Cosmology from quantum_potential

Cosmology from quantum_potential

• 1.
  arXiv:1404.3093v3[gr-qc]29Dec2014 Cosmology from quantumpotential Ahmed Farag Ali 1,2∗ and Saurya Das 3† 1 Center for Theoretical Physics, Zewail City of Science and Technology, Giza, 12588, Egypt. 2 Dept. of Physics, Faculty of Sciences, Benha University, Benha, 13518, Egypt. and 3 Department of Physics and Astronomy, University of Lethbridge, 4401 University Drive, Lethbridge, Alberta, Canada T1K 3M4 It was shown recently that replacing classical geodesics with quantal (Bohmian) trajectories gives rise to a quantum corrected Raychaudhuri equation (QRE). In this article we derive the second order Friedmann equations from the QRE, and show that this also contains a couple of quantum correction terms, the first of which can be interpreted as cosmological constant (and gives a correct estimate of its observed value), while the second as a radiation term in the early universe, which gets rid of the big-bang singularity and predicts an infinite age of our universe. The generally accepted view of our universe (homoge- neous, isotropic, spatially flat, obeying general relativ- ity, and currently consisting of about 72% Dark Energy, likely in the form of a cosmological constant Λ, about 23% Dark Matter, and the rest observable matter), im- plies its small acceleration, as inferred from Type IA su- pernova observations, CMBR data and baryon acoustic oscillations [1–4]. However, quite a few things remain to be better understood, e.g., (i) the smallness of Λ, about 10−123 in Planck units (‘the smallness problem’), (ii) the approximate equality of vacuum and matter den- sity in the current epoch (‘the coincidence problem’), (ii) the apparent extreme fine-tuning required in the early universe, to have a spatially flat universe in the current epoch (‘the flatness problem’), (iv) the true nature of dark matter, and (v) the beginning of our universe, or the so-called big- bang. In this article, we show that one may be able to get a better understanding of some of the above problems by studying the quantum correction terms in the sec- ond order Friedmann equation, derived from the quan- tum corrected Raychaudhuri equation (QRE), which in turn was obtained by replacing geodesics with quantal (Bohmian) trajectories [5] (This formulation of quan- tum mechanics gives rise to identical predictions as those of ordinary quantum mechanics). In particular, while one correction term can be interpretable as dark en- ergy, with the right density, and providing a possible explanation of the coincidence problem, the other term can be interpreted as a radiation term in the early uni- verse, preventing the formation of a big-bang type sin- ∗email: ahmed.ali@fsc.bu.edu.eg; afarag@zewailcity.edu.eg †email: saurya.das@uleth.ca gularity, and predicting an infinite age of our universe. One naturally assumes a quantum mechanical descrip- tion of the fluid or condensate filling our universe, de- scribed by a wavefunction ψ = ReiS (assumed normal- izable and single valued. Some well-studied examples in curved spacetimes, including in cosmology, include refs.[6–9]. R(xα ), S(xa ) = real functions), associated with the four velocity field ua = ( /m)∂aS, and expan- sion θ = T r(ua;b) = hab ua;b, hab = gab − uaub (with vanishing shear and twist, for simplicity. The constant ǫ1 = 1/6 for conformally invariant scalar fluid, but left arbitrary here.). We will see later in this article that a condensate composed of gravitons with a tiny mass is a natural candidate for this fluid. Then the quantum cor- rected Raychaudhuri equation follows [10] 1 dθ dλ = − 1 3 θ2 − Rcduc ud + 2 m2 hab ✷R R ;a;b + ǫ1 2 m2 hab R;a;b, (1) Note that Eq.(1) follows directly the Klein-Gordon or Dirac equation (or the Schr¨odinger equation for non-relativistic situations), and the quantum corrected geodesic equation that follows from them [10]. The sec- ond order Friedmann equation satisfied by the scale fac- tor a(t) can be derived from the above, by replacing θ = 3˙a/a , and Rcduc ud → 4πG 3 (ρ + 3p) − Λc2 /3, [11] 1 We use the metric signature (−, +, +, +) here, as opposed to (+, −, −, −) in [10], resulting in opposite sign of the 2 terms.
• 2.
  2 2 3 (here thecosmological constant Λ has dimensions of 1/(length)2 as usual.) ¨a a = − 4πG 3 (ρ + 3p) + Λc2 3 + 2 3m2 hab ✷R R ;a;b + ǫ1 2 m2 hab R;a;b , (2) where the density ρ includes visible and dark matter, and may also include additional densities that arise in massive non-linear theories of gravity [16–18]. The 2 terms in Eqs.(1) and (2) represent quantum corrections (the first of these is also known as quantum potential), which vanish in the → 0 limit, giving back the clas- sical Raychaudhuri and the Friedmann equations. Note that these additional terms are not ad-hoc or hypotheti- cal, but rather an unavoidable consequence of a quantum description of the contents of our universe. Also, since it is well known that Bohmian trajectories do not cross [19, 20], it follows that even when θ (or ˙a) → −∞, the actual trajectories (as opposed to geodesics) do not con- verge, and there is no counterpart of geodesic incomplete- ness, or the classical singularity theorems, and singular- ities such as big bang or big crunch are in fact avoided. This view is also supported by the quantum corrected geodesic deviation equation derived in [10], which sug- gested that trajectories can never actually access infinite curvatures 4 . We will return to this issue later, and con- sider the first of these terms, which naturally appears as a cosmological constant ΛQ = 2 m2c2 hab ✷R R ;a;b . (3) ΛQ depends on the amplitude R of the wavefunction ψ, which we take to be the macroscopic ground state of a condensate (more on the details of condensate in [21]). Its exact form is not important to our argument however, except that it is non-zero and spread out over the range of the observable universe. This follows from the require- ment of causality; even if matter exists beyond the hori- zon, it will have no effect on what is inside the horizon, including the wavefunction. To estimate ΛQ, one may as- sume a Gaussian form ψ ∼ exp(−r2 /L2 0), or for one which results when an interaction of strength g is included in a 2 This procedure, as well the rest of the paper assumes large scale homogeneity and isotropy. Even if there are small (perturba- tive) deviations from homogeneity, these can be absorbed in an effective density ρ. Further these do not affect the dark energy content and accelerated expansion of the universe [12, 13]. 3 Note that in [14] too, the authors studied dark energy from the Bohmian mechanics perspective, but originating in a scalar field with non-standard action. Also, recently in [15], the authors used Bohmian mechanics in the context of Wheeler-DeWitt equation, to explain inflation. 4 A similar conclusion was also arrived at by the frequency depen- dence of light paths (‘gravity’s rainbow’) picture in [22] . scalar field theory, such that ψ = ψ0 tanh(r/L0 √ 2) (g > 0) and ψ = √ 2 ψ0 sech(r/L0) (g < 0) [23], it can be easily shown that (✷R/R);a;b ≈ 1/L4 0, where L0 is the characteristic length scale in the problem, which is of the order of the Compton wavelength L0 = h/mc [24], over which the wavefunction is non-vanishing. This gives ΛQ = 1 L2 0 = mc h 2 , (4) which has the correct sign as the observed cosmological constant. Next to estimate its magnitude, we identify L0 with the current linear dimension of our observable universe, since anything outside it would not influence an accessible wavefunction. With this, m can be re- garded as the small mass of gravitons (or axions), with gravity (or Coulomb field) following a Yukawa type of force law F = −Gm1m2 r2 exp(−r/L0). Since gravity has not been tested beyond this length scale, this interpre- tation is natural, and may in fact be unavoidable [21]. If one invokes periodic boundary conditions, this is also the mass of the lowest Kaluza-Klein modes. Substitut- ing L0 = 1.4 × 1026 m, one obtains m ≈ 10−68 kg or 10−32 eV , quite consistent with the estimated bounds on graviton masses from various experiments [25], and also from theoretical considerations [26–29]. In other words, we interpret the quantum condensate as made up of these gravitons, and described by a macroscopic wavefunction. Finally, plugging in the above value of L0 in Eq.(4), we get ΛQ = 10−52 (metre)−2 (5) = 10−123 (in Planck units) , (6) which indeed matches the observed value. Also, since the size of the observable universe is about c/H0, where H0 is the current value of the Hubble parameter [31], one sees why the above value of ΛQ numerically equals H2 0 /c2 (which is 8πG/3c4 × ρcrit, the critical density), offering a viable explanation of the coincidence problem. Note that the above also implies that this equality will hold at all times during the evolution of the universe. While the above relationship may have been known, here we provide a natural explanation for it, in terms of quan- tum corrections to the Raychaudhuri equations via the wavefunction of the cosmic fluid. This also brings out the essential quantum origin of the small cosmological constant. One may also be tempted to interpret the quantum po- tential term as a perfect fluid with wQ ≡ pQ/ρQ = −1/3 (such that ρQ ∝ a−3(1+wQ) ∝ a−2 ). However, a careful analysis using chi-squared techniques, of the luminosity distances versus red-shifts of 580 union 2.1 Supernovae data points, baryon acoustic oscillations, Hubble and CM shift parameters suggest that the corresponding density does not exceed 5% of the current density of the universe (at 95% confidence level), and also does not play any sig- nificant role in early or late epochs [30]. We therefore do not consider that possibility.
• 3.
  3 Next, we considerthe second correction term in Eq.(2), which can be written in terms of H = ˙a/a, and for one species of fluid, with p/ρ = w as 5 ˙H = − 3 2 (1 + w)H2 + ǫ1 2 m2 hab R;a;b = − 3 2 (1 + w)H2 − ǫ1 2 m2 6H4 (1 + w) 6(1 + w)2 − 81 2 (1 + w) + 18 ,(7) where in the last step we have plugged in the FRW met- ric. When = 0, integration yields H → ∞ in a finite time, signifying a big-bang type of singularity. It is in- teresting to note that H4 proportional terms were also obtained from (i) the trace anomaly of a conformal field theory dual to a five-dimensional Schwarzschild-AdS ge- ometry, and which is known as holographic/conformal- anomaly Friedmann equation [33, 34], (ii) correction to Raychaudhuri equation in cosmology derived in brane world scenarios [35], and (iii) derived in spacetime ther- modynamics and the generalized uncertainty principle of quantum gravity [34]. It would be interesting to investi- gate underlying connections between the above if any. Next, to examine the presence or absence of past sin- gularities, we write Eq.(7) as ˙H = F(H) , (8) and from which the age of the universe as T = T 0 dt = HP H0 dH F(H) , (9) where H0 signifies the current epoch. For the ordinary FRW universe with no quantum corrections of the above type (i.e. ǫ1 = 0), F(H) = −(3/2)(1+w2 )H2 , the density and H → ∞ ≡ HP in the past (big-bang singularity), where | ˙H| → ∞ as well, and we get T = 2 3(1 + w)2H1 , (10) which once again, is finite. Now if corrections to the classical FRW model changes the nature of the function F(H) (e.g. the degree of the polynomial), such that now neither H nor ˙H diverges, then if HP signifies the nearest fixed point in the past, such that F(HP ) = 0, we approx- imate F(H) = F(n) (HP )(H −HP )n near the fixed point, the region which contributes most to the integral, and obtain [36] T = 1 F(n)(HP ) HP H1 dH (H − HP )n → ∞ , (11) 5 The following discussions, and in particular the conclusion fol- lowing from Eq.(11), remain valid even if the first (cosmological constant) term is retained. FIG. 1: ˙H versus H . signifying a universe without a beginning. This is pre- cisely what is expected from the no-focusing of geodesics and the quantum Raychaudhuri equation. For example, for Eq.(2), it can be easily shown that the sign of the quantum correction (H4 ) term is positive (i.e. opposite to the classical, H2 term), for −0.52 ≤ w ≤ 5.27, which covers most of the physically interesting range, including w = wR = 1/3 (radiation), which is most relevant for the very early universe, and also non-relativistic matter (w = wNR = 0). The situation is depicted in Figure (1), where it can be seen that in the above range of w, HP is indeed finite, and therefore T is infinite from Eq.(11). Thus, the second quantum correction in the Friedmann equation gets rid of the big-bang singularity. In summary, we have shown here that as for the QRE, the second order Friedmann equation derived from the QRE also contains two quantum correction terms. These terms are generic and unavoidable and follow naturally in a quantum mechanical description of our universe. Of these, the first can be interpreted as cosmological con- stant or dark energy of the correct (observed) magnitude and a small mass of the graviton (or axion). The second quantum correction term pushes back the time singu- larity indefinitely, and predicts an everlasting universe. While inhomogeneous or anisotropic perturbations are not expected to significantly affect these results, it would be useful to redo the current study with such small per- turbations to rigorously confirm that this is indeed the case. Also, as noted in the introduction, we assume it to follow general relativity, whereas the Einstein equations may themselves undergo quantum corrections, especially at early epochs, further affecting predictions. Given the robust set of starting assumptions, we expect our main results to continue to hold even if and when a fully satis- factory theory of quantum gravity is formulated. For the cosmological constant problem at late times on the other hand, quantum gravity effects are practically absent and can be safely ignored. We hope to report on these and related issues elsewhere.
• 4.
  4 Acknowledgment We thank S.Braunstein, M. W. Hossain, S. Kar, M. Sami, T. Sarkar and S. Shankaranarayanan for useful comments. We thank the anonymous referee for useful comments. This work is supported by the Natural Sci- ences and Engineering Research Council of Canada, and the Perimeter Institute for Theoretical Physics, through their affiliate program. The research of A. Farag Ali is supported by Benha University (www.bu.edu.eg) and CFP in Zewail City, Egypt. [1] S. Perlmutter et al. (The Supernova Cosmology Project) (1999), Astrophysical Journal 517 (2) (1999) 56586 [arXiv:astro-ph/9812133]. [2] A. G. Riess et al., Astron. J. 116 (1998) 1009 [arXiv:astro-ph/9805201]. [3] G. Hinshaw et al, arXiv:1212.5226 [astro-ph.CO]. [4] D. J. Eisenstein et al, Astrophys. J. 633 (2005) 560 [arXiv:astro-ph/0501171]. [5] D. Bohm, Phys. Rev. 85 (1952) 166; D. Bohm, B. J. Hiley, P. N. Kaloyerou, Phys. Rep. 144, No.6 (1987) 321. [6] S. W. Hawking, Commun. Math. Phys. 43 (1975) 199. [7] K. Melnikov, M. Weinstein, Int. J. Mod. Phys. D13 (2004) 1595 [arXiv:hep-th/0205223]. [8] J. B. Hartle, S. W. Hawking, Phys. Rev. D28 (1983) 2960. [9] R. Tumulka, arXiv:0708.0070; Gen. Rel. Grav.42 (2010) 303-346 [arXiv:0808.3060]. [10] S. Das, Phys. Rev. D89 084068 (2014) [arXiv:1311.6539]. [11] A. K. Raychaudhuri, Theoretical Cosmology, Oxford (1979). [12] E. R. Siegel, J. N. Fry, Astrophys. J. 628 (2005) L1-L4 [arXiv:astro-ph/0504421]. [13] A. Ishibashi, R. M. Wald, Class. Quant. Grav. 23 (2006) 235-250 [arXiv:gr-qc/0509108]. [14] P. F. Gonzalez-Diaz, A. Rozas-Fernandez, Phys. Lett. B641 (2006) 134 [arXiv:astro-ph/0609263]; Class. Quant. Grav. 25 (2008) 175023 [arXiv:0807.2055]; A gra- cious exit from the matter-dominated phase in a quantum cosmic phantom model, Phys.Lett.B (2014), in press. [15] D. He, D. Gao, Q. Cai, Phys. Rev. D89 (2014) 083510 [arXiv:1404.1207]. [16] G. Leon, J. Saavedra, E. N. Saridakis, Class. Quant. Grav. 30 (2013) 135001 [arXiv:1301.7419]. [17] R. Gannouji, M. W. Hossain, M. Sami, E. N. Saridakis, Phys. Rev. D87 (2013) 123536 [arXiv:1304.5095]. [18] C. de Rham, M. Fasiello, A. J. Tolley, arXiv:1410.0960. [19] P. R. Holland, The Quantum Theory of Motion, Cam- bridge (1993). [20] C. Phillipidis, C. Dewdney, B. J. Hiley, Nuovo Cimento, 52B (1979) 15-28; D. A. Deckert, D. D¨urr, P. Pickl, J. Phys. Chem. A, 111, 41 (2007) 10325; A. S. Sanz, J. Phys.: Conf. Ser. 361, 012016 (2012); A. Figalli, C. Klein, P. Markowich, C. Sparber, WKB analysis of Bohmian dynamics, arXiv:1202.3134. [21] S. Das, R. K. Bhaduri, Dark energy and dark matter from Bose-Einstein condensate, arXiv:1411.0753. [22] A. Awad, A. F. Ali, B. Majumder, JCAP 10 (2013) 052 [arXiv:1308.4343]. [23] J. Rogel-Salazar, Eur. J. Phys. 34 (2013) 247 [arXiv:1301.2073]; N. ¨Uzar, S. Deniz Han, T. T¨ufekci, E. Aydiner, arXiv: 1203.3352. [24] A. Wachter, Relativistic Quantum Mechanics, Springer (2010). [25] C. M. Will, Phys. Rev. D57 (1998) 2061; L. S. Finn, P. J. Sutton Phys. Rev. D65 (2002) 044022 [arXiv:gr-qc/0109049]; A. S. Goldhaber, M. M. Nieto, Rev. Mod. Phys. 82 (2010) 939 [arXiv:0809.1003] E. Berti, J, Gair, A. Sesana, Phys. Rev. D84 (2011) 101501(R). [26] F. Zwicky, Cosmic and terretrial tests for the rest mass of gravitons, Publications of the Astronomical Society of the Pacific, Vol. 73, No. 434, p.314. [27] J. R. Mureika, R. B. Mann, Mod. Phys. Lett. A26 (2011) 171-181 [arXiv:1005.2214]. [28] C. de Rham, G. Gabadadze, L. Heisenberg, D. Pirtskhalava, Phys. Rev. D83 (2011) 103516 [arXiv:1010.1780]; C. de Rham, G. Gabadadze, A. J. Tolley, Phys. Rev. Lett. 106 (2011) 231101 [arXiv:1011.1232]. [29] S. Majid, arXiv:1401.0673. [30] M. W. Hossain (private communication). [31] S. Weinberg, Cosmology, Oxford (2008). [32] A. G. Riess et al, Astrophys.J. 659 (2007) 98-121 [arXiv:astro-ph/0611572]; W. M. Wood-Vasey et al, As- trophys. J. 666 (1007) 694-715 [arXiv:astro-ph/0701041]; T. M. Davis et al, Astrophys.J. 666 (2007) 716-725 [arXiv:astro-ph/0701510]. [33] P. S. Apostolopoulos, G. Siopsis and N. Tetradis, Phys. Rev. Lett. 102, 151301 (2009) [arXiv:0809.3505 [hep-th]]. [34] J. E. Lidsey, Phys. Rev. D 88, 103519 (2013) [arXiv:0911.3286 [hep-th]]. [35] R. Maartens and K. Koyama, Living Rev. Rel. 13, 5 (2010) [arXiv:1004.3962 [hep-th]]. [36] A. Awad, Phys. Rev. D87 (2013) 103001 [arXiv:1303.2014].