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Bulk viscous matter and recent acceleration of the universe

Profile image of Titus K MathewTitus K Mathew

2015, The European Physical Journal C

https://doi.org/10.1140/EPJC/S10052-015-3567-6
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Abstract

The evolution of the bulk viscous matter dominated universe has been analysed using the full causal theory for the evolution of the viscous pressure in the context of the recent acceleration of the universe. The form of the viscosity is taken as ξ = αρ 1/2. We obtained analytical solutions for the Hubble parameter and scale factor of the universe. The model parameters have been computed using the observational data. The evolution of the prominent cosmological parameters was obtained. The age of the universe for the best estimated model parameters is found to be less than observational value. The viscous matter behaves like a stiff fluid in the early phase and evolves to a negative pressure fluid in the later phase. The equation of state is found to be stabilised with value ω > −1. The local as well as generalised second law of thermodynamics is satisfied. The statefinder diagnostic shows that this model is distinct from the standard ΛCDM. One of the marked deviations seen in this model to be compared with the corresponding model using the Eckart approach is that in this model the bulk viscosity decreases with the expansion of the universe, while in the Eckart formalism it increases from negative values in the early universe towards positive values.

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By means of a combined use of the type Ia supernovae and H(z) data tests, together with the study of the asymptotic properties in the equivalent phase space -through the use of the dynamical systems tools -we demonstrate that the bulk viscous matter-dominated scenario is not a good model to explain the accepted cosmological paradigm, at least, under the parametrization of bulk viscosity considered in this paper. The main objection against such scenarios is the absence of conventional radiation and matter-dominated critical points in the phase space of the model. This entails that radiation and matter dominance are not generic solutions of the cosmological equations, so that these stages can be implemented only by means of unique and very specific initial conditions, i. e., of very unstable particular solutions. Such a behavior is in marked contradiction with the accepted cosmological paradigm which requires of an earlier stage dominated by relativistic species, followed by a period of conventional non-relativistic matter domination, during which the cosmic structure we see was formed. Also, we found that the bulk viscosity is positive just until very late times in the cosmic evolution, around z < 1. For earlier epochs it is negative, been in tension with the local second law of thermodynamics.

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We analyze characteristic properties of two different cosmological models: (i) a one-component dark energy model where the bulk viscosity [Formula: see text] is associated with the fluid as a whole, and (ii) a two-component model where [Formula: see text] is associated with a dark matter component [Formula: see text] only, the dark energy component considered inviscid. Shear viscosity is omitted. We assume throughout the simple equation-of-state [Formula: see text] with [Formula: see text] a constant. In the one-component model, we consider two possibilities, either to take [Formula: see text] proportional to the scalar expansion (equivalent to the Hubble parameter), in which case the evolution becomes critically dependent on the value of the small constant [Formula: see text] and the magnitude of [Formula: see text], or we consider the case [Formula: see text], where a de Sitter final stage is reached in the future. In the two-component model, we consider only the case where the da...

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References (53)

  1. A.G. Riess et al., Supernova Search Team collaboration], Observa- tional evidence from supernovae for an accelerating universe and a cosmological constant. Astron. J. 116, 1009 (1998)
  2. S. Perlmutter et al., Supernova Cosmology Project collaboration], Measurements of Ω and Λ from 42 high redshift supernovae. Astrophys. J. 517, 565 (1999)
  3. E.J. Copeland, M. Sami, S. Tsujikawa, Dynamics of dark energy. Int. J. Mod. Phys. D 15, 1753 (2006)
  4. Y. Fujii, Origin of the gravitational constant and particle masses in scale invariant scalar tensor theory. Phys. Rev. D 26, 2580 (1982)
  5. S.M. Carroll, Quintessence and the rest of the world. Phys. Rev. Lett. 81, 3067 (1998)
  6. R.R. Cadwell, M. Kamionkowski, N.N. Weinberg, Phantom energy: dark energy with ω< -1 causes a cosmic doomsday. Phys. Rev. Lett. 91, 071301 (2003)
  7. T. Padmanabhan, S.M. Chitre, Viscous universes. Phys. Lett. B 120, 443 (1987)
  8. J.C. Fabris, S.V.B. Goncalves, R. de Sa Ribeiro, Bulk viscosity driving the acceleration of the universe. Gen. Relativ. Gravit. 38, 495 (2006)
  9. B. Li, J.D. Barrow, Does bulk viscosity create a viable unified dark matter model? Phys. Rev. D 79, 103521 (2009)
  10. W.S. Hipolito, H.E.S. Velten, W. Zimdahl, The viscous dark fluid universe. Phys. Rev. D 82, 063507 (2010)
  11. A. Avelino, U. Nucamendi, Can a matter-dominated model with constant bulk viscosity drive the accelerated expansion of the uni- verse? JCAP 04, 006 (2009)
  12. A. Avelino, U. Nucamendi, Exploring a matter-dominated model with bulk viscosity to drive the accelerated expansion of the uni- verse. JCAP 08, 009 (2010)
  13. Athira Sasidharan, Titus K. Mathew, Bulk viscous matter and recent acceleration of the universe. Eur. Phys. J. C 75, 348 (2015)
  14. Athira Sasidharan, Titus K. Mathew, Phase space analysis of bulk viscous matter dominated universe. JHEP 06, 138 (2016)
  15. I. Brevik, O. Gron, John de Haro, S.D. Odintsov, E.N. Saridakis, Viscous cosmology for early and late time universe. Int. J. Mod. Phys. D 26, 173004 (2017)
  16. K. Bamba, S. Capozeillo, S. Nojiri, S.D. Odintsove, Dark energy cosmology: the equivalent description via different theoretical models and cosmography tests. Astrophys. Space Sci. 342, 155 (2012)
  17. C. Eckart, The thermodynamics of irreversible processes. III. Rel- ativistic theory of the simple fluid. Phys. Rev. 58, 919 (1940)
  18. L.D. Landau, E.M. Lifshitz, Fluid Mechanics (Addison-Wesley, USA, 1959)
  19. W. Israel, Nonstationary irreversible thermodynamics: a causal rel- ativistic theory. Ann. Phys. (N. Y.) 100, 310 (1976)
  20. Israel, J.M. Stewart, Thermodynamics of non stationary and tran- sient effects in a relativistic gas. Phys. Lett. B 58, 213 (1976)
  21. W. Israel, J.M. Stewart, Transient relativistic thermodynamics and kinetic theory. Ann. Phys. (N. Y.) 118, 341 (1979)
  22. W. Israel, J.M. Stewart, On transient relativistic thermodynamics and kinetic theory. II. Proc. R. Soc. Lond. B 365, 43 (1979)
  23. W.A. Hiscock, J. Salmonson, Dissipative Boltzmann Robertson Walker cosmologies. Phys. Rev. D 43, 3249 (1991)
  24. A.A. Coley, R.J. van den Hoogen, Qualitative analysis of vis- cous fluid cosmological models satisfying the Israel-Stewart theory of irreversible thermodynamics. Class. Quantum Grav. 12, 1977 (1995)
  25. I. Brevik, O. Gorbunova, Dark energy and viscous cosmology. Gen. Relativ. Gravit. 37, 2039 (2005)
  26. A. Avelino, R. Garcia-Salcedo, T. Gonzalez, U. Nucamendi, I. Quiros, Bulk viscous matter dominated universe: asymptotic prop- erties. JCAP 08, 012 (2013)
  27. W.A. Hiscock, L. Lindblom, Stability and causality in dissipative relativistic fluids. Ann. Phys. (N. Y.) 151, 466 (1983)
  28. W.A. Hiscock, L. Lindblom, Generic instabilities in first-order dis- sipative relativistic fluid theories. Phys. Rev. D 31, 725 (1985)
  29. R. Maartens, V. Mendez, Nonlinear bulk viscosity and inflation. Phys. Rev. D 55, 1937 (1997)
  30. M.K. Mak, T. Harko, Exact causal viscous cosmologies. Gen. Rel- ativ. Garvit. 30, 1171 (1998)
  31. M.K. Mak, T. Harko, Full causal bulk viscous cosmological mod- els. J. Math. Phys. 39, 5458 (1998)
  32. W. Zimdahl, Cosmological particle production, causal thermody- namics and inflationary expansion. Phys. Rev. D 61, 083511 (2000)
  33. M. Zakari, D. Jou, Equations of state and transport equations in viscous cosmological models. Phys. Rev. D 48, 1597 (1993)
  34. A.A. Cooley, R.J. van den Hoogan, R. Maartens, Qualitative vis- cous cosmology. Phys. Rev. D 54, 1393 (1996)
  35. M. Cataldo, N. Cruz, S. Lepe, Viscous dark energy and phantom evolution. Phys. Lett. B 619, 005 (2005)
  36. O.F. Piattella, J.C. Fabris, W. Zimdahl, Bulk viscous cosmology with causal transport theory. JCAP 05, 029 (2011)
  37. R. Maartens, Dissipative cosmology. Class. Quant. Grav. 12, 1455 (1995)
  38. L.P. Chimento, A.S. Jacubi, Dissipative cosmological solutions. Class. Quant. Grav. 14, 1811 (1997)
  39. U. Alam, V. Sahini, A.A. Starobinsky, The case for dynamical dark energy revisited. JCAP 0406, 008 (2004)
  40. M. Tegmark et al., Cosmological constraints from the SDSS lumi- nous red galaxies. Phys. Rev. D 74, 123507 (2006)
  41. E. Carretta, R.G. Gratton, G. Clementini, F.Fusi Pecci, Distances, ages and epoch of formation of globular clusters. Astrophys. J. 533, 215 (2000)
  42. E. Komatsu et al., WMAP Collaboration, Seven year Wilkinson Microwave Anisotropy Probe (WMAP) observations: cosmologi- cal interpretation. Astrophys. J. Suppl. 192, 18 (2011)
  43. L.P. Chimanto, M.G. Richarte, Interacting dark matter and modified holographic Ricci dark energy induce a relaxed Chaplygin gas. Phys. Rev. D 84, 123507 (2011)
  44. I. Brevik, E. Elizalde, S. Nojiri, S.D. Odintsov, Viscous little rip cosmology. Phys. Rev. D 84, 103508 (2011)
  45. E.W. Kolb, M.S. Turner, The Early Universe (Addison-Wesley, California, 1990)
  46. S. Weinberg, Gravitation and Cosmology: Principles and Applica- tions of the General Theory of Relativity (Wiley, New York, 1972)
  47. G.W. Gibbons, S.W. Hawking, Cosmological event horizons, ther- modynamics, and particle creation. Phys. Rev. D 15, 2738 (1977)
  48. T.K. Mathew, R. Aiswarya, K.S. Vidya, Cosmological horizon entropy and generalized second law for flat Friedmann universe. Eur. Phys. J. C 73, 2619 (2013)
  49. A. Sheykhi, Thermodynamics of holographic interacting dark energy with the apparent horizon as an IR cutoff. Class. Quant. Grav. 27, 025007 (2010)
  50. P.C.W. Davis, Cosmological horizon and generalized second law of thermodynamics. Class. Quant. Grav. 4, L225 (1987)
  51. M.R. Setare, A. Sheykhi, Viscous dark energy and generalized sec- ond law of thermodynamics. Int. J. Mod. Phys. D 19, 1205 (2010)
  52. V. Sahni, T.D. Saini, A.A. Starobinsky, U. Alam, Statefinder-A new geometrical diagnostic of dark energy. JETP Lett. 77, 201 (2003)
  53. M. Kowalski et al., Improved cosmological constraints from new, old, and combined supernova data set. Astrophys. J. 686, 749 (2008)

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Bulk viscous matter and recent acceleration of the universe based on causal viscous theory
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Evolution of the bulk viscous matter dominated universe has been analysed using the full causal, Israel-Stewart theory for the evolution of bulk viscous pressure in the context of recent acceleration of the universe. The form of bulk viscosity is taken as ξ = αρ 1/2. We obtained analytical solutions for the Hubble parameter and scale factor of the universe. The model parameters have been computed using the Type Ia supernovae observational data. The evolution of the prominent cosmological parameters were obtained. The age of the universe for the best estimated model parameters is found to be less than observational value. The viscous matter behaves like stiff fluid in the early evolutionary phase and then evolves to a negative pressure fluid in the later phase. The equation of state is found to be stabilized with value ω > −1 and thus, the model is not showing any of the phantom behaviour during the evolution. The local as well as generalized second law of thermodynamics are satisfied in this model, while it was shown by many that the local second law is breaking in the Eckart formalism approach. The statefinder geometric diagnostic shows that the present model is distinct from the standard ΛCDM model of the universe. One of the marked deviation seen in this model compared to a corresponding model using Eckart approach is that the bulk viscosity decreases with expansion of the universe, while in it increases from negative value in the early universe towards positive values values in the Eckart formalism.

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From a hydrodynamicist’s point of view the inclusion of viscosity concepts in the macroscopic theory of the cosmic fluid would appear most natural, as an ideal fluid is after all an abstraction (exluding special cases such as superconductivity). Making use of modern observational results for the Hubble parameter plus standard Friedmann formalism, we may extrapolate the description of the universe back in time up to the inflationary era, or we may go to the opposite extreme and analyze the probable ultimate fate of the universe. In this review, we discuss a variety of topics in cosmology when it is enlarged in order to contain a bulk viscosity. Various forms of this viscosity, when expressed in terms of the fluid density or the Hubble parameter, are discussed. Furthermore, we consider homogeneous as well as inhomogeneous equations of state. We investigate viscous cosmology in the early universe, examining the viscosity effects on the various inflationary observables. Additionally, we...

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