (08:00 Los Angeles, 11:00 New York, 19:00 Moscow, 01:00 (next day) Tokyo)
Title: Group field theories for quantum spacetime
Speaker: Daniele Oriti (LMU)
Abstract:
I introduce the group field theory formalism for quantum gravity, in its basic features as well as in its links with other quantum gravity approaches. Then, I survey a few research directions and recent results, focusing in particular on continuum limit and renormalization, and on the extraction of effective cosmological dynamics from the fundamental quantum dynamics.
Thursday, January 12, 2023, 17:00 CET (08:00 Los Angeles, 11:00 New York, 19:00 Moscow, 01:00 (next day) Tokyo)
Title: Biocosmology: towards the beginning of a new scientific topic
Speaker: Andrew Liddle (Lisbon Univ.)
Abstract:
What does our biosphere have to teach to the rest of the universe? Not much, right? How can it, when our planet is tiny before the infinite vastness of the cosmos. But our recent results, founding the field of biocosmology, say otherwise - that our living biosphere contains more information, in the form of living diversity and complexity, than the rest of the Universe, supplanting even that in black holes. These results call for a dramatic review of the relationship between life and the cosmos, and even show that humans may be responsible for the dark energy that forces today’s Universe to accelerate its expansion. Now that would be the ultimate environmental catastrophe!
Work led by Marina Cortês, with Stuart Kauffman and Lee Smolin
Thursday, January 26, 2023, 17:00 CET (08:00 Los Angeles, 11:00 New York, 19:00 Moscow, 01:00 (next day) Tokyo)
Title: Scattering amplitudes for Kerr black holes and higher-spin gauge symmetry
Speaker: Henrik Johansson (Uppsala Univ.)
Abstract:
In a series of recent works it has become clear that quantum scattering amplitudes can be used to gain surprisingly useful insights to the dynamics of Kerr black holes. A simple infinite family of three-point amplitudes have been found, which describes the primary gravitational interaction of a black hole with quantum spin s. However, the corresponding Compton four-point amplitudes are not known except for a few low-spin examples. These amplitudes are needed for post-Newtonian and post-Minkowskian calculations of inspiraling binary black-hole systems. In this talk, I will show that all known Kerr amplitudes can be uniquely predicted from the principle of gauge symmetry. In particular, I will discuss the construction of a family of EFTs with Stuckelberg higher-spin fields that describe the expected dynamics of Kerr black holes. The EFTs enjoys massive higher-spin gauge symmetry, which is used as a selection principle for the non-minimal interactions. I will briefly discuss a possible interpretation of this symmetry, and why, in hindsight, it could have been anticipated.
Thursday, February 9, 2023, 17:00 CET (08:00 Los Angeles, 11:00 New York, 19:00 Moscow, 01:00 (next day) Tokyo)
Title: Hearts of Darkness: theory and phenomenology of non-singular black holes
Speaker: Francesco Di Filippo (YITP)
Abstract:
Black holes contain, deep in their interior, theoretical evidence of the failure of general relativity. A series of fundamental results, starting from the 1965 Penrose singularity theorem, proved that physically realistic initial conditions will unavoidably produce a singular black hole spacetime. I will discuss in a model-independent manner the different possibilities that singularity regularization may open. After that, I will focus on consistency issues due to an exponential blue-shift of the perturbation at the inner horizon. This phenomenon goes by the name of mass inflation instability and cast serious doubts on the viability of regular black holes as resolution to the singularity problem. Finally, I will stress the observational implications that stem from this analysis.
Thursday, February 23, 2023, 17:00 CET (08:00 Los Angeles, 11:00 New York, 19:00 Moscow, 01:00 (next day) Tokyo)
Title: A New Gravitational Action for the Trace Anomaly
Speaker: Gregory Gabadadze (New York Univ. & CCPP)
Abstract:
I will discuss the question of building a local diff-invariant effective gravitational action for the trace anomaly. General Relativity (GR) combined with the existing action for the trace anomaly is an inconsistent low energy effective field theory. This issue is addressed by extending GR into a certain scalar-tensor theory, which preserves the GR trace anomaly equation, up to higher order corrections. The extension introduces a new mass scale -- assumed to be below the Planck scale -- that governs four high dimensional terms in a local diff-invariant trace anomaly action. Such terms can be kept, while an infinite number of Planck-suppressed invariants are neglected. The resulting theory maintains two derivative equations of motion. In a certain approximation it reduces to the conformal Gallileon, which could have physical consequences.
Thursday, March 9, 2023, 17:00 CET (08:00 Los Angeles, 11:00 New York, 19:00 Moscow, 01:00 (next day) Tokyo)
Title: Quantum reference frames: a relational perspective on nonclassical spacetime
Speaker: Flaminia Giacomini (ETH, Zurich)
Abstract:
Understanding the fundamental nature of gravity at the interface with quantum theory is a major open question in theoretical physics. Recently, the study of gravitating quantum systems, namely quantum systems sourcing a gravitational field and interacting gravitationally, has attracted a lot of attention, thanks to the possibility of realising these scenarios in the laboratory in the near future. When a quantum system sources a gravitational field, spacetime is fundamentally nonclassical. In this situation, also the notion of a reference frame needs to be generalised to account for quantum features of spacetime, thus realising a quantum reference frame (QRF). In this talk, I will introduce a formalism to associate a QRF to the quantum state of a particle, and show examples of why QRFs can contribute to formulate physics on nonclassical spacetime.
Thursday, March 23, 2023, 17:00 CET (09:00 Los Angeles, 12:00 (noon) New York, 19:00 Moscow, 01:00 (next day) Tokyo)
Title: UV/IR Mixing, EFTs, Hidden Cancellations, and Origami: understanding the Higgs Mass and hierarchy problems from a string perspective
Speaker: Steve Abel (Durham University)
Abstract:
Almost all existing calculations that concern the Higgs mass are
performed within the framework of an effective field theory. While
sufficient for certain purposes, such calculations throw up problems to do
with fine-tuning and naturalness, in particular the famous hierarchy
problem. Arguably this problem has arisen because most phenomenology done within string theory has not
respected the full string symmetries that are responsible for many of the remarkable
finiteness properties for which string theory is famous. Chief among these
symmetries is worldsheet modular invariance, which is an exact symmetry of
all perturbative closed-string vacua. And yet if the UV is tamed by
this symmetry then it should be exact even today!
Thus if we believe that underlying our non-supersymmetric world there lies
a closed string theory (or something like it) then we must also believe that there is
a conspiracy of UV/IR mixing taking place across all energy scales.
In this talk I will discuss what one can learn from this fact.
Thursday, April 6, 2023, 17:00 CEST (08:00 Los Angeles, 11:00 New York, 18:00 Moscow, 00:00 (next day) Tokyo)
Title: Recent Results in Causal Set Theory
Speaker: Sumati Surya (Raman Research Institute)
Abstract:
In this talk I will review the basic features of causal set theory (CST) which is a discrete approach to quantum gravity. In CST one "quantises" the causal structure poset, thus rendering it locally finite. This latter property means that continuum spacetime must be regarded as an approximation. In order to make the correspondence commensurate with results from Lorentzian geometry, of a kinematic ensemble of random (Poisson distributed) causal sets. An important quesion is how continuum geometry and topology is encoded in the causal order. I will review results in this process of " geometric reconstruction" and related aspects, including some intriguing results on discrete entanglment entropy. I will then discuss the quantum dynamics of causal sets and summarise some recent results.
Thursday, April 13, 2023, 16:00 CEST (07:00 Los Angeles, 10:00 New York, 17:00 Moscow, 23:00 Tokyo)
Title: Going beyond the standard LCDM model by Cosmography
Speaker: Salvatore Capozziello (Universita' di Napoli "Federico II" & Scuola Superiore Meridionale)
Abstract:
The standard cosmological model is recently suffering severe
shortcomings and tensions due to the fact that, very likely, it has to be
improved at IR and UV scales. In view to reconstruct a self-consistent
cosmic history, cosmography revealed a model-independent approach capable
of fixing reliable models starting from observations. Without claim to
completeness, we are going to sketch an overview of the method and the
possible realizations towards the solutions of various cosmological
tensions.
Thursday, May 4, 2023, 17:00 CEST (08:00 Los Angeles, 11:00 New York, 18:00 Moscow, 00:00 (next day) Tokyo)
Title: The thermodynamics of Acceleration
Speaker: Ruth Gregory (King’s College London)
Abstract:
Black hole thermodynamics is a fascinating topic, which is usually approached "classically" yet reveals fundamental quantum aspects of perhaps the most fascinating object in gravitational physics. In this talk, I will discuss how we can include acceleration in this repertoire of thermodynamic charges for the black hole, first reviewing the exact solutions of accelerating black holes, then discussing their thermodynamic properties.
Abstract:
Quantum gravity is the art of showing that quantum theory and general relativity are not fundamentally incompatible, all the way to the Planck scale. It turns out that a highly fruitful strategy is to use nothing but good old quantum field theory, without any exotic ingredients, and adapt it to the situation where spacetime geometry is dynamical. It has taken us a while to address the underlying technical and conceptual challenges, but the good news is that we now have charted a path toward a theory of quantum gravity which is unitary, essentially unique and can produce "numbers" beyond perturbation theory. I will introduce the approach of Causal Dynamical Triangulations (CDT), which is to quantum gravity what lattice QCD is to nonabelian gauge theory. Its nonperturbative toolbox has allowed us to go where other approaches cannot (yet) and to extract quantitative results on quantum observables at or near the Planck scale. They reveal promising evidence for both genuine quantum effects and the existence of a classical limit, reflecting the mathematical richness of the underlying "random geometry". A breakthrough result of CDT quantum gravity in four dimensions is the emergence, from first principles, of a nonperturbative quantum spacetime with de Sitter-like properties. I will summarize these findings, highlight some structural challenges and discuss the future prospects of quantum gravity.
Thursday, June 1, 2023, 17:00 CET (08:00 Los Angeles, 11:00 New York, 19:00 Moscow, 01:00 (next day) Tokyo)
Title: Planck-scale violations of Lorentz symmetries and their implications for astrophysics and cosmology
Speaker: Giulia Gubitosi (University of Naples Federico II)
Abstract:
The Planck energy is generally considered to be the scale where quantum gravitational effects become important. In this talk, I will illustrate some of the most relevant features of models where the Planck scale governs deformations of relativistic symmetries. I will then discuss opportunities for experimental tests of such models arising in physical frameworks much below the Planck scale. This concerns some astrophysical observations, sensitive to tiny residual signatures at low energies thanks to amplification mechanisms, and cosmological observations carrying information about primordial perturbations in the early universe, which are directly influenced by Planck-scale physics. If time allows, I will comment on some recent developments on the implications of Planck-scale-deformed symmetries for the decoherence of quantum systems.
Thursday, June 8, 2023, 17:00 CET (08:00 Los Angeles, 11:00 New York, 18:00 Moscow, 00:00 (next day) Tokyo)
Title: Interacting UV fixed points - from quantum field theory to quantum gravity
Speaker: Daniel Litim (University of Sussex)
Abstract:
It is a fascinating idea that all the physics as we know it could arise
as a small perturbation of some UV conformal field theory. This necessitates
that the running couplings of the underlying quantum field theory achieve
a fixed point in the UV, such as in asymptotic freedom. In this talk, I
discuss the prospects for QFTs to develop interacting UV fixed points,
covering weakly and strongly coupled 4d QFTs, perturbatively non-renromalisable
theories such as 3d fermionic QFTs, and 4d quantum gravity. I will give an
overview of rigorous results and methodologies, and address open challenges
for quantum gravity.
Thursday, June 15, 2023, 17:00 CET (08:00 Los Angeles, 11:00 New York, 18:00 Moscow, 00:00 (next day) Tokyo)
Title: Quantum gravity and its connection to observations
Speaker: Astrid Eichhorn (University of Southern Denmark)
Abstract:
To make progress in developing a quantum theory of gravity, we need to connect candidate theories to observations. I will review ideas on connecting quantum gravity to observations in particle physics, to searches for dark matter and to observations of black holes, in particular with the (next-generation) Event Horizon Telescope.
Thursday, June 29, 2023, 17:00 CET (08:00 Los Angeles, 11:00 New York, 18:00 Moscow, 00:00 (next day) Tokyo)
Title: Causality and Scattering Time delays
Speaker: Andrew Tolley (Imperial College London)
Abstract:
Since the 1950’s it has been understood that causality imposes restrictions on the time delays incurred in scattering events, and this in turn places useful constraints on the S-matrix. I will review the classic and modern incarnations of this idea, including its realization in spherically symmetric and pp-wave spacetimes, and how it can be used to put constraints on effective field theories in a complementary way to positivity bounds.
Thursday, July 6, 2023, 17:00 CET (08:00 Los Angeles, 11:00 New York, 18:00 Moscow, 00:00 (next day) Tokyo)
Title: Black Holes, Quantum Cosmology and Negative Dimensions
Abstract:
Multidimensional spaces are widely used in mathematics and physics. This talk will discuss the emergence of negative dimensions in black hole thermodynamics and quantum cosmology.
Most black holes violate the third law of thermodynamics. The entropy of a Schwarzschild black hole is inversely proportional to the square of the temperature, and tends to infinity rather than zero when the temperature goes to zero.
We are looking for quantum statistical models with such exotic thermodynamic behavior. It is shown that a Schwarzschild black hole in D=4 spacetime dimensions corresponds to a Bose gas in space with d=-4 negative spatial dimensions. The Riemann zeta function is used to determine the entropy of a Bose gas in negative dimensions. There is also a correspondence between Bose gas in positive dimensions and black holes in negative dimensions.
One can treat the spacetime dimension as a thermodynamic quantity and add the corresponding term to the first law of thermodynamics. If we assume that the creation and evolution of the universe obey the second law of thermodynamics, and the "pre-Big Bang" state of the universe was a sphere of negative dimension, then in the semiclassical approximation to the Wheeler-DeWitt equation, the entropy reaches its maximum value at the de Sitter spaceme with the observed dimension D = 4 for de Sitter radius l=0.57. Extensions of quantum cosmological models to include negative dimensions are discussed.
The talk is based on:
I.Aref'eva and I.Volovich, ''Violation of the Third Law of Thermodynamics by Black Holes, Riemann Zeta Function and Bose Gas in Negative Dimensions,'' arXiv:2304.04695.
I.Aref'eva and I.Volovich, ''Bose Gas Modeling of the Schwarzschild Black Hole Thermodynamics,'' arXiv:2305.19827.
Thursday, July 20, 2023, 17:00 CET (08:00 Los Angeles, 11:00 New York, 19:00 Moscow, 01:00 (next day) Tokyo)
Title: A construction of unitary quantum field theory in curved spacetime and its implications for quantum gravity
Speaker: Sravan Kumar (Portsmouth University)
Abstract:
Before we ask what is the theory of quantum gravity, it is a legitimate quest to formulate a robust quantum field theory in curved spacetime (QFTCS). Several conceptual problems such as unitarity violation and information paradox have raised concerns over several decades, mainly because no S-matrix formulation is yet found in QFTCS. We argue that the institutional thinking of fixing a spacetime geometry, observers, and light cones, and then quantizing the fields in that "intuitively", or "classically", fixed spacetime is the origin of the problem. In this talk, acknowledging the fact that "time" is a parameter in quantum theory, which is different from its status in the context of General Relativity (GR), we start with a "quantum first approach" and propose a new formulation for QFTCS based on the refined understanding of discrete spacetime transformations. We will address how this new formulation of QFTCS can potentially restore unitarity and observer complementarity in de Sitter and black hole spacetimes. Finally, we shall comment on the implications of our construction to the quantum gravity research that has been happening in the last decades.
The talk is based on arXiv: 2305.06046 [hep-th], arXiv: 2305.06057 [hep-th], arXiv: 2209.03928 [hep-th], and arXiv: 2307.10345 [hep-th] in collaboration with J. Marto.
Thursday, September 21, 2023, 16:00 CEST (07:00 Los Angeles, 10:00 New York, 17:00 Moscow, 23:00 Tokyo)
Title: Astrophysical Probes of Lorentz Violation
Speaker: John Ellis (King’s College London)
Abstract:
Physical intuition suggests that the vacuum should fluctuate strongly at the Planck scale due to quantum effects, a phenomenon called space-time foam. This would be a dynamical medium that could affect the propagation of energetic particles, violating Lorentz invariance. Many of the most sensitive probes of this possibility are provided by energetic, time-varying astrophysical sources such as pulsars, gamma-ray bursters and active galactic nuclei. This talk will review available astrophysical constraints on Lorentz violation in the propagation of photons, neutrinos and gravitational waves.
Thursday, October 12, 2023, 16:00 CEST (07:00 Los Angeles, 10:00 New York, 17:00 Moscow, 23:00 Tokyo)
Title: Black Hole Information Loss Paradox and the AnaBHEL Experiment
Speaker: Pisin Chen (National Taiwan University and Stanford)
Abstract:
The question of whether Hawking evaporation violates unitarity, and therefore results in the loss of information, has remained unresolved since Hawking’s seminal discovery. To date, the investigations have remained mostly theoretical since it is almost impossible to settle this paradox through direct astrophysical black hole observations. An accelerating relativistic mirror has long been recognized as a viable setting where the physics mimics that of black hole Hawking radiation. In 2017, Chen and Mourou proposed a novel method to realize such a system by traversing an ultra-intense laser through a plasma target with a decreasing density. An international AnaBHEL (Analog Black Hole Evaporation via Lasers) Collaboration has been formed with the objectives of observing the analog Hawking radiation and shedding light on the information loss paradox. In this talk, we first provide an overview of the black hole information loss paradox, including its latest developments. We then describe the design and the status of AnaBHEL, whose R&D is in excellent progress.
Thursday, October 26, 2023, 16:00 CEST (07:00 Los Angeles, 10:00 New York, 17:00 Moscow, 23:00 Tokyo)
Title: UV-IR connections in scattering amplitudes: power of unitarity and causality
Speaker: Anna Tokareva (HIAS, Hangzhou)
Abstract:
Very basic assumptions about the structure of the fundamental theory describing the nature at high energies can allow for a set of valuable constraints on the low energy EFT. This approach provides constraints that are independent of the concrete underlying fundamental theory at high energies. The low energy physics is bounded from the set of quite natural assumptions about the structure of the full theory, such as unitarity, causality and Lorentz invariance. In this talk I will consider the EFT of photons (or other U(1) gauge field) and compare different approaches to obtain bounds on its Wilson coefficients. I will present an analytic derivation of the implications of unitarity (linear and non-linear positivity bounds) and compare these constraints with the requirement of causal propagation of the photon modes around non-trivial backgrounds generated by external sources. I will show that the low energy causality condition can give complementary constraints to the positivity bounds. Applying both constraints together can significantly reduce the allowed region of the photon EFT parameters. Besides constraints on the IR theory, the analyticity and unitarity assumptions can set bounds on the UV theory. I will show this connection using the example of the graviton mediated scattering amplitudes. Surprisingly, the form of the forward limit singularities at low energies can give an insight into the of high energy limit of graviton scattering described by the unknown physics.
Thursday, November 9, 2023, 16:00 CET (07:00 Los Angeles, 10:00 New York, 23.00 Shanghai, 00:00 (next day) Tokyo)
Title: Deriving Gauge-String Duality
Speaker: Rajesh Gopakumar (ICTS, Bangalore)
Abstract:
Gauge (or Yang-Mills) theories are the building blocks of our current physical understanding of the universe. In parallel, string theory is a framework for a consistent quantum description of gravity. Gauge-String duality a.k.a. the AdS/CFT correspondence proposes a remarkable connection between these two very different classes of theories. I will begin by discussing why it is important to arrive at a first principles understanding of the underlying mechanism of this duality relating quantum field theories and string theories (or other theories of gravity). I will then proceed to discuss a very general approach which aims to relate large N QFTs and string theories, starting from free field theories. This corresponds to a tensionless limit of the dual string theory on AdS spacetime. Finally, if time permits, I will discuss specific cases of this limit for 3d AdS (dual to 2d CFT) and 5d AdS (dual to 4d Super Yang-Mills theory), where one has begun to carry this program through to fruition, going from the string theory to the field theory and vice versa.
Thursday, November 23, 2023, 16:00 CEST (07:00 Los Angeles, 10:00 New York, 23:00 Shanghai, 00:00 (next day) Tokyo)
Title: From correlators for all Lambdas to EAdS, and back
Speaker: Massimo Taronna (Napoli Federico II & SSM)
Abstract:
I will review some recent and on-going efforts to study holographic correlation functions in de Sitter (dS) and Minkowski space. After a brief review of AdS/CFT I will start reviewing the dS story, where in the Bunch-Davies vacuum one can perturbatively rewrite correlators on the future boundary in terms of Witten diagrams in Euclidean AdS upon considering appropriate Wick rotations. I will then explain how an analogous relation holds between celestial correlation functions and Witten diagrams. Contact diagram processes are proportional to contact Witten diagrams and particle exchanges can be recast as sums, discrete or continous, of particle exchanges in EAdS. Using such relations one can then import familiar EAdS techniques to study the properties of dS and celestial correlators. In particular, we infer the analytic structure of the spectral density in the conformal partial wave expansion of correlators finding that, at least at any order in perturbation theory, it is a meromorphic function of the energy. We conclude discussing non perturbative constraints from unitarity in Euclidean Conformal Field Theory.
Thursday, December 7, 2023, 16:00 CET (07:00 Los Angeles, 10:00 New York, 23:00 Shanghai, 00:00 (next day) Tokyo)
Title: Update on Supermembrane Theory
Speaker: Hermann Nicolai (Max-Planck-Institut für Gravitationsphysik)
Abstract:
The D=11 maximally supersymmetric supermembrane is a theory
`beyond' string theory, and as such a candidate for non-perturbative
unification (`M theory'). In this talk I will review some of its basic
features, such as its reformulation as a one-dimensional gauge theory
of area preserving diffeomorhisms and the basic link to the SU(infty)
matrix model of M theory. I will also review some more recent
developments, in particular the question of how to set up a perturbative linearization suitable for a quantum mechanical path-integral treatment
and the construction of `vertex operators' for the massless states
of the theory.
Thursday, January 25, 2024, 14:00 CET (05:00 Los Angeles, 08:00 New York, 21:00 Shanghai, 22:00 Tokyo)
Title: Perspectives on Inflationary Cosmology
Speaker: Misao Sasaki (IPMU)
Abstract:
The inflationary universe has become one of the essential ingredients of the standard cosmology. In this talk I first review the basics of inflation,
focusing on the canonical single-field slow-roll model. It is emphasized that inflation is the source of all the structures in the Universe and that the predictions are perfectly consistent with observations. Nevertheless there are some hints of non-canonical features in the observational data. In addition, there is a growing interest in the gravitational waves originating from inflation, either directly or indirectly, and the associated phenomena such as the primordial black hole formation. I review recent developments in these topics.
Thursday, February 1, 2024, 15:00 CET (06:00 Los Angeles, 09:00 New York, 22:00 Shanghai, 23:00 Tokyo)
Title: The three point amplitudes and soft factors from the matrix model for M-theory
Speaker: Juan Maldacena (IAS)
Abstract:
We will review the proposal by Banks, Fischler, Shenker and Susskind for computing amplitudes in M-theory via a matrix quantum mechanics.
We then discuss a computation of the simplest amplitude, the three graviton amplitude, and apply this result to discuss the soft theorem from the matrix model point of view.
Thursday, February 15, 2024, 16:00 CET (07:00 Los Angeles, 10:00 New York, 23:00 Shanghai, 20:00 (next day) Tokyo)
Title: More on Stable Ghosts
Speaker: Alexander Vikman (CEICO)
Abstract:
I will discuss our recent works Phys.Rev.Lett. 128 (2022) 4, 041301
and e-Print: 2305.09631. There we presented a large class of
mechanical models where a canonical degree of freedom interacts with
another one with a negative kinetic term, i.e., with a ghost and yet the system is totally stable.
We proved analytically that the
classical motion of the system is finite i.e. Lagrange stable for all
initial conditions, notwithstanding that the conserved Hamiltonian is
unbounded from below and above. Moreover, there are Lyapunov stable
equilibrium configurations. Numerical computations fully supported
this. An important update is that such stable ghosts can also appear in
systems with a simple polynomial interaction.
Systems with negative kinetic terms often appear in modern
cosmology, quantum gravity, and high energy physics, and are usually
deemed as unstable. Our result demonstrates that, for mechanical
systems, this common lore can be too naïve and that a stable living
with ghosts is possible.
Thursday, February 29, 2024, 16:00 CET (07:00 Los Angeles, 10:00 New York, 23:00 Shanghai, 00:00 (next day) Tokyo)
Title: Bimetric and multimetric theories of gravity
Speaker: Fawad Hassan (Stockholm University)
Abstract:
It is well known that the interactions of the gravitational metric with other spin-2 fields are strongly constrained by the absence of ghost instabilities. In this talk I will outline the theoretical motivations for the study of such theories and then describe the ghost free theories of two interacting spin-2 fields (often called bimetric theories) and their known extensions to multiple spin-2 fields (multimetric theories). I will also describe an interesting geometrical aspect of such theories which is relevant for the existence of a consistent notion of space and time in the theory.
Thursday, March 14, 2024, 16:00 CET (07:00 Los Angeles, 10:00 New York, 23:00 Shanghai, 00:00 (next day) Tokyo)
Title: Entanglement from Hawking radiation
Speaker: Ivan Agullo (LSU)
Abstract:
It is well-known that black holes generate quantum correlations via Hawking’s pair-creation mechanism. In this talk, I’ll adapt tools from Gaussian quantum information theory to quantify the entanglement emission from black holes under different circumstances. These tools offer a remarkably good ratio of power to simplicity and permit us to demonstrate how the creation of entanglement is sensitive to the environment surrounding the horizon. In particular, ambient thermal noise (e.g., the ubiquitous CMB radiation) significantly inhibits the generation of entanglement. Additionally, I will argue that for spinning black holes, the ergoregion encompassing the horizon acts as an independent source of quantum entanglement. I'll conclude by discussing ongoing work towards the experimental verification of these ideas using analog black holes in the laboratory.
Thursday, April 4, 2024, 16:00 CEST (07:00 Los Angeles, 10:00 New York, 22:00 Shanghai, 23:00 Tokyo)
Title: Dark Matter, Inflation, and Gravitational Production of Particles with Spin
Speaker: Evan McDonough (University of Winnipeg)
Abstract:
Cosmic inflation provides not only a "cosmological collider" to generate signals of new particle species, but also a 'particle factory' to generate non-thermal relics, including dark matter. In this talk I will discuss the primordial production of particles with spin, focusing on spin-1, spin-3/2, and bosonic higher-spin fields, as well as the Kalb-Ramond antisymmetric tensor field, via gravitational particle production. Far from being a repetitive exercise, each case offers something new, such as enhanced production and/or unforeseen instabilities, which constrain the theoretically allowed dark matter parameter space and the underlying field theory.
Thursday, April 11, 2024, 16:00 CEST (07:00 Los Angeles, 10:00 New York, 22:00 Shanghai, 23:00 Tokyo)
Title: The ins and outs of cosmological correlators and the de Sitter S-matrix
Speaker: Enrico Pajer (DAMTP, Cambridge)
Abstract:
Cosmological correlators, the natural observables of the primordial universe, have been extensively studied in the past two decades using the in-in formalism pioneered by Schwinger and Keldysh for the study of dissipative open systems. Ironically, most applications in cosmology have focused on non-dissipative closed systems. We show that, for non-dissipative systems, correlators can be equivalently computed using the in-out formalism with the familiar Feynman rules. In particular, the myriad of in-in propagators is reduced to a single (Feynman) time-ordered propagator and no sum over the labelling of vertices is required. In de Sitter spacetime, this requires extending the expanding Poincare' patch with a contracting patch, which prepares the bra from the future. Our results are valid for fields of any mass and spin but assuming the absence of infrared divergences.
I will present three applications of the in-out formalism: a representation of correlators in terms of a sum over residues of Feynman propagators in the energy-momentum domain; an algebraic recursion relation that computes Minkowski correlators in terms of lower order ones; and the derivation of cutting rules from Veltman’s largest time equation, which we explicitly develop and exemplify for two-vertex diagrams to all loop orders.
The in-out formalism leads also to a natural definition of a de Sitter scattering matrix, which I will discuss in simple examples. Remarkably, I will show that our scattering matrix satisfies the standard optical theorem and the positivity that follows from it in the forward limit.
FRIDAY, April 19, 2024, 13.00 CEST (time to be fixed)
Alexei Starobinsky memorial seminar
13:00 Address by Ludmila Viktorovna, Prof. Starobinsky's widow, colleagues and friends
14:00 Varun Sahni TBA 14:30 Jun'ichi Yokoyama Stochastic inflation and beyond 15:00 David Polarski TBA 15:30 Alexander Kamneshchik The Pauli-Zeldovich cancellation of the vacuum energy divergences and 120 orders
16:00 Break
16:30 Robert Brandenberger TBA 17:00 Richard Woodard Recent Developments in Stochastic Inflation 17:30 Andrei Linde TBA 18:00 Slava Mukhanov TBA 18:30 Alexey Koshelev and Sravan KumarTBA 19:00 Closing remarks
Abstract:
TBA
Thursday, May 2, 2024, 16:00 CEST (07:00 Los Angeles, 10:00 New York, 23:00 Shanghai, 00:00 (next day) Tokyo)
Title: Asymptotic Safety Landscapes at the intersection between positivity bounds and swampland conjectures
Speaker: Alessia Platania (Perimeter Institute)
Abstract:
I will review recent progress in computing and analyzing the "landscape" of effective field theories stemming from an asymptotically safe ultraviolet completion of gravity. I will argue that this is an essential task in asymptotic safety, as well as in other approaches to quantum gravity, to test their consistency and to facilitate the comparison between their predictions. Concretely, I will focus on purely gravitational and gravity-photon systems, and I will discuss the intersections between the resulting asymptotic safety landscape, the string landscape identified by (some) swampland constraints, and positivity bounds.
Thursday, May 16, 2024, 16:00 CEST (07:00 Los Angeles, 10:00 New York, 23:00 Shanghai, 00:00 (next day) Tokyo)
Title: (Runaway) Gravitational Production of Dark Photons
Speaker: Leah Jenks (KICP, University of Chicacgo)
Abstract:
Gravitational particle production is the process by which particles are created due to the expansion of spacetime during inflation. In this talk we will discuss aspects of gravitational particle production of dark photons, a popular dark matter candidate, with a particular focus on dark photons with nonminimal couplings to gravity. I will first show that the inclusion of nonminimal couplings can lead to instabilities in the theory, namely ghosts and “runaway” production, and that the parameter space for the couplings is limited in order to avoid such instabilities. Lastly, within the allowable non-ghost and non-runaway region of parameter space, I will show that dark photons can be produced in the correct abundance to account for all of the dark matter today.
Thursday, May 30, 2024, 16:00 CEST (07:00 Los Angeles, 10:00 New York, 22:00 Shanghai, 23:00 Tokyo)
Title: Discrete Spacetime, Emergent Geometry and Computable Quantum Gravity
Speaker: Jonathan Gorard (Princeton)
Abstract:
Closely related to the question of whether spacetime should best be modeled as a discrete or a continuous mathematical structure, an important open question remains regarding the extent to which quantum gravity will end up being a computable theory. I will begin by presenting a fully discrete formalism for classical gravity, in which all of the necessary mathematical structures are a priori computable (with hypergraphs replacing Cauchy surfaces, directed acyclic graphs replacing the conformal structure of Lorentzian manifolds, hypergraph rewriting rules replacing the ADM evolution equations, hypergraph consistency conditions replacing the ADM constraint equations, etc.). I will then proceed to show how classical GR may nevertheless be recovered in the macroscale (continuum) limit of this formalism, with a particular focus on black hole spacetimes and simple gravitational collapse models as illustrative examples. Some potentially observable discrepancies from continuum GR in the mesoscale regime of the formalism will also be discussed. I will conclude with a mention of how a discrete gravitational path integral may be conjecturally formulated by allowing the rewriting dynamics to become non-deterministic and multi-threaded, and present some speculations regarding potential implications for the foundations of quantum gravity.
Thursday, June 13, 2024, 16:00 CEST (07:00 Los Angeles, 10:00 New York, 22:00 Shanghai, 23:00 Tokyo)
Title: A postquantum theory of classical gravity
Speaker: Jonathan Oppenheim (UCL)
Abstract:
I will present a consistent theory of classical systems coupled to quantum ones via the path integral formulation. In the classical limit, this is the path integral for stochastic processes like Brownian motion. We then apply the formalism to general relativity, since it's reasonable to question whether spacetime should have a quantum nature given it's status within quantum field theory. In contrast to perturbative quantum gravity, the pure gravity theory is renormalisable. The theory introduces a dimensionless coupling constant which controls the degree of diffusion in the gravitational field and runs to zero at short distances. Because of the stochastic nature of the theory, we find a deviation from general relativity at low acceleration which results in anomalous behavior of the gravitational field. This allows for both tabletop experiments and astrophysical tests of the nature of spacetime in comparison to quantum gravity and cold dark matter.
Thursday, October 10, 2024, 16:00 CEST (07:00 Los Angeles, 10:00 New York, 22:00 Shanghai, 23:00 Tokyo)
Title: Classical observables of General Relativity from scattering amplitudes
Speaker: Paolo Di Vecchia (Nordita, NBI)
Abstract:
I will be using scattering amplitudes, instead of the Lagrangian of General Relativity (GR), to compute classical observables in GR.
In the first part of the seminar I will consider the elastic scattering of two massive particles, describing two black holes, and I will show how to compute the eikonal up to two-loop order, corresponding to third Post-Minkowskian (3PM) order, that contains all the classical information. From it I will compute the first observable that is the classical deflection angle.
In the second part of the seminar I will consider inelastic processes with the emission of soft gravitons. In this case the eikonal becomes an operator containing the creation and annihilation operators of the gravitons. The case of soft gravitons can be treated following the Bloch-Nordsieck approach and, in this case, I will be computing two other observables: the zero-frequency limit (ZFL) of the spectrum dE/domega of the emitted radiation and the angular momentum loss at 2PM and 3PM. I will consider also the case in which there are static modes localised at $omega=0$.
In the third part of the seminar I will be discussing soft theorems with one graviton emission, first briefly at tree level, and then at loop level following the approach of Weinberg from 1965 and comparing it with a more recent approach by Laddha, Saha, Sahoo and Sen. I will compute the universal soft terms of the waveform, that are $ rac{1}{omega}$, $log omega$ and $omega log^2 omega$, first at the tree and one-loop level and then also at two-loop level. Then I will present a guess for the terms $omega^n log^{n+1} omega$ for $n=2, 3 …$ and I will give a formula where all these terms are resummed.
Finally, if I have time left, I will study the high energy limit. In particular, since the graviton is the massless particle with the highest spin, we expect universality at high energy in any gravitational amplitude. I will show that universality at high energy is satisfied both in the elastic and inelastic case, but this happens in the inelastic case in a very non trivial way. I will end with some conclusions and with a list of open problems.
Thursday, October 24, 2024, 16:00 CEST (07:00 Los Angeles, 10:00 New York, 22:00 Shanghai, 23:00 Tokyo)
Title: Scattering amplitudes for QCD, gravity and massive particles
Speaker: Alexander Ochirov (ShanghaiTech University)
Abstract:
I will discuss basic on-shell approach to scattering amplitudes. As an example, I will outline the derivation of two infinite families of tree-level scattering amplitudes in QCD involving a massive quark pair.
Then I will generalize one of them to gauge theory coupled to massive particles of general spin. This will allow me to discuss a novel chiral-field approach to massive higher-spin theory and applications to classical gravitational dynamics of rotating black holes.
Thursday, November 21, 2024, 16:00 CEST (07:00 Los Angeles, 10:00 New York, 22:00 Shanghai, 23:00 Tokyo)
Title: Progress on High-Energy String Collisions
Speaker: Sebastian Mizera (IAS, Princeton)
Abstract:
String theory offers a unique perspective on concrete questions about the nature of quantum gravity. I will review how such questions can be formulated in terms of the string theory S-matrix at high energies. Attempts to sharpen the S-matrix calculations reveal the need to revisit the very formulation of string perturbation theory in Lorentzian space-times. Mathematically, it relates to the open question of how to choose a compactification of the moduli space of punctured Riemann surfaces. I will review the recent progress in addressing such questions in the open and closed superstring sectors at one-loop level, resulting in concrete numerical predictions for high-energy string collisions.
Thursday, December 5, 2024, 16:00 CEST (07:00 Los Angeles, 10:00 New York, 22:00 Shanghai, 23:00 Tokyo)
Title:
Mathematical methods from scattering amplitudes to modern scientific calculus
Speaker: Pierpaola Mastrolia (Padova Univ.)
Abstract:
I recall generalised unitarity, integrand decomposition,
integration-by-parts identities, differential equations, GKZ-systems
and intersection theory, connecting Cauchy's residue theorem, Stokes'
theorem and Gauss' linking numbers,
and elaborate on the emerging crucial role played by De Rham
co-homology theory, ruling the differential and algebraic properties
of the integral functions that appear in the context of fundamental
physics.
I will recall recent applications to Feynman and Euler-Mellin
integrals in particle physics and cosmology, as well as to quantum
mechanics and Wick's theorem in QFT, concluding with the role that
Physics Informed Neural Networks may play in the context of (twisted
period) integrals evaluation.
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