Dimensional Cosmology Research Articles

Bulk Scale Factor at Very Early Universe

In this paper we propose a higher dimensional Cosmology based on FRW model and brane-world scenario. We consider the warp factor in the brane-world scenario as a scale factor in 5-dimensional generalized FRW metric, which is called as bulk scale factor, and obtain the evolution of it with space-like and time-like extra dimensions. It is then showed that, additional space-like dimensions can produce exponentially bulk scale factor under repulsive strong gravitational force in the empty universe at a very early stage.

A Four Dimensional (4D) Cosmology Model from a 5D Einstein Manifold Reduction

A Four Dimensional (4D) Cosmology Model from a 5D Einstein Manifold Reduction

Primordial nucleosynthesis in higher dimensional cosmology

AbstractWe investigate nucleosynthesis and element formation in the early universe in the framework of higher dimensional cosmology. We find that temperature decays less rapidly in higher dimensional cosmology, which we believe may have nontrivial consequences vis-a-vis primordial physics.

Implications of a decay law for the cosmological constant in higher dimensional cosmology and cosmological wormholes

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/ai)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) ≈ ax (t),xisareal negative number. 2) - In the second model, we adopt in addition to Λeffective = Ca-2 + D(b/ai)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.

Quantum cosmology with varying speed of light: Canonical approach

We investigate (n+1)-dimensional cosmology with varying speed of light. After solving corresponding Wheeler–DeWitt equation, we obtain exact solutions in both classical and quantum levels for (c−Λ)-dominated Universe. We then construct the “canonical” wave packets which exhibit a good classical and quantum correspondence. We show that arbitrary but appropriate initial conditions lead to the same classical description. We also study the situation from de-Broglie Bohm interpretation of quantum mechanics and show that the corresponding Bohmian trajectories are in good agreement with the classical counterparts.

Brane world cosmology in Jordan-Brans-Dicke theory

We consider the embedding of 3+1 dimensional cosmology in 4+1 dimensional Jordan-Brans-Dicke theory. We show that exponentially growing and power law scale factors are implied. Whereas the 4+1 dimensional scalar field is approximately constant for each, the effective 3+1 dimensional scalar field is constant for exponentially growing scale factor and time dependent for power law scale factor.

Five dimensional cosmology in Horava-Witten M theory

The cosmology in the Hubble expansion era of the Horava-Witten M theory compactified on a Calabi-Yau threefold is studied in the reduction to five-dimensions where the effects of the Calabi-Yau manifold are summarized by the volume modulus, and all perturbative potentials are included. Matter on the branes are treated as first order perturbations of the static vacuum solution, and all equations in the bulk and all boundary conditions on both end branes are imposed. It is found that for a static volume modulus and a static fifth dimension, $y$, one can recover the four-dimensional Robertson Friedmann Walker cosmology for relativistic matter on the branes, but not for nonrelativistic matter. In this case, the Hubble parameter $H$ becomes independent of $y$ to first order in matter density. This result holds also when an arbitrary number of 5-branes are included in the bulk. The five-dimensional Horava-Witten model is compared with the Randall-Sundrum phenomenology with a scalar field in the bulk where a bulk and brane potential are used so that the vacuum solutions can be rigorously obtained. (In the Appendix, the difficulty of obtaining approximate vacuum solutions for other potentials is discussed.) In this case nonrelativistic matter is accommodated by allowing the distance between the branes to vary. It is suggested that nonperturbative potentials for the vacuum solution of Horava-Witten theory are needed to remove the inconsistency that nonrelativistic matter creates.

When do colliding bubbles produce an expanding universe?

It is intriguing to consider the possibility that the Big Bang of the standard (3+1) dimensional cosmology originated from the collision of two branes within a higher dimensional spacetime, leading to the production of a large amount of entropy. In this paper we study, subject to certain well-defined assumptions, under what conditions such a collision leads to an expanding universe. We assume the absence of novel physics, so that ordinary (4+1) -dimensional Einstein gravity remains a valid approximation. It is necessary that the fifth dimension not become degenerate at the moment of collision. First the case of a symmetric collision of infinitely thin branes having a hyperbolic or flat spatial geometry is considered. We find that a symmetric collision results in a collapsing universe on the final brane unless the pre-existing expansion rate in the bulk just prior to the collision is sufficiently large in comparison to the momentum transfer in the fifth dimension. Such prior expansion may either result from negative spatial curvature or from a positive five-dimensional cosmological constant. The relevance of these findings to the Colliding Bubble Braneworld Universe scenario is discussed. Finally, results from a numerical study of colliding thick-wall branes is presented, which confirm the results of the thin-wall approximation.

Plausible upper limit on the number ofe-foldings

Based solely on the arguments relating Friedmann equation and the Cardy formula we derive a bound for the number of e-folds during inflation for a standard Friedmann-Robertson-Walker as well as non-standard four dimensional cosmology induced by a Randall-Sundrum-type model.

Strings in singular time‐dependent backgrounds

AbstractWe review the construction of time‐dependent backgrounds with space‐like singularities. We mainly consider exact CFT backgrounds. The algebraic and geometric aspects of these backgrounds are discussed. Physical issues, results and difficulties associated with such systems are reviewed. Finally, we present some new results: a two dimensional cosmology in the presence of an Abelian gauge field described within a family of ${{SL(2)\times U(1)} \over {U(1)\times Z}}$ quotient CFTs.

A new cosmological scenario in string theory

We consider new cosmological solutions with a collapsing, an intermediate and an expanding phase. The boundary between the expanding (collapsing) phase and the intermediate phase is seen by comoving observers as a cosmological past (future) horizon. The solutions are naturally embedded in string and M-theory. In the particular case of a two-dimensional cosmology, space-time is flat with an identification under boost and translation transformations. We consider the corresponding string theory orbifold and calculate the modular invariant one-loop partition function. In this case there is a strong parallel with the BTZ black hole. The higher dimensional cosmologies have a time-like curvature singularity in the intermediate region. In some cases the string coupling can be made small throughout all of space-time but string corrections become important at the singularity. This happens where string winding modes become light which could resolve the singularity. The new proposed space-time casual structure could have implications for cosmology, independently of string theory.

Holography and the generalized second law of thermodynamics in (2+1)-dimensional cosmology

The Fischler–Susskind entropy bound has been studied in (2+1)-dimensional universes with negative cosmological constant. As in all contracting universes, that bound is not satisfied. Furthermore, we found that the Fischler–Susskind bound is not compatible with a generalized second law of thermodynamics in (2+1)-dimensional cosmology, neither the classical nor the quantum version. On the other hand, the Hubble entropy bound has been constructed in (2+1)-dimensional cosmology and it is shown compatible with the generalized second law of thermodynamics.

Inflation, singular instantons, and eleven dimensional cosmology

We investigate cosmological solutions of eleven dimensional supergravity compactified on a squashed seven manifold. The effective action for the four dimensional theory contains scalar fields describing the size and squashing of the compactifying space. The potential for these fields consists of a sum of exponential terms. At early times only one such term is expected to dominate. The condition for an exponential potential to admit inflationary solutions is derived and it is shown that inflation is not possible in our model. The criterion for an exponential potential to admit a Hawking-Turok instanton is also derived. It is shown that the instanton remains singular in eleven dimensions.

Viscosity effects in (1 + 1)-dimensional cosmologies

We investigate the behaviour of the solutions to the gravitational (1 + 1)-dimensional equations with a viscous fluid source in the case of a Robertson - Walker metric. In our model the bulk viscosity coefficient is related to the energy density as and we also take into account the effects of a nonlinear viscosity . We find exact solutions of the field equations for several physically interesting cases which include Friedmann cosmologies and the time reversal of Murphy's class of cosmological solutions. We find that the global effect of bulk viscosity is to decrease the expansion rate while the nonlinear viscosity does not qualitatively change the models. On the other hand, the bulk viscosity is able to remove the initial singularities which are present in the corresponding viscosity free models as in the four-dimensional case.

MultipletSO(n+1) scalar fields driving eternal expansion in closed (D+1)-dimensional FLRW cosmologies

The possibility of having a de Sitter asymptotic stage free of choice of the value of the positive cosmological constant (“no critical λ”) is analyzed in a closed FLRW universe which starts from a “quiescent” phase of evolution and ends into a “textured” phase by taking into account multipletSO(n+1) scalar fields.

Classical N-particle cosmology in 2+1 dimensions

In 2+1 dimensional cosmology particles are topological defects in a universe that is nearly everywhere flat. The author uses a time dependent triangulation procedure of space to formulate the laws of evolution and to construct the phase space of this system. A theorem is derived from which it follows that many configurations may have a big bang in their past or a big crunch in their future. The dimensionality of phase space for N particles is 4N-11+12g, where g is the genus of 2-space. This suggests to consider 2N-6+6g pairs of canonical variables and one time variable. The quantum version of this model is speculated about.

Cosmological evolution driven by viscous dissipation and monopole condensation in ten dimensions

AbstractA ten dimensional cosmology is studied wherein the stringy state of the early universe is simulated by a combination of viscous effects and monopole condensation in the primeval ten dimensions.

Some remarks on luminosity distance and nucleosynthesis in higher dimensional cosmology

Using a previous cosmological solution of the present author, the behavior of luminosity distance is analyzed in multidimensional spacetime. It is observed that the de Sitter spacetime sets an upper limit to the absolute distance of any cosmological source where the generally accepted value of the deceleration parameter q(0) greater than -1 is taken. This generalizes an earlier result of Barnes to higher dimensions. Further, an attempt is made to investigate, in brief, the phenomenon of nucleosynthesis in the early universe. While it is premature to make a definite statement in this regard, it is found that there may be significant differences, in principle at least, from the analogous situation in four-dimensional spacetime. 15 refs.

Curvature dependent bulk viscosity in a (D + 1) dimensional cosmology leading to slow expansion

By assuming a homogeneous and isotropic (D + 1) dimensional metric and a matter content described by ultrarelativistic particles with bulk viscosity represented by a coefficient that depends on the square root of the curvature, we find that the positive curvature topology leads to a slowly varying linear expansion which may preceed the usual period of inflation commonly discussed in cosmological scenarios. The flat spatial topology leads to a cosmology expanding to an infinite radius in an inverse power law fashion.

Time variation of gravitational constant in multidimensional cosmology

In the framework of (4+N)-dimensional cosmology with an isotropic 3-space and a Ricci-flat internal space a relation between cosmological parameters and the time variation ofG is obtained. For a spatially flat universe restrictions on the density parameter are obtained.