839 research outputs found

### Gravity and Matter with Asymptotic Safety

Building a consistent Quantum Theory of Gravity is one of the most
challenging aspects of modern theoretical physics. In the past couple of years,
new attempts have been made along the path of ``asymptotic safety'' through the
use of Exact Renormalisation Group Equations, which hinge on the existence of a
non-trivial fixed point of the flow equations. We will first summarize the
major results that have been obtained along these lines, then we will consider
the effect of introducing matter fields into the theory. Our analyses show that
in order to preserve the existence of the fixed point one must satisfy some
constraints on the matter content of the theory.Comment: 5 pages, 3 figures; talk given at "Renormalization Group and
Anomalies in Gravitation and Cosmology", Ouro Preto, Brazil, March 200

### Black hole remnants due to GUP or quantum gravity?

Based on the micro-black hole \emph{gedanken} experiment as well as on
general considerations of quantum mechanics and gravity the generalized
uncertainty principle (GUP) is analyzed by using the running Newton constant.
The result is used to decide between the GUP and quantum gravitational effects
as a possible mechanism leading to the black hole remnants of about Planck
mass.Comment: 3 page

### Towards Nonperturbative Renormalizability of Quantum Einstein Gravity

We summarize recent evidence supporting the conjecture that four-dimensional
Quantum Einstein Gravity (QEG) is nonperturbatively renormalizable along the
lines of Weinberg's asymptotic safety scenario. This would mean that QEG is
mathematically consistent and predictive even at arbitrarily small length
scales below the Planck length. 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 cosmological
implications of this fixed point are discussed, and it is argued that QEG might
solve the horizon and flatness problem of standard cosmology without an
inflationary period.Comment: 10 pages, latex, 1 figur

### 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

### 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

### 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

### Bimetric Renormalization Group Flows in Quantum Einstein Gravity

The formulation of an exact functional renormalization group equation for
Quantum Einstein Gravity necessitates that the underlying effective average
action depends on two metrics, a dynamical metric giving the vacuum expectation
value of the quantum field, and a background metric supplying the coarse
graining scale. The central requirement of "background independence" is met by
leaving the background metric completely arbitrary. This bimetric structure
entails that the effective average action may contain three classes of
interactions: those built from the dynamical metric only, terms which are
purely background, and those involving a mixture of both metrics. This work
initiates the first study of the full-fledged gravitational RG flow, which
explicitly accounts for this bimetric structure, by considering an ansatz for
the effective average action which includes all three classes of interactions.
It is shown that the non-trivial gravitational RG fixed point central to the
Asymptotic Safety program persists upon disentangling the dynamical and
background terms. Moreover, upon including the mixed terms, a second
non-trivial fixed point emerges, which may control the theory's IR behavior.Comment: 35 pages, 3 figure

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