共查询到20条相似文献,搜索用时 15 毫秒
1.
S. K. Tripathy S. K. Nayak S. K. Sahu T. R. Routray 《International Journal of Theoretical Physics》2009,48(1):213-225
String cloud cosmological models are studied using spatially homogeneous and anisotropic Bianchi type VI0 metric in Saez-Ballester Scalar-Tensor theory of gravitation. The field equations are solved for massive string cloud with
particles attached to them. A more general linear equation of state of the cosmic string tension density with the proper energy
density of the universe is considered instead of taking any particular relationships like pure geometric string or the case
of the p-string. The pure geometric string and p-string solutions can be easily inferred from the models. For all viable models the possible limiting values of the linear
connection between the proper energy density and string tension density have been calculated. The physical and kinematical
properties of the models have been discussed in detail. 相似文献
2.
In this review, we discuss various cosmological issues related to our Universe from a string theoretic perspective. We analyse
the pre-big bang cosmological scenario which appears naturally in this context due to the existence of scale factor duality
symmetry in string theory. We then discuss some of the attractive and problematic features of this scenario. Finally, we introduce
a method which is powerful enough to search for cosmological solutions in various low energy limits of string theories. 相似文献
3.
Sumit R. Das 《Pramana》2007,69(1):93-108
In general relativity space-like or null singularities are common: they imply that ‘time’ can have a beginning or end. Well-known
examples are singularities inside black holes and initial or final singularities in expanding or contracting universes. In
recent times, string theory is providing new perspectives of such singularities which may lead to an understanding of these
in the standard framework of time evolution in quantum mechanics. In this article, we describe some of these approaches.
相似文献
4.
《Fortschritte der Physik》2017,65(10-11)
We study ‐dimensional half‐maximal flux backgrounds using exceptional field theory. We define the relevant generalised structures and also find the integrability conditions which give warped half‐maximal MinkowskiD and AdSD vacua. We then show how to obtain consistent truncations of type II / 11‐dimensional SUGRA which break half the supersymmetry. Such truncations can be defined on backgrounds admitting exceptional generalised structures, where , and N is the number of vector multiplets obtained in the lower‐dimensional theory. Our procedure yields the most general embedding tensors satisfying the linear constraint of half‐maximal gauged SUGRA. We use this to prove that all half‐maximal warped AdSD and MinkowskiD vacua of type II / 11‐dimensional SUGRA admit a consistent truncation keeping only the gravitational supermultiplet. We also show to obtain heterotic double field theory from exceptional field theory and comment on the M‐theory / heterotic duality. In five dimensions, we find a new SO(5, N ) double field theory with a ‐dimensional extended space. Its section condition has one solution corresponding to 10‐dimensional supergravity and another yielding six‐dimensional SUGRA. 相似文献
5.
6.
Unconstrained local Lagrangians for higher-spin gauge theories are bound to involve auxiliary fields, whose integration in the partition function generates geometric, effective actions expressed in terms of curvatures. When applied to the triplets, emerging from the tensionless limit of open string field theory, the same procedure yields interesting alternative forms of geometric Lagrangians, expressible for both bosons and fermions as squares of field-strengths. This shows that higher-spin curvatures might play a role in the dynamics, regardless of whether the Fronsdal–Labastida constraints are assumed or forgone. 相似文献
7.
A study on string theory has been done in five dimensional flat space-time. Barotropic equation of state andp-string model are discussed. Also a polynomial relation between the two scale factors is assumed. In some special cases the
solution reduces to generalized Kasner metric. Further diminision of extra dimension with the evolution of universe is exhibited.
A detailed study of phase-space analysis is done for geometric string model. 相似文献
8.
We present the gravitational coupling function and potential function in a General Scalar Tensor Theory from Noether symmetry approach. Some exact cosmological solutions are presented in the spatially flat, homogeneous and isotropic background. The solutions are exponentially expanding without any graceful exit from inflation. Also the solutions asymptotically reduce to Einstein's theory with finite value of the coupling function supporting the earlier claim of Santiago et al. Further it is shown that in a particular case the effective Newtonian constant is determined by the effective potential of the scalar field. 相似文献
9.
We compute the greybody factors for classical black holes in a domain where two kinds of charges and their anticharges are excited by the extra energy over extremality. We compare the result to the greybody factors expected from an effective string mode] which was earlier shown to give the correct entropy. In the regime where the left- and right-moving temperatures are much smaller than the square root of the effective string tension, we find a non-trivial greybody factor which agrees with the effective string mode]. However, if the temperatures are comparable with the square root of the effective string tension, the greybody factors agree only at the leading order in energy. Nevertheless, there are several interesting relations between the two results, suggesting that a modification of the effective string mode] might lead to better agreement. 相似文献
10.
11.
We review the status of string theory as a quantum theory of gravity. Our emphasis is on outstanding questions and remaining
challenges rather then on well-established results and successes. 相似文献
12.
Archan Kumar De Sisir Bhanja Subenoy Chakraborty 《General Relativity and Gravitation》2004,36(4):863-874
We present exact solutions of the gravitational field equations in the generalized Randall-Sundrum model for an anisotropic brane with Bianchi I & V geometry and string dust as the matter source. We assume the Weyl tensor in the bulk has vanishing projection on the brane and examine the different equations of state, for the string dust system. Exact analytic solutions are possible only in few cases. 相似文献
13.
String-dust cosmology in an inhomogeneous cylindrically symmetric model is considered. Solutions are obtained only for geometric
string with the separability assumption for metric coefficients. 相似文献
14.
CHEN Bin ZHANG Jun-Feng 《理论物理通讯》2005,44(3):463-472
We construct the two-flux colliding plane wave solutions in higher-dimensional gravity theory with dilaton, and two complementary fluxes. Two kinds of solutions have been obtained: Bell-Szekeres (BS) type and homogeneous type. After imposing the junction condition, we find that only the BS type solution is physically well-defined. Furthermore, we show that the future curvature singularity is always developed for our solutions. 相似文献
15.
CHEN Bin ZHANG Jun-Feng 《理论物理通讯》2005,44(9)
We construct the two-flux colliding plane wave solutions in higher-dimensional gravity theory with dilaton,and two complementary fluxes. Two kinds of solutions have been obtained: Bell-Szekeres (BS) type and homogeneous type. After imposing the junction condition, we find that only the BS type solution is physically well-defined. Furthermore, we show that the future curvature singularity is always developed for our solutions. 相似文献
16.
R. Cartas-Fuentevilla 《International Journal of Theoretical Physics》2006,45(9):1659-1666
Using covariant phase space formulations for the natural topological invariants associated with the world-surface in closed string theory, we find that certain Wilson loops defined on the world-surface and that preserve topological invariance, correspond to wave functionals for the vacuum state with zero energy. The differences and similarities with the 2-dimensional QED proposed by Schwinger early are discussed.
PACS Numbers : 81T30, 81T45 相似文献
17.
Bulk viscous fluid distribution with massive strings in LRS Bianchi type-1 space time is studied. The exact solutions of the
field equations are obtained by using the equation of state ρ=−λ and ρ=λ. We observed that the bulk viscous fluid does not survive for ρ=−λ whereas it survives for ρ=λ. Some physical and geometrical properties of the models are discussed. 相似文献
18.
Some cosmological solutions for string model are derived in higher dimensional spherically symmetric space-time, following
the techniques used by Letelier. The equations of state for strings have been used for different solutions. Also polynomial
relation between the metric coefficients has been assumed in some cases. 相似文献
19.
20.
Janadeva Maharana 《Pramana》2000,54(4):629-638
There has been many interesting developments in string theory in last couple of years. The purpose of this article is to present
a brief account of the progress made in string theory. The two invited talks by S R Das and S Mukhi in this volume contain
more detailed accounts of our understanding of black hole physics and the intimate connections between string theory and gauge
theories. 相似文献