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We discuss the possibility that we are living in a non-singular superinflationary universe and dominated by dark energy. Motivated from superstring and M-theory, the cosmology introduced here is based on the effective action of the dilaton scalar field in the presence of the matter source term. Many novel and interesting consequences are revealed through this work and discussed in some details.Keywords Higher-dimensions · Dilaton field · Scalar field · Phantom energy · Dark energy · Superinflation Following the recent advances in sting, superstring and Mtheory, we believe that extra-dimensions really exist at very high energy limit, that is, at the Planck energy regime. However, as we are living in a four-dimensional spacetime, the extra-dimension is invisible and furthermore, we expect that the later decays in time. The recent discovery of dark energy as a theoretical explanation of the accelerated expansion of the Universe is a big unattended surprise in cosmology. In fact, the recent observations of the Type SNeIa supernovae, cosmic microwave background (CMB) anisotropies, the large scale galaxies structures of the universe and SachsWolfe effects have led to the idea that our universe undergoes accelerated expansion at the present epoch tending to a flat de-Sitter space-time as predicted by inflation theory (−1.14 < w < −0.93 at 68% confidence level and has special effects that have only been detected on the largest scales of our Universe and then only in the past ten years (Alcaniz 2004). Several theoretical models, theories and possible solutions have been proposed, but the most popular ones are the cosmological constant or vacuum energy with a constant equation of state, or a variant of it known as quintessence simulated by a slowly rolling scalar field whose energymomentum tensor is dominated by the contribution of the field potential energy which drives a period of accelerated expansion. Unfortunately, both models are encountered by the huge fine-tuning problem required for its magnitude and the coincidence problem: why does the acceleration happen around a redshift of unity, at around 10 billion years? Other leading candidates are K-essence with modified kinetic energy (Armendariz-Picon et al. ), etc. Unfortunately, we still ignore which of these models are the most viable or more realistic than the others. The largest part of these models have significant drawbacks and suffer from serious finetuning problems e.g. fine tuning of parameters for different type of potentials which model quintessence and stability of radiative corrections from the matter sector. In addition, in many of these models, time varying dark energy is expected. With no doubt, the mystery of cosmic acceleration may be the richest, with broad connections to other important questions in modern cosmology and in elementary particle physics. One more serious additional problem that occurs on most of these phenomenological theories including the standard Big Bang model is the initial singularity problem. This serious problem still persists...
We discuss the possibility that we are living in a non-singular superinflationary universe and dominated by dark energy. Motivated from superstring and M-theory, the cosmology introduced here is based on the effective action of the dilaton scalar field in the presence of the matter source term. Many novel and interesting consequences are revealed through this work and discussed in some details.Keywords Higher-dimensions · Dilaton field · Scalar field · Phantom energy · Dark energy · Superinflation Following the recent advances in sting, superstring and Mtheory, we believe that extra-dimensions really exist at very high energy limit, that is, at the Planck energy regime. However, as we are living in a four-dimensional spacetime, the extra-dimension is invisible and furthermore, we expect that the later decays in time. The recent discovery of dark energy as a theoretical explanation of the accelerated expansion of the Universe is a big unattended surprise in cosmology. In fact, the recent observations of the Type SNeIa supernovae, cosmic microwave background (CMB) anisotropies, the large scale galaxies structures of the universe and SachsWolfe effects have led to the idea that our universe undergoes accelerated expansion at the present epoch tending to a flat de-Sitter space-time as predicted by inflation theory (−1.14 < w < −0.93 at 68% confidence level and has special effects that have only been detected on the largest scales of our Universe and then only in the past ten years (Alcaniz 2004). Several theoretical models, theories and possible solutions have been proposed, but the most popular ones are the cosmological constant or vacuum energy with a constant equation of state, or a variant of it known as quintessence simulated by a slowly rolling scalar field whose energymomentum tensor is dominated by the contribution of the field potential energy which drives a period of accelerated expansion. Unfortunately, both models are encountered by the huge fine-tuning problem required for its magnitude and the coincidence problem: why does the acceleration happen around a redshift of unity, at around 10 billion years? Other leading candidates are K-essence with modified kinetic energy (Armendariz-Picon et al. ), etc. Unfortunately, we still ignore which of these models are the most viable or more realistic than the others. The largest part of these models have significant drawbacks and suffer from serious finetuning problems e.g. fine tuning of parameters for different type of potentials which model quintessence and stability of radiative corrections from the matter sector. In addition, in many of these models, time varying dark energy is expected. With no doubt, the mystery of cosmic acceleration may be the richest, with broad connections to other important questions in modern cosmology and in elementary particle physics. One more serious additional problem that occurs on most of these phenomenological theories including the standard Big Bang model is the initial singularity problem. This serious problem still persists...
We investigate the late-time dynamics of a fourdimensional universe based on modified scalar field gravity in which the standard Einstein-Hilbert action R is re-We discussed two independent cases: in the first model, the scalar field potential is quartic and for this special form it was shown that the universe is dominated by dark energy with equation of state parameter w ≈ −0.2 and is accelerated in time with a scale factor evolving like a(t) ∝ t 5/3 and B + 3A ≈ 0.036. When, B + 3A → ∞ which corresponds for the purely quadratic theory, the scale factor evolves like a(t) ∝ t 1/2 whereas when B + 3A → 0 which corresponds for the purely scalar tensor theory we found when a(t) ∝ t 1.98 . In the second model, we choose an exponential potential and we conjecture that the scalar curvature and the Hubble parameter vary respectively like R = ηHφ/φ, η ∈ R and H = γφ χ , (γ, χ) ∈ R. It was shown that for some special values of χ, the universe is free from the initial singularity, accelerated in time, dominated by dark or phantom energy whereas the model is independent of the quadratic gravity corrections. Additional consequences are discussed.Keywords Modified cosmology · Quadratic correction · Quartic potential · Dark and phantom energy · Accelerated expansion · Liouville cosmology One of the most important discoveries over the past few years is that we live in an accelerated almost spatially universe with a density parameter k = −0.015 +0.020 −0.016 (within a 2% margin of error). More precisely, cosmological observations indicate that there are two periods of accelerated expansion in the history of our universe: cosmic inflation in the early universe (high energy limits) and acceleration in the current expansion of the universe (low energy limits). These facts are based on number of cosmological and astrophysical observations from the CMBR dataset of the Three-Year WMAP, data from Supernova Legacy Survey of type SNeIa and large galaxy (Riess et al
We have explored a n + 2 higher-dimensional cosmology dominated by phantom energy with a static traversable wormhole dominated by a time-dependent cosmological constant. Some interesting features are revealed and discussed in some details.Keywords Static traversable wormholes · Higher-dimensions · Phantom energy Traversable Lorentzian wormholes are topological bridges in spacetime connecting normally separated regions of a single universe, or overpasses joining two dissimilar spacetimes through a throat which is a minimal area surface. Historically, they have been in vogue ever since Morris, Thorne and Yurtsever [1] came up with the exciting possibility of constructing time machine models with these striking objects and recently by Morris and Thorne [2]. The main feature of wormhole physics is the fact that traversable wormholes are accompanied by an inevitable violation of the Null Energy Condition (NEC), i.e.T μν K μ K ν ≥ 0 where K μ is any null-vector and T μν is the stress-energy tensor. In other words, for the Lorentzian wormhole to be traversable, it necessitates exotic matter with negative energy which violate NECs. These conditions basically say that the energy density is greater than or equal to zero, for all observers. From quantum point of view, it is well-known that in the quantum regime, the violations of NEC can be easily obtained and is indeed a necessary condition for the existence of wormholes. It is noteworthy that in 1992, Hawking gave a common proof to the effect that negative energy is always required to construct a time machine in a finite region of spacetime, in particular for Cauchy horizons which are efficiently
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