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Hrvoje Štefančić Universitat de Barcelona IRGAC 2006, Barcelona July 14, 2006

Hrvoje Štefančić Universitat de Barcelona IRGAC 2006, Barcelona July 14, 2006. What is in the black box of dark energy: variable cosmological parameters or multiple (interacting) components?. Accelerating universe. accelerating universe – observationally established

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Hrvoje Štefančić Universitat de Barcelona IRGAC 2006, Barcelona July 14, 2006

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  1. Hrvoje Štefančić Universitat de Barcelona IRGAC 2006, Barcelona July 14, 2006 What is in the black box of dark energy: variable cosmological parameters or multiple (interacting) components?

  2. Accelerating universe • accelerating universe – observationally established • mechanism behind the acceleration? • dark energy • alternative mechanisms: modified gravity, braneworlds, ... • dark energy – single component which encodes all our ignorance – efficient effective description - “a black box”

  3. Cosmic coincidences observational features • w presently close to -1 • possible CC boundary crossing at small redshift • the ratio of dark energy density and matter energy density presently of order 1 • Variable cosmological parameters (phenomenology, RGE approaches, holographic DE, 4D effective description of the dynamics in higher dimensions) • composite dark energy (multiple dark energy components) S. Hannestad, E. Mortsell, JCAP 0409 (2004) 001 A. Upadhye, M. Ishak, P.J. Steinhardt, Phys. Rev. D 72 (2005) 063501 H.K. Jassal, J.S. Bagla, T. Padmanabhan, astro-ph/0601389 G-B. Zhao, J-Q. Xia, B. Feng, X. Zhang, astro-ph/0603621 proposed approaches

  4. Formalism – variable cosmological parameters • modification of GR at the level of Einstein equation • Generalized Bianchi identity • GR as a limit (for nonvariable parameters)

  5. Cosmology – variable cosmological term picture • FRW metrics – generalized Bianchi identity gives 1) 2) 3) 4) variable CC energy density matter density (possibly interacting)

  6. Variable cosmological term picture • variability of parameters with redshift • Hubble parameter

  7. Cosmology – dark energy picture dark energy matter (noninteracting) • standard formalism

  8. Matching of pictures • Effective dark energy behaviour – matching of pictures • general results J. Solà, H. Š., Mod. Phys. Lett. A 21 (2006) 479 Generalized Bianchi identity + +

  9. Matching of pictures (2) • redshift dependence of the efective dark energy EOS existence of z* close to z=0

  10. Effective dark energy EOS • slope of the effective dark energy monotonously growing with redshift phantom-like quintessence-like monotonously falling with redshift quintessence-like phantom-like

  11. Dark energy effective EOS • noninteracting (conserved – standard scaling)

  12. An example – RG motivated model J. Solà, H. Š., Phys. Lett. B 624 (2005) 147 • Hubble parameter as a RG scale

  13. Redshift dependence of the effective EOS (1)

  14. Redshift dependence of the effective EOS (2)

  15. Explanation of w(z) “coincidence” • If it exists (to be hopefuly resolved by the upcoming observational data), the CC boundary transition • should happen at small redshift • is allowed for a wide range of parameters (no special values need to be chosen) • The parameter of effective dark energy EOS is at present close to w(0)=-1 (general result) • w(z) may exibit substantial variation with the redshift

  16. Composite dark energy J. Grande, J. Solà, H. Š., gr-qc/0604057 • matter component (noninteracting) • two (interacting) dark energy components: (variable) cosmological term + additional dynamical component interaction dynamics

  17. Evolution of analytical (closed form) solution – simple expressions for =const stopping of the expansion: H(z)=0 r has a maximum! How high is it? the redshift dependence of (z)?

  18. Parameter constraints • nucleosynthesis bound: • existence of the expansion stopping • height of the maximum of r :

  19. Parametric dependence • dependence

  20. Time evolution • The peak of r(z) is less pronounced

  21. Special cases and extensions • The effect persists for . r is not bounded from below • Variable cosmological term Lambda and variable Newton coupling G – similar results J. Grande, J. Solà, H. Š., in preparation

  22. Conclusions • effective dark energy picture for the cosmological models with variable parameters • general and simple analitic results - “RG like” relation between and • counterintuitive behaviour of the effective dark energy density • explanation of w(z) “coincidences” • composite dark energy – solution of the r(z) coincidence problem • when the expansion of the universe stops, r(z) is bounded from above • r(z) may stay close to r(0) for a nonnegligible volume of the parametric space

  23. Auxilliary slides

  24. Observational evidence • Dark energy EOS w(z) – various parametrizations used • variability with the redshift • w(z) close to -1 • indication of CC boundary crossing S. Hannestad and E. Mortsell, JCAP 0409 (2004) 001 w(a)=w0 w1(aq + asq)/(aq w1+asq w0)

  25. Variable cosmological parameters – motivation (1) • Phenomenological approaches • “relaxation” of CC – solution of the CC problem • Dirac – big number hypothesis – variable G • cosmology with a decaying vacuum -K. Freese, F.C. Adams, J.A. Frieman, E. Mottola, Nucl. Phys. B 287 (1987) 797 • variable CC interacting with matter - J.M. Overduin, F.I. Cooperstock, Phys. Rev. D 58 (1998) 043506 • variable G and CC - A. Beesham, Nuovo Cim. B 96 (1986) 17

  26. Variable cosmological parameters – motivation (2) • RG cosmology • quantum field theory in curved space-time • soft decoupling -importance of the most massive fields • scale dependent effective quantum gravity action (Einstein-Hilbert truncation) • RG flow - IR fixed point hypothesis I.L. Shapiro, J. Solà, Phys. Lett. B 475 (2000) 236 I.L. Shapiro, J. Solà, JHEP 0202 (2002) 006 A. Babić, B. Guberina, R. Horvat, H. Š., Phys. Rev. D 65 (2002) 085002 I.L. Shapiro, J. Solà, C España-Bonet, P. Ruiz-Lapuente, Phys. Lett. B 574 (2003) 149 A. Bonnano, M. Reuter, Phys. Rev. D 65 (2002) 043508 A. Bonnano, M. Reuter, Phys. Lett. B 527 (2000) 9

  27. Variable cosmological parameters – motivation (3) • A.G. Cohen, D.B. Kaplan, A.E. Nelson, Phys. Rev. Lett. 82 (1999) 4971 • Effective field theory + entropy constraint = relation between UV( ) and IR (1/L) cutoffs • excluding all states that lie within their Schwarzschield radius • Holographic dark energy – M. Li, Phys. Lett. B 603 (2004) • Variable cosmological term - R. Horvat, Phys. Rev. D 70 (2004) 087301

  28. Variable cosmological parameters – motivation (4) • Higher-dimensional models (e.g. GR in 4+N dimensions) • Variability of e.g. G due to dynamics of extra (compactified) dimensions

  29. Effective dark energy EOS

  30. Redshift dependence of the effective EOS (3)

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