8,729 research outputs found
Cosmology of a covariant Galileon field
We study the cosmology of a covariant scalar field respecting a Galilean
symmetry in flat space-time. We show the existence of a tracker solution that
finally approaches a de Sitter fixed point responsible for cosmic acceleration
today. The viable region of model parameters is clarified by deriving
conditions under which ghosts and Laplacian instabilities of scalar and tensor
perturbations are absent. The field equation of state exhibits a peculiar
phantom-like behavior along the tracker, which allows a possibility to
observationally distinguish the Galileon gravity from the Lambda-CDM model.Comment: 4 pages, uses RevTe
Generalized Galileon cosmology
We study the cosmology of a generalized Galileon field with five
covariant Lagrangians in which is replaced by general scalar functions
(i=1,...,5). For these theories, the equations of motion remain
at second-order in time derivatives. We restrict the functional forms of
from the demand to obtain de Sitter solutions responsible for
dark energy. There are two possible choices for power-law functions
, depending on whether the coupling with the Ricci
scalar is independent of or depends on . The former
corresponds to the covariant Galileon theory that respects the Galilean
symmetry in the Minkowski space-time. For generalized Galileon theories we
derive the conditions for the avoidance of ghosts and Laplacian instabilities
associated with scalar and tensor perturbations as well as the condition for
the stability of de Sitter solutions. We also carry out detailed analytic and
numerical study for the cosmological dynamics in those theories.Comment: 24 pages, 10 figures, version to appear in Physical Review
Density perturbations in general modified gravitational theories
We derive the equations of linear cosmological perturbations for the general
Lagrangian density , where is a Ricci scalar,
is a scalar field, and is a field kinetic energy. We
take into account a nonlinear self-interaction term recently studied in
the context of "Galileon" cosmology, which keeps the field equations at second
order. Taking into account a scalar-field mass explicitly, the equations of
matter density perturbations and gravitational potentials are obtained under a
quasi-static approximation on sub-horizon scales. We also derive conditions for
the avoidance of ghosts and Laplacian instabilities associated with propagation
speeds. Our analysis includes most of modified gravity models of dark energy
proposed in literature and thus it is convenient to test the viability of such
models from both theoretical and observational points of view.Comment: 17 pages, no figure
Cosmological perturbation in f(R,G) theories with a perfect fluid
In order to classify modified gravity models according to their physical
properties, we analyze the cosmological linear perturbations for f(R,G)
theories (R being the Ricci scalar and G, the Gauss-Bonnet term) with a
minimally coupled perfect fluid. For the scalar type perturbations, we identify
in general six degrees of freedom. We find that two of these physical modes
obey the same dispersion relation as the one for a non-relativistic de Broglie
wave. This means that spacetime is either highly unstable or its fluctuations
undergo a scale-dependent super-luminal propagation. Two other modes correspond
to the degrees of freedom of the perfect fluid, and propagate with the sound
speed of such a fluid. The remaining two modes correspond to the entropy and
temperature perturbations of the perfect fluid, and completely decouple from
the other modes for a barotropic equation of state. We then provide a concise
condition on f(R,G) theories, that both f(R) and R+f(G) do fulfill, to avoid
the de Broglie type dispersion relation. For the vector type perturbation, we
find that the perturbations decay in time. For the tensor type perturbation,
the perturbations can be either super-luminal or sub-luminal, depending on the
model. No-ghost conditions are also obtained for each type of perturbation.Comment: 12 pages, uses RevTe
Impulsive gravitational waves of massless particles in extended theories of gravity
We investigate the vacuum pp-wave and Aichelburg-Sexl-type solutions in f(R)
and the modified Gauss-Bonnet theories of gravity with both minimal and
nonminimal couplings between matter and geometry. In each case, we obtain the
necessary condition for the theory to admit the solution and examine it for
several specific models. We show that the wave profiles are the same or
proportional to the general relativistic one
G-quartet biomolecular nanowires
We present a first-principle investigation of quadruple helix nanowires,
consisting of stacked planar hydrogen-bonded guanine tetramers. Our results
show that long wires form and are stable in potassium-rich conditions. We
present their electronic bandstructure and discuss the interpretation in terms
of effective wide-bandgap semiconductors. The microscopic structural and
electronic properties of the guanine quadruple helices make them suitable
candidates for molecular nanoelectronics.Comment: 7 pages, 3 figures, to be published in Applied Physics Letters (2002
Cosmological dynamics of fourth order gravity with a Gauss-Bonnet term
We consider cosmological dynamics in fourth order gravity with both
and correction to the Einstein gravity ( is
the Gauss-Bonnet term). The particular case for which both terms are equally
important on power-law solutions is described. These solutions and their
stability are studied using the dynamical system approach. We also discuss
condition of existence and stability of de Sitter solution in a more general
situation of power-law and .Comment: published version, references update
Massive gravity: nonlinear instability of the homogeneous and isotropic universe
We argue that all homogeneous and isotropic solutions in nonlinear massive
gravity are unstable. For this purpose, we study the propagating modes on a
Bianchi type--I manifold. We analyze their kinetic terms and dispersion
relations as the background manifold approaches the homogeneous and isotropic
limit. We show that in this limit, at least one ghost always exists and that
its frequency tends to vanish for large scales, meaning that it cannot be
integrated out from the low energy effective theory. This ghost mode is
interpreted as a leading nonlinear perturbation around a homogeneous and
isotropic background.Comment: 4 pages, uses REVTeX4.1; v2: minor update to match the published
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