Superstring theories are promising candidates for unifying gravity with all matter interactions. In this work, we discussed a particular 10-dimensional cosmology based on Gauss-Bonnet invariant term coupled to a generalized scalar-tensor theory as resulting from superstring theories. Some interesting features concerning the late-time dynamics are explored and described in some details.

This letter discusses a modified Friedmann-Robertson-Walker cosmology in which the equation of state behaves like p = 3ηa m ρn -ρ,(η,m,n) ∈ <img src="/img/revistas/bjp/v40n2/a04img01.gif" align="absmiddle">. Many interesting features are revealed in particular the manifestation of domain wall, cosmic string, stiff matter, dust and radiation/ultra-relativistic particles.

We show that superbradyons defined as superluminal particles with positive-square mass may be created in the de-Sitter region under specific conformal coupling constraints and later propagates to the Minkowskian spacetime as anti-tachyons or anti-ghosts (normal particles). It was also proved that an increasing gravitational constant with distance enriches the study where some interesting features are discussed in some details.

Higher dimensional cosmological implications ofa decay law for the cosmological constant term are analyzed. Three independent cosmological models are explored mainly: 1) - In the first model, the effective cosmological constant was chosen to decay with times like Λeffective = Ca-2 + D(b/a i)² where aiis an arbitrary scale factor characterizing the isotropic epoch which proceeds the graceful exit period. Further, the extra-dimensional scale factor decays classically like b(t) ≈ a x (t),xisareal negative number. 2) - In the second model, we adopt in addition to Λeffective = Ca-2 + D(b/a i)² the phenomenological law b(t) = a(t) exp(-Qt) as we expect that at the origin of time, there is no distinction between the visible and extra dimensions; Qis a real number. 3) -In the third model, we study a Λ-decaying extra-dimensional cosmology with a static traversable wormhole in which the four-dimensional Friedmann-Robertson-Walker spacetime is subject to the conventional perfect fluid while the extra-dimensional part is endowed by an exotic fluid violating strong energy condition and where the cosmological constant in (3+n+1) is assumed to decays like Λ(a) = 3Ca-2. The three models are discussed and explored in some details where many interesting points are revealed.

We discuss the generic features of the accelerated universe from Arnowitt-Deser-Misner renormalizable group approach applied for gravity models with variable gravitational coupling constant and cosmological constant. The universe undergoes an endless sequence of cosmic eras which starts inflating after a Big Bang and end contracting to a Big Crunch dominated by phantom energy. Presently, the universe found to be presently accelerating in time and is dominated by dark energy. Some important features and numerical results are revealed and discussed in some details.