308 research outputs found
Phantom Field with O(N) Symmetry in Exponential Potential
In this paper, we study the phase space of phantom model with O(\emph{N})
symmetry in exponential potential. Different from the model without O(\emph{N})
symmetry, the introduction of the symmetry leads to a lower bound on the
equation of state for the existence of stable phantom dominated attractor
phase. The reconstruction relation between the potential of O(\textit{N})
phantom system and red shift has been derived.Comment: 5 pages, 3 figures, replaced with the version to appear on Phys. Rev.
A social networking approach for mobile innovation in emerging countries
Thesis (S.M. in Engineering and Management)--Massachusetts Institute of Technology, Engineering Systems Division, System Design and Management Program, February 2011.Cataloged from PDF version of thesis.Includes bibliographical references (p. 121-122).Addressing the global challenges and the next billion mobile subscribers, the MIT NextLab course engages students, industry partners, entrepreneurs and the next billion mobile subscribers to develop innovative mobile services that improve the quality of life in the emerging countries. In three years, NextLab teams developed and deployed 29 projects in 14 counties, and five teams founded their own ventures after perceiving the strong demand from the vast mobile users in the developing world. However, the size and the amount of NextLab projects are limited by the schedule and the location of an academic course. The focus of this thesis is to research and develop a social networking platform that replicates the success of the NextLab course to reach out to more participants around the world. In this document, I utilized the social analysis framework to identify social processes among stakeholders in a general NextLab project, specify the possible social failures and research the possible solutions. Besides, I also reviewed the NextLab projects in 2008 and 2009 and developed the NextLab Project Development Process (NLPDP) that highlights the 12 critical stages of a NextLab project. Finally, I proposed the NextLab 2.0 Community that is integrates with the social networking solutions and the NextLab Project Development Process. The case study of the mobile logistics (m-Logistics) project is used to demonstrate how the proposed solution facilitates the collaboration and communication for a large and cross-country mobile innovation project. A number of recommendations were also discussed for further research.by Jen-Hao Yang.S.M.in Engineering and Managemen
New mechanism to cross the phantom divide
Recently, type Ia supernovae data appear to support a dark energy whose
equation of state crosses -1, which is a much more amazing problem than the
acceleration of the universe. We show that it is possible for the equation of
state to cross the phantom divide by a scalar field in the gravity with an
additional inverse power-law term of Ricci scalar in the Lagrangian. The
necessary and sufficient condition for a universe in which the dark energy can
cross the phantom divide is obtained. Some analytical solutions with or
are obtained. A minimal coupled scalar with different potentials,
including quadratic, cubic, quantic, exponential and logarithmic potentials are
investigated via numerical methods, respectively. All these potentials lead to
the crossing behavior. We show that it is a robust result which is hardly
dependent on the concrete form of the potential of the scalar.Comment: 11 pages, 5 figs, v3: several references added, to match the
published versio
Gravitational Coupling and Dynamical Reduction of The Cosmological Constant
We introduce a dynamical model to reduce a large cosmological constant to a
sufficiently small value. The basic ingredient in this model is a distinction
which has been made between the two unit systems used in cosmology and particle
physics. We have used a conformal invariant gravitational model to define a
particular conformal frame in terms of large scale properties of the universe.
It is then argued that the contributions of mass scales in particle physics to
the vacuum energy density should be considered in a different conformal frame.
In this manner, a decaying mechanism is presented in which the conformal factor
appears as a dynamical field and plays a key role to relax a large effective
cosmological constant. Moreover, we argue that this model also provides a
possible explanation for the coincidence problem.Comment: To appear in GR
Very long optical path-length from a compact multi-pass cell
The multiple-pass optical cell is an important tool for laser absorption
spectroscopy and its many applications. For most practical applications, such
as trace-gas detection, a compact and robust design is essential. Here we
report an investigation into a multi-pass cell design based on a pair of
cylindrical mirrors, with a particular focus on achieving very long optical
paths. We demonstrate a path-length of 50.31 m in a cell with 40 mm diameter
mirrors spaced 88.9 mm apart - a 3-fold increase over the previously reported
longest path-length obtained with this type of cell configuration. We
characterize the mechanical stability of the cell and describe the practical
conditions necessary to achieve very long path-lengths
Numerical study of multilayer adsorption on fractal surfaces
We report a numerical study of van der Waals adsoprtion and capillary
condensation effects on self-similar fractal surfaces. An assembly of uncoupled
spherical pores with a power-law distributin of radii is used to model fractal
surfaces with adjustable dimensions. We find that the commonly used fractal
Frankel-Halsey-Hill equation systematically fails to give the correct dimension
due to crossover effects, consistent with the findings of recent experiments.
The effects of pore coupling and curvature dependent surface tension were also
studied.Comment: 11 pages, 3 figure
Deep level emission of ZnO nanoparticles deposited inside UV opal
The temperature-dependent photoluminescence (PL) spectra of zinc oxide (ZnO)
nanocrystals deposited inside the ultraviolet (UV) opal were studied. ZnO was
grown in the voids between FCC packed silicon dioxide spheres using spray
pyrolysis under ultrasonic vibration in the solution containing a zinc nitrate
precursor. The ZnO nanoparticles inside opal matrix with UV photonic band-gap
exhibit suppression of the excitonic emission and enhancement of the deep level
emission. Suppression of the excitonic lines is due to the inhibition of
spontaneous emission, while enhancement and broadening of the DL emission in
the green spectral region is due to Purcell effect. The infiltration of ZnO
inside the photonic crystal may be a useful technique to increase its emission
efficiency in the selected spectral region.Comment: 22 pages, 4 figure
Disparate MgII Absorption Statistics towards Quasars and Gamma-Ray Bursts : A Possible Explanation
We examine the recent report by Prochter et al. (2006) that gamma-ray burst
(GRB) sight lines have a much higher incidence of strong MgII absorption than
quasar sight lines. We propose that the discrepancy is due to the different
beam sizes of GRBs and quasars, and that the intervening MgII systems are
clumpy with the dense part of each cloudlet of a similar size as the quasars,
i.e. < 10^16 cm, but bigger than GRBs. We also discuss observational
predictions of our proposed model. Most notably, in some cases the intervening
MgII absorbers in GRB spectra should be seen varying, and quasars with smaller
sizes should show an increased rate of strong MgII absorbers. In fact, our
prediction of variable MgII lines in the GRB spectra has been now confirmed by
Hao et al. (2007), who observed intervening FeII and MgII lines at z=1.48 to be
strongly variable in the multi-epoch spectra of z=4.05 GRB060206.Comment: 12 pages, 2 figures; substantially revised model calculation;
accepted for publication in Astrophysics & Space Science as a Lette
Chaotic scalar fields as models for dark energy
We consider stochastically quantized self-interacting scalar fields as
suitable models to generate dark energy in the universe. Second quantization
effects lead to new and unexpected phenomena is the self interaction strength
is strong. The stochastically quantized dynamics can degenerate to a chaotic
dynamics conjugated to a Bernoulli shift in fictitious time, and the right
amount of vacuum energy density can be generated without fine tuning. It is
numerically observed that the scalar field dynamics distinguishes fundamental
parameters such as the electroweak and strong coupling constants as
corresponding to local minima in the dark energy landscape. Chaotic fields can
offer possible solutions to the cosmological coincidence problem, as well as to
the problem of uniqueness of vacua.Comment: 30 pages, 3 figures. Replaced by final version accepted by Phys. Rev.
- …