Recovering General Relativity from a Planck scale discrete theory of quantum gravity
Fay Dowker Imperial College London
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.
Fay Dowker Imperial College London
Sravan Kumar Tokyo Institute of Technology
Mohammad M Sheikh-Jabbari Institute for Research in Fundamental Sciences (IPM)
Antonia Micol Frassino Universitat de Barcelona
Gaston Giribet Universidad de Buenos Aires
Chris Ripken Johannes Gutenberg University Mainz
Manuel Reichert University of Southern Denmark
Frank Saueressig Radboud Universiteit Nijmegen
Frank Saueressig Radboud Universiteit Nijmegen
Parampreet Singh Louisiana State University (LSU) - Department of Physics & Astronomy
Parampreet Singh Louisiana State University (LSU) - Department of Physics & Astronomy
Masanori Hanada Kyoto University
Benjamin Bahr Universität Hamburg
Bianca Dittrich Perimeter Institute for Theoretical Physics
Sebastian Steinhaus Friedrich-Schiller-Universität Jena
Timothy Budd Radboud Universiteit Nijmegen
Joren Brunekreef Radboud Universiteit Nijmegen
Benjamin Bahr Universität Hamburg
Bianca Dittrich Perimeter Institute for Theoretical Physics
Henrique Gomes University of Cambridge
Claes Uggla Karlstad University
Lee Smolin Perimeter Institute for Theoretical Physics
Roberto Percacci SISSA International School for Advanced Studies
Astrid Eichhorn University of Southern Denmark
Steve Adler Institute for Advanced Study (IAS) - School of Natural Sciences (SNS)
Renate Loll Radboud Universiteit Nijmegen
Adam Frank University of Rochester
Chopin Soo National Cheng Kung University
Sean Gryb University of Groningen
Fay Dowker Imperial College London
Avshalom Elitzur Israeli Institute for Advanced Research
Laurent Freidel Perimeter Institute for Theoretical Physics
Lee Smolin Perimeter Institute for Theoretical Physics
Joao Magueijo Imperial College London
Carlo Rovelli Centre de Physique Théorique
Jenann Ismael Columbia University
Andreas Albrecht University of California System
Stuart Kauffman Santa Fe Institute (SFI)
George Ellis University of Cape Town
Barbara Drossel Technische Universität Darmstadt
Fay Dowker Imperial College London
I will present an argument that if a theory of quantum gravity is physically discrete at the Planck scale and the theory recovers General Relativity as an approximation, then, at the current stage of our knowledge, causal sets must arise within the theory, even if they are not its basis. I will argue in particular that an apparent alternative to causal sets, viz. a certain sort of discrete Lorentzian simplicial complex, cannot recover General Relativistic spacetimes in the appropriately unique way. For it cannot discriminate between Minkowski spacetime and a spacetime with a certain sort of gravitational wave burst.
Zoom Link: https://pitp.zoom.us/j/96897602807?pwd=VkJ3VTAwYjhaTnJ3Z2ZVclZFYXErZz09
Sravan Kumar Tokyo Institute of Technology
Cosmic inflation is an important paradigm of the early Universe which is so far developed in two equivalent ways, either by geometrical modification of Einstein's general relativity (GR) or by introducing new forms of matter beyond the standard model of particle physics. Starobinsky's R+R^2 inflation based on a geometric modification of GR is one of the most observationally favorable models of cosmic inflation based on a geometric modification of GR. In this talk, I will discuss in detail the fundamental motivations for Starobinsky inflation and present how certain logical steps in the view of its UV completion lead to the emergence of a gravity theory that is non-local in nature. Then I will establish how one can perform studies of the early Universe in the context of non-local gravity and what are the observational consequences in the scope of future CMB and gravitational waves. I will discuss in detail how non-local R^2-like inflation can be observationally distinguishable from the local effective field theories of inflation.
Mohammad M Sheikh-Jabbari Institute for Research in Fundamental Sciences (IPM)
We study D dimensional pure Einstein gravity theory in a region of spacetime bounded by a generic null boundary. We show besides the graviton modes propagating in the bulk, the system is described by boundary degrees of freedom labeled by D surface charges associated with nontrivial diffeomorphisms at the boundary. We establish that the system admits a natural thermodynamical description. Using standard surface charge analysis and covariant phase space method, we formulate laws of null surface thermodynamics which are local equations over an arbitrary null surface. This thermodynamical system is generally an open system and can be closed only when there is no flux of gravitons through the null surface. Our analysis extends the usual black hole thermodynamics to a universal feature of any area element on a generic null surface in a generic diffeomorphism invariant theory of gravity.
Zoom Link: https://pitp.zoom.us/j/91590041045?pwd=UXpWY3JEd0QwK2hXanBzSkdPRC94UT09
Antonia Micol Frassino Universitat de Barcelona
In this talk, I will consider quantum effects on some specific black hole solutions and take into account their gravitational backreaction. In particular, I will describe the holographic construction of the quantum BTZ black hole (quBTZ) from an exact four-dimensional bulk solution. I will present some of the thermodynamic properties of these black holes, focus on the generalized first law and analyze the different complexity proposals for the quBTZ. Our results indicate that Action Complexity fails to account for the additional quantum contributions and does not lead to the correct classical limit. On the other hand, the Volume Complexity admits a consistent quantum expansion and agrees with known limits.
Sung-Sik Lee McMaster University
We present a model of quantum gravity in which dimension, topology and geometry of spacetime are collective dynamical variables that describe the pattern of entanglement of underlying quantum matter. As spacetimes with arbitrary dimensions can emerge, the gauge symmetry is generalized to a group that includes diffeomorphisms in general dimensions. The gauge symmetry obeys a first-class constraint operator algebra, and is reduced to a generalized hypersurface deformation algebra in states that exhibit classical spacetimes. In the semi-classical limit, we find a saddle-point solution that describes a series of (3+1)-dimensional de Sitter-like spacetimes with the Lorentzian signature bridged by Euclidean spaces in between.
Gaston Giribet Universidad de Buenos Aires
In this talk, I will revisit the calculation of infinite-dimensional symmetries that emerge in the vicinity of isolated horizons. In particular, I will focus my attention on extremal black holes, for which the isometry algebra that preserves a sensible set of asymptotic boundary conditions at the horizon strictly includes the BMS algebra. The conserved charges that correspond to this BMS sector, however, reduce to those of superrotation, generating only two copies of Witt algebra. For more general horizon isometries, in contrast, the charge algebra does include both Witt and supertranslations, being similar to BMS but s.str. differing from it. I will also show how this is extended to the case of black holes in the Einstein-Yang-Mills case, where a loop algebra associated to the gauge group is found to emerge at the horizon.
Chris Ripken Johannes Gutenberg University Mainz
A basic calculation in QFT is the construction of the Yukawa potential from a tree-level scattering amplitude. In the massless limit, this reproduces the 1/r potential. For gravity, scattering mediated by a massless graviton is thus consistent with the Newtonian potential.
In de Sitter spacetime, the cosmological constant gives rise to a mass-like term in the graviton propagator. This raises the question what the classical potential looks like when taking into account curvature effects.
In this talk, I will introduce an operator-based formalism to compute scattering amplitudes in curved spacetime, and I will show how to construct the Newtonian potential in a dS background. Remarkably, the potential gives rise to an additional repulsive force, and encodes the de Sitter horizon in a novel and non-trivial way.
Manuel Reichert University of Southern Denmark
Over the past decades, the asymptotic safety scenario has matured into a viable contender for a consistent theory of quantum gravity. However, the pressing question of unitarity is far from being settled. I will present important steps towards tackling this issue and show the first direct computation of the graviton spectral function in asymptotically safe quantum gravity with a novel Lorentzian renormalisation group approach. We find a positive graviton spectral function, showing a massless one-graviton peak and a multi-graviton continuum with an asymptotically safe scaling for large spectral values. For small spectral values, the effective field theory result is approached. I will indicate consequences for scattering amplitudes and unitarity.
Making Quantum Gravity Computable
Infrared Problems in QED and Quantum Gravity