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Dark Energy or Modified Gravity?

Dark Energy or Modified Gravity?. The Contemporary Universe. Cosmology and the Friedmann equation SNIa and cosmic acceleration CMB and closure density. WMAP (2003). Riess, et al. (2004). The Contemporary Universe. Dark energy (cosmic fuel)…

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Dark Energy or Modified Gravity?

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  1. Dark Energy or Modified Gravity?

  2. The Contemporary Universe • Cosmology and the Friedmann equation • SNIa and cosmic acceleration • CMB and closure density WMAP (2003) Riess, et al. (2004)

  3. The Contemporary Universe • Dark energy (cosmic fuel)… • E.g., vacuum energy or scalar field or condensate (quintessence)

  4. The Contemporary Universe • …versus modified gravity? • Rather than new ingredient, treat as signal of first real lack of understanding of gravity • Infrared rather than ultraviolet modifications

  5. The Contemporary Universe • How to differentiate DE from MG? • Powerful idea: using gravity to create local physics and cosmology • Can one deduce force law from cosmology? • Can one manufacture cosmological information from force law? • Powerful enough? • Well…

  6. The Contemporary Universe • Cosmology and the Friedmann equation

  7. The Contemporary Universe • Cosmology and the Friedmann equation

  8. The Contemporary Universe • Cosmology and the Friedmann equation • Pure matter domination

  9. Modified Gravity • The picture

  10. Modified Gravity • The picture

  11. Modified Gravity • Matching metrics • Inside l= l : cosmological • Outside l = l : Schwarzschild (empty) * * • Lue, Scoccimarro, Starkman PRD69:044005(2004)

  12. Modified Gravity • The Schwarzschild-like metric • Lue, Scoccimarro, Starkman PRD69:044005(2004)

  13. Modified Gravity • Governing scales • Einstein regime: H > H0 • Einstein regime: r<r * • Lue, Scoccimarro, Starkman PRD69:044005(2004)

  14. Modified Gravity • Modified Newton’s Law • Values for r : Earth 4 lightyears Sun 420 lightyears Milky Way (1012 solar masses) 4 million lightyears * • Gruzinov astro-ph/0112246; Lue, Starkman PRD67:064002(2003); Dvali, Gruzinov, Zaldarriaga PRD68:024012(2003)

  15. Modified Gravity • Observer’s point of view • Lue, Starkman PRD67:064002(2003); Lue, Scoccimarro, Starkman PRD69:044005(2004)

  16. Dark Energy versus Modified Gravity • Effective Newton’s constants • Lue, Scoccimarro, Starkman PRD69:044005(2004)

  17. Dark Energy versus Modified Gravity • Growth of large-scale structure 5-10% discrepancy • Lue, Scoccimarro, Starkman PRD69:044005(2004)

  18. Dark Energy versus Modified Gravity • Late-time ISW DE MG ~O(1) discrepancy • Lue, Scoccimarro, Starkman PRD69:044005(2004)

  19. Dark Energy versus Modified Gravity • Differentiation between DE and MG through discrepant force-law • Anomalous structure growth, even with identical expansion history • Can see both in 8 and in late-time ISW

  20. Braneworlds and Modified Gravity • Braneworld model of Dvali, Gabadadze and Porrati • Gravity theory with effective metastable graviton • Modifications to Einstein gravity occur at large (rather than short) distances • Dvali,Gabadadze,Porrati PLB485:208(2000); Dvali,Gabadadze,Kolanovic,Nitti PRD64:084004(2001), 65:024031(2002)

  21. Braneworlds and Modified Gravity • Braneworld model of Dvali, Gabadadze and Porrati • Modification of gravity occur at r • Large-scale structure affected in analogous ways • Even residual solar-system measurements (anomalous precessions) * 10% discrepancy • Lue, Starkman PRD67:064002(2003); Lue, Scoccimarro, Starkman PRD69:124015(2004)

  22. Braneworlds and Modified Gravity • Growth of large-scale structure • Lue, Scoccimarro, Starkman astro-ph/0401515

  23. Braneworlds and Modified Gravity • Linear growth • Lue, Scoccimarro, Starkman astro-ph/0401515

  24. Braneworlds and Modified Gravity • Braneworld model of Dvali, Gabadadze and Porrati • Easily accommodates weff < -1 through modification of Friedmann equation • Deffayet PLB502:199(2001); Sahni, Shtanov JCAP0311:014(2003) ; Lue, Starkman astro-ph/0408246

  25. Remarks • Modified gravity is a distinct paradigm for cosmic acceleration • Using gravity for both local attraction and cosmology a powerful idea • Distinct from dark energy paradigm • Subject to imminent observational discrimination

  26. The Contemporary Universe • Actually, rather a lot… • Dramatic successes • Intimate connection between matter, geometry and gravity Newton Einstein

  27. Braneworlds and Modified Gravity • Einstein gravity in 5-dimensions • Intrinsic curvature term • Dvali,Gabadadze,Porrati PLB485:208(2000); Dvali,Gabadadze,Kolanovic,Nitti PRD64:084004(2001), 65:024031(2002)

  28. Braneworlds and Metastable Gravitons • Disparate gravitational strengths: brane vs. bulk • Brane gravity appears metastable short distances: 4-dimensional long distances: 5-dimensional • Dvali,Gabadadze,Porrati PLB485:208(2000); Dvali,Gabadadze,Kolanovic,Nitti PRD64:084004(2001), 65:024031(2002)

  29. Braneworld Cosmology • The Friedmann equation Both phases Self-accelerating phase FLRW phase • Deffayet PLB502:199(2001)

  30. Braneworld Cosmology • The Friedmann equation • Deffayet PLB502:199(2001)

  31. Braneworld Cosmology • Global structure • Lue PRD67:064004(2003)

  32. Braneworld Cosmology • Constraints on r0 • Friedmann equation • Self-accelerating phase • Deffayet, Dvali, Gabadadze PRD65:044023(2002); Deffayet,Landau,Raux,Zaldarriaga,Astier PRD66:024019(2002)

  33. Braneworld Gravity • Modified Newton’s law mass = m • Lue PRD66:043509(2002); Gruzinov astro-ph/0112246; Porrati PLB534:209(2002); Lue, Starkman PRD67:064002(2003)

  34. Braneworld Gravity • Strong gravity, dynamic extrinsic curvature • Lue PRD66:043509(2002); Lue, Starkman PRD67:064002(2003)

  35. Physical Considerations • Orbit precession • Per orbit • DGP anomalous precession rate • Lue, Starkman PRD67:064002(2003)

  36. Physical Considerations • Orbit precession • Uncertainties: < 430 mas/year Mercury: < 1 mas/year (2009-2010) < 10 mas/year Moon: BepiColombo (ESA) < 10 mas/year Mars: • Nordtvedt PRD61:122001(2000); Will gr-qc/0103036; Milani, et al. PRD66:082001(2002)

  37. Physical Considerations • Growth of large-scale structure • Lue, Scoccimarro, Starkman astro-ph/0401515

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