Self-accelerated Universe Induced by Repulsive Effects as an Alternative to Dark Energy and Modified Gravities

The existence of current–time universe’s acceleration is usually modeled by means of two main strategies. The first makes use of a dark energy barotropic fluid entering by hand the energy–momentum tensor of Einstein’s theory. The second lies on extending the Hilbert–Einstein action giving rise to the class of extended theories of gravity. In this work, we propose a third approach, derived as an intrinsic geometrical effect of space–time, which provides repulsive regions under certain circumstances. We demonstrate that the effects of repulsive gravity naturally emerge in the field of a homogeneous and isotropic universe. To this end, we use an invariant definition of repulsive gravity based upon the behavior of the curvature eigenvalues. Moreover, we show that repulsive gravity counterbalances the standard gravitational attraction influencing both late and early times of the universe evolution. This phenomenon leads to the present speed up and to the fast expansion due to the inflationary epoch. In so doing, we are able to unify both dark energy and inflation in a single scheme, showing that the universe changes its dynamics when \({\ddot{H}\over H}=-2 \dot{H}\), at the repulsion onset time where this condition is satisfied. Further, we argue that the spatial scalar curvature can be taken as vanishing because it does not affect at all the emergence of repulsive gravity. We check the goodness of our approach through two cosmological fits involving the most recent union 2.1 supernova compilation.     

Functional Integrals and Quantum Fluctuations on Two-DimensionalNoncommutative Space-Time

The generalized Thirring model with impurity coupling is defined ontwo-dimensional noncommutative space-time, a modified propagator and freeenergy are derived by means of functional integrals method. Moreover,quantum fluctuations and excitation energies are calculated on two-dimensional black hole and soliton background.     

Space-Time Fluctuations Induced by D-Branes and Their Effects on Neutrino Oscillations

Neutrino oscillations are analyzed in the Ellis-Mavromatos-Nanopoulos-Volkov (ENMV) model, where the quantum gravitational fluctuations of the space-time background are described by virtual D branes. Such fluctuations may induce neutrino oscillations if a violation of the equivalence principle or a tiny violation of the Lorentz invariance is imposed. In this framework, the oscillation length of neutrinos turns out to be proportional to E ^–2 M, where E is the neutrino energy and M is the energy which is the scale characterizing the topological fluctuations in the vacuum.     

Functional Integrals and Quantum Fluctuations on Two-Dimensional Noncommutative Space-Time

The generalized Thirring model with impurity coupling is defined on two-dimensional noncommutative space-time, a modified propagator and free energy are derived by means of functional integrals method. Moreover, quantum fluctuations and excitation energies are calculated on two-dimensional black hole and soliton background.     

Quantum Vacuum Fluctuations in a Chromomagnetic-Like Background

In this paper, we study the effects associated to quantum vacuum fluctuations of vectorial perturbations of the Abelian SU(2) Yang-Mills field in a static and homogeneous chromomagnetic-like background field, at zero temperature. We use periodic and antiperiodic boundary conditions in order to calculate the Casimir energy by means of the frequency sum technique and of the regularization method based on zeta functions, analyzing its behavior in the weak and strong coupling regimes. We compare the obtained results with the similar ones found for scalar and spinor fields placed in an ordinary magnetic field background. We show that only in the weak coupling regime the non-trivial topology of the system encoded in the antiperiodic boundary conditions changes the nature of the Casimir force with respect to the periodic ones. Considering the weak coupling scenario, we show that the introduction of a third polarization state in the perturbations makes manifest the effects on the Casimir energy due to the coupling with the chromomagnetic-like background field, for both the boundary conditions. Finally, in the strong coupling regime, in which the quantum vacuum is not stable due to the Nielsen-Olesen instabilities, we evaluate the effects of a compact extra dimension on its stabilization.     

Large-Scale Two-Dimensional Quantum Fluctuations of Five-Dimensional and Four-Dimensional Space-Time Metrics

Large-scale two-dimensional quantum fluctuations of five-dimensional space-time metric are constructed and the effect of the fluctuations on the nested four-dimensional worlds is studied. In doing so, the fluctuations affect not all four-dimensional worlds but only a part of them. The energy-momentum tensor of four-dimensional space-time has a physical form both in the absence and in the presence of fluctuations; it means that the fluctuations can be realized by real matter. A spatial region occupied by the fluctuations constructed in this work can be infinitely large and the fluctuations can occur during a long period of time. Therefore, we refer to these fluctuations as large-scale fluctuations.     

Loop Quantum Corrections to Statefinder Parameters of Dark Energy

In this paper, we have calculated the statefinder parameters for the Friedmann-Robertson-Walker (FRW) Universe in the gravitational framework of loop quantum cosmology (LQC). As examples, we study two types of dark energy models namely Holographic dark energy and New-Agegraphic dark energy.     

Holographic Dark Energy Model with Quintessence in Bianchi Type-I Space-Time

In this paper, we have studied a homogeneous and anisotropic universe filled with matter and holographic dark energy components. Assuming deceleration parameter to be a constant, an exact solution to Einstein’s field equations in axially symmetric Bianchi type-I line element is obtained. A correspondence between the holographic dark energy models with the quintessence dark energy models is also established. Quintessence potential and the dynamics of the quintessence scalar field are reconstructed, which describe accelerated expansion of the universe.     

The Complex Symmetry Gravitational Theory as a New Alternative of Dark Energy

We propose that complex symmetry gravitational theory (CSGT) explain the accelerating expansion of universe. In this paper, universe is taken as the double complex symmetric space. Cosmological solution is obtained within CSGT. The conditions of the accelerating expansion of universe are discussed within CSGT. Moreover, the range of equation of state of matter ω_ε is given in the hyperbolic imaginary space. PACS: 98.80Es, 04.20Fy     

Inertial Mass and Vacuum Fluctuations in Quantum Field Theory

Motivated by recent works on the origin of inertial mass, we revisit the relationship between the mass of charged particles and zero-point electromagnetic fields. To this end we first introduce a simple model comprising a scalar field coupled to stochastic or thermal electromagnetic fields. Then we check if it is possible to start from a zero bare mass in the renormalization process and express the finite physical mass in terms of a cut-off. In scalar QED this is indeed possible, except for the problem that all conceivable cut-offs correspond to very large masses. For spin-1/2 particles (QED with fermions) the relation between bare mass and renormalized mass is compatible with the observed electron mass and with a finite cut-off, but only if the bare mass is not zero; for any value of the cut-off the radiative correction is very small.     

Holographic Dark Energy in Induced Gravity

Many astrophysics data show that our universe has a critical energy density, and 73% of it is dark energy, which drives the accelerating expansion of the universe. We consider the holographic dark energy in induced gravity by taking the Hubble scale, particle horizon and event horizon as the infrared cutoff. We find that only the event horizon can give accelerating expansion of our universe.     

介观电子谐振腔量子能谱与能量的量子涨落

针对介观电子谐振腔模型,在由电荷算符本征态构成的新Fock空间中,假设系统具有变换的对称性,通过求解Hamilton算符的本征值方程,给出系统的量子能谱关系.在电荷算符的Fock态下计算能量的量子涨落,分析和研究电子谐振腔的量子能谱性质.结果表明:类似于电荷的量子性,能谱明显地呈现出离散性,其大小决定于谐振腔的电参量、形状因子及栅极所加偏压等因素;而能量的量子涨落却仅与电荷量子、Planck常数以及系统自感有关.     

On Quantum Fluctuations Relations with Generalized Energy Measurements

Quantum fluctuations relations are typically derived with projective measurements of energy at the beginning and the end of the protocol. Though projective measurements are easy to treat theoretically, they may be difficult to implement in experiments. We show that recent results on the force protocol with generalized measurements remain valid with more general forms of quantum evolution. In the case considered, states of an open quantum system are transformed according to an arbitrary trace-preserving completely positive map. The energy measurements used in the protocol are prescribed as a superposition of projective ones. Such measurements can be parameterized by a continuous variable substituted into acceptance functions of the apparatuses. The role of limited measurement precision is also clarified here. As is known, the standard Jarzynski equality and the Tasaki–Crooks theorem remain valid for the case of unital quantum channels. Using characteristic functions, we recast standard relations for the scenario with generalized measurements. With respect to the Jarzynski equality, generalized measurements lead to a negative shift in observed values of the difference of free energies. A deviation of the actual quantum channel from unital ones may also generate some shift with an arbitrary sign.     

Holographic Gas as Dark Energy

We investigate the statistical nature of holographic gas, which may represent the quasi-particle excitations of a strongly correlated gravitational system. We find that the holographic entropy can be obtained by modifying degeneracy. We calculate thermodynamical quantities and investigate stability of the holographic gas. When applying to cosmology, we find that the holographic gas behaves as holographic dark energy, and the parameter c in holographic dark energy can be calculated from our model. Our model of holographic gas generally predicts c 〈 1, implying that the fate of our universe is phantom-like.     

Holographic Dark Energy Model with Interaction and Cosmological Constant in the Flat Space-Time

In this paper we consider holographic dark energy model with interaction in the flat space-time with non-zero cosmological constant. We calculate cosmic scale factor and Hubble expansion parameter by using the time-dependent dark energy density. Then, we obtain phenomenological interaction between holographic dark energy and matter. We fixed our solution by using the observational data.     

Dynamics of Multiple Scalar Fields Model of Dark Energy with Spatial Curvature

The dynamical system of multiple scalar fields in FRW universe with different spatial curvature have been analyzed in this paper. In the radiation-dominated phase, the constant curvature factor k does not work on the cosmic dynamical behaviors, including the scaling solution, energy density parameter and equation-of-state parameter. These aspects are affected by curvature factor k in the matter-dominated phase. In the special scalar field-dominated phase, the energy density parameter normalization restricts the Universe is spatial flat and the curvature factor k is not present in the dynamics. In this paper, the Universe is closed in the matter-dominated phase, and flat in the scalar field-dominated phase. The spatial flatness and the w _ϕ =−1 in the third phase are coincide with the current observations.