330 research outputs found
Fractal properties of quantum spacetime
We show that in general a spacetime having a quantum group symmetry has also
a scale dependent fractal dimension which deviates from its classical value at
short scales, a phenomenon that resembles what observed in some approaches to
quantum gravity. In particular we analyze the cases of a quantum sphere and of
\k-Minkowski, the latter being relevant in the context of quantum gravity.Comment: 4 pages, 2 figures; some minor corrections; reference adde
Signatures of gravitational fixed points at the LHC
We study quantum-gravitational signatures at the CERN Large Hadron Collider (LHC) in the context of theories with extra spatial dimensions and a low fundamental Planck scale in the TeV range. Implications of a gravitational fixed point at high energies are worked out using Wilson¿s renormalization group. We find that relevant cross sections involving virtual gravitons become finite. Based on gravitational lepton pair production we conclude that the LHC is sensitive to a fundamental Planck scale of up to 6 TeV
Asymptotically Safe Lorentzian Gravity
The gravitational asymptotic safety program strives for a consistent and
predictive quantum theory of gravity based on a non-trivial ultraviolet fixed
point of the renormalization group (RG) flow. We investigate this scenario by
employing a novel functional renormalization group equation which takes the
causal structure of space-time into account and connects the RG flows for
Euclidean and Lorentzian signature by a Wick-rotation. Within the
Einstein-Hilbert approximation, the -functions of both signatures
exhibit ultraviolet fixed points in agreement with asymptotic safety.
Surprisingly, the two fixed points have strikingly similar characteristics,
suggesting that Euclidean and Lorentzian quantum gravity belong to the same
universality class at high energies.Comment: 4 pages, 2 figure
Spectral geometry as a probe of quantum spacetime
Employing standard results from spectral geometry, we provide strong evidence
that in the classical limit the ground state of three-dimensional causal
dynamical triangulations is de Sitter spacetime. This result is obtained by
measuring the expectation value of the spectral dimension on the ensemble of
geometries defined by these models, and comparing its large scale behaviour to
that of a sphere (Euclidean de Sitter). From the same measurement we are also
able to confirm the phenomenon of dynamical dimensional reduction observed in
this and other approaches to quantum gravity -- the first time this has been
done for three-dimensional causal dynamical triangulations. In this case, the
value for the short-scale limit of the spectral dimension that we find is
approximately 2. We comment on the relevance of these results for the
comparison to asymptotic safety and Horava-Lifshitz gravity, among other
approaches to quantum gravity.Comment: 25 pages, 6 figures. Version 2: references to figures added,
acknowledgment added
An effective action for asymptotically safe gravity
Asymptotically safe theories of gravitation have received great attention in
recent times. In this framework an effective action embodying the basic
features of the renormalized flow around the non-gaussian fixed point is
derived and its implications for the early universe are discussed. In
particular, a "landscape" of a countably infinite number of cosmological
inflationary solutions characterized by an unstable de Sitter phase lasting for
a large enough number of e-folds is found.Comment: 5 pages, to appear as a Rapid Communication in Physical Review
Quasinormal modes for asymptotic safe black holes
Under the hypothesis of asymptotic safety of gravity, the static, spherically
symmetric black hole solutions in the infrared limit are corrected by
non-perturbative effects. Specifically, the metric is modified by the running
of gravitational couplings. In this work, we investigate the effects of this
correction to the quasinormal modes (QNMs) of a test scalar field propagating
in this kind of black hole background analytically and numerically. It is found
that although the quasi-period frequencies and the damping of oscillations are
respectively enhanced and weakened by the quantum correction term, the
stability of the black hole remains.Comment: 11 pages, 1 figures, accepted for publication in CQG. arXiv admin
note: text overlap with arXiv:1007.131
Quark contact interactions at the LHC
Quark contact interactions are an important signal of new physics. We
introduce a model in which the presence of a symmetry protects these new
interactions from giving large corrections in flavor changing processes at low
energies. This minimal model provides the basic set of operators which must be
considered to contribute to the high-energy processes. To discuss their
experimental signature in jet pairs produced in proton-proton colllisions, we
simplify the number of possible operators down to two. We show (for a
representative integrated luminosity of 200 pb^-1 at \surd s = 7 TeV) how the
presence of two operators significantly modifies the bound on the
characteristic energy scale of the contact interactions which is obtained by
keeping a single operator.Comment: 8 pages, 2 figure
Renormalisation group improvement of scalar field inflation
We study quantum corrections to Friedmann-Robertson-Walker cosmology with a
scalar field under the assumption that the dynamics are subject to
renormalisation group improvement. We use the Bianchi identity to relate the
renormalisation group scale to the scale factor and obtain the improved
cosmological evolution equations. We study the solutions of these equations in
the renormalisation group fixed point regime, obtaining the time-dependence of
the scalar field strength and the Hubble parameter in specific models with
monomial and trinomial quartic scalar field potentials. We find that power-law
inflation can be achieved in the renormalisation group fixed point regime with
the trinomial potential, but not with the monomial one. We study the transition
to the quasi-classical regime, where the quantum corrections to the couplings
become small, and find classical dynamics as an attractor solution for late
times. We show that the solution found in the renormalisation group fixed point
regime is also a cosmological fixed point in the autonomous phase space. We
derive the power spectrum of cosmological perturbations and find that the
scalar power spectrum is exactly scale-invariant and bounded up to arbitrarily
small times, while the tensor perturbations are tilted as appropriate for the
background power-law inflation. We specify conditions for the renormalisation
group fixed point values of the couplings under which the amplitudes of the
cosmological perturbations remain small.Comment: 17 pages; 2 figure
One Loop Beta Functions in Topologically Massive Gravity
We calculate the running of the three coupling constants in cosmological,
topologically massive 3d gravity. We find that \nu, the dimensionless
coefficient of the Chern-Simons term, has vanishing beta function. The flow of
the cosmological constant and Newton's constant depends on \nu, and for any
positive \nu there exist both a trivial and a nontrivial fixed point.Comment: 44 pages, 16 figure
Spherically symmetric ADM gravity with variable G and Lambda(c)
This paper investigates the Arnowitt--Deser--Misner (hereafter ADM) form of
spherically symmetric gravity with variable Newton parameter G and cosmological
term Lambda(c). The Newton parameter is here treated as a dynamical variable,
rather than being merely an external parameter as in previous work on closely
related topics. The resulting Hamilton equations are obtained; interestingly, a
static solution exists, that reduces to Schwarzschild geometry in the limit of
constant G, describing a Newton parameter ruled by a nonlinear differential
equation in the radial variable r. A remarkable limiting case is the one for
which the Newton parameter obeys an almost linear growth law at large r. An
exact solution for G as a function of r is also obtained in the case of
vanishing cosmological constant. Some observational implications of these
solutions are obtained and briefly discussed.Comment: 16 pages, 2 figures. The presentation has been improved in all
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