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Decrumpling or TVSD model explains why the universe is accelerating today Forough Nasseri1 Physics Department, Sabzevar University of Tarbiat Moallem, P.O.Box 397, Sabzevar, Iran 6 (February 1, 2008) 0 0 Abstract 2 n Withintheframeworkofamodeluniversewithtimevariablespace a J dimension (TVSD), known as decrumpling or TVSD model, we show 5 the present value of the deceleration parameter is negative implying 2 that the universe is accelerating today. Our study is based on a flat universe with the equation of state parameter to be ω(z = 0) 1 2 ≈ − v today. More clearly, decrumpling model tells us the universe is accel- 2 erating today due to the cosmological constant which is the simplest 1 candidate for the dark energy. 0 1 0 In recent years the exploration of the universe at redshifts of order unity 6 has provided information about the time evolution of the expansion rate of 0 / the universe. Observations indicate that the universe is presently undergoing h t aphaseofaccelerated expansion[1]. Thegoalofthisletteristoshow amodel - p universe with time variable space dimension, known as decrumpling model, e explains why the universe is accelerating today. Our approach to conclude h : thisresult is tocompute thedeceleration parameter of themodelandto show v i its present value is negative, implying the universe is accelerating today. For X more details about decrumpling or TVSD model see Refs. [2]-[9]. r a We will use the natural units system that sets k , c, and h¯ all equal B to one, so that ℓ = M−1 = √G. To read easily this letter we also use P P the notation D instead of D(t) that means the space dimension D is as a t function of time. In Refs.[2]-[9], the decrumpling or TVSD model has been studied. As- sume the universe consists of a fixed number of universal cells having a N characteristic length δ in each of their dimensions. The volume of the universe at the time t depends on the configuration of the cells. It is easily seen that vol (cell) = vol (cell)δDt−D0, (1) Dt D0 1 Email: [email protected] 1 where the t subscript in D means that D to be as a function of time, i.e. t D(t). Interpreting the radius of the universe, a, as the radius of gyration of a crumpled “universal surface”, the volume of space can be written aDt = vol (cell) N Dt = vol (cell)δDt−D0 N D0 = a D0δDt−D0 (2) 0 or a Dt a D0 = 0 = eC, (3) δ δ (cid:18) (cid:19) (cid:18) (cid:19) where C is a universal positive constant. Its value has a strong influence on the dynamics of spacetime, for example on the dimension of space, say, at the Planck time. Hence, it has physical and cosmological consequences and may be determined by observation. The zero subscript in any quantity, e.g. in a and D , denotes its present value. We coin the above relation 0 0 as a“dimensional constraint” which relates the “scale factor” of decrumpling model to the spatial dimension. We consider the comoving length of the Hubble radius at present time to be equal to one. So the interpretation of the scale factor as a physical length is valid. The dimensional constraint can be written in this form 1 1 a 1 = ln + . (4) D C a D t (cid:18) 0(cid:19) 0 It is seen that by the expansion of the universe, the space dimension de- creases. Time derivative of (3) or (4) leads to D2a˙ D˙ = t . (5) t − Ca It can be easily shown that the case of constant space dimension corresponds to when C tends to infinity. In other words, C depends on the number of fundamental cells. For C + , the number of cells tends to infinity and → ∞ δ 0. In this limit, the dependence between the space dimensions and the → radius of the universe is removed, and consequently we have a constant space dimension. 2 Table 1: Values of C and δ for some values of D . Time variation of space P dimension today has been also calculated in terms of sec−1 and yr−1. D C δ (cm) D˙ (sec−1) D˙ (yr−1) P 0 0 | | 3 + 0 0 0 4 1678∞.797 8.6158 10−216 1.7374 10−20h 5.4827 10−13h 0 0 × − × − × 10 599.571 1.4771 10−59 4.8648 10−20h 1.5352 10−12h 0 0 25 476.931 8.3810×10−42 −6.1158×10−20h −1.9299×10−12h 0 0 + 419.699 ℓ× −6.9498×10−20h −2.1931×10−12h P 0 0 ∞ − × − × We define D as the space dimension of the universe when the scale P factor is equal to the Planck length ℓ . Taking D = 3 and the scale of the P 0 universe today to be the present value of the Hubble radius H−1 and the 0 space dimension at the Planck length to be 4,10, or 25, from Kaluza-Klein and superstring theory, we can obtain from (3) and (4) the corresponding value of C and δ 1 1 ℓ 1 1 ℓ 1 P P = ln + = ln + , (6) DP C a0! D0 C H0−1! 3 δ = a e−C/D0 = H−1e−C/3. (7) 0 0 In Table 1, values of C, δ and also D˙ for some interesting values of D t 0 P are given. These values are calculated| by assuming D = 3 and H−1 = 0 0 3000h−1Mpc = 9.2503 1027h−1cm, where we take h = 1. 0 × 0 0 LetusdefinetheactionofthemodelforthespecialFriedmann-Robertson- Walker (FRW) metric in an arbitrary fixed space dimension D, and then try to generalize it to variable dimension. Now, take the metric in constant D+1 dimensions in the following form ds2 = N2(t)dt2 +a2(t)dΣ2, (8) − k where N(t) denotes the lapse function and dΣ2 is the line element for a k D-manifold of constant curvature k = +1,0, 1. The Ricci scalar is given by − D 2a¨ a˙ 2 N2k 2a˙N˙ R = +(D 1) + . (9) N2 ( a − "(cid:18)a(cid:19) a2 #− aN ) 3 Substituting from Eq.(9) in the Einstein-Hilbert action for pure gravity in constant D +1 dimensions 1 S = d(1+D)x√ gR, (10) G 2κ − Z and using the Hawking-Ellis action of a perfect fluid for the model, the following Lagrangian has been obtained [9] V a Dt a˙ 2 N2k a Dt L := Dt D (D 1) ρNV , (11) I −2κN (cid:18)a0(cid:19) t t − "(cid:18)a(cid:19) − a2 #− Dt (cid:18)a0(cid:19) where κ = 8πM −2 = 8πG, ρ the energy density, and V the volume of the P Dt space-like sections 2π(Dt+1)/2 V = , closed universe, k = +1, (12) Dt Γ[(D +1)/2] t π(Dt/2) V = χ Dt, flat universe, k = 0, (13) Dt Γ(D /2+1) c t 2π(Dt/2) V = f(χ ), open universe, k = 1, (14) Dt Γ(D /2) c − t where χ is a cut-off and f(χ ) is a function thereof (for more details see Ref. C c [9]). In the limit of constant space dimensions, or D = D , L approaches t 0 I to the Einstein-Hilbert Lagrangian which is V a D0 a˙ 2 N2k a D0 L0 := D0 D (D 1) ρNV , (15) I −2κ0N (cid:18)a0(cid:19) 0 0 − "(cid:18)a(cid:19) − a2 #− D0 (cid:18)a0(cid:19) where κ = 8πG and the zero subscript in G denotes its present value. So, 0 0 0 Lagrangian L cannot abandon Einstein’s gravity. Varying the Lagrangian I L with respect to N and a, we find the following equations of motion in the I gauge N = 1, respectively a˙ 2 k 2κρ + = , (16) a a2 D (D 1) (cid:18) (cid:19) t t − a¨ a˙ 2 k D2dlnV D (2D 1) D2 (D 1) + + t Dt 1 t t − + t t − (cid:26)a "(cid:18)a(cid:19) a2#(cid:18)− 2C dDt − − 2C(Dt −1) 2D0(cid:19)(cid:27) dlnV D D a +κp Dt t t ln +1 = 0. (17) − dD C − C a (cid:18) t 0 (cid:19) 4 Using (16) and (17), the evolution equation of the space dimension can be obtained by D4 2κρ D˙ 2 = t kδ−2e−2C/Dt . (18) t C2 "Dt(Dt 1) − # − The continuity equation of the model universe with variable space dimension can be obtained by (16) and (17) d a Dt d a Dt ρ V +p V = 0. (19) dt " (cid:18)a0(cid:19) Dt# dt "(cid:18)a0(cid:19) Dt# All above equations, from (1) to (19), are written and discussed in Refs.[2]- [9]. Let us here calculate the present value of the deceleration parameter q 0 in this model. We use (17) in the case of flat universe, i.e. k = 0. Therefore, we get aä q − 0 ≡ a˙2 (cid:12)0 = D02(cid:12)(cid:12)(cid:12)dlnVD 1 D0(2D0 −1) −2C dD − − 2C(D 1) (cid:12)(cid:12)D=D0 0 − D p D2 (cid:12) 1 dlnV 1 + 0 0 0 (cid:12) D , (20) 2 − 2ρ0 C dD (cid:12)D=D0 − D0! (cid:12) (cid:12) where p and ρ are the present value of the pr(cid:12)essure and the energy density 0 0 respectively, satisfying the present equation of state in the model p = ω ρ . (21) 0 0 0 Here ω is the parameter of the equation of state at the present time. There- 0 fore we can rewrite (20) in terms of ω as 0 D2 1 dlnV 1 D (2D 1) q = 0 (1+ω ) D + 1 0 0 − . (22) 0 0 2 −C dD (cid:12)D=D0 D0!− − 2C(D0 1) (cid:12) − (cid:12) This equation can be directly obta(cid:12)ined by the time derivative of (16) in the case of a flat universe. Concerning the dynamics in a flat universe, the authors of Ref.[10] conclude on the validity of the Cosmic Concordance version of the ΛCDM Model that is Ω 0.3, Ω 0.7, ω(z = 0) 1 and no rapid evolution of the M Λ ≈ ≈ ≈ − 5 equation of state parameter. In the limit of constant space dimension, i.e. C + , from Eq.(22) the deceleration parameter today is given by → ∞ D 0 q = (1+ω ) 1. (23) 0 0 2 − Taking ω 1 and D = 3, the value of q in the universe with constant 0 0 0 ≈ − space dimension satisfies q 1. In the case of decrumpling model, by 0 ≈ − considering ω 1 and D = 3, the value of the deceleration parameter 0 0 ≈ − satisfies 15 q 1 , (24) 0 ≈ − − 4C (see Eq.(22) and the values of C given in Table 1). Therefore, decrumpling model explains the universe is accelerating today, i.e. q < 0 or a¨ > 0 and 0 in this model the reason for the present cosmic acceleration of the universe is due to the cosmological constant which is the simplest candidate of the dark energy. In the case of matter-dominated universe, i.e. p = 0, we do not analyze Eq.(22) because supernovae searches have shown that a simplest matter-dominated and decelerating universe should be ruled out. Acknowledgments F.N. thanks Amir and Shahrokh for useful helps and also thanks her birth- placecity, Neishabour inIran,where sheresearches inphysics andcosmology. References [1] For a review of the observational evidence for the acceleration of the expansion of the universe, see, e.g. N.A. Bahcall, J.P. Ostriker, S. Perl- mutter, andP.J. Steinhardt, Science 284 (1999)1481, astro-ph/9906463. [2] F. Nasseri, Int.J.Mod.Phys.D 14 (2005) 1577, hep-th/0412070. [3] F. Nasseri and S.A. Alavi, Int. J. Mod. Phys. 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