1,168 research outputs found
The Adiabatic Instability on Cosmology's Dark Side
We consider theories with a nontrivial coupling between the matter and dark
energy sectors. We describe a small scale instability that can occur in such
models when the coupling is strong compared to gravity, generalizing and
correcting earlier treatments. The instability is characterized by a negative
sound speed squared of an effective coupled dark matter/dark energy fluid. Our
results are general, and applicable to a wide class of coupled models and
provide a powerful, redshift-dependent tool, complementary to other
constraints, with which to rule many of them out. A detailed analysis and
applications to a range of models are presented in a longer companion paper.Comment: 4 pages, 1 figur
Universality in D-brane Inflation
We study the six-field dynamics of D3-brane inflation for a general scalar
potential on the conifold, finding simple, universal behavior. We numerically
evolve the equations of motion for an ensemble of more than 7 \times 10^7
realizations, drawing the coefficients in the scalar potential from statistical
distributions whose detailed properties have demonstrably small effects on our
results. When prolonged inflation occurs, it has a characteristic form: the
D3-brane initially moves rapidly in the angular directions, spirals down to an
inflection point in the potential, and settles into single-field inflation. The
probability of N_{e} e-folds of inflation is a power law, P(N_{e}) \propto
N_{e}^{-3}, and we derive the same exponent from a simple analytical model. The
success of inflation is relatively insensitive to the initial conditions: we
find attractor behavior in the angular directions, and the D3-brane can begin
far above the inflection point without overshooting. In favorable regions of
the parameter space, models yielding 60 e-folds of expansion arise
approximately once in 10^3 trials. Realizations that are effectively
single-field and give rise to a primordial spectrum of fluctuations consistent
with WMAP, for which at least 120 e-folds are required, arise approximately
once in 10^5 trials. The emergence of robust predictions from a six-field
potential with hundreds of terms invites an analytic approach to multifield
inflation.Comment: 28 pages, 9 figure
The dynamical viability of scalar-tensor gravity theories
We establish the dynamical attractor behavior in scalar-tensor theories of
dark energy, providing a powerful framework to analyze classes of theories,
predicting common evolutionary characteristics that can be compared against
cosmological constraints. In the Jordan frame the theories are viewed as a
coupling between a scalar field, \Phi, and the Ricci scalar, R, F(\Phi)R. The
Jordan frame evolution is described in terms of dynamical variables m \equiv
d\ln F/d\ln \Phi and r \equiv -\Phi F/f, where F(\Phi) = d f(\Phi)/d\Phi. The
evolution can be alternatively viewed in the Einstein frame as a general
coupling between scalar dark energy and matter, \beta. We present a complete,
consistent picture of evolution in the Einstein and Jordan frames and consider
the conditions on the form of the coupling F and \beta required to give the
observed cold dark matter (CDM) dominated era that transitions into a late time
accelerative phase, including transitory accelerative eras that have not
previously been investigated. We find five classes of evolutionary behavior of
which four are qualitatively similar to those for f(R) theories (which have
\beta=1/2). The fifth class exists only for |\beta| < \sqrt{3}/4, i.e. not for
f(R) theories. In models giving transitory late time acceleration, we find a
viable accelerative region of the (r,m) plane accessible to scalar-tensor
theories with any coupling, \beta (at least in the range |\beta| \leq 1/2,
which we study in detail), and an additional region open only to theories with
|\beta| < \sqrt{3}/4.Comment: 24 pages, 3 figure
Cosmological Implications of a Scale Invariant Standard Model
We generalize the standard model of particle physics such it displays global
scale invariance. The gravitational action is also suitably modified such that
it respects this symmetry. This model is interesting since the cosmological
constant term is absent in the action. We find that the scale symmetry is
broken by the recently introduced cosmological symmetry breaking mechanism.
This simultaneously generates all the dimensionful parameters such as the
Newton's gravitational constant, the particle masses and the vacuum or dark
energy. We find that in its simplest version the model predicts the Higgs mass
to be very small, which is ruled out experimentally. We further generalize the
model such that it displays local scale invariance. In this case the Higgs
particle disappears from the particle spectrum and instead we find a very
massive vector boson. Hence the model gives a consistent description of
particle physics phenomenology as well as fits the cosmological dark energy.Comment: 12 pages, no figure
Future of the universe in modified gravitational theories: Approaching to the finite-time future singularity
We investigate the future evolution of the dark energy universe in modified
gravities including gravity, string-inspired scalar-Gauss-Bonnet and
modified Gauss-Bonnet ones, and ideal fluid with the inhomogeneous equation of
state (EoS). Modified Friedmann-Robertson-Walker (FRW) dynamics for all these
theories may be presented in universal form by using the effective ideal fluid
with an inhomogeneous EoS without specifying its explicit form. We construct
several examples of the modified gravity which produces accelerating
cosmologies ending at the finite-time future singularity of all four known
types by applying the reconstruction program. Some scenarios to resolve the
finite-time future singularity are presented. Among these scenarios, the most
natural one is related with additional modification of the gravitational action
in the early universe. In addition, late-time cosmology in the non-minimal
Maxwell-Einstein theory is considered. We investigate the forms of the
non-minimal gravitational coupling which generates the finite-time future
singularities and the general conditions for this coupling in order that the
finite-time future singularities cannot emerge. Furthermore, it is shown that
the non-minimal gravitational coupling can remove the finite-time future
singularities or make the singularity stronger (or weaker) in modified gravity.Comment: 25 pages, no figure, title changed, accepted in JCA
Challenges for String Cosmology
We critically assess the twin prospects of describing the observed universe
in string theory, and using cosmological experiments to probe string theory.
For the purposes of this short review, we focus on the limitations imposed by
our incomplete understanding of string theory. After presenting an array of
significant obstacles, we indicate a few areas that may admit theoretical
progress in the near future.Comment: 18 pages; contribution to a focus issue on string cosmology for
Classical and Quantum Gravit
A Statistical Approach to Multifield Inflation: Many-field Perturbations Beyond Slow Roll
We study multifield contributions to the scalar power spectrum in an ensemble
of six-field inflationary models obtained in string theory. We identify
examples in which inflation occurs by chance, near an approximate inflection
point, and we compute the primordial perturbations numerically, both exactly
and using an array of truncated models. The scalar mass spectrum and the number
of fluctuating fields are accurately described by a simple random matrix model.
During the approach to the inflection point, bending trajectories and
violations of slow roll are commonplace, and 'many-field' effects, in which
three or more fields influence the perturbations, are often important. However,
in a large fraction of models consistent with constraints on the tilt the
signatures of multifield evolution occur on unobservably large scales. Our
scenario is a concrete microphysical realization of quasi-single-field
inflation, with scalar masses of order , but the cubic and quartic couplings
are typically too small to produce detectable non-Gaussianity. We argue that
our results are characteristic of a broader class of models arising from
multifield potentials that are natural in the Wilsonian sense.Comment: 39 pages, 17 figures. References added. Matches version published in
JCA
Compactifying the state space for alternative theories of gravity
In this paper we address important issues surrounding the choice of variables
when performing a dynamical systems analysis of alternative theories of
gravity. We discuss the advantages and disadvantages of compactifying the state
space, and illustrate this using two examples. We first show how to define a
compact state space for the class of LRS Bianchi type I models in -gravity
and compare to a non--compact expansion--normalised approach. In the second
example we consider the flat Friedmann matter subspace of the previous example,
and compare the compact analysis to studies where non-compact
non--expansion--normalised variables were used. In both examples we comment on
the existence of bouncing or recollapsing orbits as well as the existence of
static models.Comment: 18 pages, revised to match published versio
Generalizing Galileons
The Galileons are a set of terms within four-dimensional effective field
theories, obeying symmetries that can be derived from the dynamics of a
3+1-dimensional flat brane embedded in a 5-dimensional Minkowski Bulk. These
theories have some intriguing properties, including freedom from ghosts and a
non-renormalization theorem that hints at possible applications in both
particle physics and cosmology. In this brief review article, we will summarize
our attempts over the last year to extend the Galileon idea in two important
ways. We will discuss the effective field theory construction arising from
co-dimension greater than one flat branes embedded in a flat background - the
multiGalileons - and we will then describe symmetric covariant versions of the
Galileons, more suitable for general cosmological applications. While all these
Galileons can be thought of as interesting four-dimensional field theories in
their own rights, the work described here may also make it easier to embed them
into string theory, with its multiple extra dimensions and more general
gravitational backgrounds.Comment: 16 pages; invited brief review article for a special issue of
Classical and Quantum Gravity. Submitted to CQ
Primordial fluctuations and non-Gaussianities from multifield DBI Galileon inflation
We study a cosmological scenario in which the DBI action governing the motion
of a D3-brane in a higher-dimensional spacetime is supplemented with an induced
gravity term. The latter reduces to the quartic Galileon Lagrangian when the
motion of the brane is non-relativistic and we show that it tends to violate
the null energy condition and to render cosmological fluctuations ghosts. There
nonetheless exists an interesting parameter space in which a stable phase of
quasi-exponential expansion can be achieved while the induced gravity leaves
non trivial imprints. We derive the exact second-order action governing the
dynamics of linear perturbations and we show that it can be simply understood
through a bimetric perspective. In the relativistic regime, we also calculate
the dominant contribution to the primordial bispectrum and demonstrate that
large non-Gaussianities of orthogonal shape can be generated, for the first
time in a concrete model. More generally, we find that the sign and the shape
of the bispectrum offer powerful diagnostics of the precise strength of the
induced gravity.Comment: 34 pages including 9 figures, plus appendices and bibliography.
Wordings changed and references added; matches version published in JCA
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