211 research outputs found
Simple generalizations of Anti-de Sitter space-time
We consider new cosmological solutions which generalize the cosmological
patch of the Anti-de Sitter (AdS) space-time, allowing for fluids with
equations of state such that . We use them to derive the associated
full manifolds. We find that these solutions can all be embedded in flat
five-dimensional space-time with signature, revealing deformed
hyperboloids. The topology and causal-structure of these spaces is therefore
unchanged, and closed time-like curves are identified, before a covering space
is considered. However the structure of Killing vector fields is entirely
different and so we may expect a different structure of Killing horizons in
these solutions.Comment: 6 Pages, 5 Figures, Corrections and additions made for publication in
Journal of Classical and Quantum Gravit
k-Essence, superluminal propagation, causality and emergent geometry
The k-essence theories admit in general the superluminal propagation of the
perturbations on classical backgrounds. We show that in spite of the
superluminal propagation the causal paradoxes do not arise in these theories
and in this respect they are not less safe than General Relativity.Comment: 34 pages, 5 figure
Stationary Configurations Imply Shift Symmetry: No Bondi Accretion for Quintessence / k-Essence
In this paper we show that, for general scalar fields, stationary
configurations are possible for shift symmetric theories only. This symmetry
with respect to constant translations in field space should either be manifest
in the original field variables or reveal itself after an appropriate field
redefinition. In particular this result implies that neither k-Essence nor
Quintessence can have exact steady state / Bondi accretion onto Black Holes. We
also discuss the role of field redefinitions in k-Essence theories. Here we
study the transformation properties of observables and other variables in
k-Essence and emphasize which of them are covariant under field redefinitions.
Finally we find that stationary field configurations are necessarily linear in
Killing time, provided that shift symmetry is realized in terms of these field
variables.Comment: 8 page
Near Scale Invariance with Modified Dispersion Relations
We describe a novel mechanism to seed a nearly scale invariant spectrum of
adiabatic perturbations during a non-inflationary stage. It relies on a
modified dispersion relation that contains higher powers of the spatial
momentum of matter perturbations. We implement this idea in the context of a
massless scalar field in an otherwise perfectly homogeneous universe. The
couplings of the field to background scalars and tensors give rise to the
required modification of its dispersion relation, and the couplings of the
scalar to matter result in an adiabatic primordial spectrum. This work is meant
to explicitly illustrate that it is possible to seed nearly scale invariant
primordial spectra without inflation, within a conventional expansion history.Comment: 7 pages and no figures. Uses RevTeX
On Power Law Inflation in DBI Models
Inflationary models in string theory which identify the inflaton with an open
string modulus lead to effective field theories with non-canonical kinetic
terms: Dirac-Born-Infeld scalar field theories. In the case of a -brane
moving in an AdS throat with a quadratic scalar field potential DBI kinetic
terms allow a novel realization of power law inflation. This note adresses the
question of whether this behaviour is special to this particular choice of
throat geometry and potential. The answer is that for any throat geometry one
can explicitly find a potential which leads to power law inflation. This
generalizes the well known fact that an exponential potential gives power law
inflation in the case of canonical kinetic terms.Comment: References and comments adde
Imperfect Dark Energy from Kinetic Gravity Braiding
We introduce a large class of scalar-tensor models with interactions
containing the second derivatives of the scalar field but not leading to
additional degrees of freedom. These models exhibit peculiar features, such as
an essential mixing of scalar and tensor kinetic terms, which we have named
kinetic braiding. This braiding causes the scalar stress tensor to deviate from
the perfect-fluid form. Cosmology in these models possesses a rich
phenomenology, even in the limit where the scalar is an exact Goldstone boson.
Generically, there are attractor solutions where the scalar monitors the
behaviour of external matter. Because of the kinetic braiding, the position of
the attractor depends both on the form of the Lagrangian and on the external
energy density. The late-time asymptotic of these cosmologies is a de Sitter
state. The scalar can exhibit phantom behaviour and is able to cross the
phantom divide with neither ghosts nor gradient instabilities. These features
provide a new class of models for Dark Energy. As an example, we study in
detail a simple one-parameter model. The possible observational signatures of
this model include a sizeable Early Dark Energy and a specific equation of
state evolving into the final de-Sitter state from a healthy phantom regime.Comment: 41 pages, 7 figures. References and some clarifying language added.
This version was accepted for publication in JCA
The Imperfect Fluid behind Kinetic Gravity Braiding
We present a standard hydrodynamical description for non-canonical scalar
field theories with kinetic gravity braiding. In particular, this picture
applies to the simplest galileons and k-essence. The fluid variables not only
have a clear physical meaning but also drastically simplify the analysis of the
system. The fluid carries charges corresponding to shifts in field space. This
shift-charge current contains a spatial part responsible for diffusion of the
charges. Moreover, in the incompressible limit, the equation of motion becomes
the standard diffusion equation. The fluid is indeed imperfect because the
energy flows neither along the field gradient nor along the shift current. The
fluid has zero vorticity and is not dissipative: there is no entropy
production, the energy-momentum is exactly conserved, the temperature vanishes
and there is no shear viscosity. Still, in an expansion around a perfect fluid
one can identify terms which correct the pressure in the manner of bulk
viscosity. We close by formulating the non-trivial conditions for the
thermodynamic equilibrium of this imperfect fluid.Comment: 23 pages plus appendices. New version includes extended discussion on
diffusion and dynamics in alternative frames, as well as additional
references. v3 reflects version accepted for publication in JHEP: minor
comments added regarding suitability to numerical approache
K fields, compactons, and thick branes
K fields, that is, fields with a non-standard kinetic term, allow for soliton
solutions with compact support, i.e., compactons. Compactons in 1+1 dimensions
may give rise to topological defects of the domain wall type and with finite
thickness in higher dimensions. Here we demonstrate that, for an appropriately
chosen kinetic term, propagation of linear perturbations is completely
suppressed outside the topological defect, confining the propagation of
particles inside the domain wall. On the other hand, inside the topological
defect the propagation of linear perturbations is of the standard type, in
spite of the non-standard kinetic term. Consequently, this compacton domain
wall may act like a brane of finite thickness which is embedded in a higher
dimensional space, but to which matter fields are constrained. In addition, we
find strong indications that, when gravity is taken into account, location of
gravity in the sense of Randall--Sundrum works for these compacton domain
walls. When seen from the bulk, these finite thickness branes, in fact, cannot
be distinguished from infinitely thin branes.Comment: some references and further remarks adde
Compact self-gravitating solutions of quartic (K) fields in brane cosmology
Recently we proposed that K fields, that is, fields with a non-standard
kinetic term, may provide a mechanism for the generation of thick branes, based
on the following observations. Firstly, K field theories allow for soliton
solutions with compact support, i.e., compactons. Compactons in 1+1 dimensions
may give rise to topological defects of the domain wall type and with finite
thickness in higher dimensions. Secondly, propagation of linear perturbations
is confined inside the compacton domain wall. Further, these linear
perturbations inside the topological defect are of the standard type, in spite
of the non-standard kinetic term. Thirdly, when gravity is taken into account,
location of gravity in the sense of Randall--Sundrum works for these compacton
domain walls provided that the backreaction of gravity does not destabilize the
compacton domain wall. It is the purpose of the present paper to investigate in
detail the existence and stability of compacton domain walls in the full K
field and gravity system, using both analytical and numerical methods. We find
that the existence of the domain wall in the full system requires a correlation
between the gravitational constant and the bulk cosmological constant, which is
thoroughly analyzed.Comment: 40 pages, 18 figures, one section on brane stability added, where the
stability under fluctuations of the scalar field is demonstrate
Cosmology with Interaction between Phantom Dark Energy and Dark Matter and the Coincidence Problem
We study a cosmological model in which phantom dark energy is coupled to dark
matter by phenomenologically introducing a coupled term to the equations of
motion of dark energy and dark matter. This term is parameterized by a
dimensionless coupling function , Hubble parameter and the energy
density of dark matter, and it describes an energy flow between the dark energy
and dark matter. We discuss two cases: one is the case where the
equation-of-state of the dark energy is a constant; the other is
that the dimensionless coupling function is a constant. We investigate
the effect of the interaction on the evolution of the universe, the total
lifetime of the universe, and the ratio of the period when the universe is in
the coincidence state to its total lifetime. It turns out that the interaction
will produce significant deviation from the case without the interaction.Comment: Latex, 17 pages including 14 figures, minor change
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