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Programme and abstracts for EGIV

Emergent Gravity IV

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Program

The conference is organized as follows. There are 7 sessions distributed over the week, see table below. Every session is followed by a longer discussion session. Unless otherwise indicated, all conference events will occur in room 182 of the Ike Barber Building.

    Quantum Information and Gravity [ Monday moring ]
    Networks [ Monday afternoon ]
    Condensed matter and AdS/ CFT Dualities [ Tuesday morning ]
    Horava-Lifshitz-Gravity [ Tuesday afternoon ]
    Quantum field theory and Gravity [ Wednesday morning ]
    Gravity Analogues/Emergent spacetimes [ Thursday all day ]
    Quantum Gravity [ Friday morning ]

 

On Thursday we will have three laboratory tours during which we will be presenting the ongoing black hole experiments at UBC. We have to devide the participants into smaller groups due to spatial restrictions in our laboratory; there will be three tours, each lasting 30 minutes. The remaining participants can enjoy an extended coffee break (from 11am till 12:30pm) at the physics department (Hennings Building, next to the conference venue) combined with a poster session.

Besides plenty of coffee breaks during the day we will offer breakfast outside the lecture theatre (Room 182 of the Ike Barber Learning Centre) from 8am till 9am throughout the week. The conference dinner will be held on Thursday evening. It is free and every participant is invited to join us! The conference will conclude with a Pub lunch on Friday. To sign up for the conference dinner, lab tours and pub lunch please see sign-up sheets at the entrance of the lecture theatre.

  Monday,
24th August 2009		 Tuesday,
25th August 2009		 Wednesday,
26th August 2009		 Thursday,
27th August 2009		 Friday,
28th August 2009
8:00 Breakfast at the conference venue

"Continental"

 Breakfast at the conference venue

"Healthy Start"

 Breakfast at the conference venue

"Deluxe"

 Breakfast at the conference venue

"Healthy Start"

 Breakfast at the conference venue

"Continental"
8:15
8:30
8:45
9:00 Kempf Affleck Arzano Schuetzhold Bojowald
9:15
9:30
9:45 Vidal Van Raamsdonk Oriti Unruh Dittrich
10:00
10:15
10:30 Coffee and cake Coffee and cake Coffee and cake Lawrence Coffee and cake
10:45
11:00 Raussendorf Semenoff Chicro Coffee and cake:
Lab tours by
Penrice
Poster session
at Hennings 318:
Foreman,Gjoni,
Gooding,Mazur		 Markopoulou
11:15
11:30 Piazza Hu
11:45 Discussion/Chair:
Schuetzhold		 Discussion/Chair:
Loll
12:00 Urban Dreyer
12:15
12:30 Lunchtime break Lunchtime break Discussion/Chair:
Visser		 Lunchtime break Discussion/Chair:
Oriti
12:45
13:00
13:15   Lunch at Mahoney and Sons Pub
Sign up!
13:30
13:45
14:00 Loll Visser Wen
14:15
14:30 Uhlmann
14:45
15:00 Conrady Brandenberger Jain  
15:15
15:30 Coffee and cake Coffee and cake Coffee and cake
15:45
16:00 Bilson-Thomson Saffin Philbin
16:15
16:30 Rideout Mei Richartz
16:45
17:00 Discussion/Chair:
Markopoulou		 Discussion/Chair:
Unruh		 Discussion/Chair:
Weinfurtner
17:15
17:30
17:45
18:00      
18:15
18:30
18:45
19:00
19:15
19:30 Dinner
Sign up!
 
 
23:00

 

Abstracts

 

Affleck
Download presentation		 Bosonization in (1+1) dimensional condensed matter physics: its successes and limitations
Bosonization provides an exact mapping of certain (1+1) dimensional field theories of interacting fermions into non-interacting bosons. It is very useful for studying certain one-dimensional condensed matter systems which exhibit an approximate Lorentz invariance. However, terms in the condensed matter Hamiltonian which break Lorentz invariance, even non-interacting terms, lead to interactions in the bosonized model which are very challenging to treat. I will discuss recent attempts to overcome this difficulty.
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Arzano
Download presentation		 Fun from none: deformed Fock space and hidden entanglement
Attempts to go beyond the framework of local quantum field theory include scenarios in which the action of external symmetries on the quantum fields Hilbert space is deformed. I will discuss how such deformed symmetries, which became very popular as proposals to introduce an observer independent energy scale in relativistic theories, can be understood as emerging from inequivalent quantizations of the corresponding classical field theory. A common feature of these models is that the quantum group symmetry of their Hilbert spaces induces additional structure in the multiparticle states which in turns reflects a non-trivial momentum-dependent statistics. The richer structure of the deformed Fock space allows for the possibility of entanglement between the field modes and, in particular, when the deformation scale is proportional to the Planck energy, it leads to a "planckian" mode-entanglement invisible to an observer that cannot probe the Planck scale.
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Bilson--Thompson
Download presentation		 What a tangled web weaves us
I shall discuss the prospect that randomly generated networks, such as those which occur in certain theories of quantum gravity, automatically lead to the emergence of matter and $ 3+1 $ dimensional spacetime.
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Bojowald
Download presentation		 Effective dynamics of loop quantum gravity
Loop quantum gravity provides a discrete formulation of space-time in terms of elementary building blocks. This talk will discuss means to approach the semiclassical limit and the role of discreteness in corrections to the classical behavior.
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Brandenberger
Download presentation		 Challenges for Horava-Lifshitz gravity
I will introduce Horava-Lifshitz gravity, a model in which general covariance and Lorentz invariance are emergent. Turning this model into a viable theory of quantum gravity with the correct Newtonian limit is, however, facing serious challenges which will be discussed.
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Chirco
Download presentation		 Non-equilibrium Thermodynamics of Spacetime: the Role of Gravitational Fluxes.
It has been shown that from the assumed proportionality of entropy and horizon area for all local acceleration horizon, the Einstein equation can be derived as a local constitutive equation for an equilibrium spacetime thermodynamics. However, more recentltly, the same approach has been extended to an f(R) modified gravity theory, showing that a curvature correction to the entropy that is polynomial in the Ricci scalar leads to a non-equilibrium thermodynamical setting. Here we argue that the non-equilibrium thermodynamical setting is the correct one even for classical GR. We show that the dissipative character leading to a non-equilibrium spacetime thermodynamics is actually related to the local fluxes of gravitational radiation, hence unavoidable. These fluxes are generally associated to the purely gravitational degrees of freedom of the theory which are fixed by the kinematics of the local spacetime causal structure through the Equivalence Principle. In this sense, the thermodynamic description seems to go beyond Einstein's theory as an intrinsic property of gravitation.
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Conrady
Download presentation		 Simplicial complexes from condensation
I will talk about work in progress on a statistical model of graphs. The configurations are arbitrary graphs with a fixed number of links. The Hamiltonian is a simple function of the graph that favors the formation of 2d simplicial complexes. Monte Carlo simulations show that at low temperatures the system is in an ordered phase, where the links condense to 2d simplicial complexes. At high temperatures one has a disordered phase and the links form graphs of high connectivity.
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Dittrich
Download presentation		 The fate of diffeomorphism symmetry in quantum gravity models
We will discuss notions of diffeomorphism symmetry in (discrete) quantum gravity models. Different scenarios are possible, one is that diffeomorphism symmetry is fundamentally broken and only emerges at a large scale limit another is that quantum gravity is diffeomorphism invariant at all scales. We will try to classify current approaches according to these scenarios and mention some consequences for the quantum gravity practician.
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Dreyer
Download presentation		 Internal Relativity
We will discuss a new approach to the problem of quantum gravity and its application to cosmology. In this new approach to quantum gravity the metric is not given a priori but instead emerges at low energies. Because the metric is inferred from the behavior of the low energy particles one can discuss the question of what the remnants of a phase transition are that produced these particles. We argue that the observed spectrum of the microwave background radiation could be a explained as a result of such a transition.
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Foreman The Casimir Effect in Layered Superconductors
Cuprate (copper-based) high-temperature superconductors are ceramics and are therefore poor conductors above the transition temperature Tc. Below Tc, they exhibit superconductivity in copper-oxide layers that are separated on the nanometer scale by material that remains poorly conducting. An open question is the origin of the superconducting condensation energy in the cuprates (and other high-temperature superconductors, many of which share a similar layered structure). Here, we ask to what extent the Casimir energy that arises as the temperature is lowered below Tc could contribute to the condensation energy. To this end, the material is modeled as consisting below Tc of parallel plasma sheets separated by vacuum, and as possessing no Casimir effect above Tc. Due to the close proximity of the superconducting planes, the system is in the regime where the Casimir effect becomes a van der Waals type effect, dominated by contributions from internal TM surface plasmons that propagate along the ab planes. Within this crude model, the Casimir energy is found to be of the same order of magnitude as the superconducting condensation energy.
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Gjoni Some Considerations on String Theory
As it is known, string theory is the arena to solve the apparent discrepancy between general relativity and quantum coherence. Progress has been done to identify the quantum microstates that are counted by the entropy of a bh macro state. Indeed, a weak coupling regime of strings, where states can be identified and counted, is dually connected with a strong coupling regime described by super gravity and where bh are expected. The consistent picture described still leaves unclear the nature of the microstates in the bh regime which are dually connected with those identified and counted in the string regime. They cannot, of course, be themselves identified with bh objects having their event horizon and thus an entropy that microstates have not. In order to clarify this issue, there have been recent attempts to extend the analysis by discussing "regular bh" microstates, with geometries that would resemble the bh ones beyond a certain radius but having no metric singularity nor event horizon. They should somehow give rise, in a classical limit, to bona fide black holes. It is yet unclear if super gravity metric configurations are sufficient to describe fuzzball sates or if it is necessary to go beyond the metric description of string states to reliably identify and distinguish the microstates in this bh regime Despite these hints, I find that it is still far from trivial to understand how an average procedure over microstates with regular metric may give rise to a macro state with singularity and event horizon. To my point of view string theory indicates that the solution of the paradox lies in its basic quantum nature due to which the very concept of space time, its metric and thus general relativity appear only in a classical limit.
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Gooding Five-dimensional black hole capture cross sections
I will present a study completed with Andrei Frolov (SFU) of scattering and capture of particles by a rotating black hole in the five-dimensional spacetime described by the Myers-Perry metric. The equations of geodesic motion are integrable, and allow us to calculate capture conditions for a free particle sent towards a black hole from infinity. We introduce a three-dimensional impact parameter describing asymptotic initial conditions in the scattering problem for a given initial velocity. The capture surface in impact parameter space is a sphere for a nonrotating black hole, and is deformed for a rotating black hole. We obtain asymptotic expressions that describe such deformations for small rotational parameters, and use numerical calculations to investigate the arbitrary rotation case, which allows us to visualize the capture surface as extremal rotation is approached.
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Hu
Download presentation		 Ways and Means for Emergence
In Leiden (2007) I introduced a new conceptual framework and listed the radically different tasks we face in viewing gravity as emergent. At MIT (2008) I talked about some key issues of emergent vs quantum gravity, such as nonlocality and stochasticity placed in the context of strongly correlated many-body systems and analyzed using nonequilibrium concepts and methods. In this talk I will dwell on the methods and processes useful for addressing emergent phenomena in physics with examples relevant to gravitation and cosmology. First is spacetime as condensate -- how to decipher the properties of the `atoms of spacetime' from the noise the condensate feels, in analog to BEC dynamics with fluctuations via the stochastic Gross-Pitavesky equation. The second is macroscopic quantum phenonena viewed more from the statistical mechanics light: Why our habitual linkage of macroscopic with classical could be too limiting or even misleading, as the IR-UV behavior is more subtle than a simplistic micro/quantum -macro/classical division. I shall illustrate this with the nPI effective action and large N expansion techniques applied to entropy generation and quantum-classical correspondence.
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Jain
Download presentation		 Numerical experiments in emergent spacetimes
Emergent spacetimes in fluid-type systems present one possibility for observing and testing the predictions of curved spacetime quantum field theory. We present the results of numerical simulations for expanding spacetime models in Bose--Einstein condensates - ultracold quantum gases with a low energy fluid description. We further discuss the evidence of quasiparticle production in such systems as an analogue of cosmological particle production, referring to both past and current work.
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Kempf
Download presentation
Download paper		 Spacetime could be simultaneously discrete and continuous, the same way that information can
At the heart of information theory is the discipline called Shannon sampling theory, which establishes the equivalence of continuous and discrete representations of information. Sampling theory has recently been generalized and applied to physical fields on curved spacetimes. Here, we show how sampling theory can be generalized and applied to spacetime itself, yielding the equivalence of continuous and lattice representations of spacetimes that possess a suitable natural ultraviolet cutoff. The Einstein action is induced. The resulting methods could be useful in a approaches approaches to quantum gravity.
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Lawrence Water channel experiments and black-hole physics
Water channel experiments have been conducted to test the notion of Schutzold and Unruh (2002) that water waves in a flowing fluid can be used to simulate phenomena around black and white (dumb) holes. A streamlined 10 cm high obstacle is placed into uniform flow down a 10 m long 0.15 m wide channel. The volumetric flow rate and the depth of flow are adjusted until the following hydraulic conditions are achieved. Upstream of the obstacle the flow is subcritical (subluminal) with respect to shallow-water (long) waves. The flow becomes critical at the crest of the obstacle and slightly supercritical (superluminal) just downstream of it. This transition is analogous to a black hole. A very weak undular jump (a relatively smooth transition from supercritical to subcritical flow) appears over the lee face of the obstacle. This transition is analogous to a white, or dumb, hole. The flow remains subcritical to the end of the channel. A packet of shallow-water waves are generated downstream of the obstacle. Given that the flow is subcritical they travel upstream. However, as they travel up the lee face of the sill they slow down both because the flow velocity in the channel increases and because their phase speed will decrease as they shoal (move into shallow water). As their wavelength decreases so too does their group velocity, eventually the packet of waves is arrested (it cannot enter the white hole), and then begins to move downstream, even though the phase velocity of individual waves is still upstream.
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Loll
Download presentation		 Self-Organized Quantum Spacetime and the Role of Time
A novel approach to the quantization of gravity, using so-called "Causal Dynamical Triangulations (CDT)", throws new light on the physical origins of spacetime. It describes the seemingly structureless, empty spacetime we see around us and its cosmological "shape" - that of an exponentially expanding universe - as resulting from the complex, dynamical interplay of an enormous number of microscopic quantum fluctuations. The underlying mechanisms of "emergence" and "self-organization" are familiar from the theory of complex systems, but had so far not found a concrete application in high-energy physics. I will summarize the basic ingredients and construction method underlying this approach, and some of its achievements and outstanding challenges. Specifically, I will comment on what it teaches us about the role of time, its non-emergence, and the relation of the proper time present in CDT quantum gravity with stochastic time recently uncovered in two spacetime dimensions.
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Markopoulou
Download presentation		 Network-based toy models for gravity
A number of different approaches to quantum gravity recently proposed are based on the idea that spacetime geometry and gravity are derivative concepts and only apply at an approximate level. Two fundamental challenges to any such approach are, at the conceptual level, the role of time in the emergent context and, technically, the fact that the lack of a spacetime makes difficult the straightforward application of well-known methods of statistical physics and quantum field theory to the problem. We study these using microscopic models for emergent space and gravity that are based on network evolution in which no a priori geometric notions are present. In this talk we present two models. In the first model, we are able to use methods from quantum information theory to derive features such as a variable speed of light from a non-geometric quantum system. The second model is a toy system for an interesting non-linear matter-geometry interaction in which geometry is defined by the behavior of matter.
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Mazur Entanglement entropy from non-linear scalar interactions during inflation
Using a simple model that explicitly follows the production of particles during inflation, we simulate the production of entanglement entropy in a cosmological scalar field during inflation due to self-interactions of the field. We find that the amount of entanglement entropy generated scales roughly as a power law S \propto \lambda^{1.75}, where \lambda is the coupling coefficient of the non-linear potential term. We also investigate how the entanglement entropy scales with the duration of inflation and compare various entanglement measures from the literature with the von Neumann entropy. Decoherence is often invoked to explain the classical nature of density distributions today, but we find that the decoherence generated through non-linear interactions is not sufficient to explain the classicality of the density fluctuations.
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Mei
Download presentation		 Solutions to Horava Gravity
Ho\v rava-Lifshitz gravity posed as a possibly much simpler alternative to other huge projects for quantum gravity, such as the string theory. Studying its classical solutions is one of the most straightforward ways to test if Einstein gravity can be recovered from the theory in the IR limit. We will present some of the results from this perspective.
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Oriti
Download presentation		 On the emergence of spacetime and matter in the Group Field Theory approach to Quantum Gravity
This talk is divided in two parts. In the first part, we introduce the Group Field Theory formalism for Quantum Gravity, and we present the arguments for considering it a promising candidate description of the microstructure of space. We also recapitulate the general strategy for investigating the emergence of a continuum spacetime from the GFT microstructures. In the second part we present the results of some work in progress showing how an effective field theory of matter on a non-commutative flat spacetime (of the ``Deformed Special relativity'' type) emerges in fact from a simple GFT, with a procedure similar to that leading to effective field theories for quasi-particles on an emergent geometry in condensed matter systems.
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Penrice Black hole experiments laboratory tour
A short 15-20 minute tour of our experimental facility. The tour will compose of a short description of the experimental setup and will then be followed by a demonstration. The demonstration will include the creation of an effective black hole and white hole horizon. Waves will then be sent into the white hole horizon where we believe that they are getting up converted and then dragged out of the system by the flow rate. This will be followed by the presentation of some preliminary results.
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Philbin
Download presentation		 Experimental demonstrations of horizons in fluid mechanics and optics
The lecture discusses the lessons learned from some of our demonstrations of analogues of the event horizon. In one experiment we studied water waves in counter-flows and in another light propagation in photonic-crystal fibres influenced by ultrashort light pulses. In both experiments, we created white-hole horizons. As white holes are time-reversed black holes, their laboratory analogues exhibit essentially the same physics. In the water-wave experiment, we observed negative-frequency waves in a regime where a horizon for surface waves is not established. In the optical experiment, we measured the frequency shifting of light at the group-velocity horizon. We have not observed Hawking radiation yet, but we are working on a modification of the experiment that will allow us to do so. In both the water-wave and the optical experiment, negative-frequency waves occur in strongly-dispersive and non-stationary regimes, cases that have been largely neglected by theory until recently.
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Piazza
Download presentation		 The IR-completion of Gravity: What happens at Hubble scales?
I will give account of a recent work in which I attempt to modify the metric-manifold structure of GR in the infra-red. The proposed modification does not contain any adjustable parameter as it is effective at length scales comparable with the inverse (extrinsic) curvature. The guiding line for this modification is a recently proposed "ultra-strong" version of the equivalence principle, according to which the vacuum expectation value of the (bare) energy momentum tensor is exactly the same as in flat space: constant everywhere and quartically divergent with the cut-off. Some cosmological consequences of this modification will be discussed.
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Raussendorf Measurement-based quantum computation with cluster states
I give an introduction to measurement based quantum computation with cluster states (MQC) [1], state the main open problems and point out where, in my opinion, overlaps with quantum gravity could arise. Cluster state quantum computation is not driven by unitary evolution but rather by projective local measurements. The scheme uses a highly entangled multi-qubit cluster state as resource for universal computation. As the computation proceeds the entanglement in the resource state is progressively destroyed. Any quantum algorithm can be implemented on a sufficiently large cluster state, by appropriate choice of the local measurement bases. The result of the computation is revealed in correlations of the individually random measurement outcomes.
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Richartz
Download presentation		 Universal superradiance
We analyse the necessary conditions for the occurance of superradiance. Starting with a very general wave equation, we show that the existence of an ergoregion is not enough; a mechanism of energy extraction (wave propagation through an event horizon for example) is also crucial. Several examples are discussed, including real and analogue rotating black holes, rapidly rotating stars and fluids with vorticity. The role of dynamical instabilities is also discussed.
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Rideout
Download presentation		 deSitter Space from Causal Set Dynamics
We present evidence that the early universe of sequential growth dynamics for causal sets begins with a phase of super-exponential expansion, followed by a deSitter-like period of expansion.
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Saffin
Download presentation		 Strong coupling issues in Horava gravity
The recent intriguing proposal of Horava that quantum gravity may be regulated at high energies by violating Lorentz symmetry raises some issues about the resoration of this symmetry at low scales. We shall discuss some work that shows the existence of an extra scalar degree of freedom that, at low energies, becomes strongly coupled, causing problems for the emergence of general relativity at low energies.
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Schuetzhold
Download presentation		 Analogue gravity
This talk reviews some qualitative and quantitative analogies between gravity and condensed matter including analogues for black/white holes as well as the expanding/contracting cosmos. By studying these analogies, we might gain a deeper understanding of the effects of (quantum) gravity such as Hawking radiation.
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Semenoff Holographic gauge theory
The most well developed application of AdS/CFT duality is to the study strongly coupled gauge field theories. In this endeavor, string theory is used in a regime where it is approximated by classical (super-)gravity and where computations in classical gravity yield information about the quantum regime of the gauge theory. In this talk, I will discuss some of the reasoning behind this duality as well as review some recent results.
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Uhlmann
Download presentation		 On the simulation of expanding spacetimes in trapped Bose-Einstein condensates
The prospect of simulating cosmic quantum effects in laboratory systems has gained much attention in recent years. Low-energy phase fluctuations (phonons) in expanding Bose-Einstein condensates for instance obey the same evolution equations as a massless scalar field in a flat Friedman-Robertson-Walker-Lemaitre spacetime, which is believed to provide an adequate description of the universe during cosmic inflation. Hence, different features of these expanding spacetimes can be identified such as the formation of an effective sonic horizon as well as freezing and amplification of phonon fluctuations. In this talk, I will address the impact of a (harmonic) trap potential on their evolution and discuss the implications for an actual experiment.
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Urban The cosmological constant from the Veneziano ghost
We suggest that the solution to the cosmological vacuum energy puzzle is linked to the infrared sector of the effective theory of gravity interacting with standard model fields, with QCD fields specifically. We work in the framework of low energy quantum gravity as an effective field theory. In particular, we compute the vacuum energy in terms of QCD parameters and the Hubble constant $ H $ such that the vacuum energy is $ \epsilon_{vac} \sim H \cdot m_q\langle\bar{q}q\rangle /m_{\eta'} \sim (3.6\cdot 10^{-3} \text{eV})^4 $, which is amazingly close to the observed value today. The QCD ghost (responsible for the solution of the $ U(1)_A $ problem) plays a crucial r\^ole in the computation of the vacuum energy, because the ghost properties at very large but finite distances slightly deviate (as $ \sim H / \Lambda_{\mathrm{QCD}} $) from their infinite volume Minkowski values. Another important prediction of this framework states that the vacuum energy owes its existence to the asymmetry of the cosmos. Indeed, this effect is a direct consequence of the embedding of our Universe on a non-trivial manifold such as a torus with (slightly) different linear sizes. Such a violation of cosmological isotropy is apparently indeed supported by WMAP, and will be confirmed (or ruled out) by future PLANCK data.
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Unruh Dumb holes and water slides
This talk will review some of the features of dumb holes, with particular emphasis on the behaviour of waves in a flume tank (the experimental situation being carried out here). In particular the alteration of the dispersion relation due to varying depth and velocity of the fluid will be examined, together with the various horizon phenomena expected.
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Raamsdonk Comments on quantum gravity and entanglement
In this talk, we attempt to provide some insights into the structure of non-perturbative descriptions of quantum gravity using known examples of gauge-theory / gravity duality in string theory. We argue that in familiar examples, a quantum description of spacetime can be associated with a manifold-like structure in which particular patches of spacetime are associated with states or density matrices in specific quantum systems. We argue that quantum entanglement between microscopic degrees of freedom plays an essential role in the emergence of a dual spacetime from the nonperturbative degrees of freedom.
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Vidal
Download presentation		 Spin networks, entanglement and the simulation of strongly interacting systems on a lattice.
A Tensor Network (TN) exploits the structure of entanglement in strongly interacting systems on a lattice to provide an efficient representation of its ground state. TN algorithms are becoming increasingly popular, thanks to their ability to simulate systems that cannot be addressed with quantum Monte Carlo techniques, such as frustrated antiferromagnets and interacting spins on a 2D lattice. In my talk I will explain how, in the presence of a symmetry, a TN reduces to a linear superposition of exponentially many Spin Networks (of the type you adore in loop quantum gravity). It follows that TN algorithms can be used to efficiently evaluate such exponentially large linear superpositions of Spin Networks.
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Visser Who's afraid of Lorentz symmetry breaking?
Is Lorentz symmetry truly fundamental? Or is it just an "accidental" low-momentum emergent symmetry? Opinions on this issue have undergone a radical mutation over the last few years. Historically, Lorentz symmetry was considered absolutely fundamental --- not to be trifled with --- but for a number of independent reasons the modern viewpoint is more nuanced. What are the benefits of Lorentz symmetry breaking? What can we do with it? Why should we care?
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Wen
Download presentation		 Emergence of helicity +/-2 modes (gravitons) from a local qbit model
We study the possibility of the emergence of helicity $ \pm 2 $ gapless excitations (the gravitons) from two quantum spin models. In the first quantum spin model (called the L-type model), the helicity +/-2 gapless excitations are shown to appear as the ONLY type of low energy excitations. Within a perturbative calculation, the dispersion of the gapless helicity +/-2 mode is found to be om ~ k^3. The appearance of the gapless helicity +/-2 modes suggests that the ground state of the quantum spin model is a new state of matter. In the second model (called the N-type model) the collective modes are strongly interacting and there is no reliable approach to understand its low energy dynamics. Using a spin-wave/quantum-freeze approach (which is shown to reproduce the correct emergent U(1) gauge theory in a quantum rotor model), we argue that the second model may contain helicity +/-2 gapless excitations as the only type of low energy excitations with a linear dispersion om ~ k. We believe that the gaplessness of the helicity +/-2 excitations in both models is topologically robust: any translation invariant perturbations cannot generate a gap for those helicity +/-2 excitations. Our results shed light on the quest to find a local bosonic model whose ground state supports helicity +/-2 gapless excitations with a linear dispersion. Such linearly dispersing helicity +/-2 excitations will correspond to emergent gravitons.
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