312 research outputs found
Bouncing cosmologies in massive gravity on de Sitter
In the framework of massive gravity with a de Sitter reference metric, we
study homogeneous and isotropic solutions with positive spatial curvature.
Remarkably, we find that bounces can occur when cosmological matter satisfies
the strong energy condition, in contrast to what happens in classical general
relativity. This is due to the presence in the Friedmann equations of
additional terms, which depend on the scale factor and its derivatives and can
be interpreted as an effective fluid. We present a detailed study of the system
using a phase space analysis. After having identified the fixed points of the
system and investigated their stability properties, we discuss the cosmological
evolution in the global physical phase space. We find that bouncing solutionsComment: 14 pages, 8 figure
Multi-disformal invariance of nonlinear primordial perturbations
We study disformal transformations of the metric in the cosmological context.
We first consider the disformal transformation generated by a scalar field
and show that the curvature and tensor perturbations on the uniform
slicing, on which the scalar field is homogeneous, are non-linearly
invariant under the disformal transformation. Then we discuss the
transformation properties of the evolution equations for the curvature and
tensor perturbations at full non-linear order in the context of spatial
gradient expansion as well as at linear order. In particular, we show that the
transformation can be described in two typically different ways: one that
clearly shows the physical invariance and the other that shows an apparent
change of the causal structure. Finally we consider a new type of disformal
transformation in which a multi-component scalar field comes into play, which
we call a "multi-disformal transformation". We show that the curvature and
tensor perturbations are invariant at linear order, and also at non-linear
order provided that the system has reached the adiabatic limit.Comment: 8 page
Cosmological disformal invariance
The invariance of physical observables under disformal transformations is
considered. It is known that conformal transformations leave physical
observables invariant. However, whether it is true for disformal
transformations is still an open question. In this paper, it is shown that a
pure disformal transformation without any conformal factor is equivalent to
rescaling the time coordinate. Since this rescaling applies equally to all the
physical quantities, physics must be invariant under a disformal
transformation, that is, neither causal structure, propagation speed nor any
other property of the fields are affected by a disformal transformation itself.
This fact is presented at the action level for gravitational and matter fields
and it is illustrated with some examples of observable quantities. We also find
the physical invariance for cosmological perturbations at linear and high
orders in perturbation, extending previous studies. Finally, a comparison with
Horndeski and beyond Horndeski theories under a disformal transformation is
made.Comment: 23 pages + Appendix, updated versio
Preeclampsia and diuresis criteria: our experience at the University of Iowa Hospitals and Clinics
Preeclampsia is a multisystem disorder that complicates approximately 5-8% of pregnancies. It is known that the clinical manifestations of preeclampsia are a result of widespread vasoconstriction. It is therefore thought that the onset of diuresis may signal a reversal of this vasoconstriction, and thus resolution of the disease. This retrospective cohort study was conducted at the University of Iowa Hospitals and Clinics (UIHC) to validate our diuresis criteria and determine if there was a trend for diuresis that was dependent on the type of preeclampsia. This study suggests that the diuresis curves for those women with mild preeclampsia and severe preeclampsia do not vary significantly after the first 5 hours We were able to separate the women with severe preeclampsia into two groups, one of which would have met our diuresis criteria prior to 24 hours and the other of which would not have met diuresis criteria at 24 hours post-delivery
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