The post-Newtonian limit in C-theories of gravitation

C-theory provides a unified framework to study metric, metric-affine and more general theories of gravity. In the vacuum weak-field limit of these theories, the parameterized post-Newtonian (PPN) parameters $\beta$ and $\gamma$ can differ from their general relativistic values. However, there are several classes of models featuring long-distance modifications of gravity but nevertheless passing the Solar system tests. Here it is shown how to compute the PPN parameters in C-theories and also in nonminimally coupled curvature theories, correcting previous results in the literature for the latter.Comment: 5 pages, no figures; To appear in PRD as a rapid communicatio

Perturbations in electromagnetic dark energy

It has been recently proposed that the presence of a temporal electromagnetic field on cosmological scales could explain the phase of accelerated expansion that the universe is currently undergoing. The field contributes as a cosmological constant and therefore, the homogeneous cosmology produced by such a model is exactly the same as that of $\Lambda$CDM. However, unlike a cosmological constant term, electromagnetic fields can acquire perturbations which in principle could affect CMB anisotropies and structure formation. In this work, we study the evolution of inhomogeneous scalar perturbations in this model. We show that provided the initial electromagnetic fluctuations generated during inflation are small, the model is perfectly compatible with both CMB and large scale structure observations at the same level of accuracy as $\Lambda$CDM.Comment: 12 pages, 3 figures. Added new comments to match the published versio

String cosmological model in the presence of a magnetic flux

A Bianchi type I string cosmological model in the presence of a magnetic flux is investigated. A few plausible assumptions regarding the parametrization of the cosmic string and magneto-fluid are introduced and some exact analytical solutions are presented.Comment: 9 pages, 4 Figure

Shining Light on Modifications of Gravity

Many modifications of gravity introduce new scalar degrees of freedom, and in such theories matter fields typically couple to an effective metric that depends on both the true metric of spacetime and on the scalar field and its derivatives. Scalar field contributions to the effective metric can be classified as conformal and disformal. Disformal terms introduce gradient couplings between scalar fields and the energy momentum tensor of other matter fields, and cannot be constrained by fifth force experiments because the effects of these terms are trivial around static non-relativistic sources. The use of high-precision, low-energy photon experiments to search for conformally coupled scalar fields, called axion-like particles, is well known. In this article we show that these experiments are also constraining for disformal scalar field theories, and are particularly important because of the difficulty of constraining these couplings with other laboratory experiments.Comment: 20 pages, 10 figures. v2: Matches version accepted by JCAP; additional discussion of the strong coupling scale. Conclusions unchange

Unifying Einstein and Palatini gravities

We consider a novel class of $f(\R)$ gravity theories where the connection is related to the conformally scaled metric $\hat g_{\mu\nu}=C(\R)g_{\mu\nu}$ with a scaling that depends on the scalar curvature $\R$ only. We call them C-theories and show that the Einstein and Palatini gravities can be obtained as special limits. In addition, C-theories include completely new physically distinct gravity theories even when $f(\R)=\R$. With nonlinear $f(\R)$, C-theories interpolate and extrapolate the Einstein and Palatini cases and may avoid some of their conceptual and observational problems. We further show that C-theories have a scalar-tensor formulation, which in some special cases reduces to simple Brans-Dicke-type gravity. If matter fields couple to the connection, the conservation laws in C-theories are modified. The stability of perturbations about flat space is determined by a simple condition on the lagrangian.Comment: 17 pages, no figure

Dirac fields in f(R)-gravity with torsion

We study f(R)-gravity with torsion in presence of Dirac massive fields. Using the Bianchi identities, we formulate the conservation laws of the theory and we check the consistency with the matter field equations. Further, we decompose the field equations in torsionless and torsional terms: we show that the non-linearity of the gravitational Lagrangian reduces to the presence of a scalar field that depends on the spinor field; this additional scalar field gives rise to an effective stress-energy tensor and plays the role of a scale factor modifying the normalization of Dirac fields. Problems for fermions regarding the positivity of energy and the particle-antiparticle duality are discussed.Comment: 14 page

Growth of perturbations in dark matter coupled with quintessence

We consider the evolution of linear perturbations in models with a nonminimal coupling between dark matter and scalar field dark energy. Growth of matter inhomogeneities in two examples of such models proposed in the literature are investigated in detail. Both of these models are based on a low-energy limit of effective string theory action, and have been previously shown to naturally lead to late acceleration of the universe. However, we find that these models can be ruled out by taking properly into account the impact of the scalar field coupling on the formation of structure in the dark matter density. In particular, when the transition to acceleration in these models begins, the interaction with dark energy enchances the small scale clustering in dark matter much too strongly. We discuss the the role of an effective small scale sound speed in such models with a coupled dark sector.Comment: 14 pages, 10 figures; v2: references adde

Unconventional cosmology on the (thick) brane

We consider the cosmology of a thick codimension 1 brane. We obtain the matching conditions leading to the cosmological evolution equations and show that when one includes matter with a pressure component along the extra dimension in the brane energy-momentum tensor, the cosmology is of non-standard type. In particular one can get acceleration when a dust of non-relativistic matter particles is the only source for the (modified) Friedman equation. Our equations would seem to violate the conservation of energy-momentum from a 4D perspective, but in 5D the energy-momentum is conserved. One could write down an effective conserved 4D energy-momentum tensor attaching a ``dark energy'' component to the energy-momentum tensor of matter that has pressure along the extra dimension. This extra component could, on a cosmological scale, be interpreted as matter-coupled quintessence. We comment on the effective 4D description of this effect in terms of the time evolution of a scalar field (the 5D radion) coupled to this kind of matter.Comment: 9 pages, v2. eq.(17) corrected, comments on effective theory change

Inducing empathy towards upper limb impairments using a physical device and virtual reality

Abstract. Empathy is the ability to understand concepts deeply and intimately from the perspective of another person. Having this empathetic understanding of different medical conditions will help make more informed decisions when designing for a particular condition and increase the motivation for providing higher quality results. However, it can be quite challenging for people to easily gain this kind of empathetic knowledge without fully comprehending the extent to which a particular impairment affects someone’s day-to-day life. One of the most popular and effective methods of inducing empathy towards impairments is the use of empathy simulations. The basic concept of empathy simulations is to realistically simulate the limitations posed by an impairment so that the participant can gain a first-hand experience of what it is like to live with the impairment. Traditionally, these simulations were created using various physical means, but lately the use of virtual reality devices in these simulations has become more common. Virtual reality is essentially technology that allows the user to embody another persons perspective, which makes it exceptionally suitable for empathy simulations. The aim of our study was to investigate the generation of empathy towards upper extremity motor impairments using a mixture of physical and virtual means. For the purposes of this study, we built an arm mobility restricting harness to mimic an upper extremity motor impairment and a virtual reality environment of a home kitchen where the simulations took place. Two groups of volunteer participants experienced the simulation by performing simple tasks in the virtual reality environment while being limited by the mobility restricting harness. The difference between the groups was in having to recite different backstories for their simulated characters. Backstory for group 1 was in first-person, and group 2 for group in third-person. The stories were thought to target affective and cognitive empathy differently. The participants’ level of empathy was measured once before the simulation and once after the simulation using a collection of standardized questionnaires. The study showed significant increase in the level of emotional contagion over all participants (p < 0.044*) suggesting that the simulation increased the participants’ level of empathy in that category. No significant difference was measured between backstories, however, the results suggest the first-person story to assist cognitive empathy. The study also showed that the group with the backstory in first-person had better scores in all categories of embodiment suggesting that the first-person backstory enabled participants to better relate to their virtual character. Despite some promising results, further studies are needed to investigate empathy generation using a mixed physical and virtual empathy simulations