A General Exact Solution of the Einstein–Dirac Equations with The
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Journal of Experimental and Theoretical Physics, Vol. 98, No. 4, 2004, pp. 619–628. Translated from Zhurnal Éksperimental’noÏ i TeoreticheskoÏ Fiziki, Vol. 125, No. 4, 2004, pp. 707–716. Original Russian Text Copyright © 2004 by Zhelnorovich. GRAVITATION, ASTROPHYSICS A General Exact Solution of the Einstein–Dirac Equations with the Cosmological Constant in Homogeneous Space V. A. Zhelnorovich Institute of Mechanics, Moscow State University, Vorob’evy gory, Moscow, 119992 Russia e-mail: [email protected] Received May 29, 2003 Abstract—We have obtained a general exact solution of the system of Einstein–Dirac equations with the cos- mological constant in homogeneous Riemannian space of the first type according to the Bianchi classification. © 2004 MAIK “Nauka/Interperiodica”. 1. INTRODUCTION ant way. Using this gauge allows the number of unknown functions in the Einstein–Dirac equations to We face two problems when integrating the Ein- be reduced by six, while keeping the equations invari- stein–Dirac equations. ant under transformation of the observer’s coordinate The first, purely technical problem stems from the system. fact that the Einstein–Dirac equations constitute a com- plex system of nonlinear partial differential equations With the tetrad gauge used here, all of the equations of the second order for 24 unknown functions. Previ- can be written as equations of the first order only for ously, some of the particular exact solutions [1–6] to two invariants of the spinor field and the Ricci rotation the Einstein–Dirac equations in homogeneous space symbols of the proper vector bases determined by the have been obtained only for diagonal metrics of the spinor field. The tetrad gauge transforms the Dirac Riemannian event space. equations to equations for the Ricci rotation symbols and for the spinor field invariants, with the Ricci rota- The second problem is fundamental in nature and tion symbols in the Dirac equations being linear and ψ stems from the fact that the spinor field functions in without derivatives. Therefore, the Dirac equations in the Riemannian event space can be determined only in homogeneous Riemannian space close the system of certain nonholonomic orthonormal bases (tetrads) Einstein equations for the Ricci rotation symbols with- which that must be specified, or a tetrad gauge is said to out using additional equations. In this case, we can first be needed. A large number of such gauges are known, integrate the first-order equations for the Ricci rotation and different authors have suggested various gauges. symbols and the spinor field invariants and then inte- All of these are either noninvariant under transforma- grate the first-order equations for the tetrad (Lamé) tion of the variables of the observer’s coordinate system coefficients. or are written in the form of differential equations, which complicates the initial system of equations. These two factors—the reduction in the number of unknown functions by six and the possibility of inte- Physically, all gauges are equivalent, because the grating the second-order equations in two steps — con- Einstein–Dirac equations are invariant under the choice siderably simplify the Einstein–Dirac equations. As a of tetrads. Mathematically, however, using a bad gauge result, it becomes possible to obtain new exact solu- (i.e., additional equations that close the Einstein–Dirac tions of these equations. equations) can greatly complicate the equations, while using a good gauge can significantly simplify them. A general exact solution of the Einstein–Dirac equa- tions in homogeneous Riemannian space of the first In many respects, the problem of choosing a reason- type according to the Bianchi classification was able tetrad gauge stems from the fact that the solutions obtained in [7–9]. Recently, new papers in which the of the Einstein–Dirac equations have previously been authors discuss models described by the Einstein equa- obtained only for diagonal metrics. Since the basis vec- tions with the cosmological constant (including those tors of a holonomic coordinate system for such metrics with the spinor fields) have appeared. In this paper, we are orthogonal, the tetrads associated with the orthogo- obtain a general exact solution to the system of Ein- nal holonomic basis of the Riemannian space can be stein–Dirac equations with the cosmological constant chosen naturally. in homogeneous Riemannian space in connection with In this paper, we use a new tetrad gauge [7] that is increasing interest in studying the role of the cosmolog- algebraic and, at the same time, is formed in an invari- ical constant. 1063-7761/04/9804-0619 $26.00 © 2004 MAIK “Nauka/Interperiodica” 620 ZHELNOROVICH 2. THE SPINOR FIELDS complex conjugation; the invariant spinor of the second IN FOUR-DIMENSIONAL RIEMANNIAN SPACE rank β is given by the equations Let V be the four-dimensional Riemannian space γ T βγ β–1 β˙ T β with metric signature (+, +, +, –) referred to a coordi- ˙ a ==– a , . (4) nate system with variables xi and with a holonomic vec- tor basis Ji (i = 1, 2, 3, 4). We specify the metric tensor In general, the four-component spinor field ψ has of space V in basis Ji by the covariant components gij; two real invariants, ρ and η, that can be determined by the connectivity is defined by the Christoffel symbols using the equation Γs . In space V, we introduce a smooth field of ij + + 5 i ρexp()iη = ψ ψ + iψ γ ψ, (5) orthonormal bases (tetrads) ea(x ) (a = 1, 2, 3, 4) in the form where i a ea ==ha Ji, Ji hi ea, (1) γ 5 1 εabcdγ γ γ γ = ------ a b c d, a i 24 where hi and ha are the scale factors. Below, we denote the indices of the tensor components calculated εabcd are the components of the four-dimensional Levi– in basis Ji by the Latin letters i, j, k, …, and the indices Civita pseudotensor, ε1234 = –1. Using the spinor field ψ of the tensor components calculated in the orthonormal and the conjugate spinor field ψ+ in Riemannian space bases ea by the first Latin letters a, b, c, d, e, and f. V, we can determine the proper orthonormal vector The differential of the vectors of the orthonormal basis e˘ of the spinor field: i a basis ea(x ) is defined by the Ricci rotation symbols b i i i i i de = ∆ , e dx , which can be expressed in terms of the π ξ σ a ia b ˘e1 ====Ji, ˘e2 Ji, e˘ 3 Ji, e˘4 u Ji. scale factors The vector components πi, ξi, σi, and ui are specified by ∆ 1[ j ()∂ ∂ the relations [7, 10] iac, = --- hc ih ja – jhia 2 (2) j ()∂ ∂ b j s ()]∂ ∂ ρπi ==Im()ψT Eγ iψ , ρξi Re()ψT Eγ iψ , – ha ih jc – jhic + hi hahc jhsb – sh jb . (6) ρσi ψ+γ iγ 5ψ ρ i ψ+γ iψ ∂ ∂ ∂ i ==, u i , Here, i = / x . i Let us define the spinor field of the first rank, y(x ), in which the spin tensors γ i = hi γ a satisfy the equation in Riemannian space V specified by the contravariant a components ψA(xi) (A = 1, 2, 3, 4) in the orthonormal i j j i ij i γ γ γ γ bases ea(x ). The spinor indices are juggled by using the +2= g I. ψA ABψ ψ ψB formulas = e B and A = eAB . In these formulas, || || –1 || AB|| ˘ i E = eAB and E = e are the covariant and contra- Clearly, the scale factors ha that correspond to the variant components of the metric spinor, respectively, proper basis e˘ are defined by the matrix given by the equations a γ T γ –1 T 1 1 1 1 a ==–E aE , E –E. (3) π ξ σ u 2 2 2 2 γ i π ξ σ Here, T is the transposition symbol; a are the four- ˘ u ha = . (7) dimensional Dirac matrices, which, by definition, sat- π3 ξ3 σ3 u3 isfy the equation π4 ξ4 σ4 u4 γ γ γ γ a b +2b a = gabI, γ β || || If the spintensors a, E, and are specified as where I is a unit four-dimensional matrix; and gab = diag(1, 1, 1, –1) are the covariant components of the metric tensor in the orthonormal basis ea. 000i 00 01 Let us also define the conjugate spinor field by the γ = 00i 0 , γ = 00–0 1 , ψ+ ψ+ 1 2 covariant components = A using the relation 0 –00i 01–00 ψ+ = ψ˙ T β , in which the dot above the letter means –000i 10 00 JOURNAL OF EXPERIMENTAL AND THEORETICAL PHYSICS Vol. 98 No. 4 2004 A GENERAL EXACT SOLUTION OF THE EINSTEIN–DIRAC EQUATIONS 621 the covariant derivatives for the partial derivatives. 00i 0 00i 0 Eqs. (11) are identically valid by the definition of ρ, η, γ 000–i γ 000i and ∆˘ , . 3 = , 4 = , (8) sij –000i i 000 0 i 00 0 i 00 3. THE EINSTEIN–DIRAC EQUATIONS WITH THE COSMOLOGICAL CONSTANT 0100 0010 Let us consider the system of equations E ==–0001 , β 0001 , γ a∇ ψ ψ 0001– 1000 a +0,m = 0010 0100 1 R – ---Rg + λg = κT , ab 2 ab ab ab then the components of the spinor ψ calculated in the (13) ˘ ρ η 1[∇ψ+γ ψ proper basis ea are defined by the invariants and as T ab = --- a b follows [7, 10]: 4 ()γ∇ ψ+ ψψ+γ ∇ ψ∇()γψ+ ψ] – b a + b a – a b . 1 2 1 i ψ˘˘==0, ψ i ---ρexp ---η , 2 2 Here, ψ is the four-component spinor field in four- (9) dimensional Riemannian event space V specified in the 3 4 1 i λ κ ψ˘˘==0, ψ i ---ρexp –---η .