5 research outputs found
The formation of voids in a universe with cold dark matter and a cosmological constant
A spherical Lagrangian hydrodynamical code has been written to study the
formation of cosmological structures in the early Universe. In this code we
take into account the presence of collisionless non-baryonic cold dark matter
(CDM), the cosmological constant and a series of physical processes present
during and after the recombination era, such as photon drag resulting from the
cosmic background radiation and hydrogen molecular production. We follow the
evolution of the structure since the recombination era until the present epoch.
As an application of this code we study the formation of voids starting from
negative density perturbations which evolved during and after the recombination
era. We analyse a set of COBE-normalized models, using different spectra to see
their influence on the formation of voids. Our results show that large voids
with diameters ranging from 10h^{-1} Mpc up to 50h^{-1} Mpc can be formed in a
universe model dominated by the cosmological constant (\Omega_\Lambda ~ 0.8).
This particular scenario is capable of forming large and deep empty regions
(with density contrasts \delta < -0.6). Our results also show that the physical
processes acting on the baryonic matter produce a transition region where the
radius of the dark matter component is greater than the baryonic void radius.
The thickness of this transition region ranges from about tens of kiloparsecs
up to a few megaparsecs, depending on the spectrum considered. Putative objects
formed near voids and within the transition region would have a different
amount of baryonic/dark matter when compared with \Omega_b/\Omega_d. If one
were to use these galaxies to determine, by dynamical effects or other
techniques, the quantity of dark matter present in the Universe, the result
obtained would be only local and not representative of the Universe as a whole.Comment: MNRAS (in press); 9 pages, no figure
State of the Art: Small Spacecraft Technology
This report provides an overview of the current state-of-the-art of small spacecraft technology, with particular emphasis placed on the state-of-the-art of CubeSat-related technology. It was first commissioned by NASAs Small Spacecraft Technology Program (SSTP) in mid-2013 in response to the rapid growth in interest in using small spacecraft for many types of missions in Earth orbit and beyond, and was revised in mid-2015 and 2018. This work was funded by the Space Technology Mission Directorate (STMD). For the sake of this assessment, small spacecraft are defined to be spacecraft with a mass less than 180 kg. This report provides a summary of the state-of-the-art for each of the following small spacecraft technology domains: Complete Spacecraft, Power, Propulsion, Guidance Navigation and Control, Structures, Materials and Mechanisms, Thermal Control, Command and Data Handling, Communications, Integration, Launch and Deployment, Ground Data Systems and Operations, and Passive Deorbit Devices
GuideView: Structured Multi-modal Delivery of Clinical Guidelines
GuideView is a system designed for structured, multimodal delivery of clinical guidelines. Clinical instructions are presented simultaneously in voice, text, pictures or video or animations. Users navigate using mouse-clicks and voice commands. An evaluation study performed at a medical simulation laboratory found that voice and video instructions were rated highly