891 research outputs found
Teleparallel Theories of Gravity: Illuminating a Fully Invariant Approach
Teleparallel gravity and its popular generalization gravity can be
formulated as fully invariant (under both coordinate transformations and local
Lorentz transformations) theories of gravity. Several misconceptions about
teleparallel gravity and its generalizations can be found in the literature,
especially regarding their local Lorentz invariance. We describe how these
misunderstandings may have arisen and attempt to clarify the situation. In
particular, the central point of confusion in the literature appears to be
related to the inertial spin connection in teleparallel gravity models. While
inertial spin connections are commonplace in special relativity, and not
something inherent to teleparallel gravity, the role of the inertial spin
connection in removing the spurious inertial effects within a given frame of
reference is emphasized here. The careful consideration of the inertial spin
connection leads to the construction of a fully invariant theory of
teleparallel gravity and its generalizations. Indeed, it is the nature of the
spin connection that differentiates the relationship between what have been
called good tetrads and bad tetrads and clearly shows that, in principle, any
tetrad can be utilized. The field equations for the fully invariant formulation
of teleparallel gravity and its generalizations are presented and a number of
examples using different assumptions on the frame and spin connection are
displayed to illustrate the covariant procedure. Various modified teleparallel
gravity models are also briefly reviewed.Comment: v2: 72 pages, revised version, references added, matches published
versio
Scalar Field Cosmologies with Barotropic Matter: Models of Bianchi class B
We investigate in detail the qualitative behaviour of the class of Bianchi
type B spatially homogeneous cosmological models in which the matter content is
composed of two non-interacting components; the first component is described by
a barotropic fluid having a gamma-law equation of state, whilst the second is a
non-interacting scalar field (phi) with an exponential potential V=Lambda exp(k
phi). In particular, we study the asymptotic properties of the models both at
early and late times, paying particular attention on whether the models
isotropize (and inflate) to the future, and we discuss the genericity of the
cosmological scaling solutions.Comment: 18 pages, 1 figure, uses revtex and epsf to insert figur
The stability of cosmological scaling solutions
We study the stability of cosmological scaling solutions within the class of
spatially homogeneous cosmological models with a perfect fluid subject to the
equation of state p_gamma=(gamma-1) rho_gamma (where gamma is a constant
satisfying 0 < gamma < 2) and a scalar field with an exponential potential. The
scaling solutions, which are spatially flat isotropic models in which the
scalar field energy density tracks that of the perfect fluid, are of physical
interest. For example, in these models a significant fraction of the current
energy density of the Universe may be contained in the scalar field whose
dynamical effects mimic cold dark matter. It is known that the scaling
solutions are late-time attractors (i.e., stable) in the subclass of flat
isotropic models. We find that the scaling solutions are stable (to shear and
curvature perturbations) in generic anisotropic Bianchi models when gamma <
2/3. However, when gamma > 2/3, and particularly for realistic matter with
gamma >= 1, the scaling solutions are unstable; essentially they are unstable
to curvature perturbations, although they are stable to shear perturbations. We
briefly discuss the physical consequences of these results.Comment: AMSTeX, 7 pages, re-submitted to Phys Rev Let
Brane-world Cosmologies with non-local bulk effects
It is very common to ignore the non-local bulk effects in the study of
brane-world cosmologies using the brane-world approach. However, we shall
illustrate through the use of three different scenarios, that the non-local
bulk-effect does indeed have significant impact on both the
initial and future behaviour of brane-world cosmologies.Comment: 17 pages, no figures, iopart.cls, submitted to CQ
The Dynamics of Multi-Scalar Field Cosmological Models and Assisted Inflation
We investigate the dynamical properties of a class of spatially homogeneous
and isotropic cosmological models containing a barotropic perfect fluid and
multiple scalar fields with independent exponential potentials. We show that
the assisted inflationary scaling solution is the global late-time attractor
for the parameter values for which the model is inflationary, even when
curvature and barotropic matter are included. For all other parameter values
the multi-field curvature scaling solution is the global late-time attractor
(in these solutions asymptotically the curvature is not dynamically
negligible). Consequently, we find that in general all of the scalar fields in
multi-field models with exponential potentials are non-negligible in late-time
behaviour, contrary to what is commonly believed. The early-time and
intermediate behaviour of the models is also studied. In particular, n-scalar
field models are investigated and the structure of the saddle equilibrium
points corresponding to inflationary m-field scaling solutions and
non-inflationary m-field matter scaling solutions are also studied (where m<n),
leading to interesting transient dynamical behaviour with new physical
scenarios of potential importance.Comment: 27 pages, uses REVTeX Added an appendix illustrating some of the
details needed to compute the stability of the assisted inflationary solutio
Qualitative Viscous Cosmology
The Full (non--truncated) Israel--Stewart theory of bulk viscosity is applied
to dissipative FRW spacetimes. Dimensionless variables and dimensionless
equations of state are used to write the Einstein--thermodynamic equations as a
plane autonomous system and the qualitative behaviour of this system is
determined. Entropy production in these models is also discussed.Comment: 13 pages, REVTeX, accepted for publication in Physical Review
Local and non-local measures of acceleration in cosmology
Current cosmological observations, when interpreted within the framework of a
homogeneous and isotropic Friedmann-Lemaitre-Robertson-Walker (FLRW) model,
strongly suggest that the Universe is entering a period of accelerating
expansion. This is often taken to mean that the expansion of space itself is
accelerating. In a general spacetime, however, this is not necessarily true. We
attempt to clarify this point by considering a handful of local and non-local
measures of acceleration in a variety of inhomogeneous cosmological models.
Each of the chosen measures corresponds to a theoretical or observational
procedure that has previously been used to study acceleration in cosmology, and
all measures reduce to the same quantity in the limit of exact spatial
homogeneity and isotropy. In statistically homogeneous and isotropic
spacetimes, we find that the acceleration inferred from observations of the
distance-redshift relation is closely related to the acceleration of the
spatially averaged universe, but does not necessarily bear any resemblance to
the average of the local acceleration of spacetime itself. For inhomogeneous
spacetimes that do not display statistical homogeneity and isotropy, however,
we find little correlation between acceleration inferred from observations and
the acceleration of the averaged spacetime. This shows that observations made
in an inhomogeneous universe can imply acceleration without the existence of
dark energy.Comment: 19 pages, 10 figures. Several references added or amended, some minor
clarifications made in the tex
Viral sensing by epithelial cells involves PKR- and caspase-3-dependent generation of gasdermin E pores
Viral sensing in myeloid cells involves inflammasome activation leading to gasdermin pore formation, cytokine release, and cell death. However, less is known about viral sensing in barrier epithelial cells, which are critical to the innate immune response to RNA viruses. Here, we show that poly(I:C), a mimic of viral dsRNA, is sensed by NLRP1 in human bronchial epithelial cells, leading to inflammasome-dependent gasdermin D (GSDMD) pore formation via caspase-1. DsRNA also stimulated a parallel sensing pathway via PKR which activated caspase-3 to cleave gasdermin E (GSDME) to form active pores. Influenza A virus (IAV) infection of cells caused GSDME activation, cytokine release, and cell death, in a PKR-dependent but NLRP1-independent manner, involving caspase-8 and caspase-3. Suppression of GSDMD and GSDME expression increased IAV replication. These data clarify mechanisms of gasdermin cleavage in response to viral sensing and reveal that gasdermin pore formation is intrinsically antiviral in human epithelial cells.</p
Comparison of two percutaneous tracheostomy techniques, guide wire dilating forceps and Ciaglia Blue Rhino: a sequential cohort study
INTRODUCTION: To evaluate and compare the peri-operative and postoperative complications of the two most frequently used percutaneous tracheostomy techniques, namely guide wire dilating forceps (GWDF) and Ciaglia Blue Rhino (CBR). METHODS: A sequential cohort study with comparison of short-term and long-term peri-operative and postoperative complications was performed in the intensive care unit of the University Medical Centre in Nijmegen, The Netherlands. In the period 1997–2000, 171 patients underwent a tracheostomy with the GWDF technique and, in the period 2000–2003, a further 171 patients with the CBR technique. All complications were prospectively registered on a standard form. RESULTS: There was no significant difference in major complications, either peri-operative or postoperative. We found a significant difference in minor peri-operative complications (P < 0.01) and minor late complications (P < 0.05). CONCLUSION: Despite a difference in minor complications between GWDF and CBR, both techniques seem equally reliable
Whole genome sequencing of human metapneumoviruses from clinical specimens using MinION nanopore technology
Human metapneumovirus (HMPV), a member of the Pneumoviridae family, is a causative agent of respiratory illness in young children, the elderly, and immunocompromised individuals. Globally, viruses belonging to two main genetic lineages circulate, A and B, which are further divided into four genetic sublineages (A1, A2, B1, B2). Classical genotyping of HMPV is based on the sequence of the fusion (F) and attachment (G) glycoprotein genes, which are under direct antibody-mediated immune pressure. Whole genome sequencing provides more information than sequencing of subgenomic fragments and is therefore a powerful tool for studying virus evolution and disease epidemiology and for identifying transmission events and nosocomial outbreaks. Here, we report a robust method to obtain whole genome sequences for HMPV using MinION Nanopore technology. This assay is able to generate HMPV whole genome sequences from clinical specimens with good coverage of the highly variable G gene and is equally sensitive for strains of all four genetic HMPV sublineages. This method can be used for studying HMPV genetics, epidemiology, and evolutionary dynamics.</p
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