Fri - 05/03/24

Arrows of time in bouncing cosmologies

  • Authors: Marco de Cesare
  • Subjects: General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th)
  • Arxiv link: https://arxiv.org/abs/2405.01380
  • Abstract Different approaches to quantum gravity, such as loop quantum cosmology and group field theory, predict the resolution of the initial cosmological singularity via a ‘bounce’: a regular spacetime region that connects the expanding branch of the universe to a contracting branch. The cosmological arrow of time, which by definition points in the direction of cosmic expansion, is reversed at the bounce. Nonetheless, it is still possible to discriminate between the two branches by considering different arrows, as defined for instance by the growth of perturbations. After reviewing general aspects of the time arrow problem in cosmology, we examine the properties of different arrows of time in bouncing cosmologies, focusing on the loop quantum cosmology bounce as a case study. We also present a new exact solution to the effective Friedmann equations of loop quantum cosmology with pressureless dust and a cosmological constant, which is a simplified version of the $\Lambda$CDM bounce scenario, where these issues can be examined in detail.

Investigations on Lorentzian Spin-foams and Semiclassical Space-times

  • Authors: José Diogo Simão
  • Subjects: General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th)
  • Arxiv link: https://arxiv.org/abs/2405.01360
  • Abstract This thesis is developed in the context of the spin-foam approach to quantum gravity; all results are concerned with the Lorentzian theory and with semiclassical methods. A correspondence is given between Majorana 2-spinors and time-like hypersurfaces in Minkowski 3-space based on complexified quaternions. It is shown that the former suggest a symplectic structure on the spinor phase space which, together with an area-matching constraint, yields a symplectomorphism to $T^*\mathrm{SU}(1,1)$. A complete 3-dimensional Lorentzian spin-foam amplitude for both space- and time-like triangles is proposed. It is shown to asymptote to Regge theory in the semiclassical regime. The asymptotic limit of the 4-dimensional Conrady-Hnybida model for general polytopes is scrutinized. Minkowski’s theorem on convex polyhedra is generalized to Lorentzian signature, and new boundary states for time-like polygons are introduced. It is found that the semiclassical amplitude for such polygons is insufficiently constrained. A method for the asymptotic evaluation of integrals subject to external parameters is discussed. The method is developed in detail for the special problem of spin-foam gluing constraints away from their dominant critical points. A relation to the gluing constraints of effective spin-foams is suggested.

  1. [2405.01212] - Non-perturbative Quantum Gravity in Fock representations - Thomas Thiemann

  2. [2405.01407] - Graviton-photon oscillations as a probe of quantum gravity - Andrea Palessandro

  3. [2405.00845] - Subleading analysis for $S^3$ partition functions of $\mathcal{N}=2$ holographic SCFTs - Seppe Geukens, Junho Hong

  4. [2405.01084] - A positive metric over DGKT vacua - Eran Palti, Nicolò Petri

  5. [2405.01253] - 2d Ising Critical Couplings from Quantum Gravity - Valentin Bonzom, Etera R. Livine

Thu - 05/02/24

Towards quantum gravity with neural networks: Solving quantum Hamilton constraints of 3d Euclidean gravity in the weak coupling limit

  • Authors: Hanno Sahlmann, Waleed Sherif
  • Subjects: General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th); Computational Physics (physics.comp-ph)
  • Arxiv link: https://arxiv.org/abs/2405.00661
  • Abstract We consider 3-dimensional Euclidean gravity in the weak coupling limit of Smolin and show that it is BF-theory with $\text{U(1)}^3$ as a Lie group. The theory is quantised using loop quantum gravity methods. The kinematical degrees of freedom are truncated, on account of computational feasibility, by fixing a graph and deforming the algebra of the holonomies to impose a cutoff on the charge vectors. This leads to a quantum theory related to $\text{U}_q \text{(1)}^3$ BF-theory. The effect of imposing the cutoff on the charges is examined. We also implement the quantum volume operator of 3d loop quantum gravity. Most importantly we compare two constraints for the quantum model obtained: a master constraint enforcing curvature and Gauss constraint, as well as a combination of a quantum Hamilton constraint constructed using Thiemann’s strategy and the Gauss master constraint. The two constraints are solved using the neural network quantum state ansatz, demonstrating its ability to explore models which are out of reach for exact numerical methods. The solutions spaces are quantitatively compared and although the forms of the constraints are radically different, the solutions turn out to have a surprisingly large overlap. We also investigate the behavior of the quantum volume in solutions to the constraints.

There is no new related paper today

  1. [2405.00594] - Anomalous diffusion and factor ordering in (1+1)-dimensional Lorentzian quantum gravity - Elijah Sanderson, Rachel Lash Maitra, AJ Liberatore

  2. [2405.00083] - Density of States, Black Holes and the Emergent String Conjecture - Alek Bedroya, Rashmish K. Mishra, and Max Wiesner

  3. [2405.00114] - Gravitational entropy is observer-dependent - Julian De Vuyst, Stefan Eccles, Philipp A. Hoehn, Josh Kirklin

  4. [2405.00674] - Universal Bounds on CFT Distance Conjecture - Hirosi Ooguri, Yifan Wang

Wed - 05/01/24

There is no new related paper today

There is no new related paper today

  1. [2404.18956] - Quantum Field Theory of Black Hole Perturbations with Backreaction: I. General framework - Thomas Thiemann

  2. [2404.19526] - Scale and Conformal Invariance in 2d Sigma Models, with an Application to N=4 Supersymmetry - George Papadopoulos, Edward Witten

Tue - 04/30/24

Asymptotically safe – canonical quantum gravity junction

  • Authors: Thomas Thiemann
  • Subjects: High Energy Physics - Theory (hep-th); General Relativity and Quantum Cosmology (gr-qc)
  • Arxiv link: https://arxiv.org/abs/2404.18220
  • Abstract The canonical (CQG) and asymptotically safe (ASQG) approach to quantum gravity share to be both non-perturbative programmes. However, apart from that they seem to differ in several aspects such as: 1. Signature: CQG is Lorentzian while ASQG is mostly Euclidian. 2. Background Independence (BI): CQG is manifesly BI while ASQG is apparently not. 3. Truncations: CQG is apparently free of truncations while ASQG makes heavy use of them. The purpose of the present work is to either overcome actual differences or to explain why apparent differences are actually absent. Thereby we intend to enhance the contact and communication between the two communities. The focus of this contribution is on conceptual issues rather than deep technical details such has high order truncations. On the other hand the paper tries to be self-contained in order to be useful to researchers from both communities. The point of contact is the path integral formulation of Lorentzian CQG in its reduced phase space formulation which yields the formal generating functional of physical (i.e. gauge invariant) either Schwinger or Feynman N-point functions for (relational) observables. The corresponding effective actions of these generating functionals can then be subjected to the ASQG Wetterich type flow equations which serve in particular to find the rigorous generating fuctionals via the inverse Legendre transform of the fixed pointed effective action.

Relational Lorentzian Asymptotically Safe Quantum Gravity: Showcase model

  • Authors: Renata Ferrero, Thomas Thiemann
  • Subjects: High Energy Physics - Theory (hep-th); General Relativity and Quantum Cosmology (gr-qc)
  • Arxiv link: https://arxiv.org/abs/2404.18224
  • Abstract In a recent contribution we identified possible points of contact between the asymptotically safe and canonical approach to quantum gravity. The idea is to start from the reduced phase space (often called relational) formulation of canonical quantum gravity which provides a reduced (or physical) Hamiltonian for the true (observable) degrees of freedom. The resulting reduced phase space is then canonically quantised and one can construct the generating functional of time ordered Wightman (i.e. Feynman) or Schwinger distributions respectively from the corresponding time translation unitary group or contraction semigroup respectively as a path integral. For the unitary choice that path integral can be rewritten in terms of the Lorentzian Einstein Hilbert action plus observable matter action and a ghost action. The ghost action depends on the Hilbert space representation chosen for the canonical quantisation and a reduction term that encodes the reduction of the full phase space to the phase space of observavbles. This path integral can then be treated with the methods of asymptically safe quantum gravity in its {\it Lorentzian} version. We also exemplified the procedure using a concrete, minimalistic example namely Einstein-Klein-Gordon theory with as many neutral and massless scalar fields as there are spacetime dimensions. However, no explicit calculations were performed. In this paper we fill in the missing steps. Particular care is needed due to the necessary switch to Lorentzian signature which has strong impact on the convergence of ``heat’’ kernel time integrals in the heat kernel expansion of the trace involved in the Wetterich equation and which requires different cut-off functions than in the Euclidian version. As usual we truncate at relatively low order and derive and solve the resulting flow equations in that approximation.

There is no new related paper today

  1. [2404.17624] - Phase transitions, critical behavior and microstructure of the FRW universe in the framework of higher order GUP - Zhong-Wen Feng, Shi-Yu Li, Xia Zhou, Haximjan Abdusattart

  2. [2404.18741] - Torsion-induced axions in string theory, quantum gravity and the cosmological tensions - Nick E. Mavromatos, Panagiotis Dorlis, Sotirios-Neilos Vlachos

  3. [2404.18811] - Weak equivalence principle and nonrelativistic limit of general dispersion relations - Manuel Hohmann, Christian Pfeifer, Fabian Wagner

  4. [2404.18898] - Celestial Optical Theorem - Reiko Liu, Wen-Jie Ma

Mon - 04/29/24

There is no new related paper today

Partial absence of cosine problem in 3d Lorentzian spin foams

  • Authors: Alexander F. Jercher, José Diogo Simão, Sebastian Steinhaus
  • Subjects: General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th)
  • Arxiv link: https://arxiv.org/abs/2404.16943
  • Abstract We study the semi-classical limit of the recently proposed coherent spin foam model for (2+1) Lorentzian quantum gravity. Specifically, we analyze the gluing equations derived from the stationary phase approximation of the vertex amplitude. Typically these exhibit two solutions yielding a cosine of the Regge action. However, by inspection of the algebraic equations as well as their geometrical realization, we show in this note that the behavior is more nuanced: when all triangles are either spacelike or timelike, two solutions exist. In any other case, only a single solution is obtained, thus yielding a single Regge exponential.

  1. [2404.17071] - Bootstrapping the Abelian Lattice Gauge Theories - Zhijin Li, Shutong Zhou

  2. [2404.17556] - Curvature Correlators in Nonperturbative 2D Lorentzian Quantum Gravity - J. van der Duin, R. Loll

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