Research Article | Open Access

Rong-Jia Yang, Shuang Nan Zhang, Bohai Chen, Jing-Zhao Qi, "An Old Universe in K-Essence Cosmology", *International Scholarly Research Notices*, vol. 2011, Article ID 523617, 5 pages, 2011. https://doi.org/10.5402/2011/523617

# An Old Universe in K-Essence Cosmology

**Academic Editor:**P. Marziani

#### Abstract

Models in which the Lagrangian contains only a kinetic factor and does not depend explicitly on the field itself in k-essence cosmology are considered. In the case of a constant potential, we obtain an exact analytic solution of k-essence, , under a simple hypothesis, , but without any assumption about the form of . In purely kinetic k-essence model, the acceleration can only be induced after the matter-dominated epoch; the Universe is about Gyr old now and about 3.1 Gyr old at which is consistent with the fact of quasar observation, while this observation contradicts with the prediction of ΛCDM model, and the year where a transition from deceleration to acceleration expansion is about from 18.43 Gyr to 25.2 Gyr after the beginning of the Universe. These results may offer clues to test observationally the k-essence scenario in the future.

#### 1. Introduction

According to several different kinds of astrophysical studies including supernova (SN) Ia [1, 2], large-scale structure [3], and Cosmic Microwave Background (CMB) [4], the present Universe energy content seems to consist of approximately 4% baryons, 26% dark matter, and a little amount of radiation. The remaining around 70% of its energy, with a positive energy density but a negative pressure, is homogeneously distributed in the universe and is causing the accelerated expansion of the Universe. The simplest candidate of dark energy is the vacuum energy (or the cosmological constant ) [5]. This scenario is in general agreement with various data but suffers from two very difficult problems: first, why the vacuum energy is so much smaller than its expected natural value (the cosmological constant problem) [6]; second, why the matter and dark energy densities are of precisely the same order of magnitude today (the coincidence problem). Recently, it was shown that CDM model may also suffer from age problem [7].

It is thus natural to pursue alternative possibilities to explain the mystery of dark energy, such as quintessence [8], phantom [9, 10], -essence [11], tachyon [12–14], DGP [15], and spatial Ricci scalar dark energy model [16]. K-essence, a simple approach toward constructing a model for an accelerated expansion of the Universe, is to work with the idea that the unknown dark energy component is due to a minimally coupled scalar field with noncanonical kinetic energy [17]. A feature of k-essence models is that the negative pressure results from the nonlinear kinetic energy of the scalar field. Secondly, because of the dynamical attractor behavior, cosmic evolution is insensitive to initial conditions in k-essence theories. Thirdly, k-essence changes its speed of evolution in dynamic response to changes in the background EoS.

K-essence scenario has received much attention, it was originally proposed as a model for inflation [18] and then as a model for dark energy [17]. The stability of k-essence was studied in [19]. Dynamics of k-essence were discussed in [20]. Conditions for stable tracker solutions for k-essence in a general cosmological background were derived in [21]. Slow-roll conditions for thawing k-essence were obtained in [22]. The geometrical diagnostic for purely kinetic k-essence dark energy was discussed in [23]. In several cases, k-essence cannot be observationally distinguished from quintessence [24]. Models of k-essence unified dark matter were discussed in [25–27]. Theoretical and observational Constraints on k-essence dark energy models were discussed in [14, 28, 29]. A model-independent method to reconstruct the Lagrangian of the k-essence field by the parameterized Hubble parameter was studied in detail in [30]. With assumptions on the EoS of k-essence as functions of the scale factor , the forms of the Lagrangians were discussed in [31]. With some assumptions on the EoS of k-essence as functions of the kinetic energy , the evolution of purely kinetic k-essence is investigated in [32]. Ermakov invariant of k-essence and its observational probes for the early universe are discussed in [33].

In this paper, we will reconsider in k-essence cosmology the simplest k-essence models put forward by Scherrer [25] in which the Lagrangian contains only a kinetic factor and does not depend explicitly on the field itself. Sticking with a constant k-field potential, we obtain a general equation of state of k-essence under a simple hypothesis. This equation of state indicates that the acceleration expansion of the Universe can only be induced after the matter-dominated epoch in a very old Universe of about Gyr old, and the year where a transition from deceleration to acceleration expansion is about from 18.4 Gyr to 25.2 Gyr after the beginning of the Universe.

#### 2. K-Essence Cosmology

As a candidate of dark energy, k-essence is defined as a scalar field with nonlinear kinetic terms which appears generically in the effective action in string and supergravity theories; its action minimally coupled with gravity generically may be expressed as [17, 18, 34, 35]

where , and we take throughout this paper. The Lagrangian depends on the specific-particle theory model. In this paper, we consider only factorial Lagrangian of the form

and is a function of the kinetic energy . The name of the model suggests that the field should be driven only by its kinetic energy. For this to be strictly true, one should impose . Using the perfect fluid analogy, the energy density is given by

where . The corresponding equation of state parameter, , is just

The effective sound speed is given by

with . This definition comes from the equation describing the evolution of linear perturbations in a k-essence dominated Universe [34], and therefore is relevant when studying the stability of the theory.

From the conservation equation in a flat Friedmann-Robertson-Walker (FRW) spacetime, one finds the equation for the k-field [35]:

If at some , the sound speed must diverge, then the equation is singular and reduces to first-order equation which gives a constraint on the potential which must have the form: [36]. As discussed in [37], this case may violates the causality.

In (2), a variety of functional forms for and have been investigated [11, 35]. Note that the sound speed, , and the state parameter, , do not depend explicitly on in any case. Without losing generality, we take the to be a constant which was extensively discussed in [14, 25, 28–32, 36]; in other words, we consider a purely kinetic model in which

From (6), we get a new equation for :

In [36] (see also [25, 29]), a theoretical constraint on purely kinetic k-essence was obtained

where is a constant of integration. This Equation can be rewritten as [29]

where is a constant of integration.

Like in [25], we take k-essence as unified dark matter. Then in a flat homogeneous FRW space-time, one of Einstein field equations can be written as

Inserting (9) or (10) into (11), we can have

where is a constant. This is an exact analytic solution for generic forms of . Assuming [38] and (during one certain epoch (see dust dominated era), and can be seen as a constant), we obtain

here , is the present value of the scale factor, and is a constant. Comparing (13) with (12), we get . From (9), we can have

where is a constant. For or , the right side of (14) is zero, meaning that is a constant which is just zero (because ); these cases are not considered anymore in the rest of this paper. Integrating (14), we get the equation of state, :

where is a constant. Inserting (15) into (3), (4), (5), we get, respectively,

Consequently, we obtain a set of equations of purely kinetic k-essence without any assumption for the form of . To keep at any time, we must require that if , then , or, if , then . As explained above, we also require that . An interesting feature of our result is that the sound speed, , and the state parameter, , are exactly equal, whose values only depend on the parameter . For any value of , one may always have , which means that purely kinetic k-essence does not violate the strong energy condition.

Obviously, for , the energy density runs to zero, but the pressure does not, so the equation of state and the sound speed diverge. This case can be considered as an unphysical state and does not violate causality [39]. The way to stop running into these divergences is if relativistic dynamics intervenes to stop the field from running to [40].

For (), one can have and , which are the characteristics of the matter-dominated epoch. In this case-purely kinetic k-essence evolves asymptotically like a dark matter component. Another interesting result in this case is that the sound speed is near zero (). This property can suppress the integrated Sachs-Wolfe (ISW) effect at large angular scales [41].

When , k-essence is characterized by negative pressure which means that Cosmic acceleration can only be induced after the matter-dominated epoch in our model. This transition of k-essence from positive to negative pressure is automatically triggered by dynamics.

Figure 1 shows the evolution of the equation of state parameter of k-essence as a function of redshift in purely kinetic k-essence cosmology and the evolution of the equation of state parameter of standard CDM cosmology as a function of redshift . Taking the present (zero redshift) value of the equation of state parameter of dark energy without *a priori* assumption of a flat Universe, [42], and the normalized density for matter [43], we obtain for purely kinetic k-essence cosmology, while for standard CDM cosmology; this difference can be tested to distinguish purely kinetic k-essence cosmology from standard CDM cosmology. In this case, one may find ; this makes a definite prediction to the local expansion rate of our Universe and thus may be tested observationally. Any model which agrees with the SN Ia observations and has entered an accelerating phase in the recent epoch will have this feature [38]. Taking [44], one can easily obtain the age of the Universe which is about Gyr in purely kinetic k-essence cosmology; this result does not conflict with the stellar age bound: 11-12 Gyr.

As the equation of state of k-essence takes the value of , the Universe is just under a transition from deceleration to acceleration of expansion, which is constrained at redshift [45] or [46], that is, about from 18.4 Gyr (corresponding to point a) to 25.2 Gyr (corresponding to point b) after the beginning of the Universe in purely kinetic k-essence cosmology

From the observation of the very old quasar APM the Universe is more than 2.1 Gyr old [47] at , which contradicts the age ( Gyr) predicted by standard CDM cosmology [7]. In fact, the age of the quasar APM is still in debate, which is dependent on the Fe/O abundance ratio. The age of this quasar was initially estimated to be around 2-3 Gyr [48] and reevaluated to be 2.1 Gyr by using a chemodynamical model for the evolution of spheroids [47]. The age of 2.1 Gyr is set by the condition that Fe/O abundance ratio of the model reaches 3.3, which is the best-fitting value obtained in [49] (for detail see [7]). Of course, new and more reliable determination of the age of APM are also needed. We mention in passing that when Dunlop et al. paper first came out, the 3.5 Gyr age of 53W091 at was a significant problem for a non- cosmology [50]; however, soon thereafter papers came out that did spectral energy distribution (SED) fitting of its Keck spectrum that allowed new age estimates of around 1.8 Gyr [51, 52]. In addition, as discussed in [7], to understand the age problem better, we must be taking into account more recent observations on the value of the Hubble constant and more recent results on iron nucleosynthesis.

As seen in Figure 1, the equal-age line for 2.1 Gyr requires that . Since physically must be true, therefore the age of the Universe at must be significantly older than 2.1 Gyr, in agreement with data. In fact, the equal-age line for 2.1 Gyr crosses at , implying that astrophysical objects at the same age as the quasar APM may be found up to , since at such high redshift the Universe should be dominated by dusk-like matter, that is, . This prediction may also be tested with high redshift observations of galaxies and quasars in the future.

We note, however, that after the dark-energy-dominated epoch, k-essence must be characterized by imaginary sound speed, this seems somewhat unreasonable. However, is not a sufficient condition for a theory to be stable [53]. When the generation of shear in the fluid is taken into account, the perturbation growth for can be stabilized [54]. This issue is worthy of further studying.

#### 3. Conclusions and Discussions

To summarize, an equation of state of purely kinetic k-essence is obtained under a simple hypothesis but without any assumptions about the form of in k-essence cosmology. This equation indicates that cosmic acceleration can only be induced after the dark-matter-dominated epoch. Because of dynamical reasons, there does not exist an epoch completely filled up by dark matter in purely kinetic k-essence cosmology. In this model of k-essence, the age of the Universe is about 33.5 Gyr now, the epoch when a transition from deceleration to acceleration expansion happened is about 18.4 to 25.2 Gyr after the beginning of the Universe, the Universe is much older than 2.1 Gyr at , and very old astrophysical objects with ages of around 2.1 Gyr exist up to redshift of 5.3. These results can be tested observationally in the future.

#### Acknowledgments

This study is supported in part by National Natural Science Foundation of China under Grant no. 11147028, Hebei Provincial Natural Science Foundation of China under Grant no. A2011201147, and Research Fund for Doctoral Programs of Hebei University under Grant no. 2009-155.

#### References

- A. G. Riess, A. V. Filippenko, P. Challis et al., “Observational evidence from supernovae for an accelerating universe and a cosmological constant,”
*Astronomical Journal*, vol. 116, no. 3, pp. 1009–1038, 1998. View at: Google Scholar - S. Perlmutter, G. Aldering, G. Goldhaber et al., “Measurements of Ω and Λ from 42 high-redshift Supernovae,”
*Astrophysical Journal*, vol. 517, no. 2, pp. 565–586, 1999. View at: Google Scholar - M. Tegmark, M. A. Strauss, M. R. Blanton et al., “Cosmological parameters from SDSS and WMAP,”
*Physical Review D*, vol. 69, no. 10, Article ID 103501, 2004. View at: Publisher Site | Google Scholar - D. N. Spergel, L. Verde, H. V. Peiris et al., “First-year Wilkinson Microwave Anisotropy Probe (WMAP) observations: determination of cosmological parameters,”
*Astrophysical Journal, Supplement Series*, vol. 148, no. 1, pp. 175–194, 2003. View at: Publisher Site | Google Scholar - S. Weinberger, “The cosmological constant problem,”
*Reviews of Modern Physics*, vol. 61, no. 1, pp. 1–23, 1989. View at: Google Scholar - V. Sahni and A. Starobinsky, “The case for a postive cosmological
*λ*-term,”*International Journal of Modern Physics D*, vol. 9, no. 4, pp. 373–443, 2000. View at: Google Scholar - R. J. Yang and S. N. Zhang, “The age problem in the ΛCDM model,”
*Monthly Notices of the Royal Astronomical Society*, vol. 407, no. 3, pp. 1835–1841, 2010. View at: Publisher Site | Google Scholar - B. Ratra and P. J. E. Peebles, “Cosmological consequences of a rolling homogeneous scalar field,”
*Physical Review D*, vol. 37, no. 12, pp. 3406–3427, 1988. View at: Publisher Site | Google Scholar - R. R. Caldwell, “A phantom menace? Cosmological consequences of a dark energy component with super-negative equation of state,”
*Physics Letters, Section B*, vol. 545, no. 1-2, pp. 23–29, 2002. View at: Publisher Site | Google Scholar - S. M. Carroll, M. Hoffman, and M. Trodden, “Can the dark energy equation-of-state parameter w be less than −1?”
*Physical Review D*, vol. 68, no. 2, Article ID 023509, 2003. View at: Publisher Site | Google Scholar - C. Armendariz-Picon, V. Mukhanov, and P. J. Steinhardt, “Dynamical solution to the problem of a small cosmological constant and late-time cosmic acceleration,”
*Physical Review Letters*, vol. 85, no. 21, pp. 4438–4441, 2000. View at: Publisher Site | Google Scholar - T. Padmanabhan, “Accelerated expansion of the universe driven by tachyonic matter,”
*Physical Review D*, vol. 66, no. 2, Article ID 021301, 2002. View at: Publisher Site | Google Scholar - Z. Keresztes, L. Á. Gergely, V. Gorini, U. Moschella, and A. Y. Kamenshchik, “Tachyon cosmology, supernovae data, and the big brake singularity,”
*Physical Review D*, vol. 79, no. 8, Article ID 083504, 2009. View at: Publisher Site | Google Scholar - R. J. Yang, S. N. Zhang, and Y. Liu, “Constraints on the generalized tachyon field models from latest observational data,”
*Journal of Cosmology and Astroparticle Physics*, vol. 2008, no. 1, article 017, 2008. View at: Publisher Site | Google Scholar - G. Dvali, G. Gabadadze, and M. Porrati, “4D gravity on a brane in 5D Minkowski space,”
*Physics Letters, Section B*, vol. 485, no. 1–3, pp. 208–214, 2000. View at: Publisher Site | Google Scholar - R. -J. Yang, Z. -H. Zhu, and F. Wu, “Spatial Ricci scalar dark energy model,”
*International Journal of Modern Physics A*, vol. 26, no. 2, pp. 317–329, 2011. View at: Publisher Site | Google Scholar - C. Armendariz-Picon, V. Mukhanov, and P. J. Steinhardt, “Dynamical solution to the problem of a small cosmological constant and late-time cosmic acceleration,”
*Physical Review Letters*, vol. 85, no. 21, pp. 4438–4441, 2000. View at: Publisher Site | Google Scholar - C. Armendáriz-Picón, T. Damour, and V. Mukhanov, “k-inflation,”
*Physics Letters, Section B*, vol. 458, no. 2-3, pp. 209–218, 1999. View at: Google Scholar - L. R. Abramo and N. Pinto-Neto, “Stability of phantom k-essence theories,”
*Physical Review D*, vol. 73, no. 6, Article ID 063522, pp. 1–4, 2006. View at: Publisher Site | Google Scholar - A. D. Rendall, “Dynamics of k-essence,”
*Classical and Quantum Gravity*, vol. 23, no. 5, pp. 1557–1569, 2006. View at: Publisher Site | Google Scholar - R. Das, T. W. Kephart, and R. J. Scherrer, “Tracking quintessence and k-essence in a general cosmological background,”
*Physical Review D*, vol. 74, no. 10, Article ID 103515, 2006. View at: Publisher Site | Google Scholar - T. Chiba, S. Dutta, and R. J. Scherrer, “Slow-roll k-essence,”
*Physical Review D*, vol. 80, no. 4, Article ID 043517, 2009. View at: Publisher Site | Google Scholar - X. T. Gao and R. J. Yang, “Geometrical diagnostic for purely kinetic k-essence dark energy,”
*Physics Letters, Section B*, vol. 687, no. 2-3, pp. 99–102, 2010. View at: Publisher Site | Google Scholar - M. Malquarti, E. J. Copeland, A. R. Liddle, and M. Trodden, “A new view of k-essence,”
*Physical Review D*, vol. 67, no. 12, Article ID 123503, 2003. View at: Publisher Site | Google Scholar - R. J. Scherrer, “Purely kinetic k essence as unified dark matter,”
*Physical Review Letters*, vol. 93, no. 1, Article ID 011301, 4 pages, 2004. View at: Publisher Site | Google Scholar - D. Bertacca, S. Matarrese, and M. Pietroni, “Unified dark matter in scalar field cosmologies,”
*Modern Physics Letters A*, vol. 22, no. 38, pp. 2893–2907, 2007. View at: Publisher Site | Google Scholar - N. Bose and A. S. Majumdar, “K-essence model of inflation, dark matter, and dark energy,”
*Physical Review D*, vol. 79, no. 10, Article ID 103517, 2009. View at: Publisher Site | Google Scholar - R. J. Yang and X. T. Gao, “Observational constraints on purely kinetic k-essence dark energy models,”
*Chinese Physics Letters*, vol. 26, no. 8, article 089501, 2009. View at: Publisher Site | Google Scholar - D. Zhang, “A natural topology for fuzzy numbers,”
*Journal of Mathematical Analysis and Applications*, vol. 264, no. 2, pp. 344–353, 2001. View at: Publisher Site | Google Scholar - A. A. Sen, “Reconstructing k-essence,”
*Journal of Cosmology and Astroparticle Physics*, no. 3, pp. 177–187, 2006. View at: Publisher Site | Google Scholar - R. de Putter and E. V. Linder, “Kinetic k-essence and quintessence,”
*Astroparticle Physics*, vol. 28, no. 3, pp. 263–272, 2007. View at: Publisher Site | Google Scholar - R. J. Yang, J. Z. Qi, and B. Z. Yang, “Restrictions on purely kinetic k-essence,”
*Chinese Physics Letters*, vol. 28, no. 10, Article ID 109502, 2011. View at: Publisher Site | Google Scholar - D. Gangopadhyay and S. Mukherjee, “On a cosmological invariant as an observational probe in the early universe,”
*Gravitation and Cosmology*, vol. 17, no. 4, pp. 349–354, 2011. View at: Publisher Site | Google Scholar - J. Garriga and V. F. Mukhanov, “Perturbations in k-inflation,”
*Physics Letters, Section B*, vol. 458, no. 2-3, pp. 219–225, 1999. View at: Google Scholar - T. Chiba, “Tracking k-essence,”
*Physical Review D*, vol. 66, no. 6, Article ID 063514, 2002. View at: Publisher Site | Google Scholar - L. P. Chimento and R. Lazkoz, “Atypical k-essence cosmologies,”
*Physical Review D*, vol. 71, no. 2, pp. 023505-1–023505-7, 2005. View at: Publisher Site | Google Scholar - C. Bonvin, C. Caprini, and R. Durrer, “No-Go theorem for k-essence dark energy,”
*Physical Review Letters*, vol. 97, no. 8, Article ID 081303, 2006. View at: Publisher Site | Google Scholar - T. Padmanabhan and T. R. Choudhury, “Can the clustered dark matter and the smooth dark energy arise from the same scalar field?”
*Physical Review D*, vol. 66, no. 8, Article ID 081301, pp. 813011–813014, 2002. View at: Google Scholar - S. Hannestad, “Constraints on the sound speed of dark energy,”
*Physical Review D*, vol. 71, no. 10, Article ID 103519, 9 pages, 2005. View at: Google Scholar - L. R. Abramo and N. Pinto-Neto, “Stability of phantom k-essence theories,”
*Physical Review D*, vol. 73, no. 6, Article ID 063522, pp. 1–4, 2006. View at: Publisher Site | Google Scholar - R. Dave, R. R. Caldwell, and P. J. Steinhardt, “Sensitivity of the cosmic microwave background anisotropy to initial conditions in quintessence cosmology,”
*Physical Review D*, vol. 66, no. 2, Article ID 023516, 2002. View at: Publisher Site | Google Scholar - D. N. Spergel, R. Bean, O. Doré et al., “Three-year Wilkinson Microwave Anisotropy Probe (WMAP) observations: implications for cosmology,”
*Astrophysical Journal, Supplement Series*, vol. 170, no. 2, pp. 377–408, 2007. View at: Publisher Site | Google Scholar - T. R. Choudhury and T. Padmanabhan, “Cosmological parameters from supernova observations: a critical comparison of three data sets,”
*Astronomy and Astrophysics*, vol. 429, no. 3, pp. 807–818, 2005. View at: Publisher Site | Google Scholar - W. L. Freedman, B. F. Madore, B. K. Gibson et al., “Final results from the Hubble Space Telescope key project to measure the Hubble constant,”
*Astrophysical Journal Letters*, vol. 553, no. 1, pp. 47–72, 2001. View at: Publisher Site | Google Scholar - A. G. Riess, L. G. Sirolger, J. Tonry et al., “Type Ia supernova discoveries at z > 1 from the hubble space telescope: evidence for past deceleration and constraints on dark energy evolution,”
*Astrophysical Journal*, vol. 607, no. 2, pp. 665–687, 2004. View at: Publisher Site | Google Scholar - C. Firmani, G. Ghisellini, G. Ghirlanda, and V. Avila-Reese, “A new method optimized to use gamma-ray bursts as cosmic rulers,”
*Monthly Notices of the Royal Astronomical Society*, vol. 360, no. 1, pp. L1–L5, 2005. View at: Publisher Site | Google Scholar - A. C. S. Friaça, J. S. Alcaniz, and J. A. S. Lima, “An old quasar in a young dark energy-dominated universe?”
*Monthly Notices of the Royal Astronomical Society*, vol. 362, no. 4, pp. 1295–1300, 2005. View at: Publisher Site | Google Scholar - S. Komossa and G. Hasinger, “The X-ray evolving universe: (ionized) absorption and dust, from nearby Seyfert galaxies to high-redshift quasars,” in
*XEUS Studying the Evolution of the Universe*, G. Hasinger, Th. Baller, and A. N. Parmar, Eds., p. 281, MPE, Garching, Germany, 2003, MPE Report. View at: Google Scholar - G. Hasinger, N. Schartel, and S. Komossa, “Discovery of an ionized Fe K edge in the z = 3.91 broad absorption line quasar APM 08279 + 5255 with XMM-Newton,”
*Astrophysical Journal*, vol. 573, no. 2, pp. L77–L80, 2002. View at: Publisher Site | Google Scholar - J. Dunlop, J. Peacock, H. Spinrad et al., “A 3.5-Gyr-old galaxy at redshift 1.55,”
*Nature*, vol. 381, no. 6583, pp. 581–584, 1996. View at: Google Scholar - S. Yi, T. M. Brown, S. Heap et al., “On the age estimation of LBDS 53W091,”
*Astrophysical Journal*, vol. 533, no. 2, pp. 670–681, 2000. View at: Google Scholar - A. G. Bruzual and C. G. Magris, “A revised age for the z = 1.55 galaxy LBDS 53W091,” in
*Cosmological Parameters and the Evolution of the Universe*, K. Sato, Ed., Proceedings IAU Symposium No. 183, p. 152, Kluwer Academic Publishers, Dordrecht, The Netherlands, 1999. View at: Google Scholar - S. M. Carroll, M. Hoffman, and M. Trodden, “Can the dark energy equation-of-state parameter w be less than −1?”
*Physical Review D*, vol. 68, no. 2, Article ID 023509, 2003. View at: Publisher Site | Google Scholar - T. Koivisto and D. F. Mota, “Dark energy anisotropic stress and large scale structure formation,”
*Physical Review D*, vol. 73, no. 8, Article ID 083502, pp. 1–12, 2006. View at: Publisher Site | Google Scholar

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Copyright © 2011 Rong-Jia Yang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.