1,158 research outputs found
Dachau—Remembering the Unforgettable
Ethical research provides great benefit to the public, but ethical research is not guaranteed. Research can go terribly wrong when research subjects are not protected. Egos of scientists and others in power can cause disastrous results, and that is what happened at the Dachau concentration camp in Germany. Because of incidents such as this, medical research at universities is now reviewed by Institutional Review Boards to protect subjects. But not so long ago, in the Dachau concentration camp, researchers were free to impose pain and death upon prisoners in the name of research. Prisoners were mistreated to glean knowledge; many suffered for the advancement of science and man’s ego. The history of such events is haunting. It must never happen again, and so mankind studies the past to protect the future. For if it is true that history repeats itself, mankind must be armed with the knowledge of history
Domain Bubbles of Extra Dimensions
``Dimension bubbles'' of the type previously studied by Blau and Guendelman
[S.K. Blau and E.I. Guendelman, Phys. Rev. D40, 1909 (1989)], which effectively
enclose a region of 5d spacetime and are surrounded by a region of 4d
spacetime, can arise in a 5d theory with a compact extra dimension that is
dimensionally reduced to give an effective 4d theory. These bubbles with thin
domain walls can be stabilized against total collapse in a rather natural way
by a scalar field which, as in the case with ``ordinary'' nontopological
solitons, traps light scalar particles inside the bubble.Comment: 13 pages, no figures; to appear in Phys.Rev.
Ghost Condensation and a Consistent Infrared Modification of Gravity
We propose a theoretically consistent modification of gravity in the
infrared, which is compatible with all current experimental observations. This
is an analog of Higgs mechanism in general relativity, and can be thought of as
arising from ghost condensation--a background where a scalar field \phi has a
constant velocity, = M^2. The ghost condensate is a new kind of
fluid that can fill the universe, which has the same equation of state, \rho =
-p, as a cosmological constant, and can hence drive de Sitter expansion of the
universe. However, unlike a cosmological constant, it is a physical fluid with
a physical scalar excitation, which can be described by a systematic effective
field theory at low energies. The excitation has an unusual low-energy
dispersion relation \omega^2 \sim k^4 / M^2. If coupled to matter directly, it
gives rise to small Lorentz-violating effects and a new long-range 1/r^2 spin
dependent force. In the ghost condensate, the energy that gravitates is not the
same as the particle physics energy, leading to the possibility of both sources
that can gravitate and antigravitate. The Newtonian potential is modified with
an oscillatory behavior starting at the distance scale M_{Pl}/M^2 and the time
scale M_{Pl}^2/M^3. This theory opens up a number of new avenues for attacking
cosmological problems, including inflation, dark matter and dark energy.Comment: 42 pages, LaTeX 2
Gauged Dimension Bubbles
Some of the peculiar electrodynamical effects associated with gauged
``dimension bubbles'' are presented. Such bubbles, which effectively enclose a
region of 5d spacetime, can arise from a 5d theory with a compact extra
dimension. Bubbles with thin domain walls can be stabilized against total
collapse by the entrapment of light charged scalar bosons inside the bubble,
extending the idea of a neutral dimension bubble to accommodate the case of a
gauged U(1) symmetry. Using a dielectric approach to the 4d dilaton-Maxwell
theory, it is seen that the bubble wall is almost totally opaque to photons,
leading to a new stabilization mechanism due to trapped photons. Photon
dominated bubbles very slowly shrink, resulting in a temperature increase
inside the bubble. At some critical temperature, however, these bubbles
explode, with a release of radiation.Comment: 14 pages, no figures; to appear in Phys.Rev.
Acoustic Black Holes from Abelian Higgs Model with Lorentz Symmetry Breaking
In this paper we derive acoustic black hole metrics in the (3+1) and
(2+1)-dimensional Abelian Higgs model with Lorentz symmetry breaking. In this
set up the sound waves lose the Lorentz boost invariance and suffer a
'birefringence' effect. We have found acoustic black holes and respective
Hawking temperatures depending on the Lorentz violating parameter. Furthermore,
we obtain an acoustic Kerr-like black hole metric with the Lorentz violating
term affecting its rate of loss of mass. We also have shown that for suitable
values of the Lorentz violating parameter a wider spectrum of particle wave
function can be scattered with increased amplitude by the acoustic black hole.Comment: 12 pages, Latex, no figures, version accepted to Phys. Lett.
Probing Yukawian Gravitational Potential by Numerical Simulations. II. Elliptical Galaxies
Since the Newtonian gravitation is largely used to model with success the
structures of the universe, such as galaxies and clusters of galaxies, for
example, a way to probe and constrain alternative theories, in the weak field
limit, is to apply them to model the structures of the universe. We then
modified the well known Gadget-2 code to probe alternative theories of
gravitation through galactic dynamics. In particular, we modified the Gadget-2
code to probe alternatives theories whose weak field limits have a Yukawa-like
gravitational potential. As a first application of this modified Gadget-2 code
we simulate the evolution of elliptical galaxies. These simulations show that
galactic dynamics can be used to constrain the parameters associated with
alternative theories of gravitation.Comment: 6 pages, 5 figures - To appear in General Relativity and Gravitatio
DBI Galileon and Late time acceleration of the universe
We consider 1+3 dimensional maximally symmetric Minkowski brane embedded in a
1+4 dimensional maximally symmetric Minkowski background. The resulting 1+3
dimensional effective field theory is of DBI (Dirac-Born-Infeld) Galileon type.
We use this model to study the late time acceleration of the universe. We study
the deviation of the model from the concordance \Lambda CDM behaviour. Finally
we put constraints on the model parameters using various observational data.Comment: 16 pages, 7 eps figures, Latex Style, new references added, corrected
missing reference
Conditions for the cosmological viability of the most general scalar-tensor theories and their applications to extended Galileon dark energy models
In the Horndeski's most general scalar-tensor theories with second-order
field equations, we derive the conditions for the avoidance of ghosts and
Laplacian instabilities associated with scalar, tensor, and vector
perturbations in the presence of two perfect fluids on the flat
Friedmann-Lemaitre-Robertson-Walker (FLRW) background. Our general results are
useful for the construction of theoretically consistent models of dark energy.
We apply our formulas to extended Galileon models in which a tracker solution
with an equation of state smaller than -1 is present. We clarify the allowed
parameter space in which the ghosts and Laplacian instabilities are absent and
we numerically confirm that such models are indeed cosmologically viable.Comment: 18 pages, 6 figure
Static Observers in Curved Spaces and Non-inertial Frames in Minkowski Spacetime
Static observers in curved spacetimes may interpret their proper acceleration
as the opposite of a local gravitational field (in the Newtonian sense). Based
on this interpretation and motivated by the equivalence principle, we are led
to investigate congruences of timelike curves in Minkowski spacetime whose
acceleration field coincides with the acceleration field of static observers of
curved spaces. The congruences give rise to non-inertial frames that are
examined. Specifically we find, based on the locality principle, the embedding
of simultaneity hypersurfaces adapted to the non-inertial frame in an explicit
form for arbitrary acceleration fields. We also determine, from the Einstein
equations, a covariant field equation that regulates the behavior of the proper
acceleration of static observers in curved spacetimes. It corresponds to an
exact relativistic version of the Newtonian gravitational field equation. In
the specific case in which the level surfaces of the norm of the acceleration
field of the static observers are maximally symmetric two-dimensional spaces,
the energy-momentum tensor of the source is analyzed.Comment: 28 pages, 4 figures
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