Cosmologies with General Non-Canonical Scalar Field †Wei Fang1, ‡H.Q.Lu1, Z.G.Huang2 1Department of Physics, Shanghai University, Shanghai, 200444, P.R.China 2Department of Mathematics and Physics, 7 Huaihai Institute of Technology, Lianyungang, 222005, P.R.China 0 0 2 n a Abstract J 9 We generally investigate the scalar field model with the lagrangian L = F(X) 1 − V(φ), which we call it General Non-Canonical Scalar Field Model. We find that it 3 v is a special square potential(with a negative minimum) that drives the linear field 8 solution(φ = φ t) while in K-essence model(with the lagrangian L = V(φ)F(X)) 8 0 − 1 the potential should be taken as an inverse square form. Hence their cosmological 0 1 evolution are totally different. We further find that this linear field solutions are 6 0 highly degenerate, and their cosmological evolutions are actually equivalent to the / h divergent model where its sound speed diverges. We also study the stability of the t - linear field solution. With a simple form of F(X) = 1 √1 2X we indicate that p − − e our model may be considered as a unified model of dark matter and dark energy. h : Finally we study the case when the baryotropic index γ is constant. It shows that, v Xi unlike the K-essence, the detailed form of F(X) depends on the potential V(φ). We r analyze the stability of this constant γ solution and find that they are stable for a 0 γ 1. Finally we simply consider the constant c2 case and get an exact solution 0 ≤ s for F(X). Keywords: Non-Canonical Scalar Field; Linear field solution; Dark Energy; K- essence; Cosmology. PACS: 98.80.Cq, 04.65.+e, 11.25.-w xiaoweifang−[email protected] † Alberthq−lu@staff.shu.edu.cn ‡ 1 1 Introduction Dark energy problem may be one of the biggest issues in current theoretical physics andcosmology(seeRef[1]forarecentreview). Thebuildingoftheoreticalmodelsaswellas the breakthrough at astrophysical observations have never halted since 1998. Technically speaking, one can operate on either the r.h.s or the l.h.s of the Einstein equation to get a reasonable interpretation of the accelerating expansion of the universe. Many candidates of dark energy have been proposed, such as the cosmological constant[2], quintessence[3], K-essence[4], phantom[5], modifying gravity[6] and so on. Among these models the scalar field models are undoubtedly the most important class of theoretical models. Generally speaking, the lagrangian of the scalar field model can be generally represented as[7] L = f(φ)F(X) V(φ) (1) − where X = 1 φ µφ = 1φ˙2 for a spatially homogeneous scalar field. Eq.(1) has included 2∇µ ∇ 2 all the popular single scalar field models. It describes K-essence when V(φ) = 0 and standard quintessence when f(φ) =constant and F(X) = X. The idea of K-essence was firstly introduced as a possible model for inflation[8] and Later was considered as a possible modelfordarkenergy[4,9]. L.P.Chimento foundthefirst integralof theK-essence field equation for any function F(X) when the potential is taken as inverse square form or a constant[10]. In Ref[11], it was found that every quintessence model can be view as a K-essence model generated by a kinetic linear F(X) function, and some K-essence potentials and their quintessence correspondence are also found. In this paper, we will focus on another class of models with its lagrangian L = F(X) − V(φ),whichwethinkisasimportantasK-essencemodelbutwhosegeneralcharactersand roles in cosmology is far beyond clear. Here we should point out that, from the original literature’s point of view[8,9], our lagrangian(Eq.(2)) also belongs to K-essence model, which ischaractered by a lagrangianL = L(X,φ). However asfar aswe know, most works about K-essence model are just based on the lagrangian form L = V(φ)F(X). Therefore in this paper we call the model with lagrangian L = F(X) V(φ) Non-Canonical Scalar − Field Model while specify the model with the lagrangian L = V(φ)F(X) as K-essence model. The paper is organized as follows: Section 2 is the theoretical framework. In section 3 we investigate the linear field solution and find the potential driving this linear field solution. The divergent model with its speed of sound diverge(c2 ) is considered s → ∞ in section 4. Some solvable general non-canonical scalar field solution are discussed in section 5. Section 6 is the conclusion. 2 2 Basic Framework Let us restrict ourselves for the time being to the cosmological setting corresponding to the flat universe described by the FRW metric. We consider the spatially homogeneous real scalar field φ with a non-canonical kinetic energy term. The lagrangian density is L = F(X) V(φ) (2) − where V(φ) is a potential and F(X) is an arbitrary function of X. Obviously above equa- tion is a special case of Eq.(1) when the function f(φ) =constant. It includes quintessence [F(X) = X] and a phantom field [F(X) = X]. In fact this form of lagrangian has ap- − peared in Refs[1,12]. We can easily get the following equations: p = L = F(X) V(φ) (3) − ρ = 3H2 = 2L X L = 2XF F(X)+V(φ) (4) ,X ,X − − F c 2 = p /ρ = [1+2X ,XX]−1 (5) s ,X ,X F ,X Wherewetake8πG = 1forconvenience. FromEqs.(3,4)wegettherelationρ+p = 2XF ,X and this yields the state equation ω larger than -1 if F > 0(ω < 1 if F < 0). φ ,X φ ,X − Vikman has argued that it is impossible for ω to cross the phantom line divide(ω = 1) φ φ − in single scalar field theory[13]. However it is argued that this result holds only for models withoutconsidering higherderivative terms[14]. Eq.(5)describes theeffective soundspeed of the perturbations. c 2 1 if F = 0 is satisfied. s ,XX ≡ The motion equations of the general non-linear scalar field are ρ φ¨+3c 2Hφ˙ + ,φ = 0 (6) s ρ ,X ¨ ˙ (F +2XF )φ+3HF φ+V = 0 (7) ,X ,XX ,X ,φ γ γ V ( )· +3H(1 γ)( )+ ,φ = 0 (8) φ˙ − φ˙ 3H2 where ”f ” denotes the derivative with respect to subscript index x and γ = (ρ + p)/ρ ,x is the baryotropic index. Eqs.(6,7,8) are different forms of the motion equation and they are equivalent to each other. 3 The Linear Field Model In this section we will investigate a special case that the field possesses a linear field solution φ = φ t. We find that the form of potential V(φ) which drives this evolution 0 3 is a square potential with a negative minimum. We also show that the usual linear field solution in non-linear scalar field theory leads to an entirely different universe comparing with the universe in K-essence model. For the linear field solution, X = 1φ˙2 1φ 2 is a constant and φ¨ = 0, then we get 2 ≡ 2 0 following equation from Eqs.(4,7): V 2 ,φ V(φ)+F(X) φ 2F = 0 (9) 3F 2φ 2 − − 0 ,X ,X 0 F and F are only the function of X and therefore they both are constant. We set ,X F = F(X ) and F = F (X ) by evaluating them at X = X = 1φ 2. Solving Eq.(9) 0 0 ,0 ,X 0 0 2 0 and Eq.(4) we get the exact solutions for potential V(φ) and scale factor a: 3 V(φ) = F 2φ 2(φ+c)2 +F F φ 2 (10) ,0 0 0 ,0 0 4 − F a = a exp[ ,0(φ t+c)2] (11) 0 0 − 4 Therefore the linear field solution leads to a square potential Eq.(10) and a evolution of scale factor Eq.(11). It is worthwhile to compare the same case in K-essence model. It is argued[15]that thesamelinear fieldsolutioninK-essence modelleads toaninverse square potential and a power law expansion of scale factor. It is interesting that the same linear field solution lead to different cosmological evolution and therefore different cosmological implication. We should emphasize that the potentialis exactly derived fromEqs.(4,9) and its form is unique. Moreover in this case the different forms of F(X) degenerate to only two cases: F > 0 and F < 0 and respectively correspond to non-phantom(ω > 1) and ,0 ,0 − phantom case(ω < 1). The phantom case(F < 0) describes a universe from contracting ,0 − phase to expanding phase and is excluded easily by current observation. So we restrict F > 0 for next discussion. From Eq.(4) we get ρ = F φ 2 F +V(φ). To ensure the ,0 ,0 0 0 − energy density ρ has a positive kinetic energy term we demand F φ 2 F > 0 and this ,0 0 0 − immediately leads to the square potential with a negative minimum value F F φ 2[see 0 ,0 0 − Eq.(10)]. In fact this result is well-intelligible in an expanding universe. Because if the square potential has a non-negative minimum it is well-known that the scalar field φ will roll down the potential and finally cease at the minimum position φ = c and the linear − field solution φ = φ t will be no long valid. On the other hand it is argued that for a 0 potential with a negative minimum the scalar field can oddly roll up the potential from its minimum(see Fig.1) and the universe enters a contracting phase from an expanding phase[16] and therefore the scalar field can evolve to . ∞ It is very interesting that the universe in our model can avoid a beginning singularity. If we think the classic cosmology is valid when energy scale is below Plank scale, then the scale factor at the beginning is very small[a = a e−ρpl/(3F,0φ02), where ρ is the energy 0 pl 4 density at plank time] but does not equal zero. However this universe can not escape from a collapse in future. This evolutive behaviors are completely different from the same linear field solution case(φ = φ t) in K-essence model where the scale factor behaves as 0 a tn[10] and the universe was birth from a singularity and expand for ever. ∝ One of our concerns is whether our model can describe a suitable universe with a phase of accelerating expansion. The answer is positive because we have a¨ (ρ + ∝ − 3p) = 3F φ 2[2 F (φ + c)2] and ρ + 3p < 0 for φ < φ or φ > φ , where φ = −2 ,0 0 − ,0 1 2 1 c 2 ,φ = c + 2 . The potential and the evolutive behaviors of universe − − qF,0 2 − qF,0 are showed in Fig.1. We can see from Fig.1, the field rolls down the potential from an initial value and the universe undergoes an accelerating expansion. When the field evolutes to φ , the universe enters a decelerating expansion and finally becomes zero 1 expansion rate when the field arrives at φ = c. When the field crosses the point φ = − c the expansion rate H dramatically changes it sign from H > 0 to H < 0 and the − field rolls up the potential from its minimum. When the field rolls from c to φ the 2 − universe undergoes an accelerating contraction. After going over φ , the universe enters 2 a decelerating contraction and collapses to singularity finally. Fig.1: Potential and the evolution of universe V( da/dt>0 da/dt<0 2 2 2 2 da/dt<0 da/dt<0 2 2 2 2 da/dt>0 da/dt>0 t -c 0 In addition, since we get the potential(Eq.(10)) just fromthe linear field solution with- out any other assumption, another non-trivial question is whether an arbitrary square potential with a negative constant can always lead to the linear field solution. Unfortu- nately, we will demonstrate that it is not the case: not all the square potential with a negative constant can possess the linear field solution. Given a arbitrary potential: V(φ) = A(φ+c)2 B (12) − where A, B are arbitrary positive constants. In order to have a linear field solution, from Eq.(10) A and B should satisfy A = 3F 2φ 2,B = F F φ 2. So we get the following 4 ,0 0 0 − ,0 0 5 constraint equation: 4A = 3F (F B) (13) ,0 0 − It means that A and B are not arbitrary parameters. In other words, given a value of A, the value of B is determined by Eq.(13). i.e, only the square potential with its parameters satisfying Eq.(13) can lead to a linear field solution. From Eq.(13) we find anotherinteresting character forthelinear fieldsolution. Sincethereonlyappearsthefirst two coefficients (F ,F ) of the series expansion of the function F(X) around X = X (we 0 ,0 0 can expand function F(X) as F + F (X X ) + around X ). The evolution of 0 ,0 0 0 − ··· universe will be thought of as equivalent as long as the function F(X) has the same first two coefficients of its expansion, disregarding the rest of the higher order terms. This means that the linear filed solution model possesses a high degenerate character. The last question we ask is, how stable are the linear field solutions. Since for the linear field solution we have 3HF φ +V = 0, we can rewrite Eq.(7) as follows: ,0 0 ,φ ˙ dφ 3H ˙ = − (F φ F φ ) (14) ,X ,0 0 dt F +2XF − ,X ,XX For the case that c2 = costant 0, Eq.(14) becomes s ≥ ˙ dφ = 3Hc2(φ˙ φ ) (15) dt − s − 0 Integrating Eq.(15), we have c ˙ 1 φ = φ + (16) 0 a3c2s ˙ Eq.(16) shows that, if the universe expands for ever, φ has an asymptotic limit φ and 0 the solution φ = φ t is stable. However, as we known from Eq.(11), after expanding to 0 a maximum scale factor the universe will contract to a singularity eventually. Therefore this linear field solution can not be stable. 4 The Divergent Model In this section we investigate another interesting model where the speed of sound is divergent(c2 ). From Eq.(5), we have: s → ∞ F ,XX 1+2X = 0 (17) F ,X Integrating Eq.(17) we get the form of function F(X): 1 F(X) = c2X2 +c3 (18) 6 where c ,c are the integral constants. From Eq.(18), the sound speed of this special form 2 3 ofF(X)diverges(c 2 ). ThesameformoffunctionF(X)isalsoobtainedinK-essence s → ∞ model with the lagrangian being V(φ)F(X)[10,15]. Recently This type lagrangian is − thoroughly investigated and exploited in model building[17], where this type lagrangian is called as Cuscuton. From Eq.(4) and Eq.(18), we have 3H2 = V(φ) c (19) 3 − From Eq.(7) and Eq.(19) we get 1 c 3(V(φ) c )+dV(φ)/dφ = 0 (20) 2 3 2 − p Therefore we can immediately get the potential V(φ): 3 V(φ) = c2(φ c )2 +c (21) 8 2 − 4 3 where c is an integral constant. For the model building the concrete form of function 4 F(X) and the potential V(φ) can be constructed respectively, however, it is very inter- esting that the divergent model with the special lagrangian Eq.(18) determine the unique form of potential Eq.(21). This maybe means that the divergent model with the speed of sound c2 = has some special implications. Let us recall the result obtained in Section s ∞ 3: if the square potential satisfies the constraint Eq.(10), the solution of the scalar field will be linear field solution. From Eqs.(10, 18), we have 3F 2φ 2 = 3c2,F F φ 2 = c , 4 ,0 0 8 2 0− ,0 0 3 which just coincides with Eq.(21). This means that the divergent model and the linear field solution are degenerate. Namely the divergent model and the linear field solution model are kinetically isomorphic and share the same evolution of scale factor a and scalar field φ: F φ = φ t, a = a exp[ ,0(φ t+c)2] (22) 0 0 0 − 4 However it remains to clarify why this could happen. From the mathematical point of view, we maybe get the interpretation from the motion equation of field Eq.(7) since both the divergent model and the linear field model lead the first term of Eq.(7) to vanish. Therefore they share the same motion equation and then lead to the same cosmological evolution. But what is the physical implication that the scalar field theory with an infinite speed of sound is degenerate with the linear field model? There also exists the same situation in the K-essence model that the linear field solution and the divergent model(c2 = ) are isomorphic[18]. s ∞ 7 5 Solvable General Non-canonical Scalar Field Cos- mologies In this section we will focus on some special cases when Eq.(8) exists a first integral or can be solved exactly. Though what we consider are quite simple, they can also lead to some important results. Additional, we will try to find the relationships and the differences between our General Non-Canonical Scalar Field model and K-essence model. A. V(φ) = V 0 When the potential V(φ) is a constant(= V ), Eq.(8) exists a first integral 0 γ c 5 = (23) φ˙ a3H2 where c is an arbitrary integral constant. Eq.(23) is very similar with the first integral 5 obtainedinK-essence modelwitha constant potential[10,19]. Fora constant potentialthe K-essence lagrangian can be written as L = V F (X) while the lagrangian in our model k 0 k − is L = F (X) V . They are actually equivalent if we define F (X) = V (1 F (X)). g g 0 g 0 g − − So our model can easily reproduce the K-essence models with constant potential. The K-essence models with a constant potential are hotly studied for its exquisite role in unifying thedarkmatter anddarkenergy[20]. Letwe consider aspecialformoflagrangian L = 1 √1 2X V(φ), which is considered as a Non-Linear Born-Infeld(NLBI) scalar g − − − field theory in Ref[21]. When the potential is constant V , we can find the exact solutions: 0 c φ˙2 = 6 (24) c +a6 6 c 6 ρ = 1+ +(V 1) (25) r a6 0 − a6 c 2 = 1 φ˙2 = (26) s − c +a6 6 Where c is an integral constant. From Eqs.(25,26) we know that, the energy density 6 behaves as dark matter at early time(for small a, ρ a−3 and c 2 0) and dark energy s ∝ ≃ at late time(for large a, ρ V = const and c 2 1). So, our model can also play the 0 s ≃ ≃ same role that unifies the dark matter and dark energy. B. γ =constant In this subsection we will assume that the baryotropic index γ = γ =constant. Then 0 the state equation ω = γ 1 = γ 1, is also a constant. The constant γ kinematically 0 − − leads to the cosmological solution 4a3γ0 a = a t2/3γ0, ρ = 0 /a3γ0 (27) 0 φ 3γ2 0 8 From the relation ρ+p 2H˙ 2XF ,X γ = = = (28) 0 ρ −3H2 2XF F(X)+V(φ) ,X − We get the following equation: 2(γ 1) 0 − XF F(X)+V(φ) = 0 (29) ,X γ − 0 γ0 InRef[18],itisshowed thatF(X) = X2(γ0−1) canleadtotheconstantγ withanypotential in K-essence model. Here we show that in our model the form of F(X) depends on the potential(see Eq.(29)). Namely, to admit the cosmological solution Eq.(27), the kinetic term F(X) and potential term V(φ) must satisfy Eq.(29). Only for a constant potential V ,we get a similar function of F(X): 0 γ0 F(X) = c7X2(γ0−1) +V0 (30) where c is an integral constant. 7 It is quite interesting to consider whether the solution with constant γ is stable. 0 To answer this question, we let γ vary with time. Differentiating the equation of the baryotropic index γ, we get p˙ γ˙ = (γ 1)(3Hγ + ) (31) − p We immediately get two critical points: γ 1 = 0 or γ satisfies Eq.(32): 0 0 − p˙ 3Hγ + = 0 (32) 0 p When this stationary condition Eq.(32) holds, the potential V(φ) and function F(X) will satisfy the relation: c 8 p = F(X) V(φ) = (33) − a3γ0 From Eqs.(31,32), we have γ˙ = 3H(γ 1)(γ γ ) (34) 0 − − Integrating Eq(34) we get γ a3(1−γ0) c 0 9 γ = − (35) a3(1−γ0) c9 − where c , c is an integral constant. Eq.(35) indicates that, for the expanding universe 8 9 and γ < 1 the baryotropic index γ has the asymptotic limit γ . However for γ > 1 the 0 0 0 baryotropic index γ will approach the asymptotic limit 1. The case γ = 1 should be 0 considered apart. For γ = 1, the solution is 0 c 10 γ = 1 (36) − 3lna 9 Where c is an integral constant. Eq.(36) shows that the solution with γ = 1 is also 10 0 stable in an expanding universe. Therefore, we can conclude that the solutions with constant baryotropic index are attractors in the case γ 1 and the γ = 1 solutions 0 0 ≤ separate stable from unstable regions in the phase space. C. c2 =constant s For the dark energy behaving as a fluid, the speed of sound c is another important s parameter in addition to the equation of state ω. The speed of sound c is the propagation s of the perturbation of the background scalar field, which can affects the CMB power spectrum. Therefore the effective sound speed c of dark energy would provide crucial s information which is complementary to the equation of state ω. In this subsection we will study the simple case that the speed of sound is constant. From Eq.(5), we obtain the equation as follows: 2c2XF = (1 c2)F (37) s ,XX − s ,X Integrating Eq.(26), we have 2c2 1+c2s F(X) = 1+sc2c11X 2c2s +c12 (38) s where c , c is the integral constants. 11 12 6 Conclusion Inthispaperwehavegenerallyinvestigatedthegeneralnon-canonicalscalarfieldmodel as a candidate of dark energy. We found that it was a special square potential(with a negative minimum) that drove the linear field solution in our model while the potential shouldbetakenasaninverse squareforminK-essencemodel. Ourresults showedthatthe linear field solution was highly degenerate, and shared the same cosmological evolution with the divergent model where its sound speed c2 . We pointed out that our model s → ∞ with a constant potential was actually equivalent to the K-essence model also with a constant potential. With a simple form of F(X) we indicated that our model can be also considered as a unified model of dark matter and dark energy. In addition we studied the constant γ case. The results showed that, unlike the K-essence model, the detailed form 0 of F(X) depended on the potential. We find that the constant γ solution is stable for 0 γ 1. We also found the form of F(X) which possessed the constant c2 solution. Our 0 ≤ s work may throw light on the study of the scalar field theory and the exploration of dark energy. 10