9,209 research outputs found
Tradeoff between Smoother and Sooner "Little Rip"
There exists dark energy models that predict the occurrence of "little rip".
At the point of little rip the Hubble rate and its cosmic time derivative
approach infinity, which is quite similar to the big rip singularity except
that the former happens at infinite future while the latter at a finite cosmic
time; both events happen in the future and at high energies. In the case of the
big rip, a combination of ultra-violet and infra-red effects can smooth its
doomsday. We therefore wonder if the little rip can also be smoothed in a
similar way. We address the ultra-violet and infra-red effects in general
relativity through a brane-world model with a Gauss-Bonnet term in the bulk and
an induced gravity term on the brane. We find that the little rip is
transformed in this case into a sudden singularity, or a "big brake". Even
though the big brake is smoother than the little rip in that the Hubble rate is
finite at the event, the trade-off is that it takes place sooner, at a finite
cosmic time. In our estimate, the big brake would happen at roughly 1300Gyr.Comment: 9 pages, 4 figures. RevTex4-1. Title modified and discussion
expanded. Version accepted in European Physical Journal
Constraints on single entity driven inflationary and radition eras
We present a model that attempts to fuse the inflationary era and the
subsequent radiation dominated era under a unified framework so as to provide a
smooth transition between the two. The model is based on a modification of the
generalized Chaplygin gas. We constrain the model observationally by mapping
the primordial power spectrum of the scalar perturbations to the latest data of
WMAP7. We compute as well the spectrum of the primordial gravitational waves as
would be measured today.Comment: 4 pages, 6 figures. Contribution to the Spanish Relativity Meeting
(ERE2011), Madrid, Spai
The Impact of Sibling Sex Composition on Women's Educational Achievements: A Unique Natural Experiment by Twins Gender Shocks
In a pro-male biased society, brothers may reduce the parental investment received by female siblings, if parents face time or financial constraints. But brothers may also cause positive externalities. Using more than 12,000 firstborn twins from a highly sex-imbalanced economy, Taiwan, we test if women have fewer opportunities to attend college if they have a brother rather than a sister. To minimize the problem of sex selection, we exploit the fact that twin sex is random given the sex of the other twin, once we limit the data to time periods in which abortion was illegal and technology was unavailable to abort one of the two twins. We show that the birth of a male sibling, relative to a female, has almost no impact on women's or men's college enrollments on the average. If there is any effect, it is small and imprecise. Our results point to the importance of accounting for positive externalities (e.g., decreasing family size) created by a son's birth, in studies on sibling rivalry.education, son preference, sibling rivalry, sibling spillover, sex selective abortion, within-family allocation of resources
Cosmological Imprints of a Generalized Chaplygin Gas Model for the Early Universe
We propose a phenomenological model for the early universe where there is a
smooth transition between an early "quintessence" phase and a radiation
dominated era. The matter content is modelled by an appropriately modified
Chaplygin gas for the early universe. We constrain the model observationally by
mapping the primordial power spectrum of the scalar perturbations to the latest
data of WMAP7. We compute as well the spectrum of the primordial gravitational
waves as would be measured today. We show that the high frequencies region of
the spectrum depends on the free parameter of the model and most importantly
this region of the spectrum can be within the reach of future gravitational
waves detectors.Comment: 10 pages, 8 figures. RevTex
Constrained path planning of unmanned vehicles
The application of unmanned system performing large-scale tasks, for instance, long-term surveillance/reconnaissance, large area sensing/mapping, and long distance materials handling is a relatively new and exciting topic. However, developing a practical system is still challenging due to complex models and hardware restriction. This manuscript explores various path planning missions from a more realistic perspective, such as point-to-point obstacle avoiding, multi-targets trajectory finding, informative motion planning, and multi-Hamiltonian Path Problem (mHPP) with two types of unmanned vehicles, Unmanned Ariel Vehicles (UAVs) and Unmanned Ground Vehicles (UGVs). These problems are formulated as classical optimization problems with constraints representing the environment and kinematic limitations, and then solved by proposed numerical or heuristic optimization approaches. The selected methods are used to handle nonlinear, discontinuous, and multi-objective formulations of the constrained mission planning problems. The feasibility and effectiveness of the proposed algorithms are inspected by the performance and comparison with other proposed methods in literature. The resulting simulations and experimental tests obtained from all the methods are demonstrated and discussed
Control of Networked Robotic Systems
With the infrastructure of ubiquitous networks around the world, the study of robotic systems over communication networks has attracted widespread attention. This area is denominated as networked robotic systems. By exploiting the fruitful technological developments in networking and computing, networked robotic systems are endowed with potential and capabilities for several applications. Robots within a network are capable of connecting with control stations, human operators, sensors, and other robots via digital communication over possibly noisy channels/media. The issues of time delays in communication and data losses have emerged as a pivotal issue that have stymied practical deployment. The aim of this dissertation is to develop control algorithms and architectures for networked robotic systems that guarantee stability with improved overall performance in the presence of time delays in communication.
The first topic addressed in this dissertation is controlled synchronization that is utilized for networked robotic systems to achieve collective behaviors. Exploiting passivity property of individual robotic systems, the proposed control schemes and interconnections are shown to ensure stability and convergence of synchronizing errors. The robustness of the control algorithms to constant and time-varying communication delays is also studied. In addition to time delays, the number of communication links, which prevents scalability of networked robotic systems, is another challenging issue. Thus, a synchronizing control with practically feasible constraints of network topology is developed.
The problem of networked robotic systems interacting with human operators is then studied subsequently. This research investigates a teleoperation system with heterogeneous robots under asymmetric and unknown communication delays. Sub-task controllers are proposed for redundant slave robot to autonomously achieve additional tasks, such as singularity avoidance, joint angle limits, and collision avoidance. The developed control algorithms can enhance the efficiency of teleoperation systems, thereby ameliorating the performance degradation due to cognitive limitations of human operator and incomplete information about the environment.
Compared to traditional robotic systems, control of robotic manipulators over networks has significant advantages; for example, increased flexibility and ease of maintenance. With the utilization of scattering variables, this research demonstrates that transmitting scattering variables over delayed communications can stabilize an otherwise unstable system. An architecture utilizing delayed position feedback in conjunction with scattering variables is developed for the case of time-varying communication delays. The proposed control architecture improves tracking performance and stabilizes robotic manipulators with input-output communication delays. The aforementioned control algorithms and architectures for networked robotic systems are validated via numerical examples and experiments
New Constructions of Zero-Correlation Zone Sequences
In this paper, we propose three classes of systematic approaches for
constructing zero correlation zone (ZCZ) sequence families. In most cases,
these approaches are capable of generating sequence families that achieve the
upper bounds on the family size () and the ZCZ width () for a given
sequence period ().
Our approaches can produce various binary and polyphase ZCZ families with
desired parameters and alphabet size. They also provide additional
tradeoffs amongst the above four system parameters and are less constrained by
the alphabet size. Furthermore, the constructed families have nested-like
property that can be either decomposed or combined to constitute smaller or
larger ZCZ sequence sets. We make detailed comparisons with related works and
present some extended properties. For each approach, we provide examples to
numerically illustrate the proposed construction procedure.Comment: 37 pages, submitted to IEEE Transactions on Information Theor
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