10,041 research outputs found
Proper Time Flow Equation for Gravity
We analyze a proper time renormalization group equation for Quantum Einstein
Gravity in the Einstein-Hilbert truncation and compare its predictions to those
of the conceptually different exact renormalization group equation of the
effective average action. We employ a smooth infrared regulator of a special
type which is known to give rise to extremely precise critical exponents in
scalar theories. We find perfect consistency between the proper time and the
average action renormalization group equations. In particular the proper time
equation, too, predicts the existence of a non-Gaussian fixed point as it is
necessary for the conjectured nonperturbative renormalizability of Quantum
Einstein Gravity.Comment: 11 pages, revtex4, no figures, bibte
Renormalization of the Topological Charge in Yang-Mills Theory
The conditions leading to a nontrivial renormalization of the topological
charge in four--dimensional Yang--Mills theory are discussed. It is shown that
if the topological term is regarded as the limit of a certain nontopological
interaction, quantum effects due to the gauge bosons lead to a finite
multiplicative renormalization of the theta--parameter while fermions give rise
to an additional shift of theta. A truncated form of an exact renormalization
group equation is used to study the scale dependence of the theta--parameter.
Possible implications for the strong CP--problem of QCD are discussed.Comment: 31 pages, late
The role of Background Independence for Asymptotic Safety in Quantum Einstein Gravity
We discuss various basic conceptual issues related to coarse graining flows
in quantum gravity. In particular the requirement of background independence is
shown to lead to renormalization group (RG) flows which are significantly
different from their analogs on a rigid background spacetime. The importance of
these findings for the asymptotic safety approach to Quantum Einstein Gravity
(QEG) is demonstrated in a simplified setting where only the conformal factor
is quantized. We identify background independence as a (the ?) key prerequisite
for the existence of a non-Gaussian RG fixed point and the renormalizability of
QEG.Comment: 2 figures. Talk given by M.R. at the WE-Heraeus-Seminar "Quantum
Gravity: Challenges and Perspectives", Bad Honnef, April 14-16, 2008; to
appear in General Relativity and Gravitatio
Running Gauge Coupling in Asymptotically Safe Quantum Gravity
We investigate the non-perturbative renormalization group behavior of the
gauge coupling constant using a truncated form of the functional flow equation
for the effective average action of the Yang-Mills-gravity system. We find a
non-zero quantum gravity correction to the standard Yang-Mills beta function
which has the same sign as the gauge boson contribution. Our results fit into
the picture according to which Quantum Einstein Gravity (QEG) is asymptotically
safe, with a vanishing gauge coupling constant at the non-trivial fixed point.Comment: 27 page
Renormalization Group Flow of Quantum Gravity in the Einstein-Hilbert Truncation
The exact renormalization group equation for pure quantum gravity is used to
derive the non-perturbative \Fbeta-functions for the dimensionless Newton
constant and cosmological constant on the theory space spanned by the
Einstein-Hilbert truncation. The resulting coupled differential equations are
evaluated for a sharp cutoff function. The features of these flow equations are
compared to those found when using a smooth cutoff. The system of equations
with sharp cutoff is then solved numerically, deriving the complete
renormalization group flow of the Einstein-Hilbert truncation in . The
resulting renormalization group trajectories are classified and their physical
relevance is discussed. The non-trivial fixed point which, if present in the
exact theory, might render Quantum Einstein Gravity nonperturbatively
renormalizable is investigated for various spacetime dimensionalities.Comment: 58 pages, latex, 24 figure
Quantum Gravity effects near the null black hole singularity
The structure of the Cauchy Horizon singularity of a black hole formed in a
generic collapse is studied by means of a renormalization group equation for
quantum gravity. It is shown that during the early evolution of the Cauchy
Horizon the increase of the mass function is damped when quantum fluctuations
of the metric are taken into account.Comment: 15 Pages, one figure. Minor changes in the presentation, to appear on
Phys.Rev.
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
CO oxidation on Pd(100) at technologically relevant pressure conditions: A first-principles kinetic Monte Carlo study
The possible importance of oxide formation for the catalytic activity of
transition metals in heterogenous oxidation catalysis has evoked a lively
discussion over the recent years. On the more noble transition metals (like Pd,
Pt or Ag) the low stability of the common bulk oxides suggests primarily
sub-nanometer thin oxide films, so-called surface oxides, as potential
candidates that may be stabilized under gas phase conditions representative of
technological oxidation catalysis. We address this issue for the Pd(100) model
catalyst surface with first-principles kinetic Monte Carlo (kMC) simulations
that assess the stability of the well-characterized (sqrt{5} x sqrt{5})R27
surface oxide during steady-state CO oxidation. Our results show that at
ambient pressure conditions the surface oxide is stabilized at the surface up
to CO:O2 partial pressure ratios just around the catalytically most relevant
stoichiometric feeds (p(CO):p(O2) = 2:1). The precise value depends sensitively
on temperature, so that both local pressure and temperature fluctuations may
induce a continuous formation and decomposition of oxidic phases during
steady-state operation under ambient stoichiometric conditions.Comment: 13 pages including 5 figures; related publications can be found at
http://www.fhi-berlin.mpg.de/th/th.htm
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