16,174 research outputs found
String theoretic axion coupling and the evolution of cosmic structures
We examine the effects of the axion coupling to on the evolution
of cosmic structures. It is shown that the evolutions of the scalar- and
vector-type perturbations are not affected by this axion coupling. However the
axion coupling causes an asymmetric evolution of the two polarization states of
the tensor-type perturbation, which may lead to a sizable polarization
asymmetry in the cosmological gravitational wave if inflation involves a period
in which the axion coupling is important. The polarization asymmetry produced
during inflation are conserved over the subsequent evolution as long as the
scales remain in the large-scale limit, and thus this may lead to an observable
trace in the cosmic microwave background radiation.Comment: 10 pages, REVte
Singularities in scalar-tensor gravity
The analysis of certain singularities in scalar-tensor gravity contained in a
recent paper is completed, and situations are pointed out in which these
singularities cannot occur.Comment: 6 pages, LaTe
Conserved cosmological structures in the one-loop superstring effective action
A generic form of low-energy effective action of superstring theories with
one-loop quantum correction is well known. Based on this action we derive the
complete perturbation equations and general analytic solutions in the
cosmological spacetime. Using the solutions we identify conserved quantities
characterizing the perturbations: the amplitude of gravitational wave and the
perturbed three-space curvature in the uniform-field gauge both in the
large-scale limit, and the angular-momentum of rotational perturbation are
conserved independently of changing gravity sector. Implications for
calculating perturbation spectra generated in the inflation era based on the
string action are presented.Comment: 5 pages, no figure, To appear in Phys. Rev.
No-boundary measure and preference for large e-foldings in multi-field inflation
The no-boundary wave function of quantum gravity usually assigns only very
small probability to long periods of inflation. This was a reason to doubt
about the no-boundary wave function to explain the observational universe. We
study the no-boundary proposal in the context of multi-field inflation to see
whether the number of fields changes the situation. For a simple model, we find
that indeed the no-boundary wave function can give higher probability for
sufficient inflation, but the number of fields involved has to be very high.Comment: 16 pages, 2 figure
Cosmological Gravitational Wave in a Gravity with Quadratic Order Curvature Couplings
We present a set of equations describing the cosmological gravitational wave
in a gravity theory with quadratic order gravitational coupling terms which
naturally arise in quantum correction procedures. It is known that the
gravitational wave equation in the gravity theories with a general term
in the action leads to a second order differential equation with the only
correction factor appearing in the damping term. The case for a
term is completely different. The gravitational wave is described by a fourth
order differential equation both in time and space. However, curiously, we find
that the contributions to the background evolution are qualitatively the same
for both terms.Comment: 4 pages, revtex, no figure
Extended quintessence, inflation, and stable de Sitter spaces
A new gauge-invariant criterion for stability against inhomogeneous
perturbations of de Sitter space is applied to scenarios of dark energy and
inflation in scalar-tensor gravity. The results extend previous studies.Comment: 16 pages, LaTeX, to appear in Class. Quantum Gra
In-situ measurements of the optical absorption of dioxythiophene-based conjugated polymers
Conjugated polymers can be reversibly doped by electrochemical means. This
doping introduces new sub-bandgap optical absorption bands in the polymer while
decreasing the bandgap absorption. To study this behavior, we have prepared an
electrochemical cell allowing measurements of the optical properties of the
polymer. The cell consists of a thin polymer film deposited on gold-coated
Mylar behind which is another polymer that serves as a counterelectrode. An
infrared transparent window protects the upper polymer from ambient air. By
adding a gel electrolyte and making electrical connections to the
polymer-on-gold films, one may study electrochromism in a wide spectral range.
As the cell voltage (the potential difference between the two electrodes)
changes, the doping level of the conjugated polymer films is changed
reversibly. Our experiments address electrochromism in
poly(3,4-ethylene-dioxy-thiophene) (PEDOT) and
poly(3,4-dimethyl-propylene-dioxy-thiophene) (PProDOT-Me). This closed
electrochemical cell allows the study of the doping induced sub-bandgap
features (polaronic and bipolaronic modes) in these easily oxidized and highly
redox switchable polymers. We also study the changes in cell spectra as a
function of polymer thickness and investigate strategies to obtain cleaner
spectra, minimizing the contributions of water and gel electrolyte features
Hypernetwork functional image representation
Motivated by the human way of memorizing images we introduce their functional
representation, where an image is represented by a neural network. For this
purpose, we construct a hypernetwork which takes an image and returns weights
to the target network, which maps point from the plane (representing positions
of the pixel) into its corresponding color in the image. Since the obtained
representation is continuous, one can easily inspect the image at various
resolutions and perform on it arbitrary continuous operations. Moreover, by
inspecting interpolations we show that such representation has some properties
characteristic to generative models. To evaluate the proposed mechanism
experimentally, we apply it to image super-resolution problem. Despite using a
single model for various scaling factors, we obtained results comparable to
existing super-resolution methods
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