49 research outputs found
The Gravitational Two-Loop Counterterm is Asymptotically Safe
Weinberg's asymptotic safety scenario provides an elegant mechanism to
construct a quantum theory of gravity within the framework of quantum field
theory based on a non-Gau{\ss}ian fixed point of the renormalization group
flow. In this work we report novel evidence for the validity of this scenario,
using functional renormalization group techniques to determine the
renormalization group flow of the Einstein-Hilbert action supplemented by the
two-loop counterterm found by Goroff and Sagnotti. The resulting system of beta
functions comprises three scale-dependent coupling constants and exhibits a
non-Gau{\ss}ian fixed point which constitutes the natural extension of the one
found at the level of the Einstein-Hilbert action. The fixed point exhibits two
ultraviolet attractive and one repulsive direction supporting a low-dimensional
UV-critical hypersurface. Our result vanquishes the longstanding criticism that
asymptotic safety will not survive once a "proper perturbative counterterm" is
included in the projection space.Comment: 7 pages, 1 figur
Rigid Supersymmetry from Conformal Supergravity in Five Dimensions
We study the rigid limit of 5d conformal supergravity with minimal
supersymmetry on Riemannian manifolds. The necessary and sufficient condition
for the existence of a solution is the existence of a conformal Killing vector.
Whenever a certain curvature becomes abelian the backgrounds define a
transversally holomorphic foliation. Subsequently we turn to the question under
which circumstances these backgrounds admit a kinetic Yang-Mills term in the
action of a vector multiplet. Here we find that the conformal Killing vector
has to be Killing. We supplement the discussion with various appendices.Comment: 23 page
Slow-roll inflation in gravity
In the framework of gravity theory, the
slow-roll approximation of the cosmic inflation is investigated, where is
the trace of the energy-momentum tensor , and are the
Ricci scalar and tensor, respectively. After obtaining the equations of motion
of the gravitational field from the action principle in the spatially flat FLRW
metric, the fundamental equations of this theory are received by introducing
the inflation scalar field as the matter and taking into account only the
minimum curvature-inflation coupling term. Remarkably, after taking the
slow-roll approximation, the identical equations as in gravity with a
mixing term are derived. Several potentials of interest in different
domains are evaluated individually, calculating the slow-roll parameter and the
e-folding number . Finally, we analyze the behavior of the inflation scalar
field under perturbation while ignoring the effect of metric perturbations.
This research complements the slow-roll inflation in the modified theory of
gravity.Comment: 14 pages, 1 figur
Four Point Functions of the Stress Tensor and Conserved Currents in AdS_4/CFT_3
We compute four point functions of the stress tensor and conserved currents
in AdS_4/CFT_3 using bulk perturbation theory. We work at treel level in the
bulk theory, which we take to be either pure gravity or Yang Mills theory in
AdS. We bypass the tedious evaluation of Witten diagrams using recently
developed recursion relations for these correlators. In this approach, the four
point function is obtained as the sum of residues of a rational function at
easily identifiable poles. We write down an explicit formula for the four point
correlator with arbitrary external helicities and momenta. We verify that,
precisely as conjectured in a companion paper, the Maximally Helicity Violating
(MHV) amplitude of gravitons or gluons appears as the coefficient of a
specified singularity in the MHV stress-tensor or current correlator. We
comment on the remarkably simple analytic structure of our answers in momentum
space.Comment: 44 pages. Zipped source includes a Mathematica notebook (v2)
discussion section revise
Lorentz symmetry is relevant
We set up a covariant renormalisation group equation on a foliated spacetime
which preserves background diffeomorphism symmetry. As a first application of
the new formalism, we study the effect of quantum fluctuations in Lorentz
symmetry breaking theories of quantum gravity. It is found that once a small
breaking is introduced e.g. at the Planck scale, quantum fluctuations enhance
this breaking at low energies. A numerical analysis shows that the
magnification is of order unity for trajectories compatible with a small
cosmological constant. The immediate consequence is that the stringent
observational constraints on Lorentz symmetry breaking are essentially
scale-independent and must be met even at the Planck scale.Comment: 10 pages, 1 figur
xTras: a field-theory inspired xAct package for Mathematica
We present the tensor computer algebra package xTras, which provides
functions and methods frequently needed when doing (classical) field theory.
Amongst others, it can compute contractions, make Ans\"atze, and solve
tensorial equations. It is built upon the tensor computer algebra system xAct,
a collection of packages for Mathematica.Comment: 29 pages. The package can be downloaded from
http://www.xact.es/xtras