Scientific Series

A proposal is made for a fundamental theory,  in which the history of the universe is constituted of views of itself.  Views are attributes of events, and the theory's only be-ables; they comprise information about energy and momentum transferred to an event from its causal past. 

The  theory is called the causal theory of views (CTV) and is a candidate for a completion of QM.  It is partly based on energetic causal sets (ECS),   an approach developed  with Marina Cortes.  A key result that applies also here is that spacetime is emergent from the ECS dynamics.  This implies that the fundamental dynamics involve no notion of space, distance or derivatives.  Instead I propose that a measure of similarity of views replaces derivatives as the basic measure of change and difference.

A measure of the diversity of views in a  causal network is introduced, called the variety (originally invented with Julian  Barbour).  I postulate a dynamics for CTV based on an action involving the variety and show that in an appropriate limit, it reduces to Schrodinger quantum mechanics.  A key result is that  the variety reduces to Bohm's quantum potential.

Based on arXiv:1307.6167,  arXiv:1308.2206 , arXiv:1712.0479 and a paper in preparation.

The subject of equivariant elliptic cohomology finds itself at the interface of topology, string theory, affine representation theory, singularity theory and integrable systems. These connections were already known to the founders of the discipline, Grojnowski, Segal, Hopkins, Devoto, but have come into sharper focus in recent years with a number of remarkable developments happening simultaneously: First, there are the geometric constructions, due to Kitchloo, Rezk and Spong, Berwick-Evans and Tripathy, building on the older program of Stolz-Teichner, and of course Segal. These are now all known to be equivalent to Grojnowski's original definition, thanks to work of Spong. Second, there are new applications, to integrable systems (Aganagic-Okounkov, Felder-Rimanyi-Varchenko-Tarasov) and to representation theory (Yang-Zhao-Zhong, G-Ram). Finally, there is the work of Weber-Rimanyi-Kumar, who build on the techniques of Borisov and Libgober to define Schubert classes in equivariant elliptic cohomology (with an added dynamical parametre), recovering the stable envelopes in type A. This talk will give a gentle introduction to the topic and attempt an overview over these recent developments.

Compact binaries may be formed dynamically in globular clusters, with large (close to unity) orbital eccentricity and emitting gravitational waves within the detection band of ground based detectors. The gravitational waves from such sources resemble more a discrete set of bursts than the continuous signal of their quasi-circular counterparts. I here discuss the construction of new analytic waveforms for such systems, which rely on treating the problem as a perturbation of a parabolic fly-by. I will discuss the results of parameter estimation with these waveforms for binary black holes, and the inclusion of tidal effects for binary neutron stars.

In a quantum measurement process, classical information about the measured system spreads through the environment. In contrast, quantum information about the system becomes inaccessible to local observers. In this talk, I will present a result about quantum channels indicating that an aspect of this phenomenon is completely general. We show that for any evolution of the system and environment, for everywhere in the environment excluding an O(1)-sized region we call the "quantum Markov blanket," any locally accessible information about the system must be approximately classical, i.e. obtainable from some fixed measurement. The result strengthens the earlier result of arXiv:1310.8640 in which the excluded region was allowed to grow with total environment size.  I will also discuss applications to many-body physics.

Rolling scalar fields play an important role in understanding cosmology within a particle physics framework. Coupling a rolling scalar field to light degrees of freedom gives rise to a thermal friction which, if large enough, induces a thermal bath. In the context of inflation the presence of such a thermal bath has compelling consequences as it significantly alters the usual observables, leading to a suppression of the tensor-to-scalar ratio r and a unique prediction for non-gaussianities. In my talk, I will illuminate why the axion of a non-Abelian gauge group is the ideal candidate for generating the thermal friction and how it sets the stage for a minimal setup of warm inflation, as well as a potential solution to the Hubble tension.

Gravitational waves (GWs) have already proved immensely powerful for constraining cosmological extensions of GR, both from data-driven and theoretical perspectives. However, GWs really come into their own when used in combination with complementary electromagnetic data. I’ll start by reviewing some of the bounds on extended gravity theories from GW detections to date. I'll introduce the formalism, the phenomenology, and the astrophysical pitfalls of these tests. Finally, we'll explore the impact of future experiments like LISA and accompanying galaxy surveys on the remaining parameter space of modified gravity theories.

This talk starts by reviewing known examples of how topological materials generate new kinds of electrodynamic couplings and effects.  Three-dimensional topological insulators realize a particular electromagnetic coupling known as “axion electrodynamics”, and understanding this leads to an improved understanding of magnetoelectricity in all materials.  We then turn to how topological Weyl and Dirac semimetals can show unique electromagnetic responses; we argue that in linear response the main observable effect solves an old problem via the orbital moment of Bloch electrons, and how in nonlinear optics there should be a new quantized effect, which may have been seen experimentally.  This nonlinear effect has a natural quantum e^3/h^2 and appears in chiral Weyl semimetals over a finite range of frequencies.  We discuss interaction and disorder corrections to nonlinear responses in closing.

The EDELWEISS direct detection experiment uses cryogenic Ge semiconductor detectors to search for sub-GeV dark matter (DM) particles. With its low gap energy, germanium is an excellent target to explore DM particles interactions with nucleons in the sub-GeV range, and DM-electron interactions in the MeV range, as well as search for the absorption of eV-scale dark photons. The collaboration has recently obtained the first Ge-based constraints on sub-MeV dark DM particles interacting with electrons using a 33.4 g Ge cryogenic detector prototype with a 0.53 electron-hole pair (rms) resolution, operated underground at the Laboratoire Souterrain de Modane.
These results will be presented, as well as the EDELWEISS-SubGeV plans to achieve lower threshold and more efficient particle identification at low energy.

Using a process-theoretic formalism, we introduce the notion of a causal-inferential theory: a triple consisting of a theory of causal influences, a theory of inferences (of both the Boolean and Bayesian varieties), and a specification of how these interact. Recasting the notions of operational and realist theories in this mold clarifies what a realist account of an experiment offers beyond an operational account. It also yields a novel characterization of the assumptions and implications of standard no-go theorems for realist representations of operational quantum theory, namely, those based on Bell’s notion of locality and those based on generalized noncontextuality. Moreover, our process-theoretic characterization of generalised noncontextuality is shown to be implied by an even more natural principle which we term Leibnizianity. Most strikingly, our framework offers a way forward in a research program that seeks to circumvent these no-go results. Specifically, we argue that if one can identify axioms for a realist causal-inferential theory such that the notions of causation and inference can differ from their conventional (classical) interpretations, then one has the means of defining an intrinsically quantum notion of realism, and thereby a realist representation of operational quantum theory that salvages the spirit of locality and of noncontextuality.

We have tentative evidence of massive stars that disappear without a bright transient. It is commonly argued that this massive stars have low angular momentum and can collapse into a black hole without significant feedback. In this talk I will make use of general-relativistic hydrodynamical simulations to understand the flow around a newly-formed black hole. I will discuss the angular momentum needed in order for the infalling material to be accreted into the black hole without forming a centrifugally supported structure, thus generating no effective feedback. If the feedback from the black hole is significant, the collapse can be halted and, as a result, it is likely followed by a bright transient. With the results from the simulation, I will constrain the maximum rotation rate for the disappearing massive progenitors know, and set a limit on the rate of expected disappearing stars.

"Analogue" Hamiltonian simulation involves engineering a Hamiltonian of
interest in the laboratory and studying its properties experimentally.
Large-scale Hamiltonian simulation experiments have been carried out in
optical lattices, ion traps and other systems for two decades. Despite
this, the theoretical basis for Hamiltonian simulation is surprisingly
sparse. Even a precise definition of what it means to simulate a
Hamiltonian was lacking.

AdS/CFT duality postulates that quantum gravity in a d-dimensional
anti-de-Sitter bulk space is equivalent to a strongly interacting field
theory on its d-1 dimensional boundary. Recently, connections between
AdS/CFT duality and quantum error-correcting codes have led (amongst
other things) to tensor network toy models that capture important aspects
of this holographic duality. However, these toy models struggle to
encompass dualities between bulk and boundary energy scales and dynamics.

On the face of it, these two topics seem to have nothing whatsoever to do
with one another.

In my talk, I will explain how we put analogue Hamiltonian simulation on
a rigorous theoretical footing, by drawing on techniques from Hamiltonian
complexity theory and Jordan algebras. I will show how this proved far
more fruitful than a mere mathematical tidying-up exercise, leading to
the discovery of universal quantum Hamiltonians [Science, 351:6 278,
p.1180, 2016], [Proc. Natl. Acad. Sci. 115:38 p.9497, 2018]. And I will
explain how this new Hamiltonian simulation formalism, together with
hyperbolic Coxeter groups, allowed us to extend the toy models of AdS/CFT
to encompass energy scales, dynamics, and even (toy models of) black hole
formation [arXiv:1810.08992].

A dark matter candidate lighter than about 30 eV exhibits wave behavior in a typical galactic environment. Examples include the QCD axion as well as other axion-like-particles. We review the particle physics motivations, and discuss experimental and observational implications of the wave dynamics, including interference substructures, vortices, soliton condensation and black hole hair.