63 research outputs found
D-brane dynamics near compactified NS5-branes
We examine the dynamics of a -brane in the background of coincident,
parallel 5-branes which have had one of their common transverse directions
compactified. We find that for small energy, bound orbits can exist at
sufficiently large distances where there will be no stringy effects. The orbits
are dependent upon the energy density, angular momentum and electric field. The
analysis breaks down at radial distances comparable with the compactification
radius and we must resort to using a modified form of the harmonic function in
this region.Comment: Latex, 20 pages, 6 figs, references adde
D-Brane Dynamics and NS5 Rings
We consider the classical motion of a probe D-brane moving in the background
geometry of a ring of NS5 branes, assuming that the latter are non-dynamical.
We analyse the solutions to the Dirac-Born-Infield (DBI) action governing the
approximate dynamics of the system. In the near horizon (throat) approximation
we find several exact solutions for the probe brane motion. These are compared
to numerical solutions obtained in more general cases. One solution of
particular interest is when the probe undergoes oscillatory motion through the
centre of the ring (and perpendicular to it). By taking the ring radius
sufficiently large, this solution should remain stable to any stringy
corrections coming from open-strings stretching between the probe and the
NS5-branes along the ring.Comment: 17 pages, Latex, 8 figures; References adde
THz emission from Fe/Pt spintronic emitters with L1-FePt alloyed interface
Recent developments in nanomagnetism and spintronics have enabled the use of
ultrafast spin physics for terahertz (THz) emission. Spintronic THz emitters,
consisting of ferromagnetic FM / non-magnetic (NM) thin film heterostructures,
have demonstrated impressive properties for the use in THz spectroscopy and
have great potential in scientific and industrial applications. In this work,
we focus on the impact of the FM/NM interface on the THz emission by
investigating Fe/Pt bilayers with engineered interfaces. In particular, we
intentionally modify the Fe/Pt interface by inserting an ordered L1-FePt
alloy interlayer. Subsequently, we establish that a Fe/L1-FePt (2\,nm)/Pt
configuration is significantly superior to a Fe/Pt bilayer structure, regarding
THz emission amplitude. The latter depends on the extent of alloying on either
side of the interface. The unique trilayer structure opens new perspectives in
terms of material choices for the next generation of spintronic THz emitters
Spinflation from Geometric Tachyon
We study the assisted inflation scenario from the rolling of BPS D3-brane
into the NS5-branes, on a transverse geometry of , coupled to
four dimensional gravity. We assume that the branes are distributed along
and the probe D3-branes spin along plane. Qualitatively this process is
similar to that of N-tachyon assisted inflation on unstable D-branes. We
further study the spinflation scenario numerically and analyze its effect.Comment: 18pages, 9 figures, added clarifications, to appear in JHE
A solution of the coincidence problem based on the recent galactic core black hole mass density increase
A mechanism capable to provide a natural solution to two major cosmological
problems, i.e. the cosmic acceleration and the coincidence problem, is
proposed. A specific brane-bulk energy exchange mechanism produces a total dark
pressure, arising when adding all normal to the brane negative pressures in the
interior of galactic core black holes. This astrophysically produced negative
dark pressure explains cosmic acceleration and why the dark energy today is of
the same order to the matter density for a wide range of the involved
parameters. An exciting result of the analysis is that the recent rise of the
galactic core black hole mass density causes the recent passage from cosmic
deceleration to acceleration. Finally, it is worth mentioning that this work
corrects a wide spread fallacy among brane cosmologists, i.e. that escaping
gravitons result to positive dark pressure.Comment: 14 pages, 3 figure
Solving the Hierarchy Problem without Supersymmetry or Extra Dimensions: An Alternative Approach
In this paper, we propose a possible new approach towards solving the gauge
hierarchy problem without supersymmetry and without extra spacetime dimensions.
This approach relies on the finiteness of string theory and the conjectured
stability of certain non-supersymmetric string vacua. One crucial ingredient in
this approach is the idea of ``misaligned supersymmetry'', which explains how
string theories may be finite even without exhibiting spacetime supersymmetry.
This approach towards solving the gauge hierarchy problem is therefore
complementary to recent proposals involving both large and small extra
spacetime dimensions. This approach may also give a new perspective towards
simultaneously solving the cosmological constant problem.Comment: 33 pages, LaTeX, 3 figure
A Quintessentially Geometric Model
We consider string inspired cosmology on a solitary -brane moving in the
background of a ring of branes located on a circle of radius . The motion of
the -brane transverse to the plane of the ring gives rise to a radion field
which can be mapped to a massive non-BPS Born-Infeld type field with a cosh
potential. For certain bounds of the brane tension we find an inflationary
phase is possible, with the string scale relatively close to the Planck scale.
The relevant perturbations and spectral indices are all well within the
expected observational bounds. The evolution of the universe eventually comes
to be dominated by dark energy, which we show is a late time attractor of the
model. However we also find that the equation of state is time dependent, and
will lead to late time Quintessence.Comment: 11 pages, 3 figures. References and comments adde
Modified Gravity and Cosmology
In this review we present a thoroughly comprehensive survey of recent work on
modified theories of gravity and their cosmological consequences. Amongst other
things, we cover General Relativity, Scalar-Tensor, Einstein-Aether, and
Bimetric theories, as well as TeVeS, f(R), general higher-order theories,
Horava-Lifschitz gravity, Galileons, Ghost Condensates, and models of extra
dimensions including Kaluza-Klein, Randall-Sundrum, DGP, and higher
co-dimension braneworlds. We also review attempts to construct a Parameterised
Post-Friedmannian formalism, that can be used to constrain deviations from
General Relativity in cosmology, and that is suitable for comparison with data
on the largest scales. These subjects have been intensively studied over the
past decade, largely motivated by rapid progress in the field of observational
cosmology that now allows, for the first time, precision tests of fundamental
physics on the scale of the observable Universe. The purpose of this review is
to provide a reference tool for researchers and students in cosmology and
gravitational physics, as well as a self-contained, comprehensive and
up-to-date introduction to the subject as a whole.Comment: 312 pages, 15 figure
Cold atoms in space: community workshop summary and proposed road-map
We summarise the discussions at a virtual Community Workshop on Cold Atoms in Space concerning the status of cold atom technologies, the prospective scientific and societal opportunities offered by their deployment in space, and the developments needed before cold atoms could be operated in space. The cold atom technologies discussed include atomic clocks, quantum gravimeters and accelerometers, and atom interferometers. Prospective applications include metrology, geodesy and measurement of terrestrial mass change due to, e.g., climate change, and fundamental science experiments such as tests of the equivalence principle, searches for dark matter, measurements of gravitational waves and tests of quantum mechanics. We review the current status of cold atom technologies and outline the requirements for their space qualification, including the development paths and the corresponding technical milestones, and identifying possible pathfinder missions to pave the way for missions to exploit the full potential of cold atoms in space. Finally, we present a first draft of a possible road-map for achieving these goals, that we propose for discussion by the interested cold atom, Earth Observation, fundamental physics and other prospective scientific user communities, together with the European Space Agency (ESA) and national space and research funding agencies.publishedVersio
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