177,135 research outputs found
Graviweak Unification
No full textThe coupling of chiral fermions to gravity makes use only of the selfdual S U (2) subalgebra of the (complexified) SO(3, 1) algebra. It is possible to identify the antiselfdual subalgebra with the S U (2)L isospin group that appears in the standard model, or with its right-handed counterpart SU(2)R that appears in some extensions. Based on this observation, we describe a form of unification of the gravitational and weak interactions. We also discuss models with fermions of both chiralities, the inclusion of strong interactions, and the way in which these unified models of gravitational and gauge interactions avoid conflict with the Coleman–Mandula theorem
Ultraviolet properties of f(R) Gravity
No full textWe discuss the existence and properties of a nontrivial fixed point in f(R)-gravity, where f is a polynomial of order up to six. Within this seven-parameter class of theories, the fixed point has three ultraviolet-attractive and four ultraviolet-repulsive directions; this brings further support to the hypothesis that gravity is nonperturbatively renormalizabile
F (R, Rμν2) at one loop
No full textWe compute the one-loop divergences in a theory of gravity with a Lagrangian of the general form f(R,RμνRμν), on an Einstein background. We also establish that the one-loop effective action is invariant under a duality that consists of changing certain parameters in the relation between the metric and the quantum fluctuation field. Finally, we discuss the unimodular version of such a theory and establish its equivalence at one-loop order with the general case
On Target-Space Duality in p-Branes
No full textWe study the target space duality transformations in p-branes as transformations which mix the world volume field equations with Bianchi identities. We consider an (m+p+1)-dimensional space-time with p + 1 dimensions compactified, and a particular form of the background fields. We find that while a GL(2) = SL(2) x R group is realized when m = 0, only a two-parameter group is realized when m > 0
One loop beta functions and fixed points in higher derivative sigma models
No full textWe calculate the one loop beta functions for nonlinear sigma models in four dimensions containing general two and four derivative terms. In the O(N) model there are four such terms and nontrivial fixed points exist for all N geq 4. In the chiral SU(N) models there are in general six couplings, but only five for N=3 and four for N=2; we find fixed points only for N=2,3. In the approximation considered, the four derivative couplings are asymptotically free but the coupling in the two derivative term has a nonzero limit. These results support the hypothesis that certain sigma models may be asymptotically safe
GAUSS LAW COMMUTATOR IN THE CHIRALLY GAUGED WESS-ZUMINO-WITTEN MODEL
No full textWe evaluate the commutator of the Gauss law constraints starting from the chirally gauged Wess-Zumino-Witten action. The calculations are done at tree level, i.e. by evaluating corresponding Poisson brackets. The results are compared with commutators obtained by others directly from the gauged fermionic theory, and with Faddeev's results based on cohomology
The renormalization group, systems of units and the hierarchy problem
No full textIn 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
Unimodular quantum gravity and the cosmological constant
No full textIt is shown that the one-loop effective action of unimodular gravity is the same as that of ordinary gravity, restricted to unimodular metrics. The only difference is in the treatment of the global scale degree of freedom and of the cosmological term. A constant vacuum energy does not gravitate, addressing one aspect of the cosmological constant problem
Fixed Points of Higher Derivative Gravity
No full textWe recalculate the beta functions of higher derivative gravity in four dimensions using the one--loop approximation to an Exact Renormalization Group Equation. We reproduce the beta functions of the dimensionless couplings that were known in the literature but we find new terms for the beta functions of Newton's constant and of the cosmological constant. As a result, the theory appears to be asymptotically safe at a non--Gaussian Fixed Point, rather than perturbatively renormalizable and asymptotically free
Towards metric-affine quantum gravity
No full textI review here some motivations to consider a theory of gravity based on independent metric and connection, and its status as a quantum theory
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