scholarly journals On the Structure of the Vacuum in Quantum Gravity: A View from the Asymptotic Safety Scenario

Universe ◽  
2019 ◽  
Vol 5 (8) ◽  
pp. 182 ◽  
Author(s):  
Alfio Bonanno
Keyword(s):  
Quantum Gravity ◽  
Degrees Of Freedom ◽  
Vacuum State ◽  
Critical Surface ◽  
Asymptotic Safety ◽  
Quantum Chromo Dynamics ◽  
Full Theory ◽  
Higher Derivative ◽  
Quadratic Gravity ◽  
Non Gaussian

Although the Asymptotic Safety scenario is one of the most promising approaches to quantum gravity, little attention has been devoted to the issue of the vacuum state. Higher derivative operators often appear on the ultraviolet critical surface around the non-Gaussian fixed point generating additional degrees of freedom which can render the standard vacuum unstable. When this happens, translation and rotational symmetries can be spontaneously broken and a new set of symmetries can show up at the level of the effective action. In this work, it will be argued that a “kinetic condensate” characterizes the vacuum state of asymptotically safe quadratic gravity theories. If this scenario is realized in the full theory, the vacuum state of gravity is the gravitational analogous to the Savvidy vacuum in Quantum Chromo-Dynamics (QCD).

Renormalization and unitarity in higher derivative and nonlocal gravity theories

2015 ◽  
Vol 30 (03n04) ◽  
pp. 1540005 ◽  
Author(s):  
E. T. Tomboulis
Keyword(s):  
Quantum Gravity ◽  
Fixed Points ◽  
Fourth Order ◽  
Entire Functions ◽  
Gravitational Action ◽  
Asymptotic Safety ◽  
Higher Derivative ◽  
Fixed Order ◽  
Gravity Theories

We review and discuss higher derivative and nonlocal theories of quantum gravity focusing on their UV and unitarity properties. We first consider the general fourth-order gravitational action, then actions containing derivatives up to any given fixed order, and discuss their UV divergences, fixed points and concomitant unitarity issues. This leads to a more general discussion of "asymptotic safety" and unitarity, which motivates the introduction of nonlocal theories containing derivatives to all orders arising from the expansion of entire functions. For such theories good UV behavior is visible at any finite truncation, but unitarity emerges only when derivatives to all orders are included.

scholarly journals Consistency of matter models with asymptotically safe quantum gravity

2015 ◽  
Vol 93 (9) ◽  
pp. 988-994 ◽  
Author(s):  
Pietro Donà ◽  
Astrid Eichhorn ◽  
Roberto Percacci
Keyword(s):  
Renormalization Group ◽  
Standard Model ◽  
Quantum Gravity ◽  
Degrees Of Freedom ◽  
Experimental Tests ◽  
Matter Content ◽  
Asymptotic Safety ◽  
The Standard Model ◽  
Renormalization Group Flows ◽  
Matter Fields

We discuss the compatibility of quantum gravity with dynamical matter degrees of freedom. Specifically, we present bounds we obtained in Donà et al. (Phys. Rev. D, 89, 084035 (2014) doi:10.1103/PhysRevD.89.084035 ) on the allowed number and type of matter fields within asymptotically safe quantum gravity. As a novel result, we show bounds on the allowed number of spin-3/2 (Rarita–Schwinger) fields (e.g., the gravitino). These bounds, obtained within truncated renormalization group flows, indicate the compatibility of asymptotic safety with the matter fields of the standard model. Further, they suggest that extensions of the matter content of the standard model are severely restricted in asymptotic safety. This means that searches for new particles at colliders could provide experimental tests for this particular approach to quantum gravity.

scholarly journals Exorcising ghosts in quantum gravity

2020 ◽  
Vol 135 (10) ◽  
Author(s):  
Iberê Kuntz
Keyword(s):  
Field Theory ◽  
Quantum Field Theory ◽  
Quantum Gravity ◽  
Boundary Conditions ◽  
Finite Number ◽  
Integration Contour ◽  
Quantum Field ◽  
Curvature Invariants ◽  
Higher Derivative ◽  
Quadratic Gravity

AbstractWe remark that Ostrogradsky ghosts in higher-derivative gravity, with a finite number of derivatives, are fictitious as they result from an unjustified truncation performed in a complete theory containing infinitely many curvature invariants. The apparent ghosts can then be projected out of the quadratic gravity spectrum by redefining the boundary conditions of the theory in terms of an integration contour that does not enclose the ghost poles. This procedure does not alter the renormalizability of the theory. One can thus use quadratic gravity as a quantum field theory of gravity that is both renormalizable and unitary.

EVIDENCE FOR PHASE TRANSITIONS CAUSED BY RADIAL FLUCTUATIONS IN AN SU(2)×U(1) LATTICE GAUGE-HIGGS THEORY

1988 ◽  
Vol 03 (01) ◽  
pp. 23-31 ◽  
Author(s):  
Yasuko Munehisa
Keyword(s):  
Monte Carlo ◽  
Phase Transitions ◽  
Degrees Of Freedom ◽  
Gauge Coupling ◽  
Higgs Field ◽  
Critical Surface ◽  
Monte Carlo Techniques ◽  
Full Theory ◽  
Lattice Gauge ◽  
Symmetric Region

Using Monte Carlo techniques, we study the phase structure of a radially active SU(2)×U(1) lattice gauge-fundamental Higgs theory in its strong U(1) gauge-coupling limit. Our results give first evidence for a critical surface induced by radial fluctuations of the Higgs field, which would extend inside the symmetric region of the full SU(2)×U(1) gauge-Higgs theory. The weak SU (2) gauge-coupling limit of the full theory, the radially active version of the SU(2)global×U(1)local gauge-Higgs theory, is also studied numerically. In this limit, we found no essential effect due to the radial degrees of freedom of the Higgs field.

scholarly journals Black holes within asymptotic safety

2014 ◽  
Vol 29 (08) ◽  
pp. 1430011 ◽  
Author(s):  
Benjamin Koch ◽  
Frank Saueressig
Keyword(s):  
Black Holes ◽  
Black Hole ◽  
Quantum Gravity ◽  
Space Time ◽  
Asymptotic Safety ◽  
Angular Momenta ◽  
Higher Derivative ◽  
The Status ◽  
Time Singularities ◽  
Characteristic Features

Black holes are probably among the most fascinating objects populating our universe. Their characteristic features found within general relativity, encompassing space–time singularities, event horizons, and black hole thermodynamics, provide a rich testing ground for quantum gravity ideas. We review the status of black holes within a particular proposal for quantum gravity, Weinberg's asymptotic safety program. Starting from a brief survey of the effective average action and scale setting procedures, an improved quantum picture of the black hole is developed. The Schwarzschild black hole and its generalizations including angular momenta, higher-derivative corrections and the implications of extra dimensions are discussed in detail. In addition, the quantum singularity emerging for the inclusion of a cosmological constant is elucidated and linked to the phenomenon of a dynamical dimensional reduction of space–time.

scholarly journals QUANTUM GRAVITY AND HIGHER CURVATURE ACTIONS

2007 ◽  
Vol 04 (01) ◽  
pp. 25-52 ◽  
Author(s):  
MARTIN BOJOWALD ◽  
AURELIANO SKIRZEWSKI
Keyword(s):  
Quantum Gravity ◽  
Degrees Of Freedom ◽  
General Scheme ◽  
Hamiltonian Formalism ◽  
Classical Type ◽  
Physical Situation ◽  
Quantum Theories ◽  
Higher Derivative ◽  
Physical Information

Effective equations are often useful to extract physical information from quantum theories without having to face all technical and conceptual difficulties. One can then describe aspects of the quantum system by equations of classical type, which correct the classical equations by modified coefficients and higher derivative terms. In gravity, for instance, one expects terms with higher powers of curvature. Such higher derivative formulations are discussed here with an emphasis on the role of degrees of freedom and on differences between Lagrangian and Hamiltonian treatments. A general scheme is then provided which allows one to compute effective equations perturbatively in a Hamiltonian formalism. Here, one can expand effective equations around any quantum state and not just a perturbative vacuum. This is particularly useful in situations of quantum gravity or cosmology where perturbations only around vacuum states would be too restrictive. The discussion also demonstrates the number of free parameters expected in effective equations, used to determine the physical situation being approximated, as well as the role of classical symmetries such as Lorentz transformation properties in effective equations. An appendix collects information on effective correction terms expected from loop quantum gravity and string theory.

scholarly journals Fakeons and the classicization of quantum gravity: the FLRW metric

2019 ◽  
Author(s):  
Damiano Anselmi
Keyword(s):  
Quantum Theory ◽  
Quantum Gravity ◽  
Classical Limit ◽  
Degrees Of Freedom ◽  
Vacuum Energy ◽  
Asymptotic Series ◽  
General Setting ◽  
Vacuum Energy Density ◽  
Field Equations ◽  
Higher Derivative

Under certain assumptions, it is possible to make sense of higher derivative theories by quantizing the unwanted degrees of freedom as fakeons, which are later projected away. Then the true classical limit is obtained by classicizing the quantum theory. Since quantum field theory is formulated perturbatively, the classicization is also perturbative. After deriving a number of properties in a general setting, we consider the theory of quantum gravity that emerges from the fakeon idea and study its classicization, focusing on the FLRW metric. We point out cases where the fakeon projection can be handled exactly, which include radiation, the vacuum energy density and the combination of the two, and cases where it cannot, which include dust. Generically, the classical limit shares many features with the quantum theory it comes from, including the impossibility to write down complete, “exact” field equations, to the extent that asymptotic series and nonperturbative effects come into play.

Gauge invariance and unitarity in higher-derivative quantum gravity

1986 ◽  
Vol 33 (10) ◽  
pp. 2756-2779 ◽  
Author(s):  
I. Antoniadis ◽  
E. T. Tomboulis
Keyword(s):  
Quantum Gravity ◽  
Gauge Invariance ◽  
Higher Derivative

scholarly journals ASYMPTOTIC SAFETY IN HIGHER-DERIVATIVE GRAVITY

2009 ◽  
Vol 24 (28) ◽  
pp. 2233-2241 ◽  
Author(s):  
DARIO BENEDETTI ◽  
PEDRO F. MACHADO ◽  
FRANK SAUERESSIG
Keyword(s):  
Fixed Point ◽  
Renormalization Group ◽  
Renormalization Group Flow ◽  
Functional Renormalization Group ◽  
Asymptotic Safety ◽  
Higher Derivative ◽  
Gravity Theories ◽  
Group Flow ◽  
Nonperturbative Renormalization

We study the nonperturbative renormalization group flow of higher-derivative gravity employing functional renormalization group techniques. The nonperturbative contributions to the β-functions shift the known perturbative ultraviolet fixed point into a nontrivial fixed point with three UV-attractive and one UV-repulsive eigendirections, consistent with the asymptotic safety conjecture of gravity. The implication of this transition on the unitarity problem, typically haunting higher-derivative gravity theories, is discussed.

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