Deriving Gauge Symmetry and Spontaneous Lorentz Violation

We consider a class of field theories with a four-vector field $A_{\mu}(x)$ in addition to other fields supplied with a global charge symmetry - theories which have partial gauge symmetry in the sense of only imposing it on those terms in the Lagrangian density which have derivatives as factors in them. We suppose that spontaneous Lorentz invariance breaking occurs in such a theory due to the four-vector field taking a non-zero vacuum expectation value. Under some very mild assumptions, we show that this Lorentz violation is not observable and the whole theory is practically gauge invariant. A very important presupposition for this theorem is that an initial condition is imposed on the no-derivative expressions corresponding to the early Universe being essentially in a vacuum state. This condition then remains true forever and can be interpreted as a gauge constraint. We formulate the conditions under which the spontaneous Lorentz violation becomes observable. Spontaneously broken Lorentz invariance could be seen by some primordially existing or created "fossil" charges with the property of moving through the Universe with a fixed velocity.Comment: Extended versio

Remarks on the Causality, Unitarity and Supersymmetric Extension of the Lorentz and CPT-Violating Maxwell-Chern-Simons Model

The gauge-invariant Chern-Simons-type Lorentz- and CPT-breaking term is here re-assessed and issues like causality, unitarity, spontaneous gauge-symmetry breaking are investigated. Moreover, we obtain a minimal extension of such a system to a supersymmetric environment. We comment on resulting peculiar self-couplings for the gauge sector, as well as on background contribution for gaugino masses.Comment: 5 pages, NPB style, talk presented at "Renormalization Group and Anomalies in Gravity and Cosmology", Ouro Preto, Brazil, March 200

An extension of the cosmological standard model with a bounded Hubble expansion rate

The possibility of having an extension of the cosmological standard model with a Hubble expansion rate $H$ constrained to a finite interval is considered. Two periods of accelerated expansion arise naturally when the Hubble expansion rate approaches to the two limiting values. The new description of the history of the universe is confronted with cosmological data and with several theoretical ideas going beyond the standard cosmological model.Comment: 10 pages, 4 figures. Minor revisio

Breaking CPT by mixed non-commutativity

The mixed component of the non-commutative parameter \theta_{\mu M}, where \mu = 0,1,2,3 and M is an extra dimensional index may violate four-dimensional CPT invariance. We calculate one and two-loop induced couplings of \theta_{\mu 5} with the four-dimensional axial vector current and with the CPT odd dim=6 operators starting from five-dimensional Yukawa and U(1) theories. The resulting bounds from clock comparison experiments place a stringent constraint on \theta_{\mu 5}, |\theta_{\mu 5}|^{-1/2} > 5\times 10^{11} GeV. The orbifold projection and/or localization of fermions on a 3-brane lead to CPT-conserving physics, in which case the constraints on \theta{\mu 5} are softened.Comment: 4 pages, latex, 1 figur

Spontaneously Generated Tensor Field Gravity

An arbitrary local theory of a symmetric two-tensor field $H_{\mu \nu}$ in Minkowski spacetime is considered, in which the equations of motion are required to be compatible with a nonlinear length-fixing constraint $H_{\mu \nu}^{2}=\pm M^{2}$ leading to spontaneous Lorentz invariance violation, SLIV ($M$ is the proposed scale for SLIV). Allowing the parameters in the Lagrangian to be adjusted so as to be consistent with this constraint, the theory turns out to correspond to linearized general relativity in the weak field approximation, while some of the massless tensor Goldstone modes appearing through SLIV are naturally collected in the physical graviton. In essence the underlying diffeomophism invariance emerges as a necessary condition for the tensor field $H_{\mu \nu}$ not to be superfluously restricted in degrees of freedom, apart from the constraint due to which the true vacuum in the theory is chosen by SLIV. The emergent theory appears essentially nonlinear, when expressed in terms of the pure Goldstone tensor modes and contains a plethora of new Lorentz and $CPT$ violating couplings. However, these couplings do not lead to physical Lorentz violation once this tensor field gravity is properly extended to conventional general relativity.Comment: 27 pages, published version, to appear in Nuclear Physics

The nearly Newtonian regime in Non-Linear Theories of Gravity

The present paper reconsiders the Newtonian limit of models of modified gravity including higher order terms in the scalar curvature in the gravitational action. This was studied using the Palatini variational principle in [Meng X. and Wang P.: Gen. Rel. Grav. {\bf 36}, 1947 (2004)] and [Dom\'inguez A. E. and Barraco D. E.: Phys. Rev. D {\bf 70}, 043505 (2004)] with contradicting results. Here a different approach is used, and problems in the previous attempts are pointed out. It is shown that models with negative powers of the scalar curvature, like the ones used to explain the present accelerated expansion, as well as their generalization which include positive powers, can give the correct Newtonian limit, as long as the coefficients of these powers are reasonably small. Some consequences of the performed analysis seem to raise doubts for the way the Newtonian limit was derived in the purely metric approach of fourth order gravity [Dick R.: Gen. Rel. Grav. {\bf 36}, 217 (2004)]. Finally, we comment on a recent paper [Olmo G. J.: Phys. Rev. D {\bf 72}, 083505 (2005)] in which the problem of the Newtonian limit of both the purely metric and the Palatini formalism is discussed, using the equivalent Brans--Dicke theory, and with which our results partly disagree.Comment: typos corrected, replaced to match published versio

Neutron stars in generalized f(R) gravity

Quartic gravity theory is considered with the Einstein-Hilbert Lagrangean $R+aR^{2}+bR_{\mu \nu}R^{\mu \nu},$ $R_{\mu \nu}$ being Ricci\'s tensor and R the curvature scalar. The parameters $a$ and $b$ are taken of order 1 km$^{2}.$ Arguments are given which suggest that the effective theory so obtained may be a plausible approximation of a viable theory. A numerical integration is performed of the field equations for a free neutron gas. As in the standard Oppenheimer-Volkoff calculation the star mass increases with increasing central density until about 1 solar mass and then decreases. However a dramatic difference exists in the behaviour of the baryon number, which increases monotonically. The calculation suggests that the theory allows stars in equilibrium with arbitrary baryon number, no matter how large.Comment: Keywords: stars, neutron stars; gravity; modified gravity Accepted in Astrophysics and Space Scienc

Linearized Treatment of Scalar perturbations in the Asymptotic Cosmological Model

In this paper the implications of a recently proposed phenomenological model of cosmology, the Asymptotic Cosmological Model (ACM), on the behavior of scalar perturbations are studied. Firstly we discuss new fits of the ACM at the homogeneous level, including fits to the Type Ia Supernovae UNION dataset, first CMB peak of WMAP5 and BAOs. The linearized equations of scalar perturbations in the FRW metric are derived. A simple model is used to compute the CMB temperature perturbation spectrum. The results are compared with the treatment of perturbations in other approaches to the problem of the accelerated expansion of the universe.Comment: 14 pages, 5 figures. Presentation in sections 2 and 3 clarified, references added. Final version to appear in Astroparticle Physic

Standard Model with Partial Gauge Invariance

We argue that an exact gauge invariance may disable some generic features of the Standard Model which could otherwise manifest themselves at high energies. One of them might be related to the spontaneous Lorentz invariance violation (SLIV) which could provide an alternative dynamical approach to QED and Yang-Mills theories with photon and non-Abelian gauge fields appearing as massless Nambu-Goldstone bosons. To see some key features of the new physics expected we propose partial rather than exact gauge invariance in an extended SM framework. This principle applied, in some minimal form, to the weak hypercharge gauge field B_{mu} and its interactions leads to SLIV with B field components appearing as the massless Nambu-Goldstone modes, and provides a number of distinctive Lorentz beaking effects. Being naturally suppressed at low energies they may become detectable in high energy physics and astrophysics. Some of the most interesting SLIV processes are considered in significant detail.Comment: 32 pages, extended version, to appear in Eur.Phys.J.

Patterns of 6-mercaptopurine and azathioprine maintenance therapy among a cohort of commercially insured individuals diagnosed with Crohn's disease in the United States

Background and aims: Thiopurines, including 6-mercaptopurine (6-MP) and azathioprine (AZA), are the mainstay of maintenance therapy for Crohn's disease (CD). However, studies examining their effectiveness in routine practice among diverse patient populations are lacking. Among a cohort of new users of 6MP/AZA, we described treatment patterns and changes in subsequent therapy. Methods: Using the Truven Health Analytics databases, we identified all individuals diagnosed with CD and initiating 6-MP/AZA monotherapy from 2001-2008 (n=3,657). We estimated the proportion of CD patients remaining on 6-MP/AZA monotherapy, using Kaplan-Meier methods, and identified predictors of treatment noncontinuation, using multivariable Cox regression. Among the "noncontinuers," we described subsequent patterns of maintenance therapy and summarized the diagnosis and procedure codes and prescription drug claims preceding treatment discontinuation. Results: The 1-year 6-MP/AZA treatment continuation rate was 42%. Children (age ?18 years) and individuals with no prior anti-tumor necrosis factor (TNF) use were more likely to continue 6-MP/AZA, while those dispensed more (>4) outpatient prescriptions for any drug before initiation of 6-MP/AZA were less likely to continue maintenance treatment. Overall, 1,128 (39%) and 105 (4%) individuals experienced a clinical event potentially indicating active disease or 6-MP/AZA-intolerance prior to discontinuation, respectively. Most patients discontinued therapy; among the remaining patients who failed to continue 6-MP/AZA, most augmented with an anti-TNF. Conclusion: Most patients initiating 6-MP/AZA monotherapy did not continue beyond 1 year. In contrast to trial evidence showing 1-year remission rates of 40%-80%, this study observed a lower effectiveness of 6-MP/AZA treatment, possibly due to differences in disease severity, patient demographics, comorbidity, adherence, and health care utilization