Hindawi Publishing Corporation Advances in Mathematical Physics Volume 2013, Article ID 892361, 5 pages http://dx.doi.org/10.1155/2013/892361
Research Article LRS Bianchi Type II Massive String Cosmological Models with Magnetic Field in Lyraβs Geometry Raj Bali,1 Mahesh Kumar Yadav,2 and Lokesh Kumar Gupta1 1 2
Department of Mathematics, University of Rajasthan, Jaipur 302004, India Department of Mathematics, Dr. H.S. Gour Central University, Sagar 470003, India
Correspondence should be addressed to Raj Bali;
[email protected] Received 10 May 2013; Accepted 17 September 2013 Academic Editor: Shri Ram Copyright Β© 2013 Raj Bali et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Bianchi type II massive string cosmological models with magnetic field and time dependent gauge function (ππ ) in the frame work of Lyraβs geometry are investigated. The magnetic field is in YZ-plane. To get the deterministic solution, we have assumed that the shear (π) is proportional to the expansion (π). This leads to π
= ππ , where R and S are metric potentials and n is a constant. We find that the models start with a big bang at initial singularity and expansion decreases due to lapse of time. The anisotropy is maintained throughout but the model isotropizes when π = 1. The physical and geometrical aspects of the model in the presence and absence of magnetic field are also discussed.
1. Introduction Bianchi type II space time successfully explains the initial stage of evolution of universe. Asseo and Sol [1] have given the importance of Bianchi type II space time for the study of universe. The string theory is useful to describe an event at the early stage of evolution of universe in a lucid way. Cosmic strings play a significant role in the structure formation and evolution of universe. The presence of string in the early universe has been explained by Kibble [2], Vilenkin [3], and Zelβdovich [4] using grand unified theories. These strings have stress energy and are classified as massive and geometric strings. The pioneer work in the formation of energy momentum tensor for classical massive strings is due to Letelier [5] who explained that the massive strings are formed by geometric strings (Stachel [6]) with particle attached along its extension. Letelier [5] first used this idea in obtaining some cosmological solutions for massive string for Bianchi type I and Kantowski-Sachs space-times. Many authorsβ namely, Banerjee et al. [7], Tikekar and Patel [8, 9], Wang [10], and Bali et al. [11β14], have investigated string cosmological models in different contexts. Einstein introduced general theory of relativity to describe gravitation in terms of geometry and it helped
him to geometrize other physical fields. Motivated by the successful attempt of Einstein, Weyl [15] made one of the best attempts to generalize Riemannian geometry to unify gravitation and electromagnetism. Unfortunately Weylβs theory was not accepted due to nonintegrability of length. Lyra [16] proposed a modification in Riemannian geometry by introducing gauge function into the structureless manifold. This modification removed the main obstacle of the Weyl theory [15]. Sen [17] formulated a new scalar tensor theory of gravitation and constructed an analogue of Einstein field equations based on Lyra geometry. Halford [18] pointed out that the constant vector field (π½) in Lyra geometry plays a similar role of cosmological constant (Ξ) in general theory of relativity. The scalar tensor theory of gravitation in Lyra geometry predicts the same effects within the observational limits as in the Einstein theory. The main difference between the cosmological theories based on Lyra geometry and Riemannian geometry lies in the fact that the constant displacement vector (π½) arises naturally from the concept of gauge in Lyra geometry whereas the cosmological constant (Ξ) was introduced by Einstein in an ad hoc manner to find static solution of his field equations. Many authors, namely, Beesham [19], T. Singh and G. P. Singh [20], Chakraborty and Ghosh [21], Rahaman and Bera [22], Pradhan et al. [23, 24],
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Bali and Chandnani [25, 26], and Ram et al. [27], have studied cosmological models in the frame work of Lyraβs geometry. The present day magnitude of magnetic field is very small as compared to estimated matter density. It might not have been negligible during early stage of evolution of universe. Asseo and Sol [1] speculated a primordial magnetic field of cosmological origin. Vilenkin [3] has pointed out that cosmic strings may act as gravitational lensing. Therefore, it is interesting to discuss whether it is possible to construct an analogue of cosmic string in the presence of magnetic field in the frame work of Lyraβs geometry. Recently, Bali et al. [28] investigated Bianchi type I string dust magnetized cosmological model in the frame work of Lyraβs geometry. In this paper, we have investigated LRS Bianchi type II massive string cosmological models with magnetic field in Lyraβs geometry. We find that it is possible to construct an analogue of cosmic string solution in presence of magnetic field in the frame work of Lyra geometry. The physical and geometrical aspects of the model together with behavior of the model in the presence and absence of magnetic field are also discussed.
π
The electromagnetic field tensor πΈπ given by Lichnerowicz [29] is given as 1 π π πΈπ = π [|β|2 (Vπ Vπ + ππ ) β βπ βπ ] , 2
with π being the magnetic permeability and βπ the magnetic flux vector defined by βπ =
πΉππ;π + πΉππ;π + πΉππ;π = 0
ππ 2 = πππ ππ ππ ,
(1)
where π11 = π22 = π33 = 1, π1 = π
ππ₯,
(2)
4
π = π
ππ§,
π = ππ‘.
2
ππ = β ππ‘ + π
ππ₯
(9)
which leads to
2 2
2
2
+ π (ππ¦ β π₯ ππ§) + π
ππ§ ,
π
ππ = πVπ Vπ β ππ₯π π₯π + πΈπ .
β1 =
(3)
where π
and π are functions of π‘ alone. π Energy momentum tensor ππ for string dust in the presence of magnetic field is given by π
ππΉ23 =0 πt (since πΉ23 is the only nonvanishing component
(10)
For π = 1, (7) leads to 2
2
(8)
leads to
πΉ23 = constant = π» (say) .
Thus the metric (1) leads to 2
(7)
and πΉππ = β πΉππ )
π44 = (β1) ,
π2 = π (ππ¦ β π₯ ππ§) ,
3
ββπ β πΉπβ Vπ , 2π πππβ
where πΉπβ is the electromagnetic field tensor and βπππβ the Levi-Civita tensor density. We assume that the current is flowing along the π₯-axis, so magnetic field is in π¦π§-plane. Thus β1 =ΜΈ 0, β2 = 0 = β3 = β4 , and πΉ23 is the only nonvanishing component of πΉππ . This leads to πΉ12 = 0 = πΉ13 by virtue of (7). We also find πΉ14 = 0 = πΉ24 = πΉ34 due to the assumption of infinite electrical conductivity of the fluid (Maartens [30]). A cosmological model which contains a global magnetic field is necessarily anisotropic since the magnetic vector specifies a preferred spatial direction (Bronnikov et al. [31]). The Maxwellβs equation
2. The Metric and Field Equations We consider Locally Rotationally Symmetric (LRS) Bianchi type II metric as
(6)
(4)
π» . ππ
(11)
π
Now the components of πΈπ corresponding to the line element (3) are as follows: πΈ11 = β
π»2 = βπΈ22 = βπΈ33 = πΈ44 . 2ππ
2 π2
(12)
Einsteinβs modified field equation in normal gauge for Lyraβs manifold obtained by Sen [17] is given by
Now the modified Einsteinβs field equations (5) for the metric (3) lead to
1 π 3 3 π π π π
π β π
ππ + ππ ππ β ππ ππ ππ = βππ 2 2 4 (in geometrized units, where 8ππΊ = 1, π = 1) ,
π
π π
π π2 πΎ 3 + 4 4 + 44 + 44 + π½2 = π + 2 2 4π
4 π
π π
π 4 π
π 2 2π
3π2 π
πΎ 3 β 4 + 42 + 44 + π½2 = β 2 2 4π
π
π
4 π
π π
2 3 2π
π πΎ π2 β 4 + 4 4 + 42 β π½2 = π + 2 2 , 4π
π
π π
4 π
π
π
(5)
where Vπ = (0, 0, 0, β1); V Vπ = β1; ππ = (0, 0, 0, π½(π‘)); V4 = β1; V4 = 1 β
π is the matter density, π the cloud stringβs tension π density, Vπ the fluid flow vector, π½ the gauge function, πΈπ the π electromagnetic field tensor and π₯ the space like 4-vectors representing the stringβs direction.
(13) (14) (15)
where πΎ = π»2 /2π and the direction of string is only along the π₯-axis so that π₯1 π₯1 = 1, π₯2 π₯2 = 0 = π₯3 π₯3 .
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3 π
The energy conservation equation ππ;π = 0 leads to π4 + π (
2π
4 π4 π
+ )βπ 4 =0 π
π π
Now we assume that (16)
1 π 3 3 π β π
ππ ) + (ππ ππ );π β (ππ ππ ππ );π = 0 2 2 4 ;π
(25)
π44 = ππσΈ ,
(26)
ππ . ππ
(27)
Thus
and conservation of left hand side of (5) leads to π (π
π
π4 = π (π) .
(17) where
which again leads to
πσΈ =
π
ππ ππ 3 3 π + πβ Ξβπ ] + ππ [ ππ β πβ Ξππβ ] π[ 2 π ππ₯π 2 ππ₯ ππ 3 π π β ππ ππ [ ππ + πβ Ξβπ ] 4 ππ₯
Therefore, (24) leads to (18)
ππ2 3π β 2 2 + π ππ π
πππ 3 π β ππ ππ [ π β πβ Ξππβ ] = 0. 4 ππ₯
3 β4π+3 3πΌ2 β4πβ1 πΎ β2πβ1 = β β π π π 4π 4π π
Equation (18) is automatically satisfied for π = 1, 2, 3. For π = π = 4, (18) leads to
which again leads to π2 =
3 π 4 ] π½ [ (π44 π4 ) + π4 Ξ44 2 ππ‘ ππ 3 4 ] + π44 π4 [ 4 β π4 Ξ44 2 ππ‘ ππ 3 4 ] β π44 π4 [ 4 + π4 Ξ44 4 ππ‘
3 π4β4π 4π (2 β π)
(19)
Equation (29) leads to π=(
βπΏπ4 β ππ2π + π ππ , )= ππ‘ π2π
2π
π 3 3 π½π½4 + π½2 ( 4 + 4 ) = 0 2 2 π
π
(20)
ππ = (0, 0, 0, π½ (π‘)) .
(21)
where
For the complete determination of the model of the universe, we assume that the shear tensor (π) is proportional to the expansion (π) which leads to
ππ 2 = β (
(22)
=
3 β4π+3 3πΌ β4πβ1 πΎ β2πβ1 β β π . π π 4π 4π π
π
= ππ ,
(32)
ππ‘ 2 2 ) ππ + π
2 ππ₯2 ππ 2
2
2
(33) 2
+ π (ππ¦ β π₯ππ§) + π
ππ§ which again leads to
(23) ππ 2 = β
with πΌ being constant of integration. Using (22) and (23) in (14), we have
2
(31)
where π = π. Using (30) and (32), the metric (3) leads to
From (20), we have
π2 2π44 + (3π β 2) 4 π
π
= ππ which leads to
3. Solution of Field Equations
πΌ , π
2 π
(30)
where πΏ = 3/(4π(2βπ)), π = 3πΌ2 /(4π(π+2)), π = πΎ/(π(πβ 2)), π = π, a new coordinate is used, and πΌ =ΜΈ 0. By (22), we have
which again leads to
π½=
(29)
3πΌ2 πΎ πβ4π β πβ2π . + 4π (π + 2) π (π β 2)
ππ 3 4 ]=0 β π44 π44 π4 [ 4 β π4 Ξ44 2 ππ‘
π
= ππ .
(28)
ππ2 πΏπ4β4π β ππβ2π + ππβ4π 2π
2
2
2
(34) 2
+ π (ππ₯ + ππ§ ) + π (ππ¦ β π₯ππ§) , (24)
where the cosmic time π‘ is defined as π‘=β«
ππ . πΏπ4β4π β ππβ2π + ππβ4π
(35)
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4. Some Physical and Geometrical Features
5. Model in Absence of Magnetic Field
Using (22), (23), (30), and (32) in (15), we have
To discuss the model in the absence of the magnetic field, we put πΎ = 0 in (29) and have
π = π΄π2β4π + π΅πβ2πβ2 ,
(36)
where π΄ = (π + 1)/(2 β π) and π΅ = 2ππ/(2 β π). Similarly from (15), the string tension density π is given as π = ππ2β4π + ππβ2πβ2 + ππβ4πβ2 ππ = π β π = (π΄ β π) π(2β4π)
(38)
π½=
3 , 4π (2 β π)
πΌ π2π+1
.
π=
2π
4 π4 + π
π
(39)
(40)
(41)
1 π
4 π4 ( β ) β3 π
π
(42)
which leads to (π β 1) β 4 πΏ π β ππ2π + π. β3 π2π+1
(43)
The deceleration parameter π is given by π= β
π
44 /π
π
42 /π
2
(44)
which leads to π= 2+
1 ππ2π (π + 2) + 2π [ ] π πΏπ4 β ππ2π + π
1 ππ2π (π + 2) + 2π ]. = 2+ [ π πΏπ4 β ππ2π + π
(47)
(48)
(49)
Using (48) and (49) in metric (3), we get ππ‘ 2 2 ) ππ + π
2 ππ₯2 ππ 2
2
(50) 2
+ π (ππ¦ β π₯ππ§) + π
ππ§ which again leads to
ππ2 + π2π ππ₯2 + ππβ4π 2 + π2 (ππ¦ β π₯ππ§) + π2π ππ§2 . πΏπ4β4π
(51)
In this case, the energy density (π), the string tension density (π), gauge function (π½), the expansion (π), shear (π), and deceleration parameter (π) are given by π + 1 β4πβ2 π 2βπ (2π β 3) (π + 1) β4π+2 π= π 2π (π β 2) ππ = π β π πΌ π½= 2 π
π πΌ = 2π+1 π 2π
4 π4 π= + π
π (2π + 1) β 4 πΏπ + π = π2π+1 1 π
4 π4 π= ( β ) β3 π
π (π β 1) β 4 πΏπ + π = β3π2π+1 π
/π
π = β 244 2 π
4 /π
π=
Shear (π) is given by π=
3πΌ2 . 4π (π + 2)
π
= ππ .
ππ 2 = β
which leads to (2π + 1) β 4 πΏπ β ππ2π + π. π2π+1
π=
βπΏπ4 + π ππ , = π4 = ππ‘ π2π
2
The expansion (π) is given as
π =
πΏ=
ππ 2 = β (
Equation (23) gives
π=
where
where π = π, and a new coordinate is used. By (22), we have
where πΎ (3 β π) (π + 1) (3 β 2π) , π= , 2π (2 β π) (π β 2) 3πΌ2 (π + 4) . π= (π + 2)
(46)
Equation (45) leads to (37)
+ (π΅ β π) π(β2πβ2) β ππ(β4πβ2) ,
π=
π2 = πΏπ4β4π + ππβ4π ,
(45)
= π2 + π (
π β πΏπ4 ). π + πΏπ4
(52)
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6. Discussion Model (34) in the presence of magnetic field starts with a big bang at π = 0 and the expansion in the model decreases as π increases. The spatial volume increases as π increases. Thus inflationary scenario exists in the model. The model has point-type singularity at π = 0 where π > 0. Since π/π =ΜΈ 0, hence anisotropy is maintained throughout. However, if π = 1, then the model isotropizes. The displacement vector π½ is initially large but decreases due to lapse of time where 2π+1 > 0; however, π½ increases continuously when 2π + 1 < 0. The matter density π > 0 when 0 < π < 2. Model (51) starts with a big bang at π = 0 when π = β1/2 and the expansion in the model decreases as time increases. The displacement vector (π½) is initially large but decreases due to lapse of time. The model (51) has point-type singularity at π = 0, where π > 0. Since π/π =ΜΈ 0, hence anisotropy is maintained throughout. However, if π = 1, then the model isotropizes. Thus it is possible to construct globally regular Bianchi type II solutions with displacement vector (π½) using geometric condition shear which is proportional to expansion.
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