Triangle Seminars
Monday, 2 Mar 2026
Lonti: classical and quantum energy conditions (3/4)
📍 London
Eleni Kontou
(KCL)
Abstract:
Energy conditions were originally formulated as pointwise bounds on contractions of the stress–energy tensor and have played a central role as assumptions in many foundational results of classical general relativity, most notably the singularity theorems. However, these conditions are generically violated by quantum fields, which admit states with locally negative energy density. Such violations are nevertheless constrained: quantum energy inequalities impose bounds on the magnitude and duration of negative energy.
In this course, I will first introduce the classical energy conditions and review their physical motivation and known violations. Then I will provide a brief introduction to quantum field theory on curved spacetimes and demonstrate how quantum energy inequalities can be derived. Finally, I will discuss in detail the average null energy condition and the limitations it imposes to causality violating spacetimes.
Course plan:
Lecture 1: Classical energy conditions and their violations
Lecture 2: Quantum field theory on curved spacetimes
Lecture 3: A derivation of a quantum energy inequality
Lecture 4: The average null energy condition
Energy conditions were originally formulated as pointwise bounds on contractions of the stress–energy tensor and have played a central role as assumptions in many foundational results of classical general relativity, most notably the singularity theorems. However, these conditions are generically violated by quantum fields, which admit states with locally negative energy density. Such violations are nevertheless constrained: quantum energy inequalities impose bounds on the magnitude and duration of negative energy.
In this course, I will first introduce the classical energy conditions and review their physical motivation and known violations. Then I will provide a brief introduction to quantum field theory on curved spacetimes and demonstrate how quantum energy inequalities can be derived. Finally, I will discuss in detail the average null energy condition and the limitations it imposes to causality violating spacetimes.
Course plan:
Lecture 1: Classical energy conditions and their violations
Lecture 2: Quantum field theory on curved spacetimes
Lecture 3: A derivation of a quantum energy inequality
Lecture 4: The average null energy condition
Posted by: Damian Galante
Tuesday, 3 Mar 2026
The Battle Against the Underdetermination of Dark Energy
📍 London
Pedro Ferreira
(Oxford)
Abstract:
Cosmological data has opened up new vistas on fundamental physics yet it is limited in its scope. While it has given us tantalizing hints at how the Universe might be expanding, it is unclear whether it can ever be used to find the microphysical structure of whatever is driving this expansion. I will discuss the evidence for what is, by far, the most thoroughly explored proposal - dynamical dark energy driven by a scalar field - pointing out what we can and cannot say about its fundamental nature. I will argue that it is unlikely (but not impossible) that it is normal quintessence, that there is strong evidence that there is some type of non-minimal coupling but which brings with it a host of undesirable consequences. I will try to look at the various loopholes and present what I think is the current status of cosmology as a probe of fundamental physics.
Cosmological data has opened up new vistas on fundamental physics yet it is limited in its scope. While it has given us tantalizing hints at how the Universe might be expanding, it is unclear whether it can ever be used to find the microphysical structure of whatever is driving this expansion. I will discuss the evidence for what is, by far, the most thoroughly explored proposal - dynamical dark energy driven by a scalar field - pointing out what we can and cannot say about its fundamental nature. I will argue that it is unlikely (but not impossible) that it is normal quintessence, that there is strong evidence that there is some type of non-minimal coupling but which brings with it a host of undesirable consequences. I will try to look at the various loopholes and present what I think is the current status of cosmology as a probe of fundamental physics.
Posted by: Sebastian Cespedes
Wednesday, 4 Mar 2026
Post-Newtonian theory and radiation reaction
📍 London
David Trestini
(University of Southampton)
Abstract:
After reviewing the main aspects of post-Newtonian theory and the current state of the art, I will discuss recent advances with regards to radiation reaction. In particular, I will discuss the difference between conservative and binding energy at fourth post-Newtonian order and its impact on waveform modeling.
After reviewing the main aspects of post-Newtonian theory and the current state of the art, I will discuss recent advances with regards to radiation reaction. In particular, I will discuss the difference between conservative and binding energy at fourth post-Newtonian order and its impact on waveform modeling.
Posted by: Riccardo Gonzo
TBA
📍 London
Suman Kundu
(SISSA)
Abstract:
TBA
TBA
Posted by: Jesse van Muiden
Scattering on the Coulomb Branch of N=4 SYM
📍 London
Kelian Haring
(University of Amsterdam)
Abstract:
I will discuss scattering on the Coulomb branch of planar N=4 SYM at finite ’t Hooft coupling. This describes a family of classical open-string S-matrices that smoothly interpolates between perturbative parton scattering at weak coupling and flat-space string scattering at strong coupling. I will focus on the four-point amplitude and discuss its remarkably rich structure: nonlinear Regge trajectories, dual conformal invariance, an intricate spectrum of bound states with an accumulation point, and a two-particle cut. Using dispersion relations and S-matrix bootstrap techniques, these properties can be incorporated to constrain the amplitude at finite ’t Hooft coupling, and I will discuss bounds on Wilson coefficients, couplings to bound states, and the overall shape of the amplitude.
This is based on https://arxiv.org/abs/2510.19909.
I will discuss scattering on the Coulomb branch of planar N=4 SYM at finite ’t Hooft coupling. This describes a family of classical open-string S-matrices that smoothly interpolates between perturbative parton scattering at weak coupling and flat-space string scattering at strong coupling. I will focus on the four-point amplitude and discuss its remarkably rich structure: nonlinear Regge trajectories, dual conformal invariance, an intricate spectrum of bound states with an accumulation point, and a two-particle cut. Using dispersion relations and S-matrix bootstrap techniques, these properties can be incorporated to constrain the amplitude at finite ’t Hooft coupling, and I will discuss bounds on Wilson coefficients, couplings to bound states, and the overall shape of the amplitude.
This is based on https://arxiv.org/abs/2510.19909.
Posted by: Andrew Svesko
Entanglement Matters: Entanglement as an organizing principle of quantum matter
📍 London
Frank Verstraete
(U Cambridge)
Abstract:
"Entanglement as an organizing principle of quantum matter"
A century after the formulation of the Schrödinger equation, the exponential complexity of many-body systems remains the central challenge of modern physics. Entanglement theory has recently broken this stalemate through the development of tensor networks. This colloquium explores these methods as a powerful arsenal for simulating strongly correlated matter and will demonstrate their utility in identifying the emergent (generalized) symmetries that define and classify quantum phases.
see more info:
https://lims.ac.uk/
https://lims.ac.uk/event/entanglement-as-an-organising-principle-of-quantum-matter/
"Entanglement as an organizing principle of quantum matter"
A century after the formulation of the Schrödinger equation, the exponential complexity of many-body systems remains the central challenge of modern physics. Entanglement theory has recently broken this stalemate through the development of tensor networks. This colloquium explores these methods as a powerful arsenal for simulating strongly correlated matter and will demonstrate their utility in identifying the emergent (generalized) symmetries that define and classify quantum phases.
see more info:
https://lims.ac.uk/
https://lims.ac.uk/event/entanglement-as-an-organising-principle-of-quantum-matter/
Posted by: JUVEN WANG
Thursday, 5 Mar 2026
Color-kinematics duality from an algebra of superforms
📍 London
Olaf Hohm
(Humboldt)
Abstract:
I give an introduction to an ongoing research program to find a first-principle and off-shell derivation of color-kinematics duality and the double-copy nature of gravity directly from field theory,
using the framework of homotopy algebra. I focus on recent progress that maps the homotopy algebra of (color-stripped) Yang-Mills theory to the (de Rham) algebra of differential forms on a simple superspace.
I give an introduction to an ongoing research program to find a first-principle and off-shell derivation of color-kinematics duality and the double-copy nature of gravity directly from field theory,
using the framework of homotopy algebra. I focus on recent progress that maps the homotopy algebra of (color-stripped) Yang-Mills theory to the (de Rham) algebra of differential forms on a simple superspace.
Posted by: Nathan Moynihan