Dark Energy In Universe Research Articles

Noether gauge symmetry approach in Gauss–Bonnet dilatonic theory of gravity

The approach of Noether symmetries with gauge term in the Gauss–Bonnet dilatonic theory of gravity is used for different values of the state parameter, ω, of the background matter. It is found that for ω = –1 (dark energy Universe), ω = 0 (matter-dominated Universe), and ω = 1 (the case of stiff matter), for the existence of Noether gauge symmetries, the potential V([Formula: see text]) and the coupling of the Gauss–Bonnet term with gravity Λ([Formula: see text]) are obtained as exponential functions of the scalar field [Formula: see text]. For the case of arbitrary ω the coupling parameter Λ([Formula: see text]) is found as a linear function of the scalar field [Formula: see text] and the potential is a constant, for the existence of Noether gauge symmetries. Here it is observed that both the potential and coupling parameter depend on the evolutionary history of the Universe.

Sterile neutrino models and nonminimal cosmologies

Cosmological measurements are affected by the energy density of both active and sterile massive neutrinos. We extend here a recent analysis of current cosmological data to non minimal cosmologies. Several possible scenarios are examined: a constant w \neq -1 dark energy equation of state, a non flat universe, a time varying dark energy component and coupled dark matter dark energy universes or modified gravity scenarios. When considering cosmological data only, (3+2) massive neutrino models with ~0.5 eV sterile species are allowed at 95% CL. This scenario has been shown to reconcile reactor, LSND and MiniBooNE positive signals with null results from other searches. Big Bang Nucleosynthesis bounds could compromise the viability of (3+2) models if the two sterile species are fully thermalized states at decoupling.

Universal procedure to cure future singularities of dark energy models

A systematic search for different viable models of the dark energy universe, all of which give rise to finite-time, future singularities, is undertaken, with the purpose to try to find a solution to this common problem. After some work, a universal procedure to cure all future singularities is developed and carefully tested with the help of explicit examples corresponding to each one of the four different types of possible singularities, as classified in the literature. The cases of a fluid with an equation of state which depends on some parameter, of modified gravity non-minimally coupled to a matter Lagrangian, of non-local gravity, and of isotropic turbulence in a dark fluid universe theory are investigated in detail.

Fate of the phantom dark energy universe in semiclassical gravity

The fate of the phantom dark energy universe in semiclassical gravity is investigated. Quantum corrections coming from massless fields conformally coupled with gravity are considered, to see if they can lead to avoidance of the big rip singularity, which shows up in a flat Friedmann-Robertson-Walker universe, filled with phantom dark energy and modeled by an equation of state of the form $p=\ensuremath{\omega}\ensuremath{\rho}$ with $\ensuremath{\omega}<\ensuremath{-}1$. The dynamics of the model are discussed for all values of the two parameters, named $\ensuremath{\alpha}>0$ and $\ensuremath{\beta}<0$, which come from quantum corrections. It is concluded that, when $\ensuremath{-}1<\frac{\ensuremath{\beta}}{3\ensuremath{\alpha}}<0$, almost all solutions develop future singularities (the corresponding scale factor and energy density go down to zero in finite time). However, when $\ensuremath{-}1>\frac{\ensuremath{\beta}}{3\ensuremath{\alpha}}$, almost all solutions describe a universe bouncing infinitely many times (an oscillating universe).

The foundation of the theory of the universe dark energy and its nature

Surprisingly recent astronomical observations have provided strong evidence that our universe is not only expanding, but also is expanding at an accelerating rate. This paper pre- sents a basis of the theory of universe space- time dark energy, a solution of Einstein’s cosmological constant problem, physical interpretation of universe dark energy and Einstein’s cosmological constant Lambda and its value ( = 0.29447 × 10-52 m-2), values of universe dark energy density 1.2622 × 10-26 kg/m3 = 6.8023 GeV, universe critical density 1.8069 × 10-26 kg/m3 = 9.7378 GeV, universe matter density 0.54207 × 10-26 kg/m3 = 2.9213 GeV, and universe radiation density 2.7103 × 10-31 kg/m3 = 1.455 MeV. The interpretation in this paper is based on geometric modeling of space-time as a perfect four- dimensional continuum cosmic fluid and the momentum generated by the time. In this modeling time is considered as a mechanical variable along with other variables and treated on an equal footing. In such a modeling, time is considered to have a mechanical nature so that the momentum associated with it is equal to the negative of the universe total energy. Since the momentum associated with the time as a mechanical variable is equal to the negative system total energy, the coupling in the time and its momentum leads to maximum increase in the space-time field with 70.7% of the total energy. Moreover, a null paraboloid is obtained and interpreted as a function of the momentum generated by time. This paper presents also an interpretation of space-time tri-dipoles, gravity field waves, and gravity carriers (the gravitons). This model suggests that the space-time has a polarity and is composed of dipoles which are responsible for forming the orbits and storing the space-time energy-momentum. The tri-di- poles can be unified into a solo space-time dipole with an angle of 45 degrees. Such a result shows that the space-time is not void, on the contrary, it is full of conserved and dynamic energy-momentum structure. Furthermore, the gravity field waves is modeled and assumed to be carried by the gravitons which move in the speed of light. The equivalent mass of the graviton (rest mass) is found to be equal to 0.707 of the equivalent mass of the light photons. Such a result indicates that the lightest particle (up to the author’s knowledge) in the nature is the graviton and has an equivalent mass equals to 2.5119 x 10-52 kg. Based on the fluidic nature of dark energy, a fourth law of thermodynamics is proposed and a new physical interpretation of Kepler’s Laws are presented. Additionally, based on the fact that what we are observing is just the history of our universe, on the Big Bang Theory, Einstein’s General Relativity, Hubble Parameter, cosmic inflation theory and on NASA’s observation of supernova 1a, then a second-order (parabolic) parametric model is obtained in this proposed paper to describe the accelerated ex- pansion of the universe. This model shows that the universe is approaching the universe cosmic horizon line and will pass through a critical point that will influence significantly its fate. Considering the breaking symmetry model and the variational principle of mechanics, then the universe will witness an infinitesimally stationary state and a symmetry breaking. As result of that, our universe will experience in the near future, a very massive impulse force in the order 1083 N. Subsequently, the universe will collapse. Finally, simulation results are demonstrated to verify the analytical results.

Singularity of spherically-symmetric spacetime in quintessence/phantom dark energy universe

We consider ideal fluid and equivalent scalar field dark energy universes where all four known types of finite time, future singularities occur at some parameter values. It is demonstrated that pressure/energy density of such quintessence/phantom dark energy diverges in spherically-symmetric spacetime at finite radius or at the center. This may cause the instability of the relativistic star or black hole in such universe. The resolution of the problem via the extra modification of the equation of state is briefly discussed.

Dark matter and dark energy in the Universe: Astrophysical reasons and theoretical models

The general review on the problem of dark matter and dark energy in observable Universe is given. The problem is treated from the point of view of research fields on the boundary of which this fundamental task is situated, namely, the astrophysics and theoretical physics. We emphasize that interdisciplinary nature of “dark sector” of Universe problem can assist in its resolution. The paper does not describe the history of the question, but is centred on revealing the main trends and perspectives.

The future of the universe in modified gravitational theories: approaching a finite-timefuture singularity

We investigate the future evolution of the dark energy universe in modified gravities, includingF(R) gravity, and string-inspired scalar Gauss–Bonnet and modified Gauss–Bonnet ones,and ideal fluid with an inhomogeneous equation of state (EoS). The modifiedFriedmann–Robertson–Walker dynamics for all of these theories may be presented in auniversal form by using the effective ideal fluid with an inhomogeneous EoS withoutspecifying its explicit form. We construct several examples of a modified gravity whichproduces accelerating cosmologies ending at the finite-time future singularities ofall four known types by applying a reconstruction program. Some scenarios forresolving a finite-time future singularity are presented. Among these scenarios,the most natural one is related to additional modification of the gravitationalaction in the early universe. In addition, late-time cosmology in the non-minimalMaxwell–Einstein theory is considered. We investigate the forms of non-minimalgravitational coupling which generate finite-time future singularities and the generalconditions for this coupling such that the finite-time future singularities cannot emerge.Furthermore, it is shown that the non-minimal gravitational coupling can remove thefinite-time future singularities or make the singularity stronger (or weaker) in modifiedgravity.

DARK ENERGY AS A RELIC OF THE VACUUM-ENERGY CANCELLATION?

We analyze the dynamical implications of an exponential Lagrangian density for the gravitational field, as referred to an isotropic FRW Universe. Then, we discuss the features of the generalized de Sitter phase, predicted by the new Friedmann equation. The existence of a consistent de Sitter solution arises only if the ratio between the vacuum energy density and that associated with the fundamental length of the theory acquires a tantalizing negative character. This choice allows us to explain the present Universe dark energy as a relic of the vacuum-energy cancellation due to the cosmological constant intrinsically contained in our scheme. The corresponding scalar-tensor description of the model is addressed too, and the behavior of the scalar field is analyzed for both negative and positive values of the cosmological term. In the first case, the Friedmann equation is studied both in vacuum and in the presence of external matter, while, in the second case, the quantum regime is approached in the framework of "repulsive" properties of the gravitational interaction, as described in recent issues in loop quantum cosmology. In particular, in the vacuum case, we find a pure non-Einsteinian effect, according to which a negative cosmological constant provides an accelerating de Sitter dynamics, in the region where the series expansion of the exponential term does not hold.

Dark energy generated from a (super-) string effective action with higher-order curvature corrections and a dynamical dilaton

We investigate the possibility of a dark energy universe emerging from an action with higher-order string loop corrections to Einstein gravity in the presence of a massless dilaton. These curvature corrections (up to $R^4$ order) are different depending upon the type of (super)string model which is considered. We find in fact that Type II, heterotic, and bosonic strings respond differently to dark energy. A dark energy solution is shown to exist in the case of the bosonic string, while the other two theories do not lead to realistic dark energy universes. Detailed analysis of the dynamical stability of the de-Sitter solution is presented for the case of a bosonic string. A general prescription for the construction of a de-Sitter solution for the low-energy (super)string effective action is also indicated. Beyond the low-energy (super)string effective action, when the higher-curvature correction coefficients depend on the dilaton, the reconstruction of the theory from the universe expansion history is done with a corresponding prescription for the scalar potentials.

The foundation of the fourth law of thermodynamics: Universe dark energy and its nature: Can dark energy be generated?

The foundation of the fourth law of thermodynamics: Universe dark energy and its nature: Can dark energy be generated?

Transition from a matter-dominated era to a dark energy universe

We develop a general program of the unification of a matter-dominated era with an acceleration epoch for scalar-tensor theory or a dark fluid. The general reconstruction of the scalar-tensor theory is fulfilled. The explicit form of the scalar potential for which the theory admits a matter-dominated era, a transition to an acceleration, and an (asymptotically de Sitter) acceleration epoch consistent with Wilkinson Microwave Anisotropy Probe data is found. The interrelation of the epochs of deceleration-acceleration transition and matter dominance-dark energy transition for dark fluids with a general equation of state (EOS) is investigated. We give several examples of such models with explicit EOS (using redshift parametrization) where matter-dark energy domination transition may precede the deceleration-acceleration transition. As a by-product, the reconstruction scheme is applied to scalar-tensor theory to define the scalar potentials which may produce the dark matter effect. The obtained modification of Newton potential may explain the rotation curves of galaxies.

Quantum cosmology: expectations and results

We study cosmologies based on low-energy effective string theory with higher-order string corrections to a tree-level action and with a modulus scalar field (dilaton or compactification modulus). In the presence of such corrections it is possible to construct nonsingular cosmological solutions in the context of Pre-Big-Bang and Ekpyrotic universes. We review the construction of nonsingular bouncing solutions and resulting density perturbations in Pre-Big-Bang and Ekpyrotic models. We also discuss the effect of higher-order string corrections on a dark energy universe and show several interesting possibilities of the avoidance of future singularities.

Cosmologies from higher‐order string corrections

AbstractWe study cosmologies based on low‐energy effective string theory with higher‐order string corrections to a tree‐level action and with a modulus scalar field (dilaton or compactification modulus). In the presence of such corrections it is possible to construct nonsingular cosmological solutions in the context of Pre‐Big‐Bang and Ekpyrotic universes. We review the construction of nonsingular bouncing solutions and resulting density perturbations in Pre‐Big‐Bang and Ekpyrotic models. We also discuss the effect of higher‐order string corrections on a dark energy universe and show several interesting possibilities of the avoidance of future singularities.

Dark energy and viscous cosmology

Singularities in the dark energy universe are discussed, assuming that there is a bulk viscosity in the cosmic fluid. In particular, it is shown how the physically natural assumption of letting the bulk viscosity be proportional to the scalar expansion in a spatially flat FRW universe can drive the fluid into the phantom region (w < -1), even if lies in the quintessence region (w > -1) in the non-viscous case.

Inhomogeneous equation of state of the universe: Phantom era, future singularity, and crossing the phantom barrier

The dark energy universe equation of state (EOS) with inhomogeneous, Hubble parameter dependent term is considered. The motivation to introduce such a term comes from time-dependent viscosity considerations and modifications of general relativity. For several explicit examples of such EOS it is demonstrated how the type of future singularity changes, how the phantom epoch emerges and how the crossing of a phantom barrier occurs. Similar cosmological regimes are considered for the universe with two interacting fluids and for the universe with implicit EOS. For instance, the crossing of the phantom barrier is realized in an easier way, thanks to the presence of inhomogeneous term. The thermodynamical dark energy model is presented where the universe entropy may be positive even at the phantom era as a result of the crossing of the $w=\ensuremath{-}1$ barrier.

The fate of (phantom) dark energy universe with string curvature corrections

We study the evolution of (phantom) dark energy universe by taking into account the higher-order string corrections to Einstein–Hilbert action with fixed dilaton and modulus fields. While the presence of a cosmological constant gives stable de Sitter fixed points in the cases of heterotic and bosonic strings, no stable de Sitter solutions exist when a phantom fluid is present. We find that the universe can exhibit a Big Crunch singularity with a finite time for type II string, whereas it reaches a Big Rip singularity for heterotic and bosonic strings. Thus the fate of dark energy universe crucially depends upon the type of string theory under consideration.

Properties of singularities in the (phantom) dark energy universe

The properties of future singularities are investigated in the universe dominated by dark energy including the phantom-type fluid. We classify the finite-time singularities into four classes and explicitly present the models which give rise to these singularities by assuming the form of the equation of state of dark energy. We show the existence of a stable fixed point with an equation of state $w&lt;\ensuremath{-}1$ and numerically confirm that this is actually a late-time attractor in the phantom-dominated universe. We also construct a phantom dark energy scenario coupled to dark matter that reproduces singular behaviors of the Big Rip type for the energy density and the curvature of the universe. The effect of quantum corrections coming from conformal anomaly can be important when the curvature grows large, which typically moderates the finite-time singularities.

The Hubble Space Telescope H0 Key Project

As this conference has attested to, cosmology is a rapidly maturing field, currently experiencing a very healthy and vigorous confrontation between theory and experiment. This rapid progress in many different areas of cosmology has not removed the longstanding interest in measuring many of the fundamental cosmological parameters - rather, the increasingly detailed predictions of current theory highlight the critical importance of independently, accurately measuring the cosmological parameters which define the basic model for the dynamical evolution of the Universe. I present here the final results of the Hubble Space Telescope (HST) Key Project to measure the Hubble constant, summarizing our method, the results and the uncertainties. The Key Project results are based on a Cepheid calibration of several secondary distance methods applied over the range of about 60 to 400 Mpc. Based on the Key Project Cepheid calibration and its application to five secondary methods (type Ia supernovae, the Tully-Fisher relation, surface brightness fluctuations, type II supernovae, and the fundamental plane for elliptical galaxies), a combined value of H0 = 72 ± 8 km/sec/Mpc is obtained. Comparing to current estimates of the ages of Galactic globular clusters, an age conflict is avoided for this high a value of H0 if we live in a Λ-dominated (or other form of dark energy) universe.

The Universality of Seesaws

I discuss how the ideas associated with Seesaws, first introduced in the context of neutrino masses, are generally useful for understanding the very disparate scales one encounters in particle physics. From this point of view, the energy scale characterizing the Universe's dark energy presents a real challenge. A natural Seesaw explanation for this scale ensues if one imagines tying the dark energy sector to the neutrino sector, but this idea requires bold new dynamics.