Sergei D. Odintsov (ICREA and IEEC-CSIC) Misao Sasaki (YITP, Kyoto University and KIAS) Presenter : Kazuharu Bamba (KMI, Nagoya University)

Transcription

1 Screening scenario for cosmological constant in de Sitter solutions, phantom-divide crossing and finite-time future singularities in non-local gravity Reference: K. Bamba, S. Nojiri, S. D. Odintsov and M. Sasaki, arxiv: [hep-th]. The 21st workshop on General Relativity and Gravitation in Japan (JGRG21) 26th September, 2011 Sakura Hall, katahira Campus, Tohoku University Presenter : Kazuharu Bamba (KMI, Nagoya University) Collaborators : Shin'ichi Nojiri (KMI and Dep. of Phys., Nagoya University) Sergei D. Odintsov (ICREA and IEEC-CSIC) Misao Sasaki (YITP, Kyoto University and KIAS)

2 I. Introduction Recent observations of Supernova (SN) Ia confirmed that the current expansion of the universe is accelerating. [Perlmutter et al. [Supernova Cosmology Project Collaboration], Astrophys. J. 517, 565 (1999)] [Riess et al. [Supernova Search Team Collaboration], Astron. J. 116, 1009 (1998)] [Astier et al. [The SNLS Collaboration], Astron. Astrophys. 447, 31 (2006)] There are two approaches to explain the current cosmic acceleration. [Copeland, Sami and Tsujikawa, Int. J. Mod. Phys. D 15, 1753 (2006)] < Gravitational field equation > G ö G ö = ô 2 T ö Gravity [Tsujikawa, arxiv: [astro-ph.co]] Matter T ö (1) General relativistic approach (2) Extension of gravitational theory : Einstein tensor : Energy-momentum tensor : Planck mass Dark Energy No. 2

3 (1) General relativistic approach Cosmological constant Scalar fields: X matter, Quintessence, Phantom, K-essence, Tachyon. F(R) Fluid: Chaplygin gas (2) Extension of gravitational theory F(R) gravity Scalar-tensor theories : Arbitrary function of the Ricci scalar [Capozziello, Cardone, Carloni and Troisi, Int. J. Mod. Phys. D 12, 1969 (2003)] [Carroll, Duvvuri, Trodden and Turner, Phys. Rev. D 70, (2004)] [Nojiri and Odintsov, Phys. Rev. D 68, (2003)] R [Arkani-Hamed, Cheng, Luty and Mukohyama, JHEP 0405 G : Gauss-Bonnet term f(g) T : torsion scalar [Dvali, Gabadadze and Porrati, Phys. Lett B 485, 208 (2000)] Ghost condensates, 074 (2004)] Higher-order curvature term gravity DGP braneworld scenario f(t) gravity [Bengochea and Ferraro, Phys. Rev. D 79, (2009)] [Linder, Phys. Rev. D 81, (2010) [Erratum-ibid. D 82, (2010)]] [Nicolis, Rattazzi and Trincherini, Phys. Rev. D 79, (2009)] Galileon gravity No. 3

4 Non-local gravity produced by quantum effects [Deser and Woodard, Phys. Rev. Lett. 99, (2007)] There was a proposal on the solution of the cosmological constant problem by non-local modification of gravity. [Arkani-Hamed, Dimopoulos, Dvali and Gabadadze, arxiv:hep-th/ ] Recently, an explicit mechanism to screen a cosmological constant in non-local gravity has been discussed. [Nojiri, Odintsov, Sasaki and Zhang, Phys. Lett. B 696, 278 (2011)] Recent related reference: [Zhang and Sasaki, arxiv: [gr-qc]] It is known that so-called matter instability occurs in F(R) gravity. [Dolgov and Kawasaki, Phys. Lett. B 573, 1 (2003)] This implies that the curvature inside matter sphere becomes very large and hence the curvature singularity could appear. It is important to examine whether there exists the curvature singularity, i.e., the finite-time future singularities in non-local gravity. No. 4

5 No. 5 We investigate de Sitter solutions in non-local gravity. We examine a condition to avoid a ghost and discuss a screening scenario for a cosmological constant in de Sitter solutions. We explicitly demonstrate that three types of the finite-time future singularities can occur in non-local gravity and explore their properties. R 2 It is shown that the addition of an term can cure the finite-time future singularities in non-local gravity.

6 II. de Sitter solution in non-local gravity A. Non-local gravity < Action > g =det(g ö ) f : Some function : Metric tensor No. 6 Λ : Cosmological constant Non-local gravity By introducing two scalar fields and, we find By the variation of the action in the first expression over ø, we obtain (or ) Substituting this equation into the action in the first expression, one re-obtains the starting action. : Covariant derivative operator : Covariant d'alembertian : Matter Lagrangian Q : Matter fields

7 < Gravitational field equation > No. 7 The variation of the action with respect to ñ gives 0 : Energy-momentum tensor of matter (prime) : Derivative with respect to < Flat Friedmann-Lema tre-robertson-walker (FLRW) metric > a(t) : Scale factor ñ We consider the case in which the scalar fields depend on time. ñ ø and only

8 Gravitational field equations in the FLRW background: No. 8 : Hubble parameter and : Energy density and pressure of matter, respectively. For a perfect fluid of matter: < Equations of motion for ñ and ø >

9 B. de Sitter solution No. 9 We assume a de Sitter solution: H 0 : Constant We also suppose. ñ 0,ñ 1 : Constants of integration ø 0,ø 1 : Constants For the de Sitter space, a behaves as. a 0 : Constant For the matter with the constant equation of state, ú m0 : Constant Putting, we obtain

10 For, No. 10 For, There is a de Sitter solution. This means that the cosmological constant is effectively screened by. H 0 For, if we choose, can be arbitrary. Thus, can be determined by an initial condition. H 0 H 0 Since can be small or large, the theory with the function with could describe the earlytime inflation or current cosmic acceleration. Λ ø

11 C. Condition to be free of ghost To examine the ghost-free condition, we make a conformal transformation to the Einstein frame: * No. 11 A superscription (E) represents quantities in the Einstein frame. S ñ ø ñ We have discarded the term because it is a total divergence. Instead of and, we may regard and to be independent fields. * þ

12 In order to avoid a ghost, the determinant of the kinetic term must be positive, which means [Nojiri, Odintsov, Sasaki and Zhang, Phys. Lett. B 696, 278 (2011)] No. 12 This condition is assumed to be satisfied. In particular, is a necessary condition. : The ghost-free condition For, ø 0 =0, : The ghost-free condition

13 de Sitter universe is stable in a period No. 13 The length of the ghost-free period : The period is less than one e-folding time. This cannot give inflation in the early universe provided that the appearance of a ghost has to be avoided.

14 III. Finite-time future singularities in non-local gravity A. Finite-time future singularities No. 14 In the flat FLRW space-time, we analyze an asymptotic solution of the gravitational field equations in the limit of the time t s when the finite-time future singularities appear. We consider the case in which the Hubble parameter is expressed as When, Scale factor h s : Positive constant q : Non-zero constant larger than -1 We only consider the period. a s : Constant

15 By using and, No. 15 We take a form of f(ñ) as. ñ c : Integration constant, : Non-zero constants By using and, ø c : Integration constant There are three cases.,,,

16 We examine the behavior of each term of the gravitational field equations in the limit, in particular that of the leading terms, and study the condition that an asymptotic solution can be obtained. the leading term For case, ø c =1 vanishes in both gravitational field For case, equations. No. 16 Thus, the expression of the Hubble parameter can be a leading-order solution in terms of for the gravitational field equations in the flat FLRW space-time. This implies that there can exist the finite-time future singularities in non-local gravity.

17 B. Relations between the model parameters and the property of the finite-time future singularities f s and û characterize the theory of non-local gravity. h s, t q s and specify the property of the finite-time future singularity. ñ c ø c and determine a leading-order solution in terms of for the gravitational field equations in the flat FLRW space-time. When, for, for and, for, H ú eff ú s for, asymptotically becomes finite and also asymptotically approaches a finite constant value. for,,, No. 17

18 It is known that the finite-time future singularities can be classified in the following manner: Type I ( Big Rip ): Type II ( sudden ): Type III: Type IV: * * * In the limit, [Nojiri, Odintsov and Tsujikawa, Phys. Rev. D 71, (2005)],, The case in which ú eff and P eff becomes finite values at is also included.,,,,,, H Higher derivatives of diverge. ú eff Peff The case in which and/or asymptotically approach finite values is also included. No. 18

19 The finite-time future singularities described by the expression of H in nonlocal gravity have the following properties: No. 19 For, For, For, Type I ( Big Rip ) Type II ( sudden ) Type III * f(ñ) H Range and conditions for the value of parameters of,, and and ø c in order that the finite-time future singularities can exist. ñ c

20 IV. Effective equation of state for the universe and phantom-divide crossing A. Cosmological evolution of the effective equation of state for the universe The effective equation of state for the universe : No. 20 H ç < 0 H ç =0 : The non-phantom (quintessence) phase w eff > à 1 w eff = à 1 Phantom crossing, H ç > 0 : The phantom phase w eff < à 1

21 We examine the asymptotic behavior of w eff in the limit by taking the leading term in terms of. No. 21 q > 1 For [Type I ( Big Rip ) singularity], we ff evolves from the non-phantom phase or the phantom one and asymptotically approaches. w eff = à 1 0 < q < 1 For [Type III singularity], w eff evolves from the non-phantom phase to the phantom one with realizing a crossing of the phantom divide or evolves in the phantom phase. The final stage is the eternal phantom phase. à 1 < q < 0 For [Type II ( sudden ) singularity], at the final stage. w eff > 0

22 B. Estimation of the current value of the effective equation of No. 22 state parameter for non-local gravity [Komatsu et al. [WMAP Collaboration], The limit on a constant equation of state for dark energy in a flat universe has been estimated as For a time-dependent equation of state for dark energy, by using a linear form, constraints on and have been found as, Astrophys. J. Suppl. 192, 18 (2011)] by combining the data of Seven- Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations with the latest distance measurements from the baryon acoustic oscillations (BAO) in the distribution of galaxies and the Hubble constant measurement. : Current value of : Derivative of w DE w DE from the combination of the WMAP data with the BAO data, the Hubble constant measurement and the high-redshift SNe Ia data.

23 We estimate the present value of. w eff For case, * No. 23 We regard at the present time because the energy density of dark energy is dominant over that of nonrelativistic matter at the present time. : The present time has the dimension of : Current value of H, [Freedman et al. [HST Collaboration], Astrophys. J. 553, 47 (2001)] 0 < q < 1 For,. For à 1 < q < 0, w eff > 0. w eff In our models, can have the present observed value of w DE.

24 R 2 C. Cosmological consequences of adding an term We explore whether the addition of an R 2 term removes the finite-time future singularities in non-local gravity. < Action > No. 24 Gravitational field equations in the flat FLRW background: In the limit, The leading terms do not vanish. The additional R 2 term can remove the finite-time future singularity.

25 V. Summary We have studied de Sitter solutions in non-local gravity. We have explored a condition to avoid a ghost and presented a screening scenario for a cosmological constant in de Sitter solutions. We have explicitly shown that three types of the finitetime future singularities (Type I, II and III) can occur in non-local gravity and examined their properties. We have investigated the behavior of the effective equation of state for the universe when the finitetime future singularities occur. We have demonstrated that the addition of an R 2 term can remove the finite-time future singularities in non-local gravity. No. 25

26 < Further results and remarks > No. 26 We have also studied de Sitter solutions in nonlocal gravity with Lagrange constraint multiplier. It has also been suggested that the addition of an term in the framework of non-local gravity might realize unification of inflation in the early universe with the cosmic acceleration in the late time. R 2

27 Backup slides

28 In the presence of matter with, for, we may have a de Sitter solution H = H 0 even if f(ñ) is given by Therefore, the following solution exists: Screening... gravity / K. Bamba et al. / bamba@kmi.nagoya-u.ac.jp28/??

29 We may introduce a new field. Screening... gravity / K. Bamba et al. / bamba@kmi.nagoya-u.ac.jp29/??

30 C. Condition to be free of ghost To examine the ghost-free condition, we make a conformal transformation to the Einstein frame: A superscription (E) represents quantities in the Einstein frame. The term may be discarded because it is a total divergence. Screening... gravity / K. Bamba et al. / bamba@kmi.nagoya-u.ac.jp 30/32

31 ñ ø ñ Instead of and, we may regard and to be independent fields. þ In order to avoid a ghost, the determinant of the kinetic term must be positive, which means [Nojiri, Odintsov, Sasaki and Zhang, Phys. Lett. B 696, 278 (2011)] This condition is assumed to be satisfied. In particular, is a necessary condition. :The ghost-free condition Screening... gravity / K. Bamba et al. / bamba@kmi.nagoya-u.ac.jp 31/32

32 The phantom phase as well as the non-phantom phase can be realized in the Einstein frame in the context of non-local gravity. If f 0 =2, We remark that since we now consider the case in which the contribution of matter is absent, w eff is equivalent to the equation of state for dark energy. ú eff P eff ú tot P tot This is because and correspond to and, respectively, and thus can be expressed as. w eff ú tot and P tot : The total energy density and pressure of the universe, respectively. In the case, : The ghost-free condition This cannot be satisfied. Thus the solution also contains a ghost. Screening... gravity / K. Bamba et al. / bamba@kmi.nagoya-u.ac.jp32/??

33 D. Cosmology in the Einstein frame We assume the FLRW metric, and consider the case in which the contribution of matter is negligible. The variation of the action in terms of þ and ñ leads to The first FLRW equation: We investigate the case that. We suppose. ( Screening... gravity / K. Bamba et al. / bamba@kmi.nagoya-u.ac.jp 33/32

34 There exists a flat solution where. If,,. The effective equation of state for the universe h 0 > 0 h 0 < 0 : : The non-phantom (quintessence) phase w eff > à 1 : The phantom phase w eff < à 1, Screening... gravity / K. Bamba et al. / bamba@kmi.nagoya-u.ac.jp 34/32

35 The phantom phase as well as the non-phantom phase can be realized in the Einstein frame in the context of non-local gravity. If f 0 =2, In the case, : The ghost-free condition This cannot be satisfied. Thus, the solution also contains a ghost. Screening... gravity / K. Bamba et al. / bamba@kmi.nagoya-u.ac.jp 35/32

36 E. Addition of an R 2 term We examine the influence of adding an R 2 term on the stability of non-local gravity in the Einstein frame. We introduce another scalar field ð. By varying the action with respect to, we have. Substituting this equation into the action in the second expression, the starting action is re-obtained. Gravitational field equations in the flat FLRW background: ð Screening... gravity / K. Bamba et al. / bamba@kmi.nagoya-u.ac.jp 36/32

37 The effective equation of state for the universe is given by If we add an R 2 term as in the action, ú eff and P eff become Screening... gravity / K. Bamba et al. / bamba@kmi.nagoya-u.ac.jp 37/32

38 We examine the condition to avoid a ghost in the present case. By following the same procedure in Sec. II C, we perform a conformal transformation to the Einstein frame., Substitution of The mass matrix is given by. Screening... gravity / K. Bamba et al. / bamba@kmi.nagoya-u.ac.jp 38/32

39 The necessary condition to avoid a ghost is that all the eigenvalues of the mass matrix must be positive. M M The characteristic equation for is given by y denotes an eigenvalue of. M E : Unit matrix y 1 > 0, y 2 > 0, y 3 > 0 : Three eigenvalues of M If, Y 3 > 0 If, Y 1 > 0 If, Y 2 < 0 Y 1 > 0, Y 2 > 0, Y 3 > 0 (i.e., ) (i.e., ) M All the three eigenvalues of cannot be positive simultaneously. Thus, the necessary condition to avoid a ghost cannot be satisfied. Screening... gravity / K. Bamba et al. / bamba@kmi.nagoya-u.ac.jp 39/32

40 III. Finite-time future singularities in non-local gravity A. Finite-time future singularities We examine whether there exists the finite-time future singularities in non-local gravity. In the flat FLRW space-time, we analyze an asymptotic solution of the gravitational field equations in the limit of the time t s when the finite-time future singularities appear. We consider the case in which the Hubble parameter is expressed as h s : Positive constant q : Non-zero constant larger than -1 We only consider the period. When, for, for and, H is finite, but H ç becomes infinity and therefore also diverges. R Screening... gravity / K. Bamba et al. / bamba@kmi.nagoya-u.ac.jp40/??

41 a s : Constant By using and, Taking the leading term in terms of, we obtain for Leading term ñ c : Integration constant for Cf. If, Leading term Screening... gravity / K. Bamba et al. / bamba@kmi.nagoya-u.ac.jp41/??

42 B. Analysis for. We take a form of f(ñ) as., : Non-zero constants By using and, Taking the leading term in terms of, we acquire for for If û>0 If û<0 Leading term ø c : Integration constant Leading term Screening... gravity / K. Bamba et al. / bamba@kmi.nagoya-u.ac.jp42/??

43 Thus, there are three cases.,,, We examine the behavior of each term on the right-hand side (r.h.s.) of the gravitational field equations in the limit, in particular that of the leading terms, and study the condition that an asymptotic solution can be obtained. We analyze the case of and that of ñ c =0separately. Λ ú m P m When,, and can be neglected because these values are finite. Screening... gravity / K. Bamba et al. / bamba@kmi.nagoya-u.ac.jp43/??

44 IV. Effective equation of state for the universe and phantom-divide crossing A. Cosmological evolution of the effective equation of state for the universe We examine the asymptotic behavior of w eff in the limit by taking the leading term in terms of. Case I(t) w eff > à 1 : Deviation w eff of from -1 I(t) > 0 I(t)=0 w eff = à 1 If, evolves from to. I(t) I(t) < 0 I(t)=0 If, evolves from to. w eff < à 1 w eff = à 1 Screening... gravity / K. Bamba et al. / bamba@kmi.nagoya-u.ac.jp44/??

45 Case For I 0 > 0 : Constant part of I(t) If, a crossing of the phantom divide from the nonphantom phase to the phantom one can occur because the sign of the second term in I(t) is negative and the absolute value of the amplitude becomes very large. I 0 < 0 I(t) w eff < à 1 If, always evolves in the phantom phase ( ). Thus the final stage is the phantom phase and it is eternal. For If we consider Λ > 0, we have. Screening... gravity / K. Bamba et al. / bamba@kmi.nagoya-u.ac.jp45/??

46 F(R) R 2 It is known that in gravity, the addition of an term could cure the finite-time future singularities. It has been suggested that in the framework of non-local gravity combined with an R 2 term, inflation in the early universe as well as the cosmic acceleration in the late time could be realized. R 2 The additional term leads to inflation and the late-time cosmic acceleration occurs due to the term of non-local gravity in the action. Screening... gravity / K. Bamba et al. / bamba@kmi.nagoya-u.ac.jp46/??

47 We have shown that the late-time accelerating universe may be effectively the quintessence, cosmological constant or phantom-like phases. We have also demonstrated that there is a case with realizing a crossing of the phantom divide from the non-phantom (quintessence) phase to the phantom one in the limit of the appearance of a finite-time future singularity. The estimation of the current value of the effective equation of state parameter for the universe which could be phantomic one around -1 shows that its observed value could be easily realized by the appropriate choice of non-local gravity parameters. We have considered the cosmological consequences of adding an term. R 2

48 Non-local gravity with Lagrange constraint multiplier

49 III. Non-local gravity with Lagrange constraint multiplier We generalize non-local gravity by introducing Lagrange constraint multiplier and examine a de Sitter solution in non-local gravity with Lagrange constraint multiplier. < The constrained action for a scalar field ψ > Constraint: No. L2 õ : Lagrange multiplier field : Time-like vector We choose. Under the constraint, we define U 0 : Constant n, ë, í : Constants

50 < Non-local action > No. L3 Introducing two scalar fields ñ and ø We assume the following form of the metric:, ψ n can be even or odd integer. and only depends on time. We suppose ñ and ø only depend on t. We examine most simple but non-trivial case that m =2. (i.e., n =0, í =0 )

51 The variation of the action with respect to ñ gives No. L4 b =0 in the above background, after putting. The variation of the action with respect to ø leads to in the above background. For m =2, the action is expressed as By the variation of the action with respect to b, we obtain the equation corresponding to the first FLRW equation: * We have used

52 The variation of the action with respect to a gives the equation corresponding to the second FLRW equation: No. L5 We investigate if there could be a solution describing the de Sitter space. We assume. * We take. We suppose., ø 0,ø 1 : Constants of the integration

53 , No. L6 : Cubic algebraic equation with respect to Hence, there is always a real solution of. ì ì We can find the value of. Consequently, if we choose properly, there always appears the solution describing the de Sitter universe. t c t c ø 1 can be a screening of the cosmological constant. * The last term is a correction coming from the covariant like model.

54 The non-local action often appears as a quantum correction. This equation implies again that the correction effectively changes the value of the cosmological constant. ø ø 0 t ø 0 If in the early universe, where, this equation means * H 0 If corresponds to the value of the Hubble parameter in the present universe, the second term could be negligible. No. L7 If is positive and very large, the effective cosmological constant in the present universe could be very small. We explore a condition to avoid the appearance of a ghost. We make a conformal transformation to the Einstein frame:

55 * We describe the Lagrangian for the part of the Lagrange multiplier field as No. L8 :The ghost-free condition The condition to avoid a ghost is described by the following form: From a necessary condition, we find implies that the sign of f 0 is the same as that of. ì, which

56 Appendix A

57 < 7-year WMAP data on the current value of > K =0 For the flat universe, constant w : Cf. Ω Λ = (68% CL) (68% CL) (95% CL) (From w K Ω K ñ (a0 H 0 ) 2 No. A1 From [E. Komatsu et al. [WMAP Collaboration], Astrophys. J. Suppl. 192, 18 (2011) [arxiv: [astro-ph.co]]]. Hubble constant ( H 0 ) measurement Baryon acoustic oscillation (BAO) : Special pattern in the large-scale correlation function of Sloan Digital Sky Survey (SDSS) luminous red galaxies : Time delay distance : Density parameter for the curvature K =0 : Flat universe.)

58 (68% CL) (95% CL) Time-dependent w No. A2 From [E. Komatsu et al. [WMAP Collaboration], Astrophys. J. Suppl. 192, 18 (2011) [arxiv: [astro-ph.co]]]. For the flat universe, a variable EoS : w 0 : Current value of w (From 1 a = 1+z z : Redshift.),

59 Appendix B

60 < Crossing of the phantom divide > No. B1 Various observational data (SN, Cosmic microwave background radiation (CMB), BAO) imply that the effective EoS of dark energy w DE may evolve from larger than -1 (non-phantom phase) to less than -1 (phantom phase). Namely, it crosses -1 (the crossing of the phantom divide). w DE à 1 z c 0 z z c Redshift: z ñ 1 a à 1 : Red shift at the crossing of the phantom divide [Alam, Sahni and Starobinsky, JCAP 0406, 008 (2004)] [Nesseris and Perivolaropoulos, JCAP 0701, 018 (2007)] [Alam, Sahni and Starobinsky, JCAP 0702, 011 (2007)] (a) w DE > à 1 Non-phantom phase (b) w DE = à 1 Crossing of the phantom divide (c) w DE < à 1 Phantom phase

61 < Effective equation of state for the universe > No. B2 : Total energy density of the universe : Total pressure of the universe (a) H ç < 0 w eff > à 1 P DE P m P r : Pressure of dark energy : Pressure of non-relativistic matter (cold dark matter and baryon) : Pressure of radiation Non-phantom phase (b) H ç =0 w eff = à 1 Crossing of the phantom divide (c) H ç > 0 w eff < à 1 Phantom phase

62 z w(z)=w 0 + w 1 1+z < Data fitting of w(z) (1) > No. B3 From [Nesseris and L. Perivolaropoulos, JCAP 0701, 018 (2007)]. SN gold data set [Riess et al. [Supernova Search Team Collaboration], Astrophys. J. 607, 665 (2004)] SNLS data set [Astier et al. [The SNLS Collaboration], Astron. Astrophys. 447, 31 (2006)] Shaded region shows 1û error. + Cosmic microwave background radiation (CMB) data [Spergel et al. [WMAP Collaboration], Astrophys. J. Suppl. 170, 377 (2007)] SDSS baryon acoustic peak (BAO) data [Eisenstein et al. [SDSS Collaboration], Astrophys. J. 633, 560 (2005)]

63 For most observational probes (except the SNLS data), a low Ω 0m prior (0.2 < Ω 0m < 0.25) leads to an increased probability (mild trend) for the crossing of the phantom divide. Ω 0m : Current density parameter of matter [Nesseris and L. Perivolaropoulos, JCAP 0701, 018 (2007)] No. B4

64 < Data fitting of w(z) (2) > No. B5 From [Alam, Sahni and Starobinsky, JCAP 0702, 011 (2007)]. SN gold data set+cmb+bao SNLS data set+cmb+bao 2û confidence level.

65 < Data fitting of w(z) (3) > No. B6 From [Zhao, Xia, Feng and Zhang, Int. J. Mod. Phys. D 16, 1229 (2007) [arxiv:astro-ph/ ]] 68% Best-fit confidence level 95% confidence level 157 gold SN Ia data set+wmap 3-year data+sdss with/without dark energy perturbations.