Abstracts Gravity 2023

We consider quantum field theory with self-interactions in various patches of Minkowski and de Sitter spacetimes. Namely, in Minkowski spacetime we consider separately the right (left) Rindler wedge, past wedge, and future wedge. In de Sitter spacetime we consider expanding the Poincaré patch, the static patch, the contracting Poincaré patch, and the global de Sitter itself. In all cases we restrict our considerations to the isometry invariant states leading to maximally analytic propagators. We prove that loop corrections in the right (left) Rindler wedge, in the past wedge (of Minkowski spacetime), in the static patch and in the expanding Poincaré patch (of de Sitter spacetime) respect the isometries of the corresponding symmetric spacetimes. All these facts are related to the causality and analyticity properties of the propagators for the states that we consider. At the same time in the future wedge, in the contracting Poincaré patch and in the global de Sitter spacetime infrared effects violate the isometries.

It is suggested that the late time exponential expansion of our Universe is caused by the universe being bombarded by baby universes. This leads to a modified Friedmann equation that is in agreement with late-time cosmological observations without invoking a cosmological constant.

It is well known that the limit eigenvalue distribution of the scaled standard Wigner matrix is the semi-circular distribution whose $2k$th moment equals the number of non-crossing pair-partitions of $\{1,2,\ldots ,2k\}$. There are several extensions of this result in the literature, including the sparse case. We discuss extension of these results by relaxing significantly the i.i.d. assumption.
The limiting spectral distribution then involve a larger class of partitions. In the process we show how some new sets of partitions gain importance. Several existing and new results for their band and sparse versions, as well as for matrices with continuous and discretevariance profile follow as special cases.
Patterned random matrices such as the reverse circulant, the symmetric circulant, the Toeplitz and the Hankel matrices and their almost sure limiting spectral distribution (LSD), have also been studied quite extensively. Under the assumption that the entries are taken from an i.i.d. sequence with finite variance, the LSD are tied together by a common thread \textemdash{}the $2k$th moment of the limit equals a weighted sum over different types of pair-partitions of the set $\{1,2,\ldots ,2k\}$ and are universal. Some results are also known for the sparse case. Time permitting we show suitable extension of these results also, along the lines of the Wigner matrix

To be communicated.

Phantom fields have been widely invoked as a source of dark energy in cosmology, but rarely taken seriously as quantum theories. The vacuum is automatically unstable to production of negative energy ghost particles plus ordinary particles, requiring such theories to be effective only, below some UV cutoff. I will present recent cosmological constraints arising from the vacuum instability, both at the level of the homogeneous background, and the density perturbations.

In this talk I will discuss how a four-dimensional de Sitter space may be constructed as a Glauber-Sudarshan state, i.e as a coherent state, in type IIB string theory.

Superinsulators are a new state of matter, which we predicted, and which has been experimentally observed on the insulating side of the superconductor-insulator transition in superconducting films. They are the S-dual of superconductors, with infinite resistance below a critical temperature. In the superinsulating state, Cooper pairs and Cooper holes are confined into neutral electric pions by electric strings, with the Cooper pairs playing the role of quarks. The underlying mechanism of superinsulation is, thus, Polyakov’s linear confinement of Cooper pairs anti-Copper pairs via instantons in 2d and monopoles in 3d. We present direct experimental evidences of this confining mechanism realised in thin superconducting films.

The dynamics of a binary system moving in the background of a supermassive black hole is affected by tidal forces. For the Kerr black hole, I will derive in full generality the electric and magnetic tidal moments at quadrupole order. I will then make use of these moments in the scenario of a hierarchical triple system made of a Kerr black hole and a binary black hole system. I will describe how the secular dynamics of the binary system is distorted by the presence of tidal forces from a much larger Kerr black hole. The treatment includes strong gravitational effects beyond the post-Newtonian approximation for the tidal forces since the binary system is allowed to be close to the event horizon of the Kerr black hole. I will discuss how tidal forces affect the eccentricity of the binary system and its merger time.

We discuss a class of integrable holographic defect conformal field theories where the defect is generated by a brane intersection, and can be described as a boundary state in the form of a matrix product state. Integrable defects include domain walls and monopoles.

In a project with Gordon Semenoff on QCD in 1+1 dimensions many years ago (when he was my postdoc advisor), we stumbled over a method to solve Calogero-Moser-Sutherland models using gauge theories (I did not know these models at the time). Since then, these models have reappeared in different forms in many of my research projects. I plan to describe a recent such project where a second quantization of the elliptic Calogero-Sutherland model led us to a new soliton equation and a non-relativistic variant of the Coleman correspondence. (Work with Bjorn Berntson and Jonatan Lenells.)

Einstein gravity action has been describing cosmology and other phenomena quite successfully while it consists of the quite simple leading term. Recent observation data available through precise measurements suggests possibility to challenge the standard cosmology model. Among the possible candidate theories beyond the Einstein gravity theory, we will focus on the Gauss-Bonnet term, which is one of the simplest extension with the higher curvature. The properties of this theory will be explained by looking at the Black Holes, which show some characteristic differences compared with those of the Einstein gravity. We also mention the implication to the cosmology.

We evaluate the four-closed-string scattering amplitude, using the Polyakov string path integral in the proper-time gauge. By identifying the Fock space representation of the four-closed-string-vertex, we obtain a field theoretic expression of the closed string scattering amplitudes. In the zero-slope limit, the four-closed-string scattering amplitude reduces to the four-graviton-scattering amplitude of Einstein’s gravity. However, at a finite slope, the four-graviton scattering amplitude in the proper-time gauge differs not only from that of Einstein gravity, but also significantly differs from the conventional one obtained by using the vertex operator technique in string theory.

At the present time, there are measurements of a number of observables in processes described by the quark-level transitions b –> s µ+ µ- and b –> c τ νbar_τ that disagree with the predictions of the standard model. A great deal of work has gone into exploring the new-physics (NP) explanations of these semileptonic B anomalies. Now, there are also discrepancies involving hadronic B decays, but because QCD is involved, much less attention has been paid to these hadronic B anomalies. Still, on quite general grounds, one expects that, if NP is present, it will affect both types of B decays. My group is looking into NP connections between the semileptonic and hadronic B anomalies. This is work in progress, and I will present a status report.

The Su-Schrieffer-Heeger model is perhaps the simplest model with edge stages, states that are an essential aspect of materials known as topological insulators. In this talk, I will briefly describe the SSH model and will discuss edge states in a finite SSH chain in isolation and a chain with leads attached to the ends. The effect of the leads on the details of the states and in particular on the energy spectrum will be discussed.

I consider string anomalies which emerge from higher terms of the DeWitt-Seeley expansion of the heat kernel as eps x 1/eps with eps being a UV cutoff. I show they give a nontrivial contribution to the central charge, telling the Nambu-Goto and Polyakov strings apart. I compute the string susceptibility of the Nambu-Goto string at one loop.

Motivated by holographic complexity, we examine a new class of gravitational observables in asymptotically AdS space associated with codimension-one slices or with codimension-zero regions. We argue that any of these observables is an equally viable candidate as the extremal volume for a gravitational dual of complexity.

It has been pointed out that double holography seemingly allows for superluminal signalling. In this talk, I will argue that as long as the brane description of double holography is treated as an effective theory, causality violations are not visible above the associated cutoff length scale. This suggests that end-of-the-world brane models are consistent with causality and that the apparent superluminal signalling is a UV effect.

I will discuss several criteria which define regions outside which time evolution within effective theory is supposed to break down and compare them. All criteria exclude regions which could naively be affected by superluminal signalling. While the conditions agree in two dimensions, in higher dimensions they disagree, posing a question about the correction description of the causal structure on the brane.

Cats have an instinctive ability to use the connection governing parallel transport in the space of shapes to land safely on their feet. Here we argue that the concept of connection that is extensively used in general relativity and other parts of theoretical physics, also explains the impressive performance of molecular motors by enabling molecules to evade conclusions of Feynman’s ratchet- and-pawl analysis. We first demonstrate, using simple molecular models, how directed rotational motion can emerge from shape changes even without angular momentum. We then computationally design knotted polyalanine molecules and show how their shape space holonomy connection organizes individual atom thermal vibrations into collective rotational motion, independently of angular momentum. Our simulations show that rotational motion arises effortlessly even in am- bient water, making the molecule an effective theory time crystal. Our findings have potential for practical molecular motor design and engineering and can be verified through high-precision nuclear magnetic resonance measurements.

The large-N limit of the d=1+1 sigma model with an SU(N)-valued field cannot be solved by conventional field-theoretic methods. This principal chiral model (PCM) is asymptotically free and believed to have a mass gap. Exploiting the model’s integrability and conjectured S matrix, exact form factors were discovered, determining, in principle, all Green’s functions. Not very long ago, an expression for the exact two-point function of the scaling field was found. The exact asymptotic infrared and ultraviolet behavior can be determined, in striking agreement with the renomalization group result. Some new investigations of operator product expansions (OPE) for currents are presented. The goal is to find the quantum equation of motion: which would finally establish that this axiomatic field theory approach genuinely solves the SU(ꚙ) PCM.

The Schwarzschild-De Sitter solution admits two parameters, the mass and the cosmological constant, both of which can be positive or negative. Indeed, stable configurations can be found that correspond to bubbles of negative mass [1–3], however crucially, in a background energy density. It seems that positive mass attracts while negative mass repels when acting on all masses, positive or negative, due to the equivalence principle [4]. However, a simple analysis of one graviton exchange implies that the potential between like mass particles should be negative, be they of positive or negative mass, implying naively attraction. We explain how this conclusion is eschewed and does not give rise to attraction for negative mass particles. We will see that the existence of the background is crucial.

What is the gravity that string theory predicts? Alternative to the conventional answer, i.e. General Relativity, I will discuss Double Field Theory as the gravity of string theory which gravitise and geometrise the whole massless sector of closed strings.

I will review the chiral anomalous effects in several types of chiral plasmas, ranging from electron fluids in topological semimetals to magnetospheres of magnetars. Generically, such plasmas are made of (quasi-)relativistic chiral particles. The low-energy collective dynamics in such plasmas become anomalous, with the chiral charge density playing an active role. The signature effects include anomalous transport in semimetals and chiral plasma instabilities in pulsars. The former may have interesting applications in technology, and the latter may explain the engine powering fast radio bursts.

Sometime ago, a new kind of gap was discovered in P doped Si at temperature below 100mK. It is known that tiny fraction of P gets magnetic moment. There are two bottle necks in approaching the problem. One is strong Kondo coupling at low temperature and the other is randomness of the impurity distribution. We explain this gap by Kondo condensation and calculation of the fermion spectral function by holographic method.

The Correlated Worldline (CWL) theory of quantum gravity is a low-energy theory (ie., for energies << M_P) in which quantum mechanics breaks down for sufficiently massive objects because of gravitational correlations between paths in a path integral. It has been shown to be consistent (ie., it obeys all Ward identities, has a sensible classical limit, and well-defined expansions in h and G, and has no unphysical sectors).

I will explain the basic structure and physical assumptions involved, and then discuss the implications for several optomechanical experiments being planned, involving massive mirrors. ”

To realize universal quantum computation, an arbitrary operation in U(N) will have to be approximated with arbitrary precision by a sequence of operations from a finite set of gates. The gates in the Clifford group are amenable for quantum error correction, but they are not sufficient for universal computation, as the Clifford group is indeed a finite group. Using so-called teleportation and single cubit operations, the Clifford group can be extended to the Clifford hierarchy, which becomes dense in U(N) with increasing hierarchy level, maintaining the amenability to error correction. In this presentation, I discuss the structure of the third level in the Clifford hierarchy.

I will describe an implementation of Wheeler’s “it from bit” programme by formulating a purely combinatorial version of quantum gravity in terms of graph theory, without a priori reference to concepts such as “space” or “time”. I will show that a combinatorial version of the Einstein-Hilbert action drives a continuous phase transition from a random graph phase to a geometric phase in which graphs become discretizations of hyperbolic space. Matter is made of space and appears as random graph defects which diffuse in the hyperbolic geometry. I will show that Brownian motion in negative curvature is asympotitcally equivalent to free fall in positive curvature: coordinate time spontaneously emerges from the diffusion equations. Accordingly, I propose that de Sitter space-time is only an effective description of a fundamental universe with constant negative curvature. The local limit theorem in negative curvature implies that, in this effective description, the subdominant, random component of diffusion looks like quantum behaviour in an Euclidean space-time of spectral dimension 3+1, independently of the underlying topological dimension.

In this talk, I describe a number of approaches to describing cosmological physics using a microscopic quantum gravity model via the tools of holography.

Recent advances in the study of the entanglement entropy of free fermion systems will be reviewed.

Ultra-light bosons make excellent candidates for dark matter.
Axions, relaxions, and ultra light spin 1 Proca states are well-known examples. Such degrees of freedom can form gravitationally bound states known as boson stars during structure formation, and interactions (gravitational, self and mutual interactions) can modify the macroscopic properties of these objects. In this talk we review the recent efforts at studying these boson stars for defining parameters ranging from those related to QCD axions with particle masses of $10^{–5}$ electron volts to ultra-light axions with masses in the range of $10^{-22}$ electron volts. Ultra-light axions can form boson condensates the size of galactic centers while QCD axions can create condensates the size of asteroids. We explore how boson stars the size of entire galaxies can arise in multi component condensates that include several flavors of axions or multiple states of the same flavor. We explore their structural stability against collapse and possible decay due to particle number violating processes.

’t Hooft loops are disorder operators related to quantized magnetic monopoles that exist in any gauge theory. I will discuss a particular supersymmetric setup where an interplay of holography, S-duality, localization and integrability results in a coherent non-perturbative description.

In this talk, I will report some recent application of non-relativistic conformal theory in many-body quantum dynamics. Non-equilibrium quantum dynamics with conformal symmetries are one of few rare cases where dynamics are fully reversible (say without entropy production). In 3D, they can occur at fine-tuned scale symmetric atomic Feshbach resonance. Here I will discuss possible emergent infrared conformal dynamics in 1D even when interactions explicitly break the scale symmetry, and connections to the one dimensional integrability.