Modern Physics Letters A

We review a number of approaches that have been adopted to explain the origin of dark energy responsible for the late-time cosmic acceleration. This includes the cosmological constant and dynamical dark energy models such as quintessence, k-essence, Chaplygin gas, f(R) gravity, scalar-tensor theories, and braneworld models. We also discuss observational and local gravity constraints on those models and clarify which models are favored or ruled out in current observations.

We review how we can construct the gravity models which reproduces the arbitrary development of the universe. We consider the reconstruction in the Einstein gravity coupled with generalized perfect fluid, scalar-Einstein gravity, scalar-Einstein-Gauss-Bonnet gravity, Einstein--gravity, and F(R)-gravity. Very explicit formulas are given to reconstruct the models, which could be used when we find the detailed data of the development of the universe by future observations. Especially we find the formulas using e-foldings, which has a direct relation with observed redshift. As long as we observe the time development of the Hubble rate H, there exists a variety of models describing the arbitrary development of universe.

Inhomogeneity at small scales (Mpcs) of our universe is manifest from observations. In regions smaller than 8h^-1 Mpc galaxies tend to cluster instead of showing a Poisson distribution. The properties of the spherical collapse model which approximately depicts this nonlinearity of the density perturbation are investigated when there exits dark energy. The evolution of a spherical perturbation depends only on the initial critical overdensity threshold for the collapse, δ_c and its value approaches to 1.58 instead of the conventionally known 1.69 independent of the value of the equation of state of dark energy.

We study the evolution of cosmological perturbations in gravity, where the Lagrangian is the sum of a Ricci scalar R and an arbitrary function f in terms of a Gauss-Bonnet term . We derive the equations for perturbations assuming matter to be described by a perfect fluid with a constant equation of state w. We show that density perturbations in perfect fluids exhibit negative instabilities during both the radiation and the matter domination, irrespective of the form of . This growth of perturbations gets stronger on smaller scales, which is difficult to be compatible with the observed galaxy spectrum unless the deviation from General Relativity is very small. Thus cosmological models are effectively ruled out from this Ultra-Violet instability, even though they can be compatible with the late-time cosmic acceleration and local gravity constraints.

An explicit model of F(R) gravity with realizing a crossing of the phantom divide is reconstructed. In particular, it is shown that the Big Rip singularity may appear in the reconstructed model of F(R) gravity. Such a Big Rip singularity could be avoided by adding R² term or non-singular viable F(R) theory¹ to the model because phantom behavior becomes transient.

Quintom models, with its Equation of State being able to cross the cosmological constant boundary w = -1, turns out to be attractive for phenomenological study. It can not only be applicable for dark energy model for current universe, but also lead to a bounce scenario in the early universe.

In this paper we first present a complete classification of gravitational waves according to their frequencies: (i) Ultra high frequency band (above 1 THz); (ii) Very high frequency band (100 kHz–1 THz); (iii) High frequency band (10 Hz–100 kHz); (iv) Middle frequency band (0.1 Hz–10 Hz); (v) Low frequency band (100 nHz–0.1 Hz); (vi) Very low frequency band (300 pHz–100 nHz); (vii) Ultra low frequency band (10 fHz–300 pHz); (viii) Hubble (extremely low) frequency band (1 aHz–10 fHz); (ix) Infra-Hubble frequency band (below 1 aHz). After briefly discussing the method of detection for different frequency bands, we review the concept and status of space gravitational-wave missions — LISA, ASTROD, ASTROD-GW, Super-ASTROD, DECIGO and Big Bang Observer. We then address to the determination of dark energy equation, and probing the inflationary physics using space gravitational wave detectors.

It is known that Dvali-Gabadadze-Porrati (DGP) braneworld model has a self-acceleration branch. However, this cosmological solution contains a spin-2 ghost. We study the possibility of avoiding the appearance of the ghost by slightly modifying the model, introducing the second brane. As a result we find we cannot avoid the appearance of ghost by two-branes model. Then we discuss whether spin-2 ghost is really harmful or not by evaluating the rate of spontaneous particle production due to ghost instability. We explain that non-covariant cutoff momentum arises rather naturally in the case of spin-2 ghost. As a result, the particle production rate is extremely suppressed and therefore the presence of a ghost might be harmless.

The theme of the TEXONO research program is on the studies of low energy neutrino and dark matter physics. The current goals are on the development of germanium detectors with sub-keV sensitivities to realize experiments on neutrino magnetic moments, neutrino-nucleus coherent scattering, as well as WIMP dark matter searches. A threshold of 100–200 eV was achieved with prototype detectors at the Kuo-Sheng Neutrino Laboratory. New limits were placed for low-mass WIMPs. The dark matter program will move to a new underground laboratory currently under construction in Sichuan, China.

In this article I review model-independent procedures for extracting properties of Weakly Interacting Massive Particles (WIMPs) from direct Dark Matter detection experiments. Neither prior knowledge about the velocity distribution function of halo Dark Matter particles nor about their mass or cross sections on target nucleus is needed. The unique required information is measured recoil energies from experiments with different detector materials.

In the talk I give a brief review of the works related with the recent results from PAMELA, ATIC and Fermi about the cosmic electron and positron excesses. Especially I introduce constraints gotten from astrophysical observations on the DM annihilation scenario to account for the cosmic electron excess.

The anomalies in positron fraction observed in PAMELA and total flux of electrons and positron reported by Fermi can be explained by a decaying dark matter. The agreement between astrophysical background and observed data in anti-proton indicates a lep-tophilic dark matter and constrains the hadronic decay branching ratios. In this work, we study the constrains on decaying rates of a dark matter to various 2-body final states using the Fermi and HESS gamma-ray data. We find that µ⁺µ^- or τ⁺τ^- final state is preferred to simultaneously explain the excesses and meet the gamma-ray constrains.

We briefly review the supersymmetric explanation for the cosmic dark matter. Although the neutralino in the minimal supersymmetric model (MSSM), the next-to-minimal supersymmetric model (NMSSM) and the nearly minimal supersymmetric model (nMSSM) can naturally explain the dark matter relic density, the PAMELA result can hardly be explained in these popular models. In the general singlet extension of the MSSM, both the PAMELA result and the relic density can be explained by the singlino-like neutralino. Such singlino-like neutralinos annihilate into the singlet-like Higgs bosons, which are light enough to decay dominantly to muons or electrons, and the annihilation cross section can be greatly enhanced by the Sommerfeld effect via exchanging a light CP-even singlet-like Higgs boson. In this scenario, in order to meet the stringent LEP constraints, the SM-like Higgs boson tends to decay into the singlet Higgs pairs instead of and consequently it will give a multi-muon signal h_SM → aa → 4_µ or h_SM → hh → 4a → 8_µ at the LHC.

Dark matter is a focused issue in galactic evolution and cosmology. Axion is a viable particle candidate for dark matter. Its interaction with photon is an effective way to detect it, e.g., pseudoscalar-photon interaction will generate vacuum dichroism in a magnetic field. Motivated to measure the QED vacuum birefringence and to detect pseudoscalar-photon interaction, we started to build up the Q & A experiment (QED [Quantum Electrodynamics] and Axion experiment) in 1994. In this talk, we first give a brief historical account of planet hunting and dark matter evidence. We then review our 3.5 m Fabry-Perot interferometer together with our results of measuring vacuum dichroism and gaseous Cotton-Mouton effects. Our first results give (-0.2 ± 2.8) × 10^-13 rad/pass, at 2.3 T with 18,700 passes through a 0.6 m long magnet, for vacuum dichroism measurement. We are upgrading our interferometer to 7 m arm-length with a new 1.8 m 2.3 T permanent magnet capable of rotation up to 13 cycles per second. We aim at optical sensitivity with 532 nm cavity finesse around 100,000. When achieved, vacuum dichroism would be measured to 8.6 × 10^-17 rad/pass in about 50 days, and QED birefringence would be measured to 28%.

In this work, we explore the possibility of leptogenesis beyond the standard Yukawa-mediated decays by considering a scenario where effective electromagnetic dipole operators that connect the light and heavy neutrinos play an important role. Through these operators, new decay channels for the postulated heavy neutrinos are opened up, and consequently, such L- and CP-violating processes can lead to the generation of the correct baryon asymmetry of the universe. In addition, it has been found that the relationship of the leptogenesis scale to the light neutrino masses is very similar to that for the standard leptogenesis setup.

We consider the µ - τ symmetric Friedberg-Lee (FL) symmetry for the neutrino sector and show that a specific FL translation leads to the tribimaximal mixing pattern of the Maki-Nakagawa-Sakata (MNS) matrix. We also apply the symmetry to the type-I seesaw framework and address the baryon asymmetry of the universe through the leptogenesis mechanism. We try to establish a relation between the net baryon asymmetry and CP phases included in the MNS matrix.

We investigate the possibility of thermal leptogenesis under the framework of radiative seesaw model with A₄×Z₂×Z₄ leptonic flavour symmetry that leads to the tribimaximal mixing in neutrino sector. A five-dimensional effective Yukawa interaction with cutoff scale Λ which provides the deviations from tribimaximal mixing. Both A₄ × Z₄ and CP symmetries are spontaneously broken after the invented scalars generating VEVs. We consider the low-energy neutrino mixing angles to constrain the parameters in our model within three possible light neutrino spectrum in 1σ. We study the flavored effects on thermal leptogenesis in each scenario.