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A modified higher-dimensional cosmology with a static traversable wormhole and dominated by a variable modified Chaplygin gas is constructed. Many interesting features are explored and discussed in some detail.A real revolution in cosmology and high energy physics concerns the recent discovery of accelerated expansion of our spatially flat matter-dominated Universe. Those observations comprise the examinations of the dynamics of galaxies (Sahni 2000(Sahni , 2002(Sahni , 2004, Type Ia supernovae (SNIa) (Riess et al. 1998) with redshift z > 0.35, the first acoustic peak of the CMB temperature fluctuations or anisotropies (Perlmutter 1999) and the recent findings of BOOMERANG experiments (de Bernardis 2000). In reality, the fact that a flat Friedmann-Robertson-Walker (FRW) model with the cosmological constant or quintessence fits best the set of astronomical observational data creates troubles of Dark Matter (DM) of about 30% and Dark Energy (DE) of about 70%. DE is in reality characterized by a negative Equation of State Parameter (EoSP): w = p/ρ < 0 with w = −1 for the case of the cosmological constant, and −1 < w < −1/3 for a quintessence fluid, whereas w < −1 for a phantom fluid. Neither the cosmological constant nor quintessence seems to solve the A.R. El-Nabulsi ( )
A modified higher-dimensional cosmology with a static traversable wormhole and dominated by a variable modified Chaplygin gas is constructed. Many interesting features are explored and discussed in some detail.A real revolution in cosmology and high energy physics concerns the recent discovery of accelerated expansion of our spatially flat matter-dominated Universe. Those observations comprise the examinations of the dynamics of galaxies (Sahni 2000(Sahni , 2002(Sahni , 2004, Type Ia supernovae (SNIa) (Riess et al. 1998) with redshift z > 0.35, the first acoustic peak of the CMB temperature fluctuations or anisotropies (Perlmutter 1999) and the recent findings of BOOMERANG experiments (de Bernardis 2000). In reality, the fact that a flat Friedmann-Robertson-Walker (FRW) model with the cosmological constant or quintessence fits best the set of astronomical observational data creates troubles of Dark Matter (DM) of about 30% and Dark Energy (DE) of about 70%. DE is in reality characterized by a negative Equation of State Parameter (EoSP): w = p/ρ < 0 with w = −1 for the case of the cosmological constant, and −1 < w < −1/3 for a quintessence fluid, whereas w < −1 for a phantom fluid. Neither the cosmological constant nor quintessence seems to solve the A.R. El-Nabulsi ( )
We explore the late-time dynamics of a fourdimensional homogeneous and isotropic universe based on a modified Brans-Dicke scalar tensor theory in the presence of string corrections and Gauss-Bonnet curvature corrections. Many original and attractive cosmological features are revealed and discussed in some details.Keywords Modified Brans-Dicke cosmology · String corrections · Gauss-Bonnet invariant term · Accelerated expansionThe analysis of the three year WMAP observations data and the very recent one of the five-year WMAP data from Supernova Legacy Survey of type Ia and galaxy indicate that our universe is accelerating with time and is governed by a mysterious form of dark energy (DE) with negative pressure obeying the equation of state parameter (EoSP) −1.14 < w < −0.93 at 68% confidence level (Riess et al. 1998(Riess et al. , 2004. Furthermore, the observational sets suggest that the Universe is spatially flat as predicted by the inflationary scenario with k = −0.015 +0.020 −0.016 within the limits of observational accuracy based on Gaussianity and adiabaticity of the CMBR power spectrum (Riess et al. 1998(Riess et al. , 2004Perlmutter et al. 1999;Schmidt et al. 1998;Steinhardt et al. 1999;Persic et al. 1996;Cunha 2009). The distribution of energy in the universe is roughly 76% DE, 20% dark matter (DM) and only 4% baryonic matter (BM). However, distance measurement from SNIa and Baryon Acoustic Oscillation confirms the presence of 73% of dark energy together with the range of the equation of state parameter −1.33 < w < −0.79. At first sight, a positive cosmological constant seems to fit correctly the observational data, but unfortunately, including a cosmological constant in the field equations come with a price: the lambda constant problem as well as the well-known coincidence problem.In the last few years, much work has been done and several theories were constructed in order to explain this amazing scenario that could fit correctly the CMBR as well as LSS and supernovae observations. The already build models range from the simple phenomenological Cold Dark Matter model consisting a mixture of cosmological constant and cold dark matter (CDM) which must be relics of a grand unified phase of the universe to Chaplygin gas (Fabris et al. 2002), Generalized Chaplygin gas model (Bento et al. 2002), quintom model (Feng 2006 Zhang 2006), hessence (Wei et al. 2005), holographic DE scenario (Zhao 2007), dark matter-dark energy interaction and so on (ElNabulsi 2009a and references therein). However, there has been several attempts for building alternative theories motivated from non-minimal and minimal scalar tensor theories (El-Nabulsi , string and superstring theories, in particular, the GaussBonnet gravity theory, models with higher-order curvature terms (El-Nabulsi
We investigate the late-time dynamics of a fourdimensional universe based on the effective action of a Brans-Dicke scalar field in the presence of the matter source term, conformal coupling of the scalar curvature to the scalar field, a dynamical cosmological constant and Gauss-Bonnet higher-order terms in the scalar curvature. Many new interesting features are revealed and discussed in some details.Keywords Brans-Dicke cosmology · Conformal coupling · Gauss-Bonnet invariant term · Dark energy · Accelerated expansionOver the past few years, there have been considerable efforts in understanding the physics behind the accelerated expansion of the universe. In fact, several cosmological observations coming from the CMBR dataset of the Three-Year WMAP and data from Supernova Legacy Survey of type SNeIa and large galaxy clustering indicate that our universe is undergoing an amazing stage of accelerated expansion (fact shows that the accelerated expansion is due to some mysterious form of energy usually dubbed dark energy (DE) with equation of state parameterSo far, several phenomenological cosmological models have been introduced in literature to explain the nature of DE. Two natural suspects are a tiny cosmological constant A.R. El-Nabulsi ( ) characterized by w = −1 or a time-dependent quintessence field which dominates the dynamics of the universe with w > −1 at a low-redshift. However, these possibilities in principle are ruled out as a consequence of the enormous inconsistency between the theoretical and experimental values of the cosmological constant. Furthermore, both models require fine tunings of the parameters to fit correctly with observational data. Phantom energy models with concurrently w < −1 and a positive energy density have been also proposed in literature. In fact, analysis of recent observational data also support w < −1 strongly, violating strong or weak energy condition. Some other alternative dark energy models are discussed largely in literature range from scalar tensor theories (Bento et al. 2002 and references therein) to quintessence models (Peebles and Ratra 2003). One remarkable ingredient of these theories is that the nature of the DE could in fact be related with the cosmological constant problem. However, most of them are based on a particular choice of the scalar field potential and furthermore, fine tuning is also required.Truly speaking, a successful dark energy theory may be derived from either a modified gravity approach (MGA) or from string and M-theory (superstring). In reality, MGA (usually refereed as f (R), R being the scalar curvature) is extremely attractive in the applications for late accelerating universe and dark energy as it presents very natural unification of the early-time inflation and late-time acceleration. The success of such unification is due to different role of gravitational terms relevant at small and at large curvature. Moreover, various models of modified gravity are predicted by string/M-theory considerations. It may as well provide as the source for uni...
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