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Theoretical Aspects of Dark Energy Models

Theoretical Aspects of Dark Energy Models. Rong-Gen Cai Institute of Theoretical Physics Chinese Academy of Sciences CCAST, July 4, 2005. Cosmic Acceleration?. Dynamics equations:. (Violate the Strong Energy Condition: exotic energy component) . Observation Data. Dark Energy?.

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Theoretical Aspects of Dark Energy Models

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  1. Theoretical Aspects of Dark Energy Models Rong-Gen Cai Institute of Theoretical Physics Chinese Academy of Sciences CCAST, July 4, 2005

  2. Cosmic Acceleration? Dynamics equations: (Violate the Strong Energy Condition:exotic energy component)

  3. Observation Data Dark Energy? Theoretical Assumptions General Relativity Cosmo Principle Model III ModelI Model II

  4. Model I: Modifications of Gravitational Theory UV: ~ 1 mm IR: ~ solar scale 1) GR’s test UV: quantum gravity effect IR: cosmos scale Brane World Scenario 2) Modify GR

  5. Modifying GR in IR: • “ Ghost Condensation and a Consistent Infrared Modification of Gravity” • by N. arkani-Hamed et al, hep-th/0312099,JHEP 0405 (2004) 074.

  6. Consider a ghost field with a wrong-sign kinetic term:

  7. Suppose the scalar field has a constant velocity: The low-energy effective action for the fluctuation has an usual form:

  8. 2) “ Is Cosmic Speed-up due to New Gravitational Physics ” by S. M. Carroll et al. astro-ph/0306438, Phys.Rev. D70 (2004) 043528 Consider a modification becoming important at extremely low curvature

  9. Making a conformal transformation yields a scalar field with potential: (1) Eternal de Sitter; (2) power-law acceleration; (3) future singularity

  10. General case:

  11. More general case: hep-th/0410031, PRD71:063513,2005 Consider:

  12. 3) Brane World Scenario: • N. Arkani-Hamed et al, 1998 • factorizable product • 2) L. Randall and R. Sundrum, 1999 • warped product in AdS_5 y RS1: RS2: 3) DGP model, 2000 a brane embedded in a Minkovski space

  13. a) A popular model: RS scenario where = 0 Fine-Tuning

  14. 2) “Dark Energy” on the brane world scenario “Braneworld models of dark energy” by V. Sahni and Y. Shtanov, astro-ph/0202346, JCAP 0311 (2003) 014 When m=0:

  15. In general they have two branches:

  16. Current value of the effective equation of state of “dark energy”

  17. The acceleration can be a transient phenomenon: Brane 2

  18. However, w crosses –1, the phantom divide? D. Huterer and A. Cooray, astro-ph/040462; Phys.Rev. D71 (2005) 023506

  19. “Crossing w=-1 in Gauss-Bonnet Brane World with Induced Gravity ” by R.G. Cai,H.S. Zhang and A. Wang, hep-th/0505186 Consider the model

  20. The equations of motion:

  21. The effective equation of state of “dark energy”: Where the Gauss-Bonnet term in the bulk and bulk mass play a curial role.

  22. Model III: Back Reaction of Fluctuations • “Cosmological influence of super-Hubble perturbations” • by E.W. Kolb, S. Matarrese, A. Notari and A. Riotto, astro-ph/0410541; • “Primordial inflation explains why the universe is accelerating today” • by E.W. Kolb, S. Matarrese, A. Notari and A. Riotto, hep-th//0503117; • “On cosmic acceleration without dark energy” • by E.W. Kolb, S. Matarrese, and A. Riotto, astro-ph/0506534

  23. Inflation produces super-horizon perturbations!

  24. Consider the presence of cosmic perturbations, Split the gravitational potential to two parts

  25. A local observer within the Hubble volume will see cosmologicalconstant

  26. which indicates the SHCDM with is indistinguishable from LCDM model.

  27. Another scenario:

  28. Beyond the super-horizon mode’s cut-off, the bulk universe is There is a super-horizon sized underdense bubble containing the observable universe, with matter density equal to the average matter density we measure locally

  29. Model II:Various Dark Energy Models: Acts as Source of E’eq • Cosmological constant: w=-1 • (2) Holographic energy • (3) Quintessence: -1<w<0 • (4) K-essence: -1 <w<0 • (5) Chaplygin gas: p=- A/rho (6) Phantom: w<-1 (7) Quintom (8) Chameleon, K-Chameleon various generalization and mixture……..

  30. (1) a very tiny positive cosmological constant ? QFT, a very successful theory Variable cosmological constant? Interaction?

  31. (2) Holographic Energy? V,A E,S R i) Bekenstein Bound: ii) Holographic Bound: iii) UV/IR Mixture: (Cohen et al, Hsu, Li….)

  32. (3) Quintessence:a very slowly varying scalar field? Tracker Potential: (4) K-essence (Born-Infeld Scalar Field):

  33. (5) Chaplygin gas ? Generalizations:

  34. (6) Phantom (Caldwell, 1999) -1<w<0, if s=1 w<-1, if s=-1

  35. (7) Quintom: normal scalar field plus phantom field W cross the phantom divide, w=-1 Hessence ? (8) Chameleon, K-Chameleon

  36. Distance between galaxies Acceleration Deceleration Now (13.7 billionyears) Beginning Time (Age of universe) SuperAcc. (w<-1) Acc.(-1 <w<-1/3) ? Expand, but w>0 (dark energy dominated) Closed, rho<0 Radiation + dust) Inflation (acceleration) The fate of our universe depends on the nature of dark energy, not only the geometry

  37. Thanks!

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