26,062 research outputs found
Cyclic cosmology from Lagrange-multiplier modified gravity
We investigate cyclic and singularity-free evolutions in a universe governed
by Lagrange-multiplier modified gravity, either in scalar-field cosmology, as
well as in one. In the scalar case, cyclicity can be induced by a
suitably reconstructed simple potential, and the matter content of the universe
can be successfully incorporated. In the case of -gravity, cyclicity can
be induced by a suitable reconstructed second function of a very
simple form, however the matter evolution cannot be analytically handled.
Furthermore, we study the evolution of cosmological perturbations for the two
scenarios. For the scalar case the system possesses no wavelike modes due to a
dust-like sound speed, while for the case there exist an oscillation
mode of perturbations which indicates a dynamical degree of freedom. Both
scenarios allow for stable parameter spaces of cosmological perturbations
through the bouncing point.Comment: 8 pages, 3 figures, references added, accepted for publicatio
Bounce and cyclic cosmology in extended nonlinear massive gravity
We investigate non-singular bounce and cyclic cosmological evolutions in a
universe governed by the extended nonlinear massive gravity, in which the
graviton mass is promoted to a scalar-field potential. The extra freedom of the
theory can lead to certain energy conditions violations and drive cyclicity
with two different mechanisms: either with a suitably chosen scalar-field
potential under a given Stuckelberg-scalar function, or with a suitably chosen
Stuckelberg-scalar function under a given scalar-field potential. Our analysis
shows that extended nonlinear massive gravity can alter significantly the
evolution of the universe at both early and late times.Comment: 20 pages, 5 figures, version published at JCA
A Wideband Multilayer Substrate Integrated Waveguide Cavity-Backed Slot Antenna Array
© 2017 IEEE. In this paper, a wideband 2 × 2 multilayer substrate integrated waveguide (SIW)-based cavity-backed slot array is proposed. The array element is constructed by stacking five layers of SIW cavity-backed slots and has a wide impedance bandwidth from 18 to 30 GHz. Two different feed networks based on probe coupling and slot coupling are introduced to maintain the wideband characteristic of the proposed element when employed in an array. Two 2 × 2 arrays fed by the proposed feed networks are designed, fabricated, and tested. The measured results show that both the arrays employing probe coupling and slot coupling feed networks have a wide impedance bandwidth over 30%. Within the obtained operation bands, good radiation performance is achieved. Moreover, advantages and disadvantages of the proposed two feed networks are discussed
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