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We perform a cosmological-model-independent test for the distance-duality (DD) relation η(z) = D L (z)(1 + z) −2 /D A (z), where D L and D A are the luminosity distance and angular diameter distance respectively, with a combination of observational data for D L taken from the latest Union2 SNe Ia and that for D A provided by two galaxy clusters samples compiled by De Filippis et al. and Bonamente et al.. Two parameterizations for η(z), i.e., η(z) = 1 + η 0 z and η(z) = 1 + η 0 z/(1 + z), are used. We find that the DD relation can be accommodated at 1σ confidence level (CL) for the De Filippis et al. sample and at 3σ CL for the Bonamente et al. sample. We also examine the DD relation by postulating two more general parameterizations: η(z) = η 0 + η 1 z and η(z) = η 0 + η 1 z/(1 + z), and find that the DD relation is compatible with the results from the De Filippis et al. and the Bonamente et al. samples at 1σ and 2σ CLs, respectively. Thus, we conclude that the DD relation is compatible with present observations.

We propose a novel enhancement mechanism of the curvature perturbations in the nonminimal derivative coupling inflation model with a coupling parameter related to the inflaton field. By considering a special form of the coupling parameter as a function of the inflaton, a period of ultraslow-roll inflation can be realized due to the gravitationally enhanced friction, and the resulting power spectrum of the curvature perturbations has a sharp peak, which is large enough to produce the primordial black holes. Under this mechanism, we can easily obtain a sharp mass spectrum of primordial black holes around specific masses such as O(10)M , O(10 −5 )M , and O(10 −12 )M , which can explain the LIGO events, the ultrashort-timescale microlensing events in OGLE data, and the most of dark matter, respectively.

We perform in this paper a statefinder diagnostic to a dark energy model with two scalar fields, called "quintom", where one of the scalar fields has a canonical kinetic energy term and the other has a negative one. Several kinds of potentials are discussed. Our results show that the statefinder diagnostic can differentiate quintom model with other dark energy models. PACS numbers: 98.80.Es,98.80.Cq, 98.80.Jk * Corresponding author It is widely believed nowadays that the present universe is undergoing an accelerating expansion. The converging evidences come from the analysis of data from supernova [1,2], CMB [3,4,5,6,7,8] and WMAP [9,10]. In order to explain the cosmic accelerating expansion, a modified theory of gravity or the existence of dark energy is required. Perhaps the simplest candidate of dark energy is the cosmological constant with equation of state w = p/ρ = −1. However, there exist two problems with it, i.e., why the cosmological constant is so tiny compared to the theoretical expectations and why the remnant cosmological constant is becoming visible precisely at the present time in the cosmic history and why it does not exactly vanish. The inspiration coming from inflation has suggested that dark energy models are likely to be described by the dynamics of a (multi) scalar field(s), such as quintessence []. The quintessence scalar field has a positive kinetic term in its Lagrangian, which violates the strong energy condition but not the dominant energy condition, and the evolution of its equation of state parameter w is in the range of −1 ≤ w ≤ 1. The phantom scalar field on the other hand possesses a negative kinetic term in its Lagrangian, which leads to some strange properties, such as the violation of the dominant energy condition and the occurrence of Big rip [66,67,68,69,70,71,72,73,74,75]( Let us note here that the possible avoidance of the Big Rip by various kinds of physical effects has been discussed by many authors [76,77,78,79,80,81,82,83].). The evolution of the equation of state parameter w for phantom is in the range of w < −1. Other models including the Chaplygin gas [84,85], braneworld models [86,87,88,89,90], holographic models [91] et al are also proposed to account for the present accelerating cosmic expansion.The recent analysis of various cosmological data seems to indicate that it is mildly favored that the evolution of the equation of state parameter of the dark energy changes from w > −1 to w < −1 at small redshift [92,93,94,95]. Although this result may be considered as tentative only since the standard Λ CDM model remains inside ≈ 2σ error bars for all data, it is worthwhile to explore theoretical models which can bring w crossing -1. In this regard, most models mentioned above can not do the job. "Quintom" [96,97,98], which assumes that the dark energy is composed of quintessence and phantom, on the other hand, can implement w crossing -1 and in some cases fits the cosmological data better than models with w ≥ −1. In addition "quintom" model can reconcile the coincidence ...

The extended quintessence is obtained by coupling a normal scalar field to the Ricci scalar defined in the metric formalism. In this paper, we propose a new extended quintessence dark energy by introducing a non-minimal coupling between the quintessence and gravity, but with the Ricci scalar given from the Palatini formalism rather than the metric one. We find that the equation of state of the new extended quintessence can cross the phantom divide line, and moreover, it oscillates around the −1 line. We also show that the universe driven by the new extended quintessence will enter a dark energy dominated de Sitter phase in the future. PACS numbers: 98.80.Cq, 98.70.Vc

We discuss the random motion of charged test particles driven by quantum electromagnetic fluctuations at finite temperature in both the unbounded flat space and flat spacetime with a reflecting boundary and calculate the mean squared fluctuations in the velocity and position of the test particle. We show that typically the random motion driven by the quantum fluctuations is one order of magnitude less significant than that driven by thermal noise in the unbounded flat space. However, in the flat space with a reflecting plane boundary, the random motion of quantum origin can become much more significant than that of thermal origin at very low temperature.PACS numbers:

We study the scalar induced gravitational wave (GW) background in inflation with gravitationally enhanced friction (GEF). The GEF mechanism, which is realized by assuming a nonminimal derivative coupling between the inflaton field and gravity, is used to amplify the small-scale curvature perturbations to generate a sizable amount of primordial black holes. We find that the GW energy spectra can reach the detectable scopes of the future GW projects, and the power spectrum of curvature perturbations has a power-law form in the vicinity of the peak. The scaling of the GW spectrum in the ultraviolet regions is two times that of the power spectrum slope, and has a lower bound. In the infrared regions, the slope of the GW spectrum can be described roughly by a log-dependent form. These features of the GW spectrum may be used to check the GEF mechanism if the scalar induced GWs are detected in the future.

The generalized Chaplygin gas (GCG), is studied in this paper by using the latest observational data including 182 gold sample type Ia supernovae (Sne Ia) data, the ESSENCE Sne Ia data and the distance ratio from z = 0.35 to z = 1089 (the redshift of decoupling). Our results rule out the In addition, we find that the phase transition from deceleration to acceleration occurs at redshift z q=0 ∼ 0.89 − 1.0 at a 1σ confidence level for the GCG model.

We study the cosmic expansion history by reconstructing the deceleration parameter q(z) from the SDSS-II type Ia supernova sample (SNIa) with two different light curve fits (MLCS2k2 and SALT-II), the baryon acoustic oscillation (BAO) distance ratio, the cosmic microwave background (CMB) shift parameter, and the lookback time-redshift (LT) from the age of old passive galaxies. Three parametrization forms for the equation of state of dark energy (CPL, JBP, and UIS) are considered. Our results show that, for the CPL and the UIS forms, MLCS2k2 SDSS-II SNIa+BAO+CMB and MLCS2k2 SDSS-II SNIa+BAO+CMB+LT favor a currently slowingdown cosmic acceleration, but this does not occur for all other cases, where an increasing cosmic acceleration is still favored. Thus, the reconstructed evolutionary behaviors of dark energy and the course of the cosmic acceleration are highly dependent both on the light curve fitting method for the SNIa and the parametrization form for the equation of state of dark energy. PACS numbers: 95.36.+x, 98.80.Es

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