Bekaert X.,University of Tours |
Boulanger N.,University of Mons |
Sundell P.A.,University of Mons
Reviews of Modern Physics | Year: 2012
Aiming at nonexperts, the key mechanisms of higher-spin extensions of ordinary gravities in four dimensions and higher are explained. An overview of various no-go theorems for low-energy scattering of massless particles in flat spacetime is given. In doing so, a connection between the S-matrix and the Lagrangian approaches is made, exhibiting their relative advantages and weaknesses, after which potential loopholes for nontrivial massless dynamics are highlighted. Positive yes-go results for non-Abelian cubic higher-derivative vertices in constantly curved backgrounds are reviewed. Finally, how higher-spin symmetry can be reconciled with the equivalence principle in the presence of a cosmological constant leading to the Fradkin-Vasiliev vertices and Vasiliev's higher-spin gravity with its double perturbative expansion (in terms of numbers of fields and derivatives) is outlined. © 2012 American Physical Society.
Volkov M.S.,University of Tours
Journal of High Energy Physics | Year: 2012
We present solutions describing homogeneous and isotropic cosmologies in the massive gravity theory with two dynamical metrics recently proposed in arXiv:1109.3515 and maintained to be ghost free. These solutions can be spatially open, closed, or flat, and at early times they are sourced by the perfect fluid, while the graviton mass typically manifests itself at late times by giving rise to a cosmological term. In addition, there are also exotic solutions, for which already at early times, when the matter density is high, the contribution of the graviton mass to the energy density is negative and large enough to screen that of the matter contribution. The total energy can then be negative, which may result in removing the initial singularity. For special parameter values there are also solutions for which the two metrics effectively decouple and evolve independently of each other. In the limit where one of the gravitational coupling constant vanishes, such special solutions reduce to those found in arXiv:1107.5504 within the theory where one of the metrics is flat.
Maruani A.,University of Tours
Journal of Investigative Dermatology | Year: 2014
Among the 100 initial priority topics for comparative effectiveness research, three concern topical drugs in the following dermatologic diseases: psoriasis, chronic lower-extremity wounds (CLEWs), and acne vulgaris (AV). Our objective was to explore the geometry of the corresponding networks of randomized controlled trials (RCTs). We performed a review of RCTs on topical drugs in psoriasis, CLEWs, and AV. We searched MEDLINE, Embase, and CENTRAL for published trials from 2007 to 2012 and ClinicalTrials.gov for unpublished trials registered since 2011. RCTs comparing at least one topical treatment with any active or inactive comparator, regardless of RCT design and outcomes, were eligible. We produced network graphs (each node representing a treatment and links between nodes representing trials) and tested for co-occurrence (preference or avoidance of specific comparisons). We included 60 RCTs on psoriasis (14,255 patients) and 19 registered RCTs, 50 of CLEWs (5,916 patients) and 7 registered RCTs, and 90 of AV (22,984 patients) and 21 registered RCTs. Head-to-head comparisons were made in 78%, 32%, and 57% of published RCTs of these conditions, respectively. The co-occurrence test suggested that no specific head-to-head comparison was significantly preferred or avoided (P-value=0.53, 0.20, and 0.57, respectively). This study has limitations, the main being that the search period was restricted to 5 years. In conclusion, more comparative effectiveness trials are needed for CLEWs, for which head-to-head comparisons are fewer than those for psoriasis and AV.Journal of Investigative Dermatology advance online publication, 28 August 2014; doi:10.1038/jid.2014.296.
Solodukhin S.N.,University of Tours
Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics | Year: 2016
We analyze the structure of the boundary terms in the conformal anomaly integrated over a manifold with boundaries. We suggest that the anomalies of type B, polynomial in the Weyl tensor, are accompanied with the respective boundary terms of the Gibbons-Hawking type. Their form is dictated by the requirement that they produce a variation which compensates the normal derivatives of the metric variation on the boundary in order to have a well-defined variational procedure. This suggestion agrees with recent findings in four dimensions for free fields of various spins. We generalize this consideration to six dimensions and derive explicitly the respective boundary terms. We point out that the integrated conformal anomaly in odd dimensions is non-vanishing due to the boundary terms. These terms are specified in three and five dimensions. © 2015 The Author.
Niedermaier M.,University of Tours
Nuclear Physics B | Year: 2010
The fixed point structure of the renormalization flow in Einstein gravity and higher derivative gravity is investigated in terms of the background effective action. A refined perturbative framework is developed consisting of: use of a covariant operator regularization that keeps track of powerlike divergences, a non-minimal subtraction ansatz for the originally dimensionful couplings in combination with a 'Wilsonian' matching condition, and the construction of a one-loop effective action exactly gauge-independent on-shell in regularized form. Using this framework strictly positive fixed points for the dimensionless Newton constant gN and the cosmological constant λ can be identified already in one-loop perturbation theory. The renormalization flow is asymptotically safe with respect to the nontrivial fixed points in both cases. In Einstein gravity a residual gauge dependence of the fixed points is unavoidable while in higher derivative gravity both the fixed point and the flow equations are universal. Along this flow spectral positivity of the Hessians can be satisfied, thereby meeting an essential condition for a well-defined Euclidean field theory setting. Dependence on O(10) initial data is erased to accuracy 0.5% after O(100) units of the renormalization mass scale and the flow settles on a λ(gN) orbit. © 2010 Elsevier B.V.