Palatini-Born-Infeld Gravity, Bouncing Universe, and Black Hole Formation

We consider the Palatini formalism of the Born-Infeld gravity. In the Palatini formalism, the propagating mode is only graviton, whose situation is different from that in the metric formalism. We discuss about the FRW cosmology by using an effective potential. Especially we consider the condition that the bouncing could occur. We also give some speculations about the black hole formationComment: LaTeX 10 pages, title is changed, the differences from the previous works are clarified, version to appear in Physics Letters

F(R)F(R) gravity in the early Universe: Electroweak phase transition and chameleon mechanism

It is widely believed that the screening mechanism is an essential feature for the modified gravity theory. Although this mechanism has been examined thoroughly in the past decade, their analyses are based on the classical configuration of the matter fields. In this paper, we demonstrate a new formulation of the chameleon mechanism in $F(R)$ gravity theory, to shed light on quantum-field theoretical effects on the chameleon mechanism as well as the related scalaron physics, induced by the matter sector. We show a potential absence of the chameleon mechanism in the cosmic history based on a scale-invariant-extended scenario beyond the standard model of particle physics, in which a realistic electroweak phase transition, possibly yielding the right amount of baryon asymmetry of Universe today, simultaneously breaks the scale invariance in the early Universe. Remarkably enough, the matter sector contribution to the trace of energy-momentum tensor turns out to be on the same order of magnitude as that computed in the classical perfect-fluid approximation, even though the theory involves the nontrivial electroweak-phase transition environment. We also briefly discuss the oscillation of the scalaron field and indirect generation of non-tensorial gravitational waves induced by the electroweak phase transition.Comment: 17 pages, 4 figures, version accepted in Chinese Physics

10 THz Ultrafast Function Generator - generation of rectangular and triangular pulse trains-

We report the synthesis of arbitrary optical waveforms by manipulating the spectral phases of Raman sidebands with a wide frequency spacing line-by-line. Trains of rectangular and triangular pulses are stably produced at an ultrahigh repetition rate of 10.6229 THz, reminiscent of an ultrafast function generator.Comment: 7 Pages, 5 Figure

Gravitational Waves in F(R)F(R) Gravity: Scalar Waves and the Chameleon Mechanism

We discuss the scalar mode of gravitational waves emerging in the context of $F(R)$ gravity by taking into account the chameleon mechanism. Assuming a toy model with a specific matter distribution to reproduce the environment of detection experiment by a ground-based gravitational wave observatory, we find that chameleon mechanism remarkably suppresses the scalar wave in the atmosphere of Earth, compared with the tensor modes of the gravitational waves. We also discuss the possibility to detect and constrain scalar waves by the current gravitational observatories and advocate a necessity of the future space-based observations.Comment: 15 pages, 5 figures, accepted version in Physical Review