Brane Gas Cosmology Research Articles

RADION STABILIZATION IN BRANE GAS COSMOLOGY

We address an overview on the mechanism of the radion stabilization in brane gas cosmology. We explain why it is not possible to stabilize the radion with a brane gas alone. The bulk RR field can arise an effective potential required for the radion stabilization. For a combination of (D - 3)-brane gas wrapping the extra dimension and 4-form RR flux in the unwrapped dimensions, it is possible that three dimensions can expand monotonically while the extra dimensions oscillate or sit at the minimum of the potential.

Moduli Stabilization in Brane Gas Cosmology

Recently, there have been works on dynamical compactification of string cosmology that can produce inflation consistent with current observations. Moduli, scalar fields characterizing the shape and the size of the extra dimensions, including dilaton and axion, play a key role in string cosmology. Since the vacuum expectation value of the dilaton determines the strength of the gravitational coupling, a rolling dilaton implies a changing gravitational constant. Also, since the extra dimensions should be compact and small through the cosmological evolution of the early universe, the volume modulus of the extra dimensions (radion) should be stabilized in any cosmological models based on string theory. To stabilize the moduli fields in string theory, various models and mechanisms were introduced [1]. One way to stabilize the moduli is using the framework of string or brane gas cosmology based on the mechanism of Brandenberger and Vafa [2,3]. A gas of extended objects (strings and branes) can affect the evolution of the universe in the presence of a nontrivial topology where winding modes are allowed. Strings or branes can be coupled to a cosmological background in the same way as a gas of point particles is coupled to a background of standard Einstein gravity. Many works to develop a stringy mechanism to stabilize the volume or shape moduli have been done in the brane gas formalism [4–18]. The key problem in fixing the dilaton and the radion is that their potential is flat and their vacuum expectation value cannot be determined perturbatively. Some nonperturbative effects must be introduced to stabilize them. For example, a nonperturbatively-generated potential from gaugino condensation was considered to stabilize the dilaton and the radion [19]. Some other factors

Stability of internal dimension with potential in brane gas cosmology

Stability of internal dimension with potential in brane gas cosmology

Radion effective potential in brane gas cosmology

We consider a cosmological solution which can explain anisotropic evolution of spatial dimensions and the stabilization of extra dimensions in brane gas formalism. We evaluate the effective potentials, induced by brane gas, bulk flux and supergravity particles, which govern the sizes of the observed three and the extra dimensions. It is possible that the wrapped internal volume can oscillate between two turning points or sit at the minimum of the potential while the unwrapped three-dimensional volume can expand monotonically. Including the supergravity particles makes the effective potential steeper as the internal volume shrinks.

Stabilization of the extra dimensions in brane gas cosmology with bulk flux

We consider the anisotropic evolution of spatial dimensions and the stabilization of internal dimensions in the framework of brane gas cosmology. We observe that the bulk RR field can give an effective potential which prevents the internal subvolume from collapsing. For a combination of (D−3)-brane gas wrapping the extra dimensions and 4-form RR flux in the unwrapped dimensions, it is possible that the wrapped subvolume has an oscillating solution around the minimum of the effective potential while the unwrapped subvolume expands monotonically. The flux gives a logarithmic bounce to the effective potential of the internal dimensions.

A RUNAWAY DIRECTION IN BRANE-GAS MODULI SPACE

Brane Gas Cosmology (BGC) is an M-theory motivated attempt to reconcile aspects of standard cosmology based on Einstein's theory of general relativity. The background in this framework is described by dilaton gravity, which introduces various moduli fields for the shape and size of the extra dimensions and the dilaton. Following previous successes in this field, we dimensionally reduce a gas of strings and branes to the d + 1-dimensional Einstein frame with the hopes of understanding late-time BGC. This procedure generates an effective potential for the moduli fields, which we analyze in the hopes of stabilizing all fields; however, with the inclusion of strings and branes alone we find one direction remains free to roll away.

Moduli stabilization in brane gas cosmology with superpotentials

In the context of brane gas cosmology in superstring theory, we showwhy it is impossible to simultaneously stabilize the dilaton and theradion with a general gas of strings (including massless modes) andD-branes. Although this requires invoking a different mechanism to stabilize these modulifields, we find that the brane gas can still play a crucial role in the early universe inassisting moduli stabilization. We show that a modest energy density of specific types ofbrane gas can solve the overshoot problem that typically afflicts potentials arising fromgaugino condensation.

Dynamical decompactification from brane gases in eleven-dimensional supergravity

Brane gas cosmology provides a dynamical decompactification mechanism that couldaccount for the number of spacetime dimensions we observe today. In this work we discussthis scenario taking into account the full bosonic sector of eleven-dimensional supergravity.We find new cosmological solutions that can dynamically explain the existence of threelarge spatial dimensions characterized by a universal asymptotic scaling behaviour and alarge number of initially unwrapped dimensions. This type of solutions enlarges thepossible initial conditions of the Universe in the Hagedorn phase and consequently canpotentially increase the probability of dynamical decompactification from anisotropicallywrapped backgrounds.

DILATON STABILIZATION IN BRANE GAS COSMOLOGY

Brane Gas Cosmology is an M-theory motivated attempt to reconcile aspects of the standard cosmology based on Einstein's theory of general relativity. Dilaton gravity, when incorporating winding p-brane states, has verified the Brandenberger–Vafa mechanism —a string-motivated conjecture which explains why only three of the nine spatial dimensions predicted by string theory grow large. Further investigation of this mechanism has argued for a hierarchy of subspaces, and has shown the internal directions to be stable to initial perturbations. These results, however, are dependent on a rolling dilaton, or varying strength of Newton's gravitational constant GN. In these proceedings we show that it is not possible to stabilize the dilaton and maintain the stability of the internal directions within the standard Brane Gas Cosmology setup.

Late time evolution of brane gas cosmology and compact internal dimensions

We study the late-time behavior of a universe in the framework of brane gas cosmology. We investigate the evolution of a universe with a gas of supergravity particles and a gas of branes. Considering the case when different dimensions are anisotropically wrapped by various branes, we have derived Friedman-like equations governing the dynamics of wrapped and unwrapped subvolumes. We point out that the compact internal dimensions are wrapped by three or higher dimensional branes.

Volume stabilization and acceleration in brane gas cosmology

We investigate toy cosmological models in(1+m+p)-dimensions withthe gas of p-braneswrapping over p-compact dimensions. In addition to winding modes, we consider the effects of momentummodes corresponding to small vibrations of branes and find that the extra dimensions aredynamically stabilized while the others expand. Adding matter, the compact volume maygrow slowly depending on the equation of state. We also obtain solutions with winding andmomentum modes where the observed space undergoes accelerated expansion.

UV perturbations in brane gas cosmology

We consider the effect of the ultraviolet (UV) or short wavelength modes on the background of Brane Gas Cosmology. We find that the string matter sources are negligible in the UV and that the evolution is given primarily by the dilaton perturbation. We also find that the linear perturbations are well behaved and the predictions of Brane Gas Cosmology are robust against the introduction of linear perturbations. In particular, we find that the stabilization of the extra dimensions (moduli) remains valid in the presence of dilaton and string perturbations.

Inflation and brane gases

We investigate a new way of realizing a period of cosmological inflation in the context of brane gas cosmology. It is argued that a gas of co-dimension one branes, out of thermal equilibrium with the rest of the matter, has an equation of state which can - after stabilization of the dilaton - lead to power-law inflation of the bulk. The most promising implementation of this mechanism might be in Type IIB superstring theory, with inflation of the three large spatial dimensions triggered by ``stabilized embedded 2-branes''. Possible applications and problems with this proposal are discussed.

Linear Perturbations in Brane Gas Cosmology

We consider the effect of string inhomogeneities on the time dependent background of Brane Gas Cosmology. We derive the equations governing the linear perturbations of the dilaton-gravity background in the presence of string matter sources. We focus on long wavelength fluctuations and find that there are no instabilities. Thus, the predictions of Brane Gas Cosmology are robust against the introduction of linear perturbations. In particular, we find that the stabilization of the extra dimensions (moduli) remains valid in the presence of dilaton and string perturbations.

Late-time dynamics of brane gas cosmology

Brane gas cosmology is a scenario inspired by string theory which proposes a simple resolution to the initial singularity problem and gives a dynamical explanation for the number of spatial dimensions of our universe. In this work we have studied analytically and numerically the late-time behaviour of these type of cosmologies taking a proper care of the annihilation of winding modes. This has help us to clarify and extend several aspects of their dynamics. We have found that the decay of winding states into non-winding states behaving like a gas of ordinary non-relativistic particles precludes the existence of a late expansion phase of the universe and obstructs the growth of three large spatial dimensions as we observe today. We propose a generic solution to this problem by considering the dynamics of a gas of non-static branes. We have also obtained a simple criterion on the initial conditions to ensure the small string coupling approximation along the whole dynamical evolution, and consequently, the consistency of an effective low-energy description. Finally, we have reexamined the general conditions for a loitering period in the evolution of the universe which could serve as a mechanism to resolve the {\sl brane problem} - a problem equivalent to the {\sl domain wall problem} in standard cosmology - and discussed the scaling properties of a self-interacting network of winding modes taking into account the effects of the dilaton dynamics.

Brane gas cosmology, M-theory and little string theory

We generalize the Brane Gas Cosmological Scenario to M-theory degrees of freedom, namely $M5$ and $M2$ branes. Without brane intersections, the Brandenberger Vafa(BV) arguments applied to M-theory degrees of freedom generically predict a large 6 dimensional spacetime. We show that intersections of $M5$ and $M2$ branes can instead lead to a large 4 dimensional spacetime. One dimensional intersections in 11D is related to (2,0) little strings (LST) on NS5 branes in type IIA. The gas regime of membranes in M-theory corresponds to the thermodynamics of LST obtained from holography. We propose a mechanism whereby LST living on the worldvolume of NS5 (M5)-branes wrapping a five dimensional torus, annihilate most efficiently in 3+1 dimensions leading to a large 3+1 dimensional spacetime. We also show that this picture is consistent with the gas approximation in M-theory.

Brane Gases on K3 and Calabi–Yau Manifolds

We initiate the study of Brane Gas Cosmology (BGC) on manifolds with nontrivial holonomy. Such compactifications are required within the context of superstring theory in order to make connections with realistic particle physics. We study the dynamics of brane gases constructed from various string theories on background spaces having a K3 submanifold. The K3 compactifications provide a stepping stone for generalizing the model to the case of a full Calabi–Yau threefold. Duality symmetries are discussed within a cosmological context. Using a duality, we arrive at an N=2 theory in four dimensions compactified on a Calabi–Yau manifold with SU(3) holonomy. We argue that the Brane Gas model compactified on such spaces maintains the successes of the trivial toroidal compactification while greatly enhancing its connection to particle physics. The initial state of the universe is taken to be a small, hot and dense gas of p-branes near thermal equilibrium. The universe has no initial singularity and the dynamics of string winding modes allow three spatial dimensions to grow large, providing a possible solution to the dimensionality problem of string theory.

Brane gas cosmology in M theory: Late time behavior

We investigate the late-time behavior of a universe containing a supergravity gas and wrapped 2-branes in the context of M theory compactified on ${T}^{10}.$ The supergravity gas tends to drive uniform expansion, while the branes impede the expansion of the directions about which they are wrapped. Assuming spatial homogeneity, we study the dynamics both numerically and analytically. At late times the radii obey power laws which are determined by the brane wrapping numbers, leading to interesting hierarchies of scale between the wrapped and unwrapped dimensions. The biggest hierarchy that could evolve from an initial thermal fluctuation produces three large unwrapped dimensions. We also study configurations corresponding to string winding, in which the M2-branes are all wrapped around the (small) 11th dimension, and show that this recovers the scenario discussed by Brandenberger and Vafa.

On T-duality in brane gas cosmology

In the context of homogeneous and isotropic superstring cosmology, the T-duality symmetry of string theory has been used to argue that for a background space–time described by dilaton gravity with strings as matter sources, the cosmological evolution of the Universe will be nonsingular. In this paper we discuss how T-duality extends to brane gas cosmology, an approximation in which the background space–time is again described by dilaton gravity with a gas of branes as a matter source. We conclude that the arguments for non-singular cosmological evolution remain valid.

Isotropization in brane gas cosmology

Brane Gas Cosmology (BGC) is an approach to unifying string theory and cosmology in which matter is described by a gas of strings and branes in a dilaton gravity background. The Universe is assumed to start out with all spatial dimensions compact and small. It has previously been shown that in this context, in the approximation of neglecting inhomogeneities and anisotropies, there is a dynamical mechanism which allows only three spatial dimensions to become large. However, previous studies do not lead to any conclusions concerning the isotropy or anisotropy of these three large spatial dimensions. Here, we generalize the equations of BGC to the anisotropic case, and find that isotropization is a natural consequence of the dynamics.