33 research outputs found
Stability of Einstein-Aether Cosmological Models
We use a dynamical systems analysis to investigate the future behaviour of
Einstein-Aether cosmological models with a scalar field coupling to the
expansion of the aether and a non-interacting perfect fluid. The stability of
the equilibrium solutions are analysed and the results are compared with the
standard inflationary cosmological solutions and previously studied
cosmological Einstein-Aether models.Comment: 13 pages, 1 figur
Principlism and citizen science: the possibilities and limitations of principlism for guiding responsible citizen science conduct
Citizen science (CS) has been presented as a novel form of research relevant for social concerns and global challenges. CS transforms the roles of participants to being actively involved at various stages of research processes, CS projects are dynamic, and pluralism arises when many non-professional researchers take an active involvement in research. Some argue that these elements all make existing research ethical principles and regulations ill-suited for guiding responsible CS conduct. However, while many have sought to highlight such challenges from CS, few have discussed principles per se providing the foundation for regulations. In this article we will investigate the possibilities of midlevel principlism in guiding responsible CS conduct. Principlism has the potential of accommodating many of the concerns taken to reduce the relevance of existing principles
Spherically symmetric Einstein-aether perfect fluid models
We investigate spherically symmetric cosmological models in Einstein-aether
theory with a tilted (non-comoving) perfect fluid source. We use a 1+3 frame
formalism and adopt the comoving aether gauge to derive the evolution
equations, which form a well-posed system of first order partial differential
equations in two variables. We then introduce normalized variables. The
formalism is particularly well-suited for numerical computations and the study
of the qualitative properties of the models, which are also solutions of Horava
gravity. We study the local stability of the equilibrium points of the
resulting dynamical system corresponding to physically realistic inhomogeneous
cosmological models and astrophysical objects with values for the parameters
which are consistent with current constraints. In particular, we consider dust
models in () normalized variables and derive a reduced (closed)
evolution system and we obtain the general evolution equations for the
spatially homogeneous Kantowski-Sachs models using appropriate bounded
normalized variables. We then analyse these models, with special emphasis on
the future asymptotic behaviour for different values of the parameters.
Finally, we investigate static models for a mixture of a (necessarily
non-tilted) perfect fluid with a barotropic equations of state and a scalar
field.Comment: 52 pages, 7 figures. Matches the published version. arXiv admin note:
text overlap with arXiv:gr-qc/0603058 by other author
Future asymptotics of tilted Bianchi type II cosmologies
In this paper we study the future asymptotics of spatially homogeneous
Bianchi type II cosmologies with a tilted perfect fluid with a linear equation
of state. By means of Hamiltonian methods we first find a monotone function for
a special tilted case, which subsequently allows us to construct a new set of
monotone functions for the general tilted type II cosmologies. In the context
of a new partially gauge invariant dynamical system, this then leads to a proof
for a theorem that for the first time gives a complete description of the
future asymptotic states of the general tilted Bianchi type II models. The
generality of our arguments suggests how one can produce monotone functions
that are useful for determining the asymptotics of other tilted perfect fluid
cosmologies, as well as for other sources.Comment: 15 page
Is There a Paradigm Shift for On-Board Computing and Processing?
Today we see the need for increased processing performance, driven by increased digitalization and new services such as Software Defined Radio, real-time Image Processing and enhanced Compression techniques, Artificial Intelligence and Machine Learning, Visual Navigation & Autonomous Control, Debris Removal etc. This, in combination with the bandwidth limitation in downlink channels present, the payload data needs to be processed and compressed on board before downlinked to the user on ground. Historically image processing in orbit has been performed using dedicated hardware and to certain extends software processing. These have been often, costly and in-flexible solutions, either using customized FPGAs or ASICs in order to achieve needed performance
The initial singularity of ultrastiff perfect fluid spacetimes without symmetries
We consider the Einstein equations coupled to an ultrastiff perfect fluid and
prove the existence of a family of solutions with an initial singularity whose
structure is that of explicit isotropic models. This family of solutions is
`generic' in the sense that it depends on as many free functions as a general
solution, i.e., without imposing any symmetry assumptions, of the
Einstein-Euler equations. The method we use is a that of a Fuchsian reduction.Comment: 16 pages, journal versio
Tilted two-fluid Bianchi type I models
In this paper we investigate expanding Bianchi type I models with two tilted
fluids with the same linear equation of state, characterized by the equation of
state parameter w. Individually the fluids have non-zero energy fluxes w.r.t.
the symmetry surfaces, but these cancel each other because of the Codazzi
constraint. We prove that when w=0 the model isotropizes to the future. Using
numerical simulations and a linear analysis we also find the asymptotic states
of models with w>0. We find that future isotropization occurs if and only if . The results are compared to similar models investigated previously
where the two fluids have different equation of state parameters.Comment: 14 pages, 3 figure
Perfect fluids and generic spacelike singularities
We present the conformally 1+3 Hubble-normalized field equations together
with the general total source equations, and then specialize to a source that
consists of perfect fluids with general barotropic equations of state.
Motivating, formulating, and assuming certain conjectures, we derive results
about how the properties of fluids (equations of state, momenta, angular
momenta) and generic spacelike singularities affect each other.Comment: Considerable changes have been made in presentation and arguments,
resulting in sharper conclusion
Rationale for a Swedish cohort consortium
We herein outline the rationale for a Swedish cohort consortium, aiming to facilitate greater use of Swedish cohorts for world-class research. Coordination of all Swedish prospective population-based cohorts in a common infrastructure would enable more precise research findings and facilitate research on rare exposures and outcomes, leading to better utilization of study participants' data, better return of funders' investments, and higher benefit to patients and populations. We motivate the proposed infrastructure partly by lessons learned from a pilot study encompassing data from 21 cohorts. We envisage a standing Swedish cohort consortium that would drive development of epidemiological research methods and strengthen the Swedish as well as international epidemiological competence, community, and competitiveness.Peer reviewe