Niayesh Afshordi, Perimeter Institute

**Is aether technically natural?**

I will discuss whether higher energy Lorentz violation should be considered a natural expectation in theories of quantum gravity with a preferred frame.

Stephon Alexander, Haverford College and Pennsylvania State University

**Quantum Gravity, Inflation and Leptogenesis**

Stephon Alexander, Haverford College and Pennsylvania State University

**Quantum Gravity and the Weak Interactions**

Giovanni Amelino-Camelia, University of Rome

**Implications of Planck-scale worldline fuzziness for quasar images and GRBs**

I give a brief critical assessment of the status of tests of Planck-scale effects using GRBs and quasar. I then report the first ever results of analysis of the propagation of quantum-mechanical particles in a noncommutative spacetime, formalizing fuzziness of worldlines that grows irreducibly with propagation distances.

Giovanni Amelino-Camelia, University of Rome

**Quantum-gravity effects as noise for gravity-wave detectors**

I discuss a mechanism that can allow Planck scale effects to manifest themselves as a source of lof-frequency noise for interferometers. The mechanism requires a discrete formulation of dynamics at the Planck scale.

Ivan Arraut, Osaka University

**Dark Matter and Dark Energy as a possible manifestations of a fundamental Scale**

If we take the idea of the Planck length as a fundamental (minimum) scale and if additionally we impose the Cosmological Constant ($Lambda$) as and infrared (IR) cut-off parameter. Then it is possible to demonstrate that Dark Matter effects can emerge as a consequence of an IR-UV mix effect. This opens the possibility of unifying the Dark Energy and Dark matter effects in a single approach.

Hugo Beauchemin,Tufts University

**Experimental search for quantum gravity - the hard facts**

The LHC has been running for over 2 years and the different experiments have accumulated enough data to find a new particle which could well be the Higgs boson, achieve Standard Model measurements with precision at the percent level, and seriously constrain a large variety of new physics scenarios, giving important hints on what the physics beyond the Standard Model can be and what it cannot be. This presentation will focus on ATLAS and CMS latest searches for new physics in events featuring signatures relevant to theories of quantum gravity. Will be presented the results on searches for large extra dimensions (direct graviton emission, virtual graviton propagation, Kaluza-Klein Randall-Sundrum resonances, etc.) and other TeV-scale gravity phenomena such as black holes (rotating and non-rotating) and string balls production and decay. Results will be reported on each of the various final states investigated for each signals. The results of some searches that are not typically interpreted into bounds on quantum gravity signals, but which could provide complementary information such as the number of possible matter fields compatible with LHC data will also be presented. An attempt to present plausible future reach of the LHC will finally be briefly discussed.

Dionigi Benincasa, Imperial College, London

**Bounding causal set non-locality**

The interplay between discreteness and Lorentz invariance in causal sets leads to a radical non-locality. This manifests itself clearly in the construction of a D'Alembertian, B, on the causal set. The requirement that fluctuations in B be small has led to the introduction of a new meso-scale lk, which is the length scale at which one expects non-local physics to manifest itself. After briefly reviewing the construction of B, I will present an estimate for lk.

Julien Bolmont, Pierre and Marie Curie University

**Lorentz Invariance Violation searches with HESS**

The high flux and variability of the blazar PKS 2155-304 as observed by H.E.S.S. during the night of 28 July 2006 allowed a very high precision search for Quantum Gravity-induced energy-dependent time lags. In this talk, I will review the results published by the H.E.S.S. collaboration on this topic. In particular, I will emphasize the latest results obtained using a likelihood fit to study individual photon data. With this method and a proper error calibration procedure, a very high precision measurement was achieved leading to the best constraints on linear and quadratic terms of the dispersion relations obtained with an AGN so far.

Alfio Bonanno, INAF Catania Astrophysical Observatory

**Can matter wave interferometry probe the quantum foam of the spacetime?**

The possibility tht advanced matter wave interferometers may detect the vacuum structure of asyptotically safe gravity is discussed.

Alfio Bonanno, INAF Catania Astrophysical Observatory

**Asymptotically safe inflation and CMB polarization**

The presence of complex critical exponents in the scaling behavior of the Newton constant and Cosmological constant has dramatic consequences at the inflation scale. In particular an infinite number of unstable de-Sitter vacua emerges from an effective quantum gravitational action. In this framework, the possibility of detecting specific signaturesof a non-gaussian fixed point of the gravitational interactions in the CMB polarization spectrum will then be discussed.

Robert Brandenberger, McGill University

**Cosmological windows such as CMB polarization and 21cm redshift surveys to probe Planck-scale physics**

Avery Broderick, Perimeter Institute

**Dancing in the Dark: Images of Quantum Black Holes**

There have recently been a number of rather surprising suggestions that the quantum nature of black holes is manifested on macroscopic scales. This raises the question of just what the image of such an object should look like. The answer is more than simply academic; with the advent of the Event Horizon Telescope (EHT), a millimetre-wave very long baseline array, it is now possible to probe a handful of supermassive black holes with angular resolutions sufficient to image their horizons. I will discuss what we might expect to see, and how in the near future we will begin to empirically probe the existence of black hole quantum states with horizon scale curvature deviations from general relativity.

Xavier Calmet, University of Sussex

**Non-thermal quantum black holes**

I discuss non-thermal quantum black holes. In models with low scale quantum gravity, these holes could be produced a the Large Hadron Collider. I stress that if they decay, the final states will be very different from those expected in the case of semi-classical holes which have been extensively studied.

Xavier Calmet, University of Sussex

**Gravity Induced Grand Unification**

Motivated by the lack of evidence for physics beyond the Standard Model in the TeV region, we discussed an alternative path for grand unification. We show that simple grand unification models based on e.g. SU(5) can work successfully even without low scale supersymmetry. In particular quantum gravitational effects could easily modify the unification conditions for the gauge and Yukawa couplings.

Antonio Di Domenico, University of Rome

**Experimental tests of CPT symmetry**

The three discrete symmetries of Quantum Mechanics, charge conjugation, parity, and time reversal, are all violated, singly or in pairs. CPT is the only combination of these symmetries which appears to be conserved in Nature. Even though this result is in agreement with the well known CPT theorem, that holds for standard quantum field theories, CPT violation in some cases might be expected and justified in the framework of a quantum theory of gravity. Here the last and more refined experimental searches for CPT violation effects will be reported with a special focus on the tests performed on neutral meson systems and the interplay between CPT symmetry and the basic principles of Quantum Mechanics. No deviation from the expectations of CPT symmetry and quantum mechanics is observed, while the precision of the measurements, in some cases, reaches the interesting Planck scale region. Finally, prospects for this kind of experimental studies will be presented.

Bianca Dittrich, Perimeter Institute

**Modelling continuum dynamics on discrete space time**

We will discuss perfect discretizations which aim at mirroring exactly continuum physics on a given lattice. Such discretizations avoid typical artifacts like Lorentz violation, energy dissipation, particle doubling and in particular breaking of diffeomorphism symmetry. Thus the question arises how to distinguish such lattice dynamics from continuum physics.

Maite Dupuis, University of Erlangen-Nuremberg

**Geometric operators in loop quantum gravity with a cosmological constant**

Loop quantum gravity is a candidate to describe the quantum gravity regime with zero cosmological constant. One of its key results is that geometric operators such as area angle volume are quantized. Not much is known when the cosmological constant is not zero. It is usually believed that to introduce this parameter in the game we need to use quantum groups. However due to the complicated algebraic structure inherent to quantum groups the geometric operators are not yet properly defined (except the area operator). I will discuss how the use of tensor operators can circumvent the difficulties and allow to construct a natural set of observables. In particular I will construct the natural geometric observables such as angle or volume and discuss some of their implications.

Astrid Eichhorn, Perimeter Institute

**Testing the consistency of quantum gravity with low-energy properties of the standard model**

Testing quantum gravity is possible by using, e.g. the available data on the properties of the standard model. I will discuss how a parameterisation of quantum gravity fluctuations in terms of metric fluctuations can be used to test the compatibility of quantum gravity with the observed low-energy properties of the standard model, such as the existence of fermions with masses much below the Planck mass.

Laurent Freidel, Perimeter Institute

**Probing the geometry of momentum space with gamma ray burst delay**

Tobias Fritz, Perimeter Institute

**Turning Weyl's tile argument into a mathematically rigorous no-go theorem**

Weyl's tile argument notes that if space was fundamentally discrete then the set of allowed velocities of a classical particle would not be isotropic. I will generalize Weyl's heuristic argument to a no-go theorem applying to any discrete periodic structure. Since this theorem does not take quantum mechanics into account it should only be regarded as the first step of a program of understanding the phenomenology of discrete spacetimes in a mathematically rigorous way.See arXiv:1109.1963

Doug Gingrich, Universityof Alberta

**Searches for Nonperturbative Gravitational States at the LHC**

Searches for black holes and string balls have recently be performed by LHC experiments. Upper limits have been placed on the production crosssections time experimental acceptance. Hard-disk production of thesestates have been ruled out over most of the current LHC energy reach.However the LHC has said little about nonperturbative states producedin model with reduced cross sections. I will discuss some popularmodels that have not yet been ruled out by the LHC experiments.

Jonathan Granot, University of Hertfordshire

**Astrophysical Searches for Quantum Gravity Signals**

Some recent searches for quantum gravity signatures using observations of distant astrophysical sources will be discussed, focusing on the search for Lorentz invariance violation (LIV) in the form of a dependence of the photon propagation speed on its energy. Fermi gamma-ray space telescope observations of ~8 keV to ~30 GeV photons from a short (< 1 s) gamma-ray burst (GRB 090510) at a cosmological distance (z = 0.903), enabled for the first time to put a direct time of flight limit on a possible linear variation of the speed of light with photon energy that is beyond the Planck scale. Parameterizing |v/c-1| = E/E_{QG} our most conservative limits are E_{QG}/E_{Planck} > 1.2, while less conservative limits are up to 1-2 orders of magnitude stricter. Other types of astrophysical searches for LIV will be briefly outlined, along with some prospects for the future.

Sabine Hossenfelder, Nordita

**Quantum superpositions of the speed of light**

If the metric is an operator, it can exist in superpositions. The simplest case one can look at is a superposition of flat spaces which differ only in the value of the speed of light. I will lay out how such superpositions can be incorporated into quantum field theory, and discuss the fate of Lorentz-invariance in this scenario.

John Kelley, University of Wisconsin-Madison

**Searching for Quantum Gravity with the IceCube Neutrino Observatory**

The IceCube Neutrino Observatory is a cubic-kilometer-scale neutrino detector built into the ice sheet at the geographic South Pole. Completed in December 2010, the detector consists of an array of photomultiplier tubes deployed along 86 cables ("strings") at depths of 1450 to 2450 m, as well as the IceTop air shower array of surface Cherenkov tanks. IceCube is detecting atmospheric neutrinos of energies above approximately 100 GeV at a rate of ~6 per hour, and is currently searching for extraterrestrial neutrinos from cosmic ray accelerators. A measurement of the atmospheric neutrino spectrum can be used to search for possible phenomenological signatures of quantum gravity (QG), such as violations of Lorentz invariance or quantum decoherence, and I present limits we have set on these phenomena in the neutrino sector. To extend the search for QG to much higher energies and cosmological baselines, we require an extraterrestrial neutrino source. In this context, I report on the status of our searches for neutrinos from gamma-ray bursts and from cosmic-ray interactions with the microwave background ("cosmogenic" neutrinos).

Jurek Kowalski-Glikman, Institute for Theoretical Physics

**Relative locality and fate of Lorentz symmetry**

In my talk I will briefly introduce the idea of relative locality, being a particular regime of quantum gravity characterized by negligible Planck length and finite Planck mass. Then I will discuss possible scenarios concerning the fate of Lorentz symmetry in this regime.

Stefano Liberati, SISSA

**An analogue model lesson for the cosmological constant**

Analogue models of gravity have been a test field for many phenomena in quantum field theory on curved spacetimes. It was recently recognized that in some situations they can also provide toy models of emergent gravity. We shall review here such a model and address within it the nature and scale of the cosmological constrant trying to draw general lessons for emergent gravity scenarios.

Stefano Liberati, SISSA

**Observational constraints on scale hierarchy in Horava-Lifshiftz gravity**

Horava-Lifshitz gravity models contain higher order operators suppressed by a characteristic scale, which is required to be parametrically smaller than the Planck scale. We show that recomputed synchrotron radiation constraints from the Crab nebula suffice to exclude the possibility that this scale is of the same order of magnitude as the Lorentz breaking scale in the matter sector. This highlights the need for a mechanism that suppresses the percolation of Lorentz violation in the matter sector and is effective for higher order operators as well.

Daniel Litim, University of Sussex

**Quantum gravity and cosmology**

In the light of upcoming high-precision data from the Planck mission and possible trans-Planckian signatures encoded in eg the microwave background radiation, and in view of possible large-distance modifications of gravity and the accelearted expansion of the universe.

Daniel Litim, University of Sussex

**New physics beyond the Standard Model of Particle Physics**

Models of new physics beyond the Standard Model of Particle Physics suggest that the quantum scale of gravity could be as low as the electro-weak or TeV energy scale. If so, they offer the exciting possibility that quantum gravity becomes accessible, experimentally, at particle colliders such as the LHC. From the theory side, low-scale gravity scenarios often require extra dimensions, or a large number of additional particle species. I propose to discuss the possibility for signatures of low-scale quantum gravity, the feasibility of concrete (quantitative) predictions based on eg. specific processes and quantum gravity set-ups, and options for experimental tests.

Seth Major, Hamilton College

**On the Observability of Discrete Spatial Geometry**

If quantum geometry is an accurate model of microscopic spatial geometry then two related questions arise, one observational and one theoretical: How and at what scale is the discreteness manifest? And, how is the general relativistic limit achieved? These questions will be discussed in the context of studies of a single atom of geometry. It will be shown that the effective scale of the discreteness could be much larger than the Planck scale. Before this scale can be predicted, the relations between discrete geometry, coherent states, and the semiclassical limit need to be clarified. Work towards this goal, using coherent states in spin foams and the spin geometry theorem of Penrose and Moussouris will be described.

Joao Magueijo, Imperial College, London

**Quantum gravity and a chiral signature in gravity waves**

I show how quantum gravity could lead to a chiral signature in the graviational wave background, proportional to the imaginary part of the Immirzi parameter. This would leave a distinctive imprint in the polarization of the cosmic microwave background. I will discuss how this isue is closely related to that of identifying the ground of base state for quantum gravity.

Joao Magueijo, Imperial College, London

**Breaking Lorentz invariance: the Universe loves it!**

I show how the local Lorentz and / or diffeomorphism invariances may be broken by a varying speed of light, softly or harshly, depending on taste. Regardless of the fundamental implications of such dramas, these smmetry breakings may be of great practical use in cosmology. They may solve the horizon and flatness problesm. A near scale-invariant sprectrum of fluctuation may arise, even without inflation. Distinct observational imprints may be left.

Antonino Marciano, Dartmouth College

**Interplay between cosmological constant and a DSR scale**

I offer brief remarks on several ways in which the cosmological constant could provide a clue toward quantum gravity. I then focus on how DSR-relativistic theories can be made compatible with spacetime expansion (possibly cosmological-constant-governed spacetime expansion), and how this interplay could manifest itself in data.

David Mattingly, University of New Hampshire

**The Irritating Persistence of Horizons**

In some approaches to quantum gravity Lorentz invariance is modified. Without Lorentz invariance one can theoretically see behind the usual Killing horizon of a black hole if, for example, one allowed for superluminal propagation. This in turn raises the possibility that one could in principle probe the singularity and the quantum gravity regime. We discuss how Lorentz violating black hole solutions in Einstein-aether theory unfortunately possess another causal boundary behind the Killing horizon that is impenetrable to any superluminal mode. We also detail progress in determining the laws of black hole mechanics and the radiation spectrum from these so-called "universal horizons". Our results suggest that even if superluminal dispersion at high frequencies did exist in nature, singularities and their associated quantum gravity resolutions may very well remain locked behind horizons.

Holger Mueller, University of California, Berkeley

**Matter-wave clocks**

De Broglie's matter wave hypothesis describes particles as oscillators at the Compton frequency $mc^2/h$, where m is the particle's mass, c is the speed of light, and h the Planck constant [1]. The concept continues to be in full use in the generalized form of the path integral, but the oscillations themselves have often been disregarded as artifacts of the theory. Here, we illustrate their physical significance through a series of atom-interferometry experiments: (i) A test of the gravitational redshift at an accuracy of 7 parts per billion [2] and its interpretation in the framework of the standard model extension [3]; (ii) A proposed gravitational Aharonov-Bohm experiment, which will reveal the gravitational redshift of the Compton frequency even in absence of a gravitational force [4]; (iii) A matter-wave clock, in which the Compton frequency of a Cs atom $ (3x10^{25} Hz)$ is divided into a conveniently measurable frequency range, based on a combination of an atom interferometer and an optical frequency comb. With an accuracy of 4 parts per billion, it can serve as a time standard based on mass or vice versa [5]. Particles are Compton clocks.

[1] L. de Broglie, Ph. D. thesis (Univ. Paris, 1924).

[2] H. M\"uller, A. Peters, and S. Chu. Nature 463, 926 (2010).

[3] M. A. Hohensee, S. Chu, A. Peters, and H. M\"uller, Phys. Rev. Lett. 106, 151102 (2011).

[4] M. A. Hohensee, B. Estey, P. Hamilton, A. Zeilinger, and H. M\"uller. Phys. Rev. Lett. 108, 230404 (2012).

[5] S.-Y. Lan, P.-C. Kuan, B. Estey, D. English, J. M. Brown, M. A. Hohensee, and H. M\"uller, submitted (2012).

Robert Nemiroff, Michigan Technological University

**A Possible Bound on Spectral Dispersion from Fermi-Detected Gamma Ray Burst 090510A}**

Three photons spanning about 30 GeV arrived within about one millisecond from the Fermi-detected GRB 090510A at a redshift of about 0.9. Although conceivably a > 3Ïƒ statistical fluctuation when taken at face value this photon bunch -- quite possibly a classic GRB pulse -- leads to a relatively tight bound on the ability of our universe to disperse high energy photons. Specifically given a generic dispersion relation where the time delay is proportional to the photon energy to the first power the limit on the dispersion strength is k1 < 1.61Ã—10-5 sec Gpc-1 GeV-1. In the context of some theories of quantum gravity this conservative bound translates into an minimum energy scale greater than 525 m_Planck suggesting that spacetime is smooth at energies perhaps three orders of magnitude over the Planck mass.

James Overduin, Towson University

**Improved test of the Equivalence Principle as a probe of quantum gravity**

Current approaches to the problems of dark energy and unification generically predict the existence of new fields (quintessence dilatons etc.) that will in principle couple with different strengths to different standard-model fields. These different coupling strengths will cause test materials of different compositions to fall at different rates in the same gravitational field violating the Equivalence Principle the foundation of General Relativity. A sufficiently sensitive measurement of the relative accelerations of different test bodies in orbit around the earth could detect or rule out these new fields complementing existing or proposed experiments in high-energy physics (colliders) and observational cosmology (space telescopes). To do this convincingly such an experiment needs at least three test materials spanning the largest possible volume in the space of atomic and molecular properties and a sensitivity to EP violations of as little as a part in $10^{18}$ (attainable only in space and at low temperatures). I discuss one such experiment the Satellite Test of the Equivalence Principle which has reached an advanced stage of prototype development and is currently awaiting a path toward a flight program.

Roberto Percacci, SISSA

**Asymptotic safety and minimal length**

Since asymptotic safety - if true - would make a quantum field theory of gavity consistent "up to arbitrarily high energy", it would seem that this notion is incompatible with the existence of a minimal length. I will argue that this is not necessarily the case, due to ambiguity in the notion of minimal length.

Martin Reuter, University of Mainz

**Dynamical dimensional reduction and Asymptotic Safety**

The effective average action approach to Quantum Einstein Gravity (QEG) is discussed as a natural framework for exploring the scale dependent Riemannian geometry and multifractal micro-structure of the effective spacetimes predicted by QEG. Their fractal properties are related to the functional RG flow on theory space, and the special role of the running cosmological constant is emphasized. The prospects of an experimental verification will also be discussed.

Martin Reuter, University of Mainz

**The Asymptotic Safety program**

The essential components of the Asymptotic Safety program are discussed as an effort which ultimately can lead to a well defined functional integral for a microscopic quantum field theory of gravity. Continuum based exact RG methods and statistical field theory approaches are compared, and it is argued that, at an intermediate stage, numerical simulations must play a role analogous to that of real experiments.

Markus Risse, University of Siegen

**The highest-energy particles of the Universe as viewed by the Pierre Auger Observatory**

One century after the seminal balloon flights of Victor Hess, the Pierre Auger Observatory aims at unveiling some of the mysteries of the highest-energy cosmic rays: what are their sources? Is there an end to the spectrum? What kind of particles are they? Are there signatures of new physics or of a violation of fundamental laws of physics? The Auger Observatory measures cosmic rays with energies of 10^20 eV or more by observing the giant air showers created when the particles hit the atmosphere. Located in Argentina, two complementary detector systems are used: an array of 1600 water-Cherenkov detectors distributed over 3000 sqkm, and fluorescence telescopes which monitor the atmosphere above the array in clear nights. Since 2005, data of unprecedented quantity and quality could be taken. In the talk, the observation principles, successes and limitations are described. Current, partly surprising results are presented. Data interpretations related to the search for violation of Lorentz invariance are mentioned.

Alejandro Satz, University of Maryland

**Quantum Gravity RG flow: a cosmological limit cycle**

I will discuss evidence for the existence of a limit cycle in the renormalization group for quantum gravity which is visible in a minisuperspace approximation. The emergence of the limit cycle can be studied through a tuning parameter representing the number of dimensions in which fluctuations of the conformal factor are suppressed. At the critical value of the tuning parameter all RG trajectories reaching the UV fixed point have an extended semiclassical regime with a small positive cosmological constant providing a possible model for a viable cosmology without fine-tuning.

Lorenzo Sindoni, Max Planck Institute for Gravitational Physics

**The cosmological constant and the emergence of the continuum**

Naturalness problems that could be signaling the necessity a completion of an effective field theory with the introduction of an otherwise overlooked ingredient. The cosmological constant problem can be seen as a signal that the EFT for gravity, general relativity, is not correctly including the gravitational properties of the vacuum. Starting from the discussion of the a possible solution to this naturalness problem for a cc-like term in a BEC analogue model, I will briefly discuss how this particular mechanism can be extended, albeit in a preliminary form, to more genuine quantum gravity models like GFT, connecting in particular the problem of the determination of the gravitational couplings (and hence the hierarchies involved) to the appearance of a semiclassical space-time.

Lee Smolin, Perimeter Institute

**Unification of gravity with the electroweak interactions**

Rafael Sorkin, Perimeter Institute

**If spacetime is a causal set then Lorentz symmetry is unbroken**

Dejan Stojkovic, SUNY

**Vanishing dimensions: theory and phenomenology**

Lower-dimensionality at higher energies has manifold theoretical advantages as recently pointed out. Moreover, it appears that experimental evidence may already exists for it - a statistically significant planar alignment of events with energies higher than TeV has been observed in some earlier cosmic ray experiments. If this alignment is not a fluke, then the LHC should be able to see effects associated with the dimensional crossover. Further, (2+1)-dimensional spacetimes have no gravitational degrees of freedom, and gravity waves cannot be produced in that epoch in the early universe. This places a universal maximum frequency at which primordial gravity waves can propagate, which may be accessible to future gravitational wave detectors such as LISA. In this talk, the theoretical motivation for "vanishing dimensions" as well as generic experimental and observational signature will be discussed.

Daniel Sudarsky, Universidad Nacional Autonoma de Mexico

**Quantum Gravity at the origin of seeds of cosmic structure?**

This meeting shows a our impatience for uncovering at long last any signal of unknown physics that might have a quantum gravitational origin. I will argue that the transition from a homogenous and isotropic state characterizing the mid-early parts of inflation ( i.e. the regime after sufficient e- folds of inflation have elapsed so that all traces of the pre-inflationary state are erased), to those eras, where the primordial inhomogeneities have appears might hold ins testing clues about the nature of quantum gravity.

Daniel Sudarsky, Universidad Nacional Autonoma de Mexico

**Quantum Gravity Phenomenology without Lorentz Invariance Violation**

Is there hope to see quantum gravity effects if the underlying theory is strictly respecting of Lorentz invariance? I will discuss a novel class of possibilities, suggested by analogy with some simple solid state physics, including one that has lead to an actual experiment, which has placed the first relevant constraints on these kind of effects.