Quantum Gravity

Quantum gravity is concerned with unifying Einstein's general theory of relativity with quantum theory into a single theoretical framework. At Perimeter Institute, researchers are actively pursuing a number of approaches to this problem including loop quantum gravity, spin foam models, asymptotic safety, emergent gravity, string theory, and causal set theory. We are also particularly interested in experimental implications of these different proposals. As the aim is a unification of the laws of physics into a single theory, the search for quantum gravity overlaps with other areas such as cosmology, particle physics and the foundations of quantum theory.

Displaying 1009 - 1020 of 1365

Canonical tensor models with local time

Naoki Sasakura Yukawa Institute for Theoretical Physics
November 15, 2012

PIRSA:12110065
Quantum Gravity
Dynamical evaporation of quantum horizons
November 08, 2012

PIRSA:12110064
Quantum Gravity
Effective field theory framework in QG
- Xavier Calmet University of Sussex
- Roberto Percacci SISSA International School for Advanced Studies
- Astrid Eichhorn Universität Heidelberg
- Martin Reuter Johannes Gutenberg University Mainz
October 25, 2012

PIRSA:12100121
Quantum Gravity
Quantum Gravity and the Weak Interactions
- Lee Smolin Perimeter Institute for Theoretical Physics
- Xavier Calmet University of Sussex
- Stephon Alexander Brown University
- Daniel Litim University of Sussex
- Doug Gingrich University of Alberta
- Dejan Stojkovic State University of New York (SUNY)
October 25, 2012

PIRSA:12100116
Quantum Gravity
Experimental search for quantum gravity - the hard facts

Hugo Beauchemin University of Oxford
October 25, 2012

PIRSA:12100115
Quantum Gravity
Lorentz symmetry: Broken, intact or deformed?
- Rafael Sorkin Perimeter Institute for Theoretical Physics
- Sabine Hossenfelder Ludwig-Maximilians-Universität München (LMU)
- David Mattingly University of New Hampshire
- Niayesh Afshordi University of Waterloo
- Daniel Sudarsky Universidad Nacional Autónoma De Mexico (UNAM)
- Stefano Liberati SISSA International School for Advanced Studies
- Joao Magueijo Imperial College London
October 24, 2012

PIRSA:12100108
Quantum Gravity
Astrophysical Searches for Quantum Gravity Signals

Jonathan Granot University of Hertfordshire
October 24, 2012

PIRSA:12100058
Quantum Gravity
Dynamical Dimensional Reduction
- Dejan Stojkovic State University of New York (SUNY)
- Astrid Eichhorn Universität Heidelberg
- Martin Reuter Johannes Gutenberg University Mainz
October 24, 2012

PIRSA:12100104
Quantum Gravity
Quantum Gravity Effects in Black Holes and Gravitational Waves
- Giovanni Amelino-Camelia Sapienza Università di Roma - Dipartimento di Fisica
- David Mattingly University of New Hampshire
- Nicolas Yunes University of Illinois Urbana-Champaign
- Avery Broderick University of Waterloo
October 23, 2012

PIRSA:12100094
Quantum Gravity
Searching for Quantum Gravity with the IceCube Neutrino Observatory

John Kelley University of Wisconsin–Madison
October 23, 2012

PIRSA:12100093
Quantum Gravity
Quantum Gravity Effects in the (Early and Late) Universe
- Stephon Alexander Brown University
- Daniel Sudarsky Universidad Nacional Autónoma De Mexico (UNAM)
- Daniel Litim University of Sussex
- Robert Brandenberger McGill University - Department of Physics
October 23, 2012

PIRSA:12100090
Quantum Gravity
Photons and Quantum Gravity in Astrophysics
- Robert Nemiroff Michigan Technological University
- Giovanni Amelino-Camelia Sapienza Università di Roma - Dipartimento di Fisica
- Niayesh Afshordi University of Waterloo
- Joao Magueijo Imperial College London
- Alfio Bonanno National Institute for Astrophysics (INAF)
- Robert Brandenberger McGill University - Department of Physics
October 22, 2012

PIRSA:12100085
Quantum Gravity

Particle Physics

Canonical tensor models with local time

Naoki Sasakura Yukawa Institute for Theoretical Physics
November 15, 2012

PIRSA:12110065
Quantum Gravity
Tensor models are generalization of matrix models, and are studied as discrete models for quantum gravity for more than two-dimensions. Among them, the rank-three tensor models can be interpreted as theories of dynamical fuzzy spaces, and they generally have the feature of
respecting symmetries. In this talk, after briefly reviewing some results on Euclidean models such as spontaneous generation of fuzzy spaces and Euclidean general relativity respecting the diffeomorphism symmetry on them, I will present a way to introduce “local” time to the rank-three tensor models by constructing “local” Hamiltonians. The consistency among the multiple ways of local time evolutions is guaranteed by the on-shell closure of the constraint algebra among the
local Hamiltonians and the symmetry generators of the tensor models. The constraint algebra is shown to approach the DeWitt algebra in a formal continuum limit. I will also discuss the two-fold uniqueness of the local Hamiltonians, and will briefly show some preliminary results on the quantization.
Dynamical evaporation of quantum horizons
November 08, 2012

PIRSA:12110064
Quantum Gravity
We describe of the evaporation process as driven by the dynamical evolution of the quantum gravitational degrees of freedom resident at the horizon, as identified by the Loop Quantum Gravity kinematics. Using a parallel with the Brownian motion, we interpret the first law of quantum dynamical horizon in terms of a fluctuation-dissipation relation applied to this fundamental discrete structure. In this way, the horizon evolution is described in terms of relaxation to an equilibrium state balanced by the excitation of Planck scale constituents of the horizon. We show how from this setting the emergence of several conservative scenarios for the final stage of the evaporation process can be microscopically derived. Namely, the leakage of part of the horizon quantum geometry information prior to the Planckian phase and the stabilization of the hole surface shrinkage forming a massive remnant, which can eventually decay, are shown to take place.
Effective field theory framework in QG
- Xavier Calmet University of Sussex
- Roberto Percacci SISSA International School for Advanced Studies
- Astrid Eichhorn Universität Heidelberg
- Martin Reuter Johannes Gutenberg University Mainz
October 25, 2012

PIRSA:12100121
Quantum Gravity
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.
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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.
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The Asymptotic Safety program
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.
Quantum Gravity and the Weak Interactions
- Lee Smolin Perimeter Institute for Theoretical Physics
- Xavier Calmet University of Sussex
- Stephon Alexander Brown University
- Daniel Litim University of Sussex
- Doug Gingrich University of Alberta
- Dejan Stojkovic State University of New York (SUNY)
October 25, 2012

PIRSA:12100116
Quantum Gravity
Quantum Gravity and the Weak Interactions
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Non-thermal quantum black holes
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.
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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.
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Models of 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.
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Unification of gravity with the electroweak interactions
Experimental search for quantum gravity - the hard facts

Hugo Beauchemin University of Oxford
October 25, 2012

PIRSA:12100115
Quantum Gravity
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.
Lorentz symmetry: Broken, intact or deformed?
- Rafael Sorkin Perimeter Institute for Theoretical Physics
- Sabine Hossenfelder Ludwig-Maximilians-Universität München (LMU)
- David Mattingly University of New Hampshire
- Niayesh Afshordi University of Waterloo
- Daniel Sudarsky Universidad Nacional Autónoma De Mexico (UNAM)
- Stefano Liberati SISSA International School for Advanced Studies
- Joao Magueijo Imperial College London
October 24, 2012

PIRSA:12100108
Quantum Gravity
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.
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If spacetime is a causal set then Lorentz symmetry is unbroken
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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.
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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.
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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.
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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.
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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
Astrophysical Searches for Quantum Gravity Signals

Jonathan Granot University of Hertfordshire
October 24, 2012

PIRSA:12100058
Quantum Gravity
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.
Dynamical Dimensional Reduction
- Dejan Stojkovic State University of New York (SUNY)
- Astrid Eichhorn Universität Heidelberg
- Martin Reuter Johannes Gutenberg University Mainz
October 24, 2012

PIRSA:12100104
Quantum Gravity
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.
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Vanishing dimension: 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
Quantum Gravity Effects in Black Holes and Gravitational Waves
- Giovanni Amelino-Camelia Sapienza Università di Roma - Dipartimento di Fisica
- David Mattingly University of New Hampshire
- Nicolas Yunes University of Illinois Urbana-Champaign
- Avery Broderick University of Waterloo
October 23, 2012

PIRSA:12100094
Quantum Gravity
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.
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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.
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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.
Searching for Quantum Gravity with the IceCube Neutrino Observatory

John Kelley University of Wisconsin–Madison
October 23, 2012

PIRSA:12100093
Quantum Gravity
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).
Quantum Gravity Effects in the (Early and Late) Universe
- Stephon Alexander Brown University
- Daniel Sudarsky Universidad Nacional Autónoma De Mexico (UNAM)
- Daniel Litim University of Sussex
- Robert Brandenberger McGill University - Department of Physics
October 23, 2012

PIRSA:12100090
Quantum Gravity
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

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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
Photons and Quantum Gravity in Astrophysics
- Robert Nemiroff Michigan Technological University
- Giovanni Amelino-Camelia Sapienza Università di Roma - Dipartimento di Fisica
- Niayesh Afshordi University of Waterloo
- Joao Magueijo Imperial College London
- Alfio Bonanno National Institute for Astrophysics (INAF)
- Robert Brandenberger McGill University - Department of Physics
October 22, 2012

PIRSA:12100085
Quantum Gravity

Particle Physics
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.

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

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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.

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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.

Title	Speaker(s)	Date	Talk Type	Info link
Canonical tensor models with local time	Naoki Sasakura	2012-11-15	Scientific Series	View details
Dynamical evaporation of quantum horizons		2012-11-08	Scientific Series	View details
Effective field theory framework in QG	Xavier Calmet, Roberto Percacci, Astrid Eichhorn, Martin Reuter	2012-10-25	Conference	View details
Quantum Gravity and the Weak Interactions	Lee Smolin, Xavier Calmet, Stephon Alexander, Daniel Litim, Doug Gingrich, Dejan Stojkovic	2012-10-25	Conference	View details
Experimental search for quantum gravity - the hard facts	Hugo Beauchemin	2012-10-25	Conference	View details
Lorentz symmetry: Broken, intact or deformed?	Rafael Sorkin, Sabine Hossenfelder, David Mattingly, Niayesh Afshordi, Daniel Sudarsky, Stefano Liberati, Joao Magueijo	2012-10-24	Conference	View details
Astrophysical Searches for Quantum Gravity Signals	Jonathan Granot	2012-10-24	Scientific Series	View details
Dynamical Dimensional Reduction	Dejan Stojkovic, Astrid Eichhorn, Martin Reuter	2012-10-24	Conference	View details
Quantum Gravity Effects in Black Holes and Gravitational Waves	Giovanni Amelino-Camelia, David Mattingly, Nicolas Yunes, Avery Broderick	2012-10-23	Conference	View details
Searching for Quantum Gravity with the IceCube Neutrino Observatory	John Kelley	2012-10-23	Conference	View details
Quantum Gravity Effects in the (Early and Late) Universe	Stephon Alexander, Daniel Sudarsky, Daniel Litim, Robert Brandenberger	2012-10-23	Conference	View details
Photons and Quantum Gravity in Astrophysics	Robert Nemiroff, Giovanni Amelino-Camelia, Niayesh Afshordi, Joao Magueijo, Alfio Bonanno, Robert Brandenberger	2012-10-22	Conference	View details

Quantum Gravity

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