1,206 research outputs found
Observational Challenges for the Standard FLRW Model
We summarise some of the main observational challenges for the standard
Friedmann-Lemaitre-Robertson-Walker cosmological model and describe how results
recently presented in the parallel session `Large--scale Structure and
Statistics' (DE3) at the `Fourteenth Marcel Grossman Meeting on General
Relativity' are related to these challenges.Comment: 17 pages; references added. Matches published version in Int. J. Mod.
Phys. D; Report on Parallel Session DE3 of MG1
G_2 Perfect-Fluid Cosmologies with a proper conformal Killing vector
We study the Einstein field equations for spacetimes admitting a maximal
two-dimensional abelian group of isometries acting orthogonally transitively on
spacelike surfaces and, in addition, with at least one conformal Killing
vector. The three-dimensional conformal group is restricted to the case when
the two-dimensional abelian isometry subalgebra is an ideal and it is also
assumed to act on non-null hypersurfaces (both, spacelike and timelike cases
are studied). We consider both, diagonal and non-diagonal metrics and find all
the perfect-fluid solutions under these assumptions (except those already
known). We find four families of solutions, each one containing arbitrary
parameters for which no differential equations remain to be integrated. We
write the line-elements in a simplified form and perform a detailed study for
each of these solutions, giving the kinematical quantities of the fluid
velocity vector, the energy-density and pressure, values of the parameters for
which the energy conditions are fulfilled everywhere, the Petrov type, the
singularities in the spacetimes and the Friedmann-Lemaitre-Robertson-Walker
metrics contained in each family.Comment: Latex, no figure
Shear-free, Irrotational, Geodesic, Anisotropic Fluid Cosmologies
General relativistic anisotropic fluid models whose fluid flow lines form a
shear-free, irrotational, geodesic timelike congruence are examined. These
models are of Petrov type D, and are assumed to have zero heat flux and an
anisotropic stress tensor that possesses two distinct non-zero eigenvalues.
Some general results concerning the form of the metric and the stress-tensor
for these models are established. Furthermore, if the energy density and the
isotropic pressure, as measured by a comoving observer, satisfy an equation of
state of the form , with , then
these spacetimes admit a foliation by spacelike hypersurfaces of constant Ricci
scalar. In addition, models for which both the energy density and the
anisotropic pressures only depend on time are investigated; both spatially
homogeneous and spatially inhomogeneous models are found. A classification of
these models is undertaken. Also, a particular class of anisotropic fluid
models which are simple generalizations of the homogeneous isotropic
cosmological models is studied.Comment: 13 pages LaTe
Kinematic Self-Similar Plane Symmetric Solutions
This paper is devoted to classify the most general plane symmetric spacetimes
according to kinematic self-similar perfect fluid and dust solutions. We
provide a classification of the kinematic self-similarity of the first, second,
zeroth and infinite kinds with different equations of state, where the
self-similar vector is not only tilted but also orthogonal and parallel to the
fluid flow. This scheme of classification yields twenty four plane symmetric
kinematic self-similar solutions. Some of these solutions turn out to be
vacuum. These solutions can be matched with the already classified plane
symmetric solutions under particular coordinate transformations. As a result,
these reduce to sixteen independent plane symmetric kinematic self-similar
solutions.Comment: 29 pages, accepted for publication in Classical Quantum Gravit
Adolescent Reproductive Knowledge, Attitudes, and Beliefs and Future Fatherhood.
PurposeWith a growing focus on the importance of men's reproductive health, including preconception health, the ways in which young men's knowledge, attitudes, and beliefs (KAB) predict their reproductive paths are understudied. To determine if reproductive KAB predicts fatherhood status, timing and residency (living with child or not).MethodsReproductive KAB and fatherhood outcomes were analyzed from the National Longitudinal Study of Adolescent Health, a 20-year, nationally representative study of individuals from adolescence into adulthood. Four measures of reproductive KAB were assessed during adolescence in waves I and II. A generalized linear latent and mixed model predicted future fatherhood status (nonfather, resident/nonresident father, adolescent father) and timing while controlling for other socio-demographic variables.ResultsOf the 10,253 men, 3,425 were fathers (686 nonresident/2,739 resident) by wave IV. Higher risky sexual behavior scores significantly increased the odds of becoming nonresident father (odds ratio [OR], 1.30; p < .0001), resident father (OR, 1.07; p = .007), and adolescent father (OR, 1.71; p < .0001); higher pregnancy attitudes scores significantly increased the odds of becoming a nonresident father (OR, 1.20; p < .0001) and resident father (OR, 1.11; p < .0001); higher birth control self-efficacy scores significantly decreased the odds of becoming a nonresident father (OR, .72; p < .0001) and adolescent father (OR, .56; p = .01).ConclusionsYoung men's KAB in adolescence predicts their future fatherhood and residency status. Strategies that address adolescent males' reproductive KAB are needed in the prevention of unintended reproductive consequences such as early and nonresident fatherhood
Longitudinal Study of Body Mass Index in Young Males and the Transition to Fatherhood.
Despite a growing understanding that the social determinants of health have an impact on body mass index (BMI), the role of fatherhood on young men's BMI is understudied. This longitudinal study examines BMI in young men over time as they transition from adolescence into fatherhood in a nationally representative sample. Data from all four waves of the National Longitudinal Study of Adolescent Health supported a 20-year longitudinal analysis of 10,253 men beginning in 1994. A "fatherhood-year" data set was created and changes in BMI were examined based on fatherhood status (nonfather, nonresident father, resident father), fatherhood years, and covariates. Though age is positively associated with BMI over all years for all men, comparing nonresident and resident fathers with nonfathers reveals different trajectories based on fatherhood status. Entrance into fatherhood is associated with an increase in BMI trajectory for both nonresident and resident fathers, while nonfathers exhibit a decrease over the same period. In this longitudinal, population-based study, fatherhood and residence status play a role in men's BMI. Designing obesity prevention interventions for young men that begin in adolescence and carry through young adulthood should target the distinctive needs of these populations, potentially improving their health outcomes
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