scholarly journals A NONSINGULAR UNIVERSE IN STRING COSMOLOGY

1999 ◽  
Vol 08 (01) ◽  
pp. 43-49 ◽  
Author(s):  
A. A. AL-NOWAISER ◽  
MURAT ÖZER ◽  
M. O. TAHA
Keyword(s):  
General Relativity ◽  
Central Charge ◽  
Energy Condition ◽  
Homogeneous Solution ◽  
Low Energy ◽  
Leading Order ◽  
Tensor Fields ◽  
Strong Energy Condition ◽  
Four Dimensions ◽  
Strong Energy

We consider the low-energy effective string action in four dimensions including the leading order-α′ terms. An exact homogeneous solution is obtained. It represents a nonsingular expanding cosmological model in which the tensor fields tend to vanish as t→∞. The scale factor a(t) of the very early universe in this model has the time dependence [Formula: see text]. The violation of the strong energy condition of classical General Relativity to avoid the initial singularity requires that the central charge deficit of the theory be larger than a certain value. The significance of this solution is discussed.

scholarly journals INHOMOGENEOUS SCALAR FIELD SOLUTIONS AND INFLATION

1999 ◽  
Vol 08 (06) ◽  
pp. 705-718 ◽  
Author(s):  
S. E. PEREZ BERGLIAFFA ◽  
K. E. HIBBERD
Keyword(s):  
Scalar Field ◽  
String Theory ◽  
General Framework ◽  
Equations Of Motion ◽  
Energy Condition ◽  
Low Energy ◽  
Strong Energy Condition ◽  
Strong Energy

We present new exact cosmological inhomogeneous solutions for gravity coupled to a scalar field in a general framework specified by the parameter λ. The equations of motion (and consequently the solutions) in this framework correspond to either low-energy string theory or Weyl integrable spacetime according to the sign of λ. We show that different inflationary behaviors are possible, as suggested by the study of the violation of the strong energy condition. Finally, by the analysis of certain curvature scalars we found that some of the solutions may be nonsingular.

scholarly journals A FOUR-DIMENSIONAL SUPERSTRING FROM THE BOSONIC STRING WITH SOME APPLICATIONS

2005 ◽  
Vol 20 (01) ◽  
pp. 99-128 ◽  
Author(s):  
B. B. DEO ◽  
L. MAHARANA
Keyword(s):  
Standard Model ◽  
Central Charge ◽  
Virasoro Algebra ◽  
Low Energy ◽  
Majorana Fermions ◽  
Modular Invariance ◽  
Model Group ◽  
Supersymmetry Algebra ◽  
Four Dimensions ◽  
Bosonic Representation

A string in four dimensions is constructed by supplementing it with 44 Majorana fermions. The later are represented by 11 vectors in the bosonic representation SO (D-1,1). The central charge is 26. The fermions are grouped in such a way that the resulting action is worldsheet supersymmetric. The energy–momentum and current generators satisfy the super-Virasoro algebra. GSO projections are necessary for proving modular invariance. Space–time supersymmetry algebra is deduced and is substantiated for specific modes of zero mass. The symmetry group of the model can descend to the low energy standard model group SU (3)× SU L(2)× U Y(1) through the Pati–Salam group.

Another Possible Abnormality of Compact Space-Time

1975 ◽  
Vol 18 (5) ◽  
pp. 695-697 ◽  
Author(s):  
K. K. Lee
Keyword(s):  
Compact Space ◽  
Energy Condition ◽  
Space Time ◽  
Energy Conditions ◽  
Strong Energy Condition ◽  
Lorentz Structure ◽  
Strong Energy

AbstractIt is shown that the Lorentz structure of a compact prespace- time M can be so chosen such that M can not satisfy the strong energy condition. Thus, combining both the causal and the strong energy conditions, a stronger case against the compact space-times as proper arenas of physics can be made.

scholarly journals USING (4+1) SPLIT AND ENERGY CONDITIONS TO STUDY THE INDUCED MATTER IN 5D RICCI-FLAT COSMOLOGY

2007 ◽  
Vol 22 (05) ◽  
pp. 985-994 ◽  
Author(s):  
YONGLI PING ◽  
HONGYA LIU ◽  
LIXIN XU
Keyword(s):  
Energy Condition ◽  
Energy Conditions ◽  
Dominant Energy Condition ◽  
Strong Energy Condition ◽  
Ricci Flat ◽  
Strong Energy ◽  
Expansion Of The Universe ◽  
Frw Models ◽  
Induced Matter ◽  
The Universe

We use (4+1) split to derive the 4D induced energy density ρ and pressure p of the 5D Ricci-flat cosmological solutions which are characterized by having a bounce instead of a bang. The solutions contain two arbitrary functions of time t and, therefore, are mathematically rich in giving various cosmological models. By using four known energy conditions (null, weak, strong, and dominant) to pick out and study physically meaningful solutions, we find that the 4D part of the 5D solutions asymptotically approaches to the standard 4D FRW models and the expansion of the universe is decelerating for normal induced matter for which all the four energy conditions are satisfied. We also find that quintessence might be normal or abnormal, depending on the parameter w of the equation of state. If -1 ≤ w < -1/3, the expansion of the universe is accelerating and the quintessence is abnormal because the strong energy condition is violated while other three are satisfied. For phantom, all the four energy conditions are violated. Before the bounce, all the four energy conditions are violated, implying that the cosmic matter before the bounce could be explained as a phantom that has a large negative pressure and makes the universe bouncing. In the early times after the bounce, the dominant energy condition is violated, while the other three are satisfied, and so the cosmic matter could be explained as a super-luminal acoustic matter.

scholarly journals NONLINEAR EQUATION OF STATE, COSMIC ACCELERATION AND DECELERATION DURING PHANTOM DOMINANCE

2009 ◽  
Vol 18 (02) ◽  
pp. 329-345 ◽  
Author(s):  
S. K. SRIVASTAVA ◽  
J. DUTTA
Keyword(s):  
Equation Of State ◽  
Nonlinear Equation ◽  
Energy Condition ◽  
Phantom Energy ◽  
Weak Energy Condition ◽  
Cosmic Acceleration ◽  
Strong Energy Condition ◽  
Strong Energy ◽  
Acceleration And Deceleration ◽  
The Universe

In this paper, the RS-II model of brane gravity is considered for the phantom universe using a nonlinear equation of state. Phantom fluid is known to violate the weak energy condition. It is found that this characteristic of phantom energy is affected drastically by the negative brane tension λ of the RS-II model. It is interesting to see that up to a certain value of energy density ρ satisfying ρ/λ < 1, the weak energy condition is violated and the universe superaccelerates. But, as ρ increases more, only the strong energy condition is violated and the universe accelerates. When 1 < ρ/λ < 2, even the strong energy condition is not violated and the universe decelerates. Expansion of the universe stops when ρ = 2 λ. This is contrary to earlier results of the phantom universe exhibiting acceleration only.

scholarly journals A NEW SINGULARITY THEOREM IN RELATIVISTIC COSMOLOGY

2000 ◽  
Vol 15 (06) ◽  
pp. 391-395 ◽  
Author(s):  
A. K. RAYCHAUDHURI
Keyword(s):  
Ricci Tensor ◽  
Energy Condition ◽  
Space Time ◽  
Relativistic Cosmology ◽  
Raychaudhuri Equation ◽  
Singularity Theorem ◽  
Strong Energy Condition ◽  
Strong Energy

It is shown that if the time-like eigenvector of the Ricci tensor is hypersurface orthogonal so that the space–time allows a foliation into space sections, then the space average of each of the scalars that appears in the Raychaudhuri equation vanishes provided that the strong energy condition holds good. This result is presented in the form of a singularity theorem.

scholarly journals Strong energy condition and the repulsive character of f(R) gravity

2017 ◽  
Vol 49 (4) ◽  
Author(s):  
Crislane S. Santos ◽  
Janilo Santos ◽  
Salvatore Capozziello ◽  
Jailson S. Alcaniz
Keyword(s):  
Energy Condition ◽  
Strong Energy Condition ◽  
Strong Energy

scholarly journals PHANTOM COSMOLOGY WITH A NONLINEAR BORN–INFELD TYPE SCALAR FIELD

2005 ◽  
Vol 14 (02) ◽  
pp. 355-362 ◽  
Author(s):  
H. Q. LU
Keyword(s):  
Dark Energy ◽  
Scalar Field ◽  
Equation Of State ◽  
Energy Condition ◽  
State Parameter ◽  
Strong Energy Condition ◽  
Strong Energy ◽  
Accelerating Expansion ◽  
Equation Of State Parameter ◽  
The Universe

Recent many physicists suggest that the dark energy in the universe might result from the Born–Infeld (B–I) type scalar field of string theory. The universe of B–I type scalar field with potential can undergo a phase of accelerating expansion. The corresponding equation of state parameter lies in the range of -1<ω<-⅓. The equation of state parameter of B–I type scalar field without potential lies in the range of 0≤ω≤1. We find that weak energy condition and strong energy condition are violated for phantom B–I type scalar field. The equation of state parameter lies in the range of ω<-1.

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