494 research outputs found
Contraints on Matter from Asymptotic Safety
Recent studies of the ultraviolet behaviour of pure gravity suggest that it
admits a non-Gaussian attractive fixed point, and therefore that the theory is
asymptotically safe. We consider the effect on this fixed point of massless
minimally coupled matter fields. The existence of a UV attractive fixed point
puts bounds on the type and number of such fields.Comment: 5 pages, 2 figures, revtex4; introduction expande
Renormalization Group Flow of the Holst Action
The renormalization group (RG) properties of quantum gravity are explored,
using the vielbein and the spin connection as the fundamental field variables.
The scale dependent effective action is required to be invariant both under
space time diffeomorphisms and local frame rotations. The nonperturbative RG
equation is solved explicitly on the truncated theory space defined by a three
parameter family of Holst-type actions which involve a running Immirzi
parameter. We find evidence for the existence of an asymptotically safe
fundamental theory, probably inequivalent to metric quantum gravity constructed
in the same way.Comment: 5 pages, 1 figur
Asymptotic Safety of Gravity Coupled to Matter
Nonperturbative treatments of the UV limit of pure gravity suggest that it
admits a stable fixed point with positive Newton's constant and cosmological
constant. We prove that this result is stable under the addition of a scalar
field with a generic potential and nonminimal couplings to the scalar
curvature. There is a fixed point where the mass and all nonminimal scalar
interactions vanish while the gravitational couplings have values which are
almost identical to the pure gravity case. We discuss the linearized flow
around this fixed point and find that the critical surface is four-dimensional.
In the presence of other, arbitrary, massless minimally coupled matter fields,
the existence of the fixed point, the sign of the cosmological constant and the
dimension of the critical surface depend on the type and number of fields. In
particular, for some matter content, there exist polynomial asymptotically free
scalar potentials, thus providing a solution to the well-known problem of
triviality.Comment: 18 pages,typeset with revtex
Is Quantum Einstein Gravity Nonperturbatively Renormalizable?
We find considerable evidence supporting the conjecture that four-dimensional
Quantum Einstein Gravity is ``asymptotically safe'' in Weinberg's sense. This
would mean that the theory is likely to be nonperturbatively renormalizable and
thus could be considered a fundamental (rather than merely effective) theory
which is mathematically consistent and predictive down to arbitrarily small
length scales. For a truncated version of the exact flow equation of the
effective average action we establish the existence of a non-Gaussian
renormalization group fixed point which is suitable for the construction of a
nonperturbative infinite cutoff-limit. The truncation ansatz includes the
Einstein-Hilbert action and a higher derivative term.Comment: 18 pages, latex, 3 figure
Renormalization of 3d quantum gravity from matrix models
Lorentzian simplicial quantum gravity is a non-perturbatively defined theory
of quantum gravity which predicts a positive cosmological constant. Since the
approach is based on a sum over space-time histories, it is perturbatively
non-renormalizable even in three dimensions. By mapping the three-dimensional
theory to a two-matrix model with ABAB interaction we show that both the
cosmological and the (perturbatively) non-renormalizable gravitational coupling
constant undergo additive renormalizations consistent with canonical
quantization.Comment: 14 pages, 3 figure
Background Independence and Asymptotic Safety in Conformally Reduced Gravity
We analyze the conceptual role of background independence in the application
of the effective average action to quantum gravity. Insisting on a background
independent renormalization group (RG) flow the coarse graining operation must
be defined in terms of an unspecified variable metric since no rigid metric of
a fixed background spacetime is available. This leads to an extra field
dependence in the functional RG equation and a significantly different RG flow
in comparison to the standard flow equation with a rigid metric in the mode
cutoff. The background independent RG flow can possess a non-Gaussian fixed
point, for instance, even though the corresponding standard one does not. We
demonstrate the importance of this universal, essentially kinematical effect by
computing the RG flow of Quantum Einstein Gravity in the ``conformally
reduced'' Einstein--Hilbert approximation which discards all degrees of freedom
contained in the metric except the conformal one. Without the extra field
dependence the resulting RG flow is that of a simple -theory. Including
it one obtains a flow with exactly the same qualitative properties as in the
full Einstein--Hilbert truncation. In particular it possesses the non-Gaussian
fixed point which is necessary for asymptotic safety.Comment: 4 figures
Vanishing Dimensions and Planar Events at the LHC
We propose that the effective dimensionality of the space we live in depends
on the length scale we are probing. As the length scale increases, new
dimensions open up. At short scales the space is lower dimensional; at the
intermediate scales the space is three-dimensional; and at large scales, the
space is effectively higher dimensional. This setup allows for some fundamental
problems in cosmology, gravity, and particle physics to be attacked from a new
perspective. The proposed framework, among the other things, offers a new
approach to the cosmological constant problem and results in striking collider
phenomenology and may explain elongated jets observed in cosmic-ray data.Comment: v1: 5 pages revtex, 1 eps figure; v2: includes extensive discussion
on violation of Lorentz invariance, featured in a Nature editorial [Nature
466 (2010) 426] http://www.nature.com/news/2010/100720/full/466426a.html; v3:
discussion expanded, matching journal versio
The Renormalization Group, Systems of Units and the Hierarchy Problem
In the context of the Renormalization Group (RG) for gravity I discuss the
role of field rescalings and their relation to choices of units. I concentrate
on a simple Higgs model coupled to gravity, where natural choices of units can
be based on Newton's constant or on the Higgs mass. These quantities are not
invariant under the RG, and the ratio between the units is scale-dependent. In
the toy model, strong RG running occurs in the intermediate regime between the
Higgs and the Planck scale, reproducing the results of the Randall-Sundrum I
model. Possible connections with the problem of the mass hierarchy are pointed
out.Comment: Plain TEX, 16 pages. Some revisions, some references adde
Fractal space-times under the microscope: A Renormalization Group view on Monte Carlo data
The emergence of fractal features in the microscopic structure of space-time
is a common theme in many approaches to quantum gravity. In this work we carry
out a detailed renormalization group study of the spectral dimension and
walk dimension associated with the effective space-times of
asymptotically safe Quantum Einstein Gravity (QEG). We discover three scaling
regimes where these generalized dimensions are approximately constant for an
extended range of length scales: a classical regime where , a
semi-classical regime where , and the UV-fixed point
regime where . On the length scales covered by
three-dimensional Monte Carlo simulations, the resulting spectral dimension is
shown to be in very good agreement with the data. This comparison also provides
a natural explanation for the apparent puzzle between the short distance
behavior of the spectral dimension reported from Causal Dynamical
Triangulations (CDT), Euclidean Dynamical Triangulations (EDT), and Asymptotic
Safety.Comment: 26 pages, 6 figure
Scale-dependent metric and causal structures in Quantum Einstein Gravity
Within the asymptotic safety scenario for gravity various conceptual issues
related to the scale dependence of the metric are analyzed. The running
effective field equations implied by the effective average action of Quantum
Einstein Gravity (QEG) and the resulting families of resolution dependent
metrics are discussed. The status of scale dependent vs. scale independent
diffeomorphisms is clarified, and the difference between isometries implemented
by scale dependent and independent Killing vectors is explained. A concept of
scale dependent causality is proposed and illustrated by various simple
examples. The possibility of assigning an "intrinsic length" to objects in a
QEG spacetime is also discussed.Comment: 52 page
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