Dark spinor models in gravitation and cosmology

We introduce and carefully define an entire class of field theories based on non-standard spinors. Their dominant interaction is via the gravitational field which makes them naturally dark; we refer to them as Dark Spinors. We provide a critical analysis of previous proposals for dark spinors noting that they violate Lorentz invariance. As a working assumption we restrict our analysis to non-standard spinors which preserve Lorentz invariance, whilst being non-local and explicitly construct such a theory. We construct the complete energy-momentum tensor and derive its components explicitly by assuming a specific projection operator. It is natural to next consider dark spinors in a cosmological setting. We find various interesting solutions where the spinor field leads to slow roll and fast roll de Sitter solutions. We also analyse models where the spinor is coupled conformally to gravity, and consider the perturbations and stability of the spinor.Comment: 43 pages. Several new sections and details added. JHEP in prin

An ecological connectivity network maintains genetic diversity of a flagship wildflower, Pulsatilla vulgaris

Ecological connectivity networks have been proposed as an efficient way to reconnect communities in fragmented landscapes. Yet few studies have evaluated if they are successful at enhancing actual functional connectivity (i.e. realized dispersal or gene flow) of focal species, or if this enhanced connectivity is enough to maintain genetic diversity and fitness of plant populations. Here we test the efficacy of an ecological connectivity network implemented in southern Germany since 1989 to reconnect calcareous grassland fragments through rotational shepherding. We genotyped 1449 individuals from 57 populations and measured fitness-related traits in 10 populations of Puisatilla vulgaris, a flagship species of calcareous grasslands in Europe. We tested if the shepherding network explained functional connectivity in P. vulgaris and if higher connectivity translated to higher genetic diversity and fitness of populations. We found that population-specific F-st was lowest in populations that had high connectivity within the shepherding network, and that well-connected populations within the network had significantly higher genetic diversity than ungrazed and more isolated grazed populations. Moreover, genetic diversity was significantly positively correlated with both seed set and seed mass. Together our results suggest that the implementation of an ecological shepherding network is an effective management measure to maintain functional connectivity and genetic diversity at the landscape scale for a calcareous grassland specialist. Populations with reduced genetic diversity would likely benefit from inclusion, or better integration into the ecological connectivity network. Our study demonstrates the often postulated but rarely tested sequence of positive associations between connectivity, genetic diversity, and fitness at the landscape scale, and provides a framework for testing the efficacy of ecological connectivity networks for focal species using molecular genetic tools.Peer reviewe

Cancer Survivors’ Health Behaviors and Outcomes: A Population-Based Study of Sexual and Gender Minorities

BACKGROUND: Most case-control studies compare cancer survivors with general population controls without considering sexual orientation or gender identity. This case-control analysis compared health risk behaviors and health outcomes among sexual and gender minority cancer survivors to those of matched sexual and gender minority participants without cancer (controls). METHODS: Using data from the 2014-2021 Behavioral Risk Factor Surveillance System, a population-based sample of 4507 cancer survivors who self-identified as transgender, gay men, bisexual men, lesbian women, or bisexual women were 1:1 propensity score matched, using age at survey, race and ethnicity, marital status, education, access to health care, and US census region. Within each sexual and gender minority group, behaviors and outcomes were compared between survivors and participants without cancer, and survivors\u27 odds ratios and 95% confidence intervals calculated. RESULTS: Gay male survivors had higher odds of depression, poor mental health, limited usual activities, difficulty concentrating, and fair or poor health. Few differences were observed between bisexual male survivors and participants without cancer. Compared with controls, lesbian female survivors had greater odds of overweight-obese status, depression, poor physical health, and fair or poor health. Bisexual female survivors had the highest rates of current smoking, depression, poor mental health, and difficulty concentrating across all sexual and gender minority groups. Statistically significantly different from transgender controls, transgender survivors had greater odds of heavy alcohol use, physical inactivity, and fair or poor health. CONCLUSIONS: This analysis revealed an urgent need to address the high prevalence of engaging in multiple health risk behaviors and not following guidelines to avoid second cancers, additional adverse outcomes, and cancer recurrences among sexual and gender minority cancer survivors

Relativistic Compact Objects in Isotropic Coordinates

We present a matrix method for obtaining new classes of exact solutions for Einstein's equations representing static perfect fluid spheres. By means of a matrix transformation, we reduce Einstein's equations to two independent Riccati type differential equations for which three classes of solutions are obtained. One class of the solutions corresponding to the linear barotropic type fluid with an equation of state $p=\gamma \rho$ is discussed in detail.Comment: 9 pages, no figures, accepted for publication in Pramana-Journal of Physic

Angular size test on the expansion of the Universe

Assuming the standard cosmological model as correct, the average linear size of galaxies with the same luminosity is six times smaller at z=3.2 than at z=0, and their average angular size for a given luminosity is approximately proportional to 1/z. Neither the hypothesis that galaxies which formed earlier have much higher densities nor their luminosity evolution, mergers ratio, or massive outflows due to a quasar feedback mechanism are enough to justify such a strong size evolution. Also, at high redshift, the intrinsic ultraviolet surface brightness would be prohibitively high with this evolution, and the velocity dispersion much higher than observed. We explore here another possibility to overcome this problem by considering different cosmological scenarios that might make the observed angular sizes compatible with a weaker evolution. One of the models explored, a very simple phenomenological extrapolation of the linear Hubble law in a Euclidean static universe, fits the angular size vs. redshift dependence quite well, which is also approximately proportional to 1/z with this cosmological model. There are no free parameters derived ad hoc, although the error bars allow a slight size/luminosity evolution. The type Ia supernovae Hubble diagram can also be explained in terms of this model with no ad hoc fitted parameter. WARNING: I do not argue here that the true Universe is static. My intention is just to discuss which theoretical models provide a better fit to the data of observational cosmology.Comment: 44 pages, accepted to be published in Int. J. Mod. Phys.

Gravitational lensing by a regular black hole

In this paper, we study a regular Bardeen black hole as a gravitational lens. We find the strong deflection limit for the deflection angle, from which we obtain the positions and magnifications of the relativistic images. As an example, we apply the results to the particular case of the supermassive black hole at the center of our galaxy.Comment: 10 pages, 4 figures. v2: Improved version, new references adde

Dark Interactions and Cosmological Fine-Tuning

Cosmological models involving an interaction between dark matter and dark energy have been proposed in order to solve the so-called coincidence problem. Different forms of coupling have been studied, but there have been claims that observational data seem to narrow (some of) them down to something annoyingly close to the $\Lambda$CDM model, thus greatly reducing their ability to deal with the problem in the first place. The smallness problem of the initial energy density of dark energy has also been a target of cosmological models in recent years. Making use of a moderately general coupling scheme, this paper aims to unite these different approaches and shed some light as to whether this class of models has any true perspective in suppressing the aforementioned issues that plague our current understanding of the universe, in a quantitative and unambiguous way.Comment: 13 pages, 9 figures, accepted for publication in JCAP. Minor corrections, one figure replaced, references adde

Classical big-bounce cosmology: dynamical analysis of a homogeneous and irrotational Weyssenhoff fluid

A dynamical analysis of an effective homogeneous and irrotational Weyssenhoff fluid in general relativity is performed using the 1+3 covariant approach that enables the dynamics of the fluid to be determined without assuming any particular form for the space-time metric. The spin contributions to the field equations produce a bounce that averts an initial singularity, provided that the spin density exceeds the rate of shear. At later times, when the spin contribution can be neglected, a Weyssenhoff fluid reduces to a standard cosmological fluid in general relativity. Numerical solutions for the time evolution of the generalised scale factor in spatially-curved models are presented, some of which exhibit eternal oscillatory behaviour without any singularities. In spatially-flat models, analytical solutions for particular values of the equation-of-state parameter are derived. Although the scale factor of a Weyssenhoff fluid generically has a positive temporal curvature near a bounce, it requires unreasonable fine tuning of the equation-of-state parameter to produce a sufficiently extended period of inflation to fit the current observational data.Comment: 34 pages, 18 figure