We study the Bondi-Sachs rockets with nonzero cosmological constant. We observe that the acceleration of the systems arises naturally in the asymptotic symmetries of (anti-) de Sitter spacetimes. Assuming the validity of the concepts of energy and ma
ss previously introduced in asymptotically flat spacetimes, we find that the emission of pure radiation energy balances the loss of the Bondi mass in certain special families of the Bondi-Sachs rockets, so in these there is no gravitational radiation.
Quintessence field is a widely-studied candidate of dark energy. There is tracker solution in quintessence models, in which evolution of the field $phi$ at present times is not sensitive to its initial conditions. When the energy density of dark ener
gy is neglectable ($Omega_phill1$), evolution of the tracker solution can be well analysed from tracker equation. In this paper, we try to study evolution of the quintessence field from full tracker equation, which is valid for all spans of $Omega_phi$. We get stable fixed points of $w_phi$ and $Omega_phi$ (noted as $hat w_phi$ and $hatOmega_phi$) from the full tracker equation, i.e., $w_phi$ and $Omega_phi$ will always approach $hat w_phi$ and $hatOmega_phi$ respectively. Since $hat w_phi$ and $hatOmega_phi$ are analytic functions of $phi$, analytic relation of $hat w_phisimhatOmega_phi$ can be obtained, which is a good approximation for the $w_phisimOmega_phi$ relation and can be obtained for the most type of quintessence potentials. By using this approximation, we find that inequalities $hat w_phi<w_phi$ and $hatOmega_phi<Omega_phi$ are statisfied if the $w_phi$ (or $hat w_phi$) is decreasing with time. In this way, the potential $U(phi)$ can be constrained directly from observations, by no need of solving the equations of motion numerically.
We use planar coordinates as well as hyperbolic coordinates to separate the de Sitter spacetime into two parts. These two ways of cutting the de Sitter give rise to two different spatial infinities. For spacetimes which are asymptotic to either half
of the de Sitter spacetime, we are able to provide definitions of the total energy, the total linear momentum, the total angular momentum, respectively. And we prove two positive mass theorems, corresponding to these two sorts of spatial infinities, for spacelike hypersurfaces whose mean curvatures are bounded by certain constant from above.
It was observed recently by Randall and Wise that, any particle in the Standard Model cannot decay to $e^+e^-$ and $gamma gamma$ final states with comparably measurable branching ratios. This is also true for most extensions of the Standard Model, wi
th the Randall-Sundrum model as an outstanding exception. In this paper, we show that two-Higgs-Doublet-Models (2HDM) yield another possible exception if certain parameters are properly chosen. In addition, we have checked that this model survives the tests of low energy processes, including the anomalous magnetic moment and electric dipole moment of leptons, lepton-flavor-violating decays $mu^- to e^- gamma$ and $e^- e^+ e^-$.
String instanton effects in Higgs physics are discussed through a type IIA model based on T^{6}/(Z^{2}times Z^{2}) orentifold compactifaction. By inclusion of rigid E2-branes, the model exhibits a MSSM-like spectrum, as well as extra mu and quartic H
iggs couplings. These extra couplings are induced via E2 instantons non-perturbatively. Setting the string scale at 10^{18} GeV, one gets interesting TeV Higgs physics. In particlular, the tree-level Higgs mass can be uplifted substantially.
An attempt has been made to address the 3sigma anomaly of the forward-backward asymmetry of b quark in LEP data via an unparticle sector. For most part of the parameter space except certain particular regions, the anomaly could not be explained away
plausibly, when constraints from other LEP observables are taken into account.
Fermionic unparticles are introduced and their basic properties are discussed. Some phenomenologies related are exploited, such as their effects on charged Higgs boson decays and anomalous magnetic moments of leptons. Also, it has been found that mea
surements of $B^0-bar B^0$ mixing could yield interesting constraints on couplings between unparticle operators and standard model fields.
