A cosmological model in Weyl-Cartan spacetime

We present a cosmological model for early stages of the universe on the basis of a Weyl-Cartan spacetime. In this model, torsion $T^{\alpha}$ and nonmetricity $Q_{\alpha \beta}$ are proportional to the vacuum polarization. Extending earlier work of one of us (RT), we discuss the behavior of the cosmic scale factor and the Weyl 1-form in detail. We show how our model fits into the more general framework of metric-affine gravity (MAG).Comment: 19 pages, 5 figures, typos corrected, uses IOP style fil

Translations and dynamics

We analyze the role played by local translational symmetry in the context of gauge theories of fundamental interactions. Translational connections and fields are introduced, with special attention being paid to their universal coupling to other variables, as well as to their contributions to field equations and to conserved quantities.Comment: 22 Revtex pages, no figures. Published version with minor correction

The Higgs sector of gravitational gauge theories

Gravitational gauge theories with de Sitter, Poincare and affine symmetry group are investigated under the aspect of the breakdown of the initial symmetry group down to the Lorentz subgroup. We review the theory of spontaneously broken de Sitter gravity by Stelle and West and apply a similar approach to the case of the Poincare and affine groups. Especially, we find that the groundstate of the metric affine theory leads to the determination of the Lorentzian signature of the metric in the groundstate. We show that the Higgs field remains in its groundstate, i.e., that the metric will have Lorentzian signature, unless we introduce matter fields that explicitely couple to the symmetric part of the connection. We also show that some features, like the necessity of the introduction of a dilaton field, that seem artificial in the context of the affine theory, appear most natural if the gauge group is taken to be the special linear group in five dimensions. Finally, we present an alternative model which is based on the spinor representation of the Lorentz group and is especially adopted to the description of spinor fields in a general linear covariant way, without the use of the infinite dimensional representations which are usually considered to be unavoidable.Comment: 50 pages, version to appear in Annals Phy

An astronomical search for evidence of new physics: Limits on gravity-induced birefringence from the magnetic white dwarf RE J0317-853

The coupling of the electromagnetic field directly with gravitational gauge fields leads to new physical effects that can be tested using astronomical data. Here we consider a particular case for closer scrutiny, a specific nonminimal coupling of torsion to electromagnetism, which enters into a metric-affine geometry of space-time. We show that under the assumption of this nonminimal coupling, spacetime is birefringent in the presence of such a gravitational field. This leads to the depolarization of light emitted from extended astrophysical sources. We use polarimetric data of the magnetic white dwarf ${RE J0317-853}$ to set strong constraints on the essential coupling constant for this effect, giving k^2 \lsim (19 {m})^2 .Comment: Statements about Moffat's NGT modified. Accepted for publication in Phys.Rev.

Physiological bases of bone regeneration II : The remodeling process

Bone remodeling is the restructuring process of existing bone, which is in constant resorption and formation. Under normal conditions, this balanced process allows the renewal of 5 ? 10% of bone volume per year. At the microscopic level, bone remodeling is produced in basic multicellular units, where osteoclasts resorb a certain quantity of bone and osteoblasts form the osteoid matrix and mineralize it to fill the previously created cavity. These units contain osteoclasts, macrophages, preosteoblasts and osteoblasts, and are controlled by a series of factors, both general and local, allowing normal bone function and maintaining the bone mass. When this process becomes unbalanced then bone pathology appears, either in excess (osteopetrosis) or deficit (osteoporosis). The purpose of this study is to undertake a revision of current knowledge on the physiological and biological mechanisms of the bone remodeling process; highlighting the role played by the regulating factors, in particular that of the growth factor

Physiological bases of bone regeneration I : Histology and physiology of bone tissue

Bone is the only body tissue capable of regeneration, allowing the restitutio ad integrum following trauma. In the event of a fracture or bone graft, new bone is formed, which following the remodeling process is identical to the pre-existing. Bone is a dynamic tissue in constant formation and resorption. This balanced phenomena, known as the remodeling process, allows the renovation of 5-15% of the total bone mass per year under normal conditions (1). Bone remodeling consists of the resorption of a certain amount of bone by osteoclasts, likewise the formation of osteoid matrix by osteoblasts, and its subsequent mineralization. This phenomenon occurs in small areas of the cortical bone or the trabecular surface, called ?Basic Multicellular Units? (BMU). Treatment in Traumatology, Orthopedics, Implantology, and Maxillofacial and Oral Surgery, is based on the biologic principals of bone regeneration, in which cells, extracellular matrix, and osteoinductive signals are involved. The aim of this paper is to provide an up date on current knowledge on the biochemical and physiological mechanisms of bone regeneration, paying particular attention to the role played by the cells and proteins of the bone matrix

Non-Linear Affine Embedding of the Dirac Field from the Multiplicity-Free SL(4,R) Unirreps

The correspondence between the linear multiplicity-free unirreps of SL(4, R) studied by Ne'eman and {\~{S}}ija{\~{c}}ki and the non-linear realizations of the affine group is worked out. The results obtained clarify the inclusion of spinorial fields in a non-linear affine gauge theory of gravitation.Comment: 13 pages, plain TeX, macros include

The Husain-Kuchar Model: Time Variables and Non-degenerate Metrics

We study the Husain-Kuchar model by introducing a new action principle similar to the self-dual action used in the Ashtekar variables approach to Quantum Gravity. This new action has several interesting features; among them, the presence of a scalar time variable that allows the definition of geometric observables without adding new degrees of freedom, the appearance of a natural non-degenerate four-metric and the possibility of coupling ordinary matter.Comment: LaTeX, 22 pages, accepted for publication in Phys. Rev.

Torsion nonminimally coupled to the electromagnetic field and birefringence

In conventional Maxwell--Lorentz electrodynamics, the propagation of light is influenced by the metric, not, however, by the possible presence of a torsion T. Still the light can feel torsion if the latter is coupled nonminimally to the electromagnetic field F by means of a supplementary Lagrangian of the type l^2 T^2 F^2 (l = coupling constant). Recently Preuss suggested a specific nonminimal term of this nature. We evaluate the spacetime relation of Preuss in the background of a general O(3)-symmetric torsion field and prove by specifying the optical metric of spacetime that this can yield birefringence in vacuum. Moreover, we show that the nonminimally coupled homogeneous and isotropic torsion field in a Friedmann cosmos affects the speed of light.Comment: Revtex, 12 pages, no figure

Growth hormone and aging

Les alteracions vasculars i degeneratives del sistema nerviós central (SNC) són dues de les causes més comunes de malaltia i de mort entre la gent gran; ambdues es correlacionen amb l'edat, amb la deficiència en GH, i poden afectar les funcions fisiològiques de la població d'edat avançada. Amb la finalitat de clarificar els efectes de la GH en el metabolisme, en els vasos i en el SNC, hem dut a terme un estudi in vivo utilitzant rates vellesWistar tractades crònicament amb GH. Les rates velles varen presentar un augment en el pes de greix i una disminució de l'índex específic de gravetat (SGI) (p < 0,05) en comparar-les amb les rates adultes no tractades. La GH va reduir el pes en greix (p < 0,05), i va mostrar també una tendència a augmentar l'SGI. Es va analitzar també la resposta de diverses substàncies vasoactives en els anells aòrtics, i es va demostrar una disminució de la vasodilatació per acetilcolina i isoprenalina (p < 0,05) en els animals vells. La contracció induïda per acetilcolina+L-NAME era més alta en els animals vells que en els adults. L'administració de GH millorava les respostes vasodilatadores (p < 0,05) mentre que tendia a reduïr les respostes vasoconstrictores. L'àrea aòrtica mitja augmentava també en les rates velles, mentre que la GH reduïa aquest paràmetre (p < 0,05). Les poblacions neuronals es reduïen en els hipocamps de les rates velles en comparar-les amb les joves. Aquesta reducció estava asociada a un augment dels nucleosomes i a una reducció de Bcl2 en el cervell. Les caspases 3 i 9 també varen augmentar. El tractament amb GH va augmentar significativament el nombre de neurones i va reduir els nucleosomes i les caspases i augmentar el Bcl2. En conclusió, el tractament perGHindueix l'aparició d'efectes beneficiosos en la composició del cos i ha restablert també les funcions cerebrals i vasculars en les rates velles.Vascular and degenerative alterations of the central nervous system (CNS) are two of the most common reasons for illness and death in elderly people; they exhibit an age-related GH deficiency that can affect their physiological functions. A study was conducted under chronic in vivo conditions using old Wistar rats, in order to clarify the effects of GH on the metabolism, vessels, and the CNS. The old rats showed an increased fat weight and a decreased Specific Gravity Index (SGI) (p < 0.05), as compared to the adult animals. GH reduced the fat weight (p < 0.05) and tended to increase the SGI (N.S.). The response to several vasoactive substances in aortic rings showed impaired vasodilatation to Acetylcholine and Isoprenaline (p < 0.05) in the old animals. Contraction, induced by Acetylcholine+ L-NAME, was higher in the old rats than in the adults. GH administration improved the vasodilatory responses (p < 0.05) and tended to reduce the constrictory responses. The aortic media area was increased in the old rats, and GH reduced this parameter (p < 0.05). The neuronal populations were reduced in the hippocampi of the old rats as compared to the young ones. This reduction was associated with an increase in nucleosomes and a reduction in Bcl2 in the brain. An increase was also detected in caspases 3 and 9. GH treatment was able to significantly enhance the number of neurons by reducing the nucleosomes and the caspases and by increasing Bcl2. In conclusion,GHtreatment was able to show beneficial effects on body composition and was able to restore both vascular and brain functions in the old rats