1,321 research outputs found
Dynamical Dark Energy and Spontaneously Generated Gravity
We study the cosmological evolution of an induced gravity model with a scale
symmetry breaking potential for the scalar field. The radiation to matter
transition, following inflation and reheating, influences the dynamics of such
a field through its non minimal coupling. We illustrate how, under certain
conditions on the potential, such a dynamics can lead to a suitable amount of
dark energy explaining the present accelerated expansion.Comment: 14 pages, 3 figure
New constraints on multi-field inflation with nonminimal coupling
We study the dynamics and perturbations during inflation and reheating in a
multi-field model where a second scalar field is nonminimally coupled to
the scalar curvature ). When is positive, the usual
inflationary prediction for large-scale anisotropies is hardly altered while
the fluctuation in sub-Hubble modes can be amplified during preheating
for large . For negative values of , however, long-wave modes of the
fluctuation exhibit exponential increase during inflation, leading to
the strong enhancement of super-Hubble metric perturbations even when
is less than unity. This is because the effective mass becomes negative
during inflation. We constrain the strength of and the initial by
the amplitude of produced density perturbations. One way to avoid nonadiabatic
growth of super-Hubble curvature perturbations is to stabilize the mass
through a coupling to the inflaton. Preheating may thus be necessary in these
models to protect the stability of the inflationary phase.Comment: 20 pages, 8 figures, submitted to Physical Review
Scaling attractors for quintessence in flat universe with cosmological term
For evolution of flat universe, we classify late time and future attractors
with scaling behavior of scalar field quintessence in the case of potential,
which, at definite values of its parameters and initial data, corresponds to
exact scaling in the presence of cosmological constant.Comment: 11 pages, 16 eps-figures, revtex4, reference with comment adde
A new twist to preheating
Metric perturbations typically strengthen field resonances during preheating.
In contrast we present a model in which the super-Hubble field resonances are
completely {\em suppressed} when metric perturbations are included. The model
is the nonminimal Fakir-Unruh scenario which is exactly solvable in the
long-wavelength limit when metric perturbations are included, but exhibits
exponential growth of super-Hubble modes in their absence. This gravitationally
enhanced integrability is exceptional, both for its rarity and for the power
with which it illustrates the importance of including metric perturbations in
consistent studies of preheating. We conjecture a no-go result - there exists
no {\em single-field} model with growth of cosmologically-relevant metric
perturbations during preheating.Comment: 6 pages, 3 figures, Version to appear in Physical Review
Loop quantum gravity effects on inflation and the CMB
In loop quantum cosmology, the universe avoids a big bang singularity and
undergoes an early and short super-inflation phase. During super-inflation,
non-perturbative quantum corrections to the dynamics drive an inflaton field up
its potential hill, thus setting the initial conditions for standard inflation.
We show that this effect can raise the inflaton high enough to achieve
sufficient e-foldings in the standard inflation era. We analyze the
cosmological perturbations generated when slow-roll is violated after
super-inflation, and show that loop quantum effects can in principle leave an
indirect signature on the largest scales in the CMB, with some loss of power
and running of the spectral index.Comment: revtex4, 5 pages, 3 figures, significant improvements in explanation
of quantization and perturbation issues; version to appear Classical and
Quantum Gravit
New femoral remains of Nacholapithecus kerioi: Implications for intraspecific variation and Miocene hominoid evolution
The middle Miocene stem kenyapithecine Nacholapithecus kerioi (16â15 Ma; Nachola, Kenya) is represented by a large number of isolated fossil remains and one of the most complete skeletons in the hominoid fossil record (KNM-BG 35250). Multiple fieldwork seasons performed by JapaneseâKenyan teams during the last part of the 20th century resulted in the discovery of a large sample of Nacholapithecus fossils. Here, we describe the new femoral remains of Nacholapithecus. In well-preserved specimens, we evaluate sex differences and within-species variation using both qualitative and quantitative traits. We use these data to determine whether these specimens are morphologically similar to the species holotype KNM-BG 35250 (which shows some plastic deformation) and to compare Nacholapithecus with other Miocene hominoids and extant anthropoids to evaluate the distinctiveness of its femur. The new fossil evidence reaffirms previously reported descriptions of some distal femoral traits, namely the morphology of the patellar groove. However, results also show that relative femoral head size in Nacholapithecus is smaller, relative neck length is longer, and neckâshaft angle is lower than previously reported for KNM-BG 35250. These traits have a strong functional signal related to the hip joint kinematics, suggesting that the morphology of the proximal femur in Nacholapithecus might be functionally related to quadrupedal-like behaviors instead of more derived antipronograde locomotor modes. Results further demonstrate that other African Miocene apes (with the exception of Turkanapithecus kalakolensis) generally fall within the Nacholapithecus range of variation, whose overall femoral shape resembles that of Ekembo spp. and Equatorius africanus. Our results accord with the previously inferred locomotor repertoire of Nacholapithecus, indicating a combination of generalized arboreal quadrupedalism combined with other antipronograde behaviors (e.g., vertical climbing)
Are Kaluza-Klein modes enhanced by parametric resonance?
We study parametric amplification of Kaluza-Klein (KK) modes in a higher
-dimensional generalized Kaluza-Klein theory, which was originally
considered by Mukohyama in the narrow resonance case. It was suggested that KK
modes can be enhanced by an oscillation of a scale of compactification by the
-dimensional sphere and by the direct product . We extend this past work to the more general case where
initial values of the scale of compactification and the quantum number of the
angular momentum of KK modes are not small. We perform analytic approaches
based on the Mathieu equation as well as numerical calculations, and find that
the expansion of the universe rapidly makes the KK field deviate from
instability bands. As a result, KK modes are not enhanced sufficiently in an
expanding universe in these two classes of models.Comment: 15 pages, 5 figure
Cosmological constraints from Gauss-Bonnet braneworld with large-field potentials
We calculate the spectral index and tensor-to-scalar ratio for patch
inflation defined by and ,
using the slow-roll expansion. The patch cosmology arisen from the Gauss-Bonnet
braneworld consists of Gauss-Bonnet (GB), Randall-Sundrum (RS), and 4D general
relativistic (GR) cosmological models. In this work, we choose large-field
potentials of to compare with the observational data. Since
second-order corrections are rather small in the slow-roll limit, the
leading-order calculation is sufficient to compare with the data. Finally, we
show that it is easier to discriminate between quadratic potential and quartic
potential in the GB cosmological model rather than the GR or RS cosmological
models.Comment: 13 pages, title changed, version to appear in JCA
Generalized Brans-Dicke theories
In Brans-Dicke theory a non-linear self interaction of a scalar field allows
a possibility of realizing the late-time cosmic acceleration, while recovering
the General Relativistic behavior at early cosmological epochs. We extend this
to more general modified gravitational theories in which a de Sitter solution
for dark energy exists without using a field potential. We derive a condition
for the stability of the de Sitter point and study the background cosmological
dynamics of such theories. We also restrict the allowed region of model
parameters from the demand for the avoidance of ghosts and instabilities. A
peculiar evolution of the field propagation speed allows us to distinguish
those theories from the LCDM model.Comment: 14 pages, 4 figures, version to appear in JCA
Brane preheating
We study brane-world preheating in massive chaotic inflationary scenario
where scalar fields are confined on the 3-brane. Assuming that quadratic
contribution in energy densities dominates the Hubble expansion rate during
preheating, the amplitude of inflaton decreases slowly relative to the standard
dust-dominated case. This leads to an efficient production of particles
via nonperturbative decay of inflaton even if its coupling is of order
. We also discuss massive particle creation heavier than inflaton,
which may play important roles for the baryo- and lepto-genesis scenarios.Comment: 6 pages, 2 figures, submitted to Physical Review
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