5,239 research outputs found
Open and Unoriented Strings from Topological Membrane - I. Prolegomena
We study open and unoriented strings in a Topological Membrane (TM) theory
through orbifolds of the bulk 3D space. This is achieved by gauging discrete
symmetries of the theory. Open and unoriented strings can be obtained from all
possible realizations of , and symmetries. The important role of
symmetry to distinguish between Dirichlet and Neumman boundary conditions is
discussed in detail.Comment: 2+43 pages, 25 figures. Major changes, mostly in subsection 4.5 and
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Assessing the use and impact of technology in physical learning environments at City University London - Learning Development Associate project
Saying ‘technology is here to stay’ implies that it is unpacking its bags for the weekend. In reality, technology has already left, and is on its way to its next destination. There is more than a little truth in the notion that only yesterday’s gadgets are available in the shops; tomorrow’s are in the laboratories or on the (digital) drawing boards.
Technology plays an important role in learning at City University London in both physical and virtual learning environments. This project examines the technology that is used in physical learning environments, and investigates why it is used. Through a series of mini case studies involving observations, interviews and surveys with key stakeholders, this study attempts to gauge the impact technology has on teaching and learning.
In general the data indicates that technology is popular with both teachers and students at the University. It can ‘add value’ by making learning more dynamic, varied and interactive. But it is not perfect: sometimes it breaks and not everybody knows that it exists or how it works, while others do not want to use some of it
Two-Stream Instability of Counter-Rotating Galaxies
The present study of the two-stream instability in stellar disks with
counter-rotating components of stars and/or gas is stimulated by recently
discovered counter-rotating spiral and S0 galaxies. Strong linear two-stream
instability of tightly-wrapped spiral waves is found for one and two-armed
waves with the pattern angular speed of the unstable waves always intermediate
between the angular speed of the co-rotating matter () and that of the
counter-rotating matter (). The instability arises from the
interaction of positive and negative energy modes in the co- and
counter-rotating components. The unstable waves are in general convective -
they move in radius and radial wavenumber space - with the result that
amplification of the advected wave is more important than the local growth
rate. For a galaxy of co-rotating stars and counter-rotating stars of
mass-fraction , or of counter-rotating gas of mass-fraction
, the largest amplification is usually for the one-armed
leading waves (with respect to the co-rotating stars). For the case of both
counter-rotating stars and gas, the largest amplifications are for , also for one-armed leading waves. The two-armed trailing
waves usually have smaller amplifications. The growth rates and amplifications
all decrease as the velocity spreads of the stars and/or gas increase. It is
suggested that the spiral waves can provide an effective viscosity for the gas
causing its accretion.Comment: 14 pages, submitted to ApJ. One table and 17 figures can be obtained
by sending address to R. Lovelace at [email protected]
Free energy and torque for superconductors with different anisotropies of H_{c2} and lambda
The free energy is evaluated for a uniaxial superconductor with the
anisotropy of the upper critical field, gamma_H = H_{c2,ab}/H_{c2,c}, different
from the anisotropy of the penetration depth gamma_{lambda} =
lambda_c/lambda_{ab}. With increasing difference between gamma_H and
gamma_{lambda}, the equilibrium orientation of the crystal relative to the
applied field may shift from theta = pi/2 (theta is the angle between the field
and the c axis) to lower angles and reach theta = 0 for large enough gamma_H.
These effects are expected to take place in MgB_2.Comment: 4 pages, 3 fig
Non-linear bigravity and cosmic acceleration
We explore the cosmological solutions of classes of non-linear bigravity
theories. These theories are defined by effective four-dimensional Lagrangians
describing the coupled dynamics of two metric tensors, and containing, in the
linearized limit, both a massless graviton and an ultralight one. We focus on
two paradigmatic cases: the case where the coupling between the two metrics is
given by a Pauli-Fierz-type mass potential, and the case where this coupling
derives from five-dimensional brane constructions. We find that cosmological
evolutions in bigravity theories can be described in terms of the dynamics of
two ``relativistic particles'', moving in a curved Lorenzian space, and
connected by some type of nonlinear ``spring''. Classes of bigravity
cosmological evolutions exhibit a ``locking'' mechanism under which the two
metrics ultimately stabilize in a bi-de-Sitter configuration, with relative
(constant) expansion rates. In the absence of matter, we find that a generic
feature of bigravity cosmologies is to exhibit a period of cosmic acceleration.
This leads us to propose bigravity as a source of a new type of dark energy
(``tensor quintessence''), exhibiting specific anisotropic features. Bigravity
could also have been the source of primordial inflation.Comment: 55 pages, 4 figures, references and comments added, final version
published in Phys. Rev.
Primordial Black Hole: Mass and Angular Momentum Evolution
The evolution of the primordial low mass black hole (PBH) in hot universe is
considered. Increase of mass and decrease of PBH spin due to the accretion of
radiation dominated matter are estimated with using of results of numerical
simulation of PBH formation and approximate relations for accretion to a
rotating black hole.Comment: Gravitation and Cosmology, accepted, 3 pages, Talk presented at the
russian summer school-seminar "Modern theoretical problems of gravitation and
cosmology" (GRACOS-2007), September 9-16, 2007, Kazan-Yalchik, Russi
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