1. String/Brane Cosmology COSMO 07 – University of Sussex C.P. Burgess

2. String/Brane Cosmology …for those who have not yet drunk the Kool-Aid with J.Blanco-Pillado, J.Cline, K. das Gupta, C. de Rham, C.Escoda, M.Gomez-Reino, D. Hoover, R.Kallosh, A.Linde,F.Quevedo, G. Tasinato and A. Tolley

3. Cosmo 07 On the shoulders of giants A. Salam, E. Sezgin, H. Nishino,G. Gibbons, S. Kachru E. Silverstein, R. Guven, C. Pope, K. Maeda, M. Sasaki, V. Rubakov, R. Gregory, I. Navarro, J. Santiago, S. Carroll, C. Guica, C. Wetterich, S. Randjbar-Daemi, F. Quevedo, Y. Aghababaie, S. Parameswaran, J. Cline, J. Matias, G. Azuelos, P-H. Beauchemin, A. Albrecht, C. Skordis, F. Ravndal, I. Zavala, G. Tasinato, J. Garriga, M. Porrati, H.P. Nilles, A. Papazoglou, H. Lee, N. Arkani-Hamad, S. Dimopoulos, N. Kaloper, R. Sundrum, D. Hoover, A. Tolley, C. de Rham, S. Forste, Z. Lalak, S. Lavingnac, C. Grojean, C. Csaki, J. Erlich, T. Hollowood, H. Firouzjahi, J. Chen, M. Luty, E. Ponton, P. Callin, D. Ghilencea, E. Copeland, O. Seto, V. Nair, S. Mukhoyama, Y. Sendouda, H. Yoshigushi, S. Kinoshita, A. Salvio, J. Duscheneau, J. Vinet, M. Giovannini, M. Graesser, J. Kile, P. Wang, P. Bostok, G. Kofinas, C. Ludeling, A. Nielsen, B. Carter, D. Wiltshire. C. K. Akama, S. Appleby, F. Arroja, D. Bailin, M. Bouhmadi-Lopez, M. Brook, R. Brown, C. Byrnes, G. Candlish, A. Cardoso, A. Chatterjee, D. Coule, S. Creek, B. Cuadros-Melgar, S. Davis, B. de Carlos, A. de Felice, G. de Risi, C. Deffayet, P. Brax, D. Easson, A. Fabbri, A. Flachi, S. Fujii, L. Gergely, C. Germani, D. Gorbunov, I. Gurwich, T. Hiramatsu, B. Hoyle, K. Izumi, P. Kanti, S. King, T. Kobayashi, K. Koyama, D. Langlois, J. Lidsey, F. Lobo, R. Maartens, N. Mavromatos, A. Mennim, M. Minamitsuji, B. Mistry, S. Mizuno, A. Padilla, S. Pal, G. Palma, L. Papantonopoulos, G. Procopio, M. Roberts, M. Sami, S. Seahra, Y. Sendouda, M. Shaeri, T. Shiromizu, P. Smyth, J. Soda, K. Stelle, Y. Takamizu, T. Tanaka, T. Torii, C. van de Bruck, D. Wands, V. Zamarias, H. Ziaeepour

4. Cosmo 07 Outline Motivation String Cosmology: Why Does it Make Sense? Branes and ‘late-Universe’ cosmology Some Dark (Energy) Thoughts String inflation A Sledgehammer for a Nutcracker? Outlook

5. Cosmo 07 Outline Motivation String Cosmology: Why Does it Make Sense? Branes and ‘late-Universe’ cosmology Some Dark (Energy) Thoughts String inflation A Sledgehammer for a Nutcracker? Outlook

6. Cosmo 07 Outline Motivation String Cosmology: Why Does it Make Sense? Branes and ‘late-Universe’ cosmology Some Dark (Energy) Thoughts String inflation A Sledgehammer for a Nutcracker? Outlook

7. Cosmo 07 Outline Motivation String Cosmology: Why Does it Make Sense? Branes and ‘late-Universe’ cosmology Some Dark (Energy) Thoughts String inflation A Sledgehammer for a Nutcracker? Outlook

8. Cosmo 07 Strings, Branes and Cosmology Why doesn’t string theory decouple from cosmology?

9. Cosmo 07 Strings, Branes and Cosmology Why doesn’t string theory decouple from cosmology? Science progresses because short- distance physics decouples from long distances.

10. Cosmo 07 Strings, Branes and Cosmology Why doesn’t string theory decouple from cosmology? Science progresses because short distance physics decouples from long distances. * Inflationary fluctuations could well arise at very high energies: M I » 10 -3 M p

11. Cosmo 07 Strings, Branes and Cosmology Why doesn’t string theory decouple from cosmology? Science progresses because short distance physics decouples from long distances. * Inflationary fluctuations could well arise at very high energies: M I » 10 -3 M p * Cosmology (inflation, quintessence, modified gravity, etc) relies on properties which can be extremely sensitive to short distances.

12. Cosmo 07 Strings, Branes and Cosmology Why doesn’t string theory decouple from cosmology? Science progresses because short distance physics decouples from long distances. * Inflationary fluctuations could well arise at very high energies: M I » 10 -3 M p * Cosmology (inflation, quintessence, modified gravity, etc) relies on properties which can be extremely sensitive to short distances. * String theory suggests important changes in the low-energy degrees of freedom: branes.

13. Cosmo 07 Strings, Branes and Cosmology Why doesn’t string theory decouple from cosmology? Why are branes important for cosmology and particle physics? D branes in string theory are surfaces on which some strings must end, ensuring their low- energy modes are trapped on the brane. Polchinski

14. Cosmo 07 Strings, Branes and Cosmology Why doesn’t string theory decouple from cosmology? Why are branes important for cosmology and particle physics? In some cases this is where the Standard Model particles live. Ibanez et al

15. Cosmo 07 Strings, Branes and Cosmology Why doesn’t string theory decouple from cosmology? Why are branes important for cosmology and particle physics? Leads to the brane-world scenario, wherein we are all brane-bound. Rubakov & Shaposhnikov

16. Cosmo 07 Strings, Branes and Cosmology Why doesn’t string theory decouple from cosmology? Why are branes important for cosmology and particle physics? Identifies hidden assumptions about low energy theory whose relaxation might help with low energy naturalness problems.

17. Cosmo 07 Naturalness Ideas for what lies beyond the Standard Model are largely driven by ‘technical naturalness’. Motivated by belief that SM is an effective field theory. + dimensionless

18. Cosmo 07 Naturalness Ideas for what lies beyond the Standard Model are largely driven by ‘technical naturalness’. Motivated by belief that SM is an effective field theory. + dimensionless BUT: effective theory can be defined at many scales

19. Cosmo 07 Naturalness Ideas for what lies beyond the Standard Model are largely driven by ‘technical naturalness’. Motivated by belief that SM is an effective field theory. + dimensionless BUT: effective theory can be defined at many scales

20. Cosmo 07 Naturalness Ideas for what lies beyond the Standard Model are largely driven by ‘technical naturalness’. Motivated by belief that SM is an effective field theory. + dimensionless BUT: effective theory can be defined at many scales Hierarchy Problem: These must cancel to 20 digits!!

21. Cosmo 07 Naturalness Ideas for what lies beyond the Standard Model are largely driven by ‘technical naturalness’. Motivated by belief that SM is an effective field theory. + dimensionless Hierarchy problem: Since the largest mass dominates, why isn’t m ~ M GUT or M p ?? Three approaches to solve the Hierarchy problem: Compositeness: H is not fundamental at energies E À M w Supersymmetry: there are new particles at E À M w and a symmetry which ensures cancellations so m 2 ~ M B 2 – M F 2 Extra Dimensions: the fundamental scale is much smaller than M p, much as G F -1/2 > M w

22. Cosmo 07 Naturalness in Crisis Ideas for what lies beyond the Standard Model are largely driven by ‘technical naturalness’. Motivated by belief that SM is an effective field theory. The Standard Model’s dirty secret: there are really two unnaturally small terms. + dimensionless

23. Cosmo 07 Naturalness in Crisis Ideas for what lies beyond the Standard Model are largely driven by ‘technical naturalness’. Motivated by belief that SM is an effective field theory. + dimensionless Can apply same argument to scales between TeV and sub-eV scales. Cosmological Constant Problem: Must cancel to 32 decimal places!!

24. Cosmo 07 Naturalness in Crisis Ideas for what lies beyond the Standard Model are largely driven by ‘technical naturalness’. Motivated by belief that SM is an effective field theory. The Standard Model’s dirty secret: there are really two unnaturally small terms. + dimensionless Harder than the Hierarchy problem: Integrating out the electron already gives too large a contribution!!

25. Cosmo 07 Naturalness in Crisis Dark energy vs vacuum energy Why must the vacuum energy be large? Seek to change properties of low-energy particles (like the electron) so that their zero-point energy does not gravitate, even though quantum effects do gravitate in atoms! Why is this seen………………but not this?

26. Cosmo 07 Naturalness in Crisis Ideas for what lies beyond the Standard Model are largely driven by ‘technical naturalness’. Motivated by belief that SM is an effective field theory. + dimensionless Cosmological constant problem: Why is  ~ 10 -3 eV rather than m e, M w, M GUT or M p ? Approaches to solve the Hierarchy problem at  ~ 10 -2 eV? Compositeness: graviton is not fundamental at energies E À  Supersymmetry: there are new particles at E À  and a symmetry which ensures cancellations so  2 ~ M B 2 – M F 2 Extra Dimensions: the fundamental scale is much smaller than M p

27. Cosmo 07 Naturalness in Crisis Ideas for what lies beyond the Standard Model are largely driven by ‘technical naturalness’. Motivated by belief that SM is an effective field theory. + dimensionless Cosmological constant problem: Why is  ~ 10 -3 eV rather than m e, M w, M GUT or M p ? Approaches to solve the Hierarchy problem at  ~ 10 -2 eV? Compositeness: graviton is not fundamental at energies E À  Supersymmetry: there are new particles at E À  and a symmetry which ensures cancellations so  2 ~ M B 2 – M F 2 Extra Dimensions: the fundamental scale is much smaller than M p ??

28. Cosmo 07 How Extra Dimensions Help 4D CC vs 4D vacuum energy Branes and scales

29. Cosmo 07 4D CC vs 4D vacuum energy Branes and scales How Extra Dimensions Help A cosmological constant

30. Cosmo 07 4D CC vs 4D vacuum energy Branes and scales How Extra Dimensions Help A cosmological constant is not distinguishable from a Lorentz invariant vacuum energy vs

31. Cosmo 07 4D CC vs 4D vacuum energy Branes and scales How Extra Dimensions Help A cosmological constant is not distinguishable* from a Lorentz invariant vacuum energy vs * in 4 dimensions…

32. Cosmo 07 How Extra Dimensions Help 4D CC vs 4D vacuum energy Branes and scales New Tools motivated by string theory (or condensed matter): Particles can be localized on surfaces (branes, or defects) within the extra dimensions Gravity is not similarly localized

33. Cosmo 07 4D CC vs 4D vacuum energy Branes and scales How Extra Dimensions Help In higher dimensions a 4D vacuum energy, if localized in the extra dimensions, can curve the extra dimensions instead of the observed four. Chen, Luty & Ponton Arkani-Hamad et al Kachru et al, Carroll & Guica Aghababaie, et al

34. Cosmo 07 4D CC vs 4D vacuum energy Branes and scales How Extra Dimensions Help To be useful it must be that extra dimensions can be as large as the observed Dark Energy density: ~ c/r » 10 -2 eV or r » 1  -metre This is possible! provided all known particles except gravity are trapped on a brane, since tests of Newton’s law allow r < 50  -metre Adelberger et al Arkani Hamed, Dvali, Dimopoulos

35. Cosmo 07 4D CC vs 4D vacuum energy Branes and scales How Extra Dimensions Help If there are extra dimensions as large as r » 1  -metre then there can only be two of them (although others could exist if they are much smaller), or else the observed strength of gravity would require the scale of extra-dimensional physics to be smaller than m w with n extra dimensions Arkani Hamed, Dvali, Dimopoulos

36. Cosmo 07 4D CC vs 4D vacuum energy Branes and scales How Extra Dimensions Help These scales are natural using standard 4D arguments.

37. Cosmo 07 4D CC vs 4D vacuum energy Branes and scales How Extra Dimensions Help These scales are natural using standard 4D arguments. Extra dimensions could start here, if there are only two of them. Arkani Hamed, Dvali, Dimopoulos

38. Cosmo 07 4D CC vs 4D vacuum energy Branes and scales How Extra Dimensions Help Only gravity gets modified over the most dangerous distance scales! Must rethink how the vacuum gravitates in 6D for these scales. SM interactions do not change at all!

39. Cosmo 07 The SLED Proposal Suppose physics is extra-dimensional above the 10 -2 eV scale. Suppose the physics of the bulk is supersymmetric. Aghababaie, CB, Parameswaran & Quevedo

40. Cosmo 07 The SLED Proposal Suppose physics is extra-dimensional above the 10 -2 eV scale. Suppose the physics of the bulk is supersymmetric. 6D gravity scale: M g ~ 10 TeV KK scale: 1/r ~ 10 -2 eV Planck scale: M p ~ M g 2 r Arkani-Hamad, Dimopoulos & Dvali

41. Cosmo 07 Suppose physics is extra-dimensional above the 10 -2 eV scale. Suppose the physics of the bulk is supersymmetric. The SLED Proposal 6D gravity scale: M g ~ 10 TeV KK scale: 1/r ~ 10 -2 eV Planck scale: M p ~ M g 2 r Choose bulk to be supersymmetric (no 6D CC allowed) Nishino & Sezgin

42. Cosmo 07 Suppose physics is extra-dimensional above the 10 -2 eV scale. Suppose the physics of the bulk is supersymmetric. The SLED Proposal 6D gravity scale: M g ~ 10 TeV KK scale: 1/r ~ 10 -2 eV Planck scale: M p ~ M g 2 r SUSY Breaking on brane: TeV in bulk: M g 2 /M p ~1/r

43. Cosmo 07 The SLED Proposal 4D graviton Particle Spectrum: 4D scalar: e  r 2 ~ const SM on brane – no partners Many KK modes in bulk

44. Cosmo 07 The SLED Proposal 4D graviton Particle Spectrum: 4D scalar: e  r 2 ~ const SM on brane – no partners Many KK modes in bulk Classical flat direction due to a scale invariance of the classical equations NOT self-tuning: response to a kick is runaway along flat direction.

45. Cosmo 07 What Needs Understanding Classical part of the argument: What choices must be made to ensure 4D flatness? Quantum part of the argument: Are these choices stable against renormalization?

46. Cosmo 07 Classical part of the argument: What choices must be made to ensure 4D flatness? Quantum part of the argument: Are these choices stable against renormalization? What Needs Understanding Search for solutions to 6D supergravity: What bulk geometry arises from a given brane configuration? What is special about the ones which are 4D flat?

47. Cosmo 07 Classical part of the argument: What choices must be made to ensure 4D flatness? Quantum part of the argument: Are these choices stable against renormalization? What Needs Understanding Search for solutions to 6D supergravity: What bulk geometry arises from a given brane configuration? What is special about the ones which are 4D flat? Bulk solutions known for most properties for 2 brane sources; Most have runaway behaviour, with extra dimensions growing or collapsing Sufficient condition for flatness is absence of brane-dilaton coupling.

48. Cosmo 07 Classical part of the argument: What choices must be made to ensure 4D flatness? Quantum part of the argument: Are these choices stable against renormalization? What Needs Understanding Search for solutions to 6D supergravity: What bulk geometry arises from a given brane configuration? What is special about the ones which are 4D flat?

49. Cosmo 07 Classical part of the argument: What choices must be made to ensure 4D flatness? Quantum part of the argument: Are these choices stable against renormalization? What Needs Understanding Search for solutions to 6D supergravity: What bulk geometry arises from a given brane configuration? What is special about the ones which are 4D flat? Chiral gauged supergravity chosen to allow extra dimensions topology of a sphere (only positive tensions):

50. Cosmo 07 Classical part of the argument: What choices must be made to ensure 4D flatness? Quantum part of the argument: Are these choices stable against renormalization? What Needs Understanding Many classes of axially symmetric solutions known Up to two singularities, corresponding to presence of brane sources Brane sources characterized by: Asymptotic near-brane behaviour is related to properties of T(  ). dT/d  nonzero implies curvature singularity

51. Cosmo 07 Classical part of the argument: What choices must be made to ensure 4D flatness? Quantum part of the argument: Are these choices stable against renormalization? What Needs Understanding Static solutions having only conical singularities are all 4D flatsolutions Unequal defect angles imply warping. Flat solutions with curvature singularities exist.solutions Static solutions exist which are 4D dS.solutions Runaways are generic. Runaways Tolley, CB, Hoover & Aghababaie Tolley, CB, de Rham & Hoover CB, Hoover & Tasinato Gibbons, Guvens & Pope

52. Cosmo 07 4D CC vs 4D vacuum energy Branes and scales What Needs Understanding Most general 4D flat solutions to chiral 6D supergravity, without matter fields. 3 nonzero gives curvature singularities at branes Gibbons, Guven & Pope

53. Cosmo 07 6D Solutions: No Branes Salam Sezgin ansatz: maximal symmetry in 4D and in 2D ds 2 = g  dx  dx + g mn dy m dy n F = f  mn dy m dy n ;  m  = 0

54. Cosmo 07 6D Solutions: No Branes Salam Sezgin ansatz: maximal symmetry in 4D and in 2D ds 2 = g  dx  dx + g mn dy m dy n F = f  mn dy m dy n ;  m  = 0 Implies: 1. g  =   2. spherical extra dimensions 3. dilaton stabilization: g 2 e  = 1/r 2

55. Cosmo 07 6D Solutions: No Branes Why a flat solution? 80’s: Unit magnetic flux leaves SUSY unbroken…

56. Cosmo 07 6D Solutions: No Branes Why a flat solution? 80’s: Unit magnetic flux leaves SUSY unbroken… …but: turns out to be 4D flat for higher fluxes as well!

57. Cosmo 07 6D Solutions: Rugby Balls Can include branes: Cut-and-paste solutions have equal-sized conical singularities at both poles; Interpret singularity as due to back-reaction of branes located at this position Solutions break supersymmetry Aghababaie, CB, Parameswaran & Quevedo

58. Cosmo 07 6D Solutions: Conical Singularities General solns with two conical singularities: Unequal defects have warped geometries in the bulk; Conical singularities correspond to absence of brane coupling to 6D dilaton (and preserve bulk scale invariance) All such (static) solutions have flat 4D geometries Gibbons, Guven & Pope Aghababaie, CB, Cline, Firouzjahi, Parameswaran, Quevedo Tasinato & Zavala

59. Cosmo 07 6D Solutions: GGP solutions General solutions with flat 4D geometry: Solutions need not have purely conical singularities at brane positions; Non-conical singularities arise when the dilaton diverges near the branes Gibbons, Guven & Pope

60. Cosmo 07 6D Solutions: Asymptotic forms General near-brane asymptotic behaviour: Solutions take power-law near-brane form as a function of the proper distance, , to the brane Field equations imply ‘Kasner-like’ relations amongst the powers: p -  =  + 3  +  =  2 + 3  2 +  2 + p 2 = 1 Lorentz invariant if:  =  Tolley, CB, Hoover & Aghababaie

61. Cosmo 07 6D Solutions: Brane matching Near-brane asymptotics and brane properties: Powers may be related to averaged conserved currents if the singular behaviour is regulated using a ‘thick brane’ Navarro & Santiago Tolley, CB, de Rham & Hoover

62. Cosmo 07 6D Solutions: Other static solutions Solutions with dS and AdS 4D geometry: Asymptotic form at one brane dictated by that at the other brane; Solutions cannot have purely conical singularities at both brane positions; Static Lorentz-breaking solutions (    ): Static solutions exist for which the time and space parts of the 4D metric vary differently within the bulk; Tolley, CB, Hoover & Aghababaie

63. Cosmo 07 6D Solutions: Time-dependence Linearized perturbations Explicit solutions are possible for conical geometries in terms of Hypergeometric functions: Solutions are marginally stable, if the perturbations are not too singular at the brane positions; Cline & Vinet Tolley, CB, de Rham & Hoover Lee & Papazoglou

64. Cosmo 07 6D Solutions: Time-dependence Nonlinear Plane-Wave Solutions: Describe eg passage of bubble-nucleation wall along the brane; Black Hole Solutions: Conical defects threaded through bulk black holes Tolley et al Kaloper et al

65. Cosmo 07 6D Solutions: Scaling solutions A broad class of exact scaling solutions Exact time-dependent solutions are possible subject to the assumption of a scaling ansatz: Likely to describe the late-time attractor behaviour of time dependent evolution; Most of these solutions describe rapid runaways with rapidly growing or shrinking dimensions. Tolley, CB, de Rham & Hoover Copeland & Seto

66. Cosmo 07 The Worries ‘Technical Naturalness’ Runaway Behaviour Stabilizing the Extra Dimensions Famous No-Go Arguments Problems with Cosmology Constraints on Light Scalars Quantum part of the argument: Are these choices stable against renormalization? So far so good, but not yet complete Brane loops cannot generate dilaton couplings if these are not initially present Bulk loops can generate such couplings, but are suppressed by 6D supersymmetry

67. Cosmo 07 What Needs Understanding Classical part of the argument: What choices must be made to ensure 4D flatness? Quantum part of the argument: Are these choices stable against renormalization?

68. Cosmo 07 What Needs Understanding Classical part of the argument: What choices must be made to ensure 4D flatness? Quantum part of the argument: Are these choices stable against renormalization? When both branes have conical singularities all static solutions have 4D minkowski geometry. Conical singularities require vanishing dilaton coupling to branes (and hence scale invariant)

69. Cosmo 07 What Needs Understanding Classical part of the argument: What choices must be made to ensure 4D flatness? Quantum part of the argument: Are these choices stable against renormalization? When both branes have conical singularities all static solutions have 4D minkowski geometry. Conical singularities require vanishing dilaton coupling to branes (and hence scale invariant) Brane loops on their own cannot generate dilaton couplings from scratch.

70. Cosmo 07 What Needs Understanding Classical part of the argument: What choices must be made to ensure 4D flatness? Quantum part of the argument: Are these choices stable against renormalization? When both branes have conical singularities all static solutions have 4D minkowski geometry. Conical singularities require vanishing dilaton coupling to branes (and hence scale invariant) Brane loops on their own cannot generate dilaton couplings from scratch. Bulk loops can generate brane-dilaton coupling but TeV scale modes are suppressed at one loop by 6D supersymmetry

71. Cosmo 07 What Needs Understanding Classical part of the argument: What choices must be made to ensure 4D flatness? Quantum part of the argument: Are these choices stable against renormalization? When both branes have conical singularities all static solutions have 4D minkowski geometry. Conical singularities require vanishing dilaton coupling to branes (and hence scale invariant) Brane loops on their own cannot generate dilaton couplings from scratch. Bulk loops can generate brane-dilaton coupling but TeV scale modes are suppressed at one loop by 6D supersymmetry Each bulk loop costs power of e  ~ 1/r 2 and so only a few loops must be checked…..

72. Cosmo 07 Prognosis Theoretical worries Observational tests

73. Cosmo 07 The Worries ‘Technical Naturalness’ Runaway Behaviour Stabilizing the Extra Dimensions Famous No-Go Arguments Problems with Cosmology Constraints on Light Scalars

74. Cosmo 07 The Worries ‘Technical Naturalness’ Runaway Behaviour Stabilizing the Extra Dimensions Famous No-Go Arguments Problems with Cosmology Constraints on Light Scalars

75. Cosmo 07 The Worries ‘Technical Naturalness’ Runaway Behaviour Stabilizing the Extra Dimensions Famous No-Go Arguments Problems with Cosmology Constraints on Light Scalars Classical part of the argument: What choices must be made to ensure 4D flatness? Quantum part of the argument: Are these choices stable against renormalization?

76. Cosmo 07 The Worries ‘Technical Naturalness’ Runaway Behaviour Stabilizing the Extra Dimensions Famous No-Go Arguments Problems with Cosmology Constraints on Light Scalars Classical part of the argument: What choices must be made to ensure 4D flatness? Now understand how 2 extra dimensions respond to presence of 2 branes having arbitrary couplings. Not all are flat in 4D, but all of those having only conical singularities are flat. (Conical singularities correspond to absence of dilaton couplings to branes) Tolley, CB, Hoover & Aghababaie Tolley, CB, de Rham & Hoover CB, Hoover & Tasinato

77. Cosmo 07 The Worries ‘Technical Naturalness’ Runaway Behaviour Stabilizing the Extra Dimensions Famous No-Go Arguments Problems with Cosmology Constraints on Light Scalars Quantum part of the argument: Are these choices stable against renormalization? So far so good, but not yet complete Brane loops cannot generate dilaton couplings if these are not initially present Bulk loops can generate such couplings, but are suppressed by 6D supersymmetry

78. Cosmo 07 The Worries ‘Technical Naturalness’ Runaway Behaviour Stabilizing the Extra Dimensions Famous No-Go Arguments Problems with Cosmology Constraints on Light Scalars

79. Cosmo 07 The Worries ‘Technical Naturalness’ Runaway Behaviour Stabilizing the Extra Dimensions Famous No-Go Arguments Problems with Cosmology Constraints on Light Scalars Most brane properties and initial conditions do not lead to anything like the universe we see around us. For many choices the extra dimensions implode or expand to infinite size. Albrecht, CB, Ravndal, Skordis Tolley, CB, Hoover & Aghababaie Tolley, CB, de Rham & Hoover

80. Cosmo 07 The Worries ‘Technical Naturalness’ Runaway Behaviour Stabilizing the Extra Dimensions Famous No-Go Arguments Problems with Cosmology Constraints on Light Scalars Most brane properties and initial conditions do not lead to anything like the universe we see around us. For many choices the extra dimensions implode or expand to infinite size. Initial condition problem: much like the Hot Big Bang, possibly understood by reference to earlier epochs of cosmology (eg: inflation) Albrecht, CB, Ravndal, Skordis Tolley, CB, Hoover & Aghababaie Tolley, CB, de Rham & Hoover

81. Cosmo 07 Prognosis Theoretical worries Observational tests

82. Cosmo 07 Observational Consequences Quintessence cosmology

83. Cosmo 07 Observational Consequences Quintessence cosmology Modifications to gravity

84. Cosmo 07 Observational Consequences Quintessence cosmology Modifications to gravity Collider physics

85. Cosmo 07 Observational Consequences Quintessence cosmology Modifications to gravity Collider physics SUSY broken at the TeV scale, but not the MSSM!

86. Cosmo 07 Observational Consequences Quintessence cosmology Modifications to gravity Collider physics Neutrino physics?

87. Cosmo 07 Observational Consequences Quintessence cosmology Modifications to gravity Collider physics Neutrino physics? And more! SUSY broken at the TeV scale, but not the MSSM!

88. Cosmo 07 Observational Consequences Quintessence cosmology Modifications to gravity Collider physics Neutrino physics Astrophysics Not the MSSM! No superpartners Bulk scale bounded by astrophysics M g ~ 10 TeV Many channels for losing energy to KK modes Scalars, fermions, vectors live in the bulk

89. Cosmo 07 Observational Consequences Quintessence cosmology Modifications to gravity Collider physics Neutrino physics Astrophysics Can there be observable signals if M g ~ 10 TeV? Must hit new states before E ~ M g. Eg: string and KK states have M KK < M s < M g

90. Cosmo 07 Observational Consequences Quintessence cosmology Modifications to gravity Collider physics Neutrino physics Astrophysics Can there be observable signals if M g ~ 10 TeV? Must hit new states before E ~ M g. Eg: string and KK states have M KK < M s < M g Dimensionless couplings to bulk scalars are unsuppressed by M g

91. Cosmo 07 SLED: Present Status Stability against loops? What choices ensure 4D flatness? Are these choices stable against renormalization? Tuned initial conditions? Do only special initial conditions lead to the Universe we see around us?

92. Cosmo 07 SLED: Present Status 4D space is not flat for arbitrary brane - bulk couplings. Stability against loops? What choices ensure 4D flatness? Are these choices stable against renormalization? Tuned initial conditions? Do only special initial conditions lead to the Universe we see around us? ABPQ

93. Cosmo 07 SLED: Present Status 4D space is not flat for arbitrary brane - bulk couplings. Most brane pairs do not produce static solutions. Stability against loops? What choices ensure 4D flatness? Are these choices stable against renormalization? Tuned initial conditions? Do only special initial conditions lead to the Universe we see around us? BQTZ, TBDH

94. Cosmo 07 SLED: Present Status 4D space is not flat for arbitrary brane - bulk couplings. Most brane pairs do not produce static solutions. In some cases these choices appear to be stable against renormalization. Stability against loops? What choices ensure 4D flatness? Are these choices stable against renormalization? Tuned initial conditions? Do only special initial conditions lead to the Universe we see around us? BH

95. Cosmo 07 SLED: Present Status Brane loops cannot generate a dilaton coupling if one is not present to begin with. Stability against loops? What choices ensure 4D flatness? Are these choices stable against renormalization? Tuned initial conditions? Do only special initial conditions lead to the Universe we see around us?

96. Cosmo 07 SLED: Present Status Brane loops cannot generate a dilaton coupling if one is not present to begin with. Bulk loops can do so, but these are suppressed by powers of e  » 1/r 2 Stability against loops? What choices ensure 4D flatness? Are these choices stable against renormalization? Tuned initial conditions? Do only special initial conditions lead to the Universe we see around us?

97. Cosmo 07 SLED: Present Status Initial conditions exist which lead to dynamics which can describe the observed Dark Energy. Stability against loops? What choices ensure 4D flatness? Are these choices stable against renormalization? Tuned initial conditions? Do only special initial conditions lead to the Universe we see around us? ABRS

98. Cosmo 07 SLED: Present Status Initial conditions exist which lead to dynamics which can describe the observed Dark Energy. Successful initial condition are scarce. Stability against loops? What choices ensure 4D flatness? Are these choices stable against renormalization? Tuned initial conditions? Do only special initial conditions lead to the Universe we see around us? TBDH

99. Cosmo 07 SLED: Present Status Initial conditions exist which lead to dynamics which can describe the observed Dark Energy. Successful initial condition are scarce. Explained by earlier dynamics (eg inflation)? Stability against loops? What choices ensure 4D flatness? Are these choices stable against renormalization? Tuned initial conditions? Do only special initial conditions lead to the Universe we see around us?

100. Cosmo 07 Summary It is too early to abandon naturalness as a fundamental criterion!

101. Cosmo 07 Summary It is too early to abandon naturalness as a fundamental criterion! It is the interplay between cosmological phenomenology and microscopic constraints which will make it possible to solve the Dark Energy problem. Technical naturalness provides a crucial clue.

102. Cosmo 07 Summary It is too early to abandon naturalness as a fundamental criterion! It is the interplay between cosmological phenomenology and microscopic constraints which will make it possible to solve the Dark Energy problem. Technical naturalness provides a crucial clue. 6D brane-worlds allow progress on technical naturalness: Vacuum energy not equivalent to curved 4D Are ‘Flat’ choices stable against renormalization?

103. Cosmo 07 Summary It is too early to abandon naturalness as a fundamental criterion! It is the interplay between cosmological phenomenology and microscopic constraints which will make it possible to solve the Dark Energy problem. Technical naturalness provides a crucial clue. 6D brane-worlds allow progress on technical naturalness: Vacuum energy not equivalent to curved 4D Are ‘Flat’ choices stable against renormalization? Tuned initial conditions Much like for the Hot Big Bang Model.

104. Cosmo 07 Summary It is too early to abandon naturalness as a fundamental criterion! It is the interplay between cosmological phenomenology and microscopic constraints which will make it possible to solve the Dark Energy problem. Technical naturalness provides a crucial clue. 6D brane-worlds allow progress on technical naturalness: Vacuum energy not equivalent to curved 4D Are ‘Flat’ choices stable against renormalization? Tuned initial conditions Much like for the Hot Big Bang Model. Enormously predictive, with many observational consequences. Cosmology at Colliders! Tests of gravity…

105. Cosmo 07 SLED: Observational Consequences Quintessence cosmology Modifications to gravity Collider physics Neutrino physics Astrophysics

106. Cosmo 07 Quintessence cosmology Modifications to gravity Collider physics Neutrino physics Astrophysics Quantum vacuum energy lifts flat direction. Specific types of scalar interactions are predicted. Includes the Albrecht- Skordis type of potential Preliminary studies indicate it is possible to have viable cosmology: Changing G; BBN;… Albrecht, CB, Ravndal & Skordis Kainulainen & Sunhede SLED: Observational Consequences

107. Cosmo 07 Quantum vacuum energy lifts flat direction. Specific types of scalar interactions are predicted. Includes the Albrecht- Skordis type of potential Preliminary studies indicate it is possible to have viable cosmology: Changing G; BBN;… Quintessence cosmology Modifications to gravity Collider physics Neutrino physics Astrophysics Albrecht, CB, Ravndal & Skordis Potential domination when: Field Dependent Couplings: SLED: Observational Consequences

108. Cosmo 07 Quantum vacuum energy lifts flat direction. Specific types of scalar interactions are predicted. Includes the Albrecht- Skordis type of potential Preliminary studies indicate it is possible to have viable cosmology: Changing G; BBN;… Quintessence cosmology Modifications to gravity Collider physics Neutrino physics Astrophysics Albrecht, CB, Ravndal & Skordis log  vs log a Radiation Matter Total Scalar SLED: Observational Consequences

109. Cosmo 07 Quantum vacuum energy lifts flat direction. Specific types of scalar interactions are predicted. Includes the Albrecht- Skordis type of potential Preliminary studies indicate it is possible to have viable cosmology: Changing G; BBN;… Quintessence cosmology Modifications to gravity Collider physics Neutrino physics Astrophysics Albrecht, CB, Ravndal & Skordis Radiation Matter Total Scalar w Parameter:  and  w vs log a   ~ 0.7 w ~ – 0.9  m ~ 0.25 SLED: Observational Consequences

110. Cosmo 07 Quantum vacuum energy lifts flat direction. Specific types of scalar interactions are predicted. Includes the Albrecht- Skordis type of potential Preliminary studies indicate it is possible to have viable cosmology: Changing G; BBN;… Quintessence cosmology Modifications to gravity Collider physics Neutrino physics Astrophysics Albrecht, CB, Ravndal & Skordis  vs log a SLED: Observational Consequences Field-dependent matter couplings

111. Cosmo 07 Quantum vacuum energy lifts flat direction. Specific types of scalar interactions are predicted. Includes the Albrecht- Skordis type of potential Preliminary studies indicate it is possible to have viable cosmology: Changing G; BBN;… Quintessence cosmology Modifications to gravity Collider physics Neutrino physics Astrophysics Albrecht, CB, Ravndal & Skordis log r vs log a SLED: Observational Consequences

112. Cosmo 07 Quintessence cosmology Modifications to gravity Collider physics Neutrino physics Astrophysics At small distances: Changes Newton’s Law at range r/2  ~ 1  m. At large distances Scalar-tensor theory out to distances of order H 0. SLED: Observational Consequences

113. Cosmo 07 Quintessence cosmology Modifications to gravity Collider physics Neutrino physics Astrophysics At small distances: Changes Newton’s Law at range r/2  ~ 1  m. At large distances Scalar-tensor theory out to distances of order H 0. SLED: Observational Consequences

114. Cosmo 07 Quintessence cosmology Modifications to gravity Collider physics Neutrino physics Astrophysics Not the MSSM! No superpartners Bulk scale bounded by astrophysics M g ~ 10 TeV Many channels for losing energy to KK modes Scalars, fermions, vectors live in the bulk SLED: Observational Consequences

115. Cosmo 07 Quintessence cosmology Modifications to gravity Collider physics Neutrino physics Astrophysics Not the MSSM! No superpartners Bulk scale bounded by astrophysics M g ~ 10 TeV Many channels for losing energy to KK modes Scalars, fermions, vectors live in the bulk Dimensionless coupling! O(0.1-0.001) from loops Azuelos, Beauchemin & CB SLED: Observational Consequences

116. Cosmo 07 Quintessence cosmology Modifications to gravity Collider physics Neutrino physics Astrophysics Not the MSSM! No superpartners Bulk scale bounded by astrophysics M g ~ 10 TeV Many channels for losing energy to KK modes Scalars, fermions, vectors live in the bulk Dimensionless coupling! O(0.1-0.001) from loops Azuelos, Beauchemin & CB SLED: Observational Consequences

117. Cosmo 07 Quintessence cosmology Modifications to gravity Collider physics Neutrino physics Astrophysics Not the MSSM! No superpartners Bulk scale bounded by astrophysics M g ~ 10 TeV Many channels for losing energy to KK modes Scalars, fermions, vectors live in the bulk Azuelos, Beauchemin & CB SLED: Observational Consequences

118. Cosmo 07 Quintessence cosmology Modifications to gravity Collider physics Neutrino physics Astrophysics Not the MSSM! No superpartners Bulk scale bounded by astrophysics M g ~ 10 TeV Many channels for losing energy to KK modes Scalars, fermions, vectors live in the bulk Azuelos, Beauchemin & CB SLED: Observational Consequences

119. Cosmo 07 Quintessence cosmology Modifications to gravity Collider physics Neutrino physics Astrophysics SLED predicts there are 6D massless fermions in the bulk, as well as their properties Massless, chiral, etc. Masses and mixings can be chosen to agree with oscillation data. Most difficult: bounds on resonant SN oscillilations. SLED: Observational Consequences Matias & CB

120. Cosmo 07 Quintessence cosmology Modifications to gravity Collider physics Neutrino physics Astrophysics SLED predicts there are 6D massless fermions in the bulk, as well as their properties Massless, chiral, etc. Masses and mixings can be naturally achieved which agree with data! Sterile bounds; oscillation experiments; 6D supergravities have many bulk fermions: Gravity: (g mn,  m, B mn, ,  ) Gauge: (A m, ) Hyper: ( ,  ) Bulk couplings dictated by supersymmetry In particular: 6D fermion masses must vanish Back-reaction removes KK zero modes eg: boundary condition due to conical defect at brane position SLED: Observational Consequences Matias & CB

121. Cosmo 07 Quintessence cosmology Modifications to gravity Collider physics Neutrino physics Astrophysics SLED predicts there are 6D massless fermions in the bulk, as well as their properties Massless, chiral, etc. Masses and mixings can be naturally achieved which agree with data! Sterile bounds; oscillation experiments; Dimensionful coupling ~ 1/M g SLED: Observational Consequences Matias & CB

122. Cosmo 07 Quintessence cosmology Modifications to gravity Collider physics Neutrino physics Astrophysics SLED predicts there are 6D massless fermions in the bulk, as well as their properties Massless, chiral, etc. Masses and mixings can be naturally achieved which agree with data! Sterile bounds; oscillation experiments; Dimensionful coupling ~ 1/M g SUSY keeps N massless in bulk; Natural mixing with Goldstino on branes; Chirality in extra dimensions provides natural L; SLED: Observational Consequences Matias & CB

123. Cosmo 07 Quintessence cosmology Modifications to gravity Collider physics Neutrino physics Astrophysics SLED predicts there are 6D massless fermions in the bulk, as well as their properties Massless, chiral, etc. Masses and mixings can be naturally achieved which agree with data! Sterile bounds; oscillation experiments; Dimensionful coupling! ~ 1/M g SLED: Observational Consequences Matias & CB

124. Cosmo 07 Quintessence cosmology Modifications to gravity Collider physics Neutrino physics Astrophysics SLED predicts there are 6D massless fermions in the bulk, as well as their properties Massless, chiral, etc. Masses and mixings can be naturally achieved which agree with data! Sterile bounds; oscillation experiments; t Dimensionful coupling! ~ 1/M g Constrained by bounds on sterile neutrino emission Require observed masses and large mixing. SLED: Observational Consequences Matias & CB

125. Cosmo 07 Quintessence cosmology Modifications to gravity Collider physics Neutrino physics Astrophysics SLED predicts there are 6D massless fermions in the bulk, as well as their properties Massless, chiral, etc. Masses and mixings can be naturally achieved which agree with data! Sterile bounds; oscillation experiments; t Dimensionful coupling! ~ 1/M g Constrained by bounds on sterile neutrino emission Require observed masses and large mixing. Bounds on sterile neutrinos easiest to satisfy if g < 10 -4. Degenerate perturbation theory implies massless states strongly mix even if g is small. This is a problem if there are massless KK modes. This is good for 3 observed flavours. Brane back-reaction can remove the KK zero mode for fermions. SLED: Observational Consequences Matias & CB

126. Cosmo 07 Quintessence cosmology Modifications to gravity Collider physics Neutrino physics Astrophysics SLED predicts there are 6D massless fermions in the bulk, as well as their properties Massless, chiral, etc. Masses and mixings can be naturally achieved which agree with data! Sterile bounds; oscillation experiments; Imagine lepton- breaking terms are suppressed. Possibly generated by loops in running to low energies from M g. Acquire desired masses and mixings with a mild hierarchy for g’/g and  ’/  Build in approximate L e – L  – L , and Z 2 symmetries. S ~ M g r SLED: Observational Consequences Matias & CB

127. Cosmo 07 Quintessence cosmology Modifications to gravity Collider physics Neutrino physics Astrophysics SLED predicts there are 6D massless fermions in the bulk, as well as their properties Massless, chiral, etc. Masses and mixings can be naturally achieved which agree with data! Sterile bounds; oscillation experiments; 1 massless state 2 next- lightest states have strong overlap with brane. Inverted hierarchy. Massive KK states mix weakly. SLED: Observational Consequences Matias & CB

128. Cosmo 07 Quintessence cosmology Modifications to gravity Collider physics Neutrino physics Astrophysics SLED predicts there are 6D massless fermions in the bulk, as well as their properties Massless, chiral, etc. Masses and mixings can be naturally achieved which agree with data! Sterile bounds; oscillation experiments; 1 massless state 2 next- lightest states have strong overlap with brane. Inverted hierarchy. Massive KK states mix weakly. Worrisome: once we choose g ~ 10 -4, good masses for the light states require:  S = k ~ 1/g Must get this from a real compactification. SLED: Observational Consequences Matias & CB

129. Cosmo 07 Quintessence cosmology Modifications to gravity Collider physics Neutrino physics Astrophysics SLED predicts there are 6D massless fermions in the bulk, as well as their properties Massless, chiral, etc. Masses and mixings can be naturally achieved which agree with data! Sterile bounds; oscillation experiments; Matias & CB Lightest 3 states can have acceptable 3- flavour mixings. Active sterile mixings can satisfy incoherent bounds provided g ~ 10 -4 or less (  i ~ g/c i ). 2 SLED: Observational Consequences

130. Cosmo 07 Quintessence cosmology Modifications to gravity Collider physics Neutrino physics Astrophysics Energy loss into extra dimensions is close to existing bounds Supernova, red-giant stars,… Scalar-tensor form for gravity may have astrophysical implications. Binary pulsars;… SLED: Observational Consequences

131. Cosmo 07 String Inflation Why try to embed inflation into string theory? Why is it hard? What have we learned?

132. Cosmo 07 String Inflation Why try to embed inflation into string theory? Why is it hard? What have we learned? Inflationary models must be embedded into a fundamental theory in order to explain:

133. Cosmo 07 String Inflation Why try to embed inflation into string theory? Why is it hard? What have we learned? Inflationary models must be embedded into a fundamental theory in order to explain: * Why the inflaton potential has its particular finely-tuned shape (and if anthropically explained, what assigns the probabilities?)

134. Cosmo 07 String Inflation Why try to embed inflation into string theory? Why is it hard? What have we learned? Inflationary models must be embedded into a fundamental theory in order to explain: * Why the inflaton potential has its particular finely-tuned shape (and if anthropically explained, what assigns the probabilities?) * What explains any special choices for initial conditions

135. Cosmo 07 String Inflation Why try to embed inflation into string theory? Why is it hard? What have we learned? Inflationary models must be embedded into a fundamental theory in order to explain: * Why the inflaton potential has its particular finely-tuned shape (and if anthropically explained, what assigns the probabilities?) * What explains any special choices for initial conditions * Why the observed particles get heated once inflation ends.

136. Cosmo 07 String Inflation Why try to embed inflation into string theory? Why is it hard? What have we learned? Inflationary models must be embedded into a fundamental theory in order to explain: * Why the inflaton potential has its particular finely-tuned shape (and if anthropically explained, what assigns the probabilities?) * What explains any special choices for initial conditions * Why the observed particles get heated once inflation ends. Can identify how robust inflationary predictions are to high-energy details, and so also what kinds of very high- energy physics might be detectable using CMB measurements.

137. Cosmo 07 String Inflation Why try to embed inflation into string theory? Why is it hard? What have we learned? String theory has many scalars having very flat potentials. These scalars (called moduli) describe the shape and size of the various extra dimensions

138. Cosmo 07 String Inflation Why try to embed inflation into string theory? Why is it hard? What have we learned? String theory has many scalars having very flat potentials. BUT their potentials are usually very difficult to calculate.

139. Cosmo 07 String Inflation Why try to embed inflation into string theory? Why is it hard? What have we learned? String theory has many scalars having very flat potentials. BUT their potentials are usually very difficult to calculate. A convincing case for inflation requires knowing the potential for all of the scalars.

140. Cosmo 07 String Inflation Why try to embed inflation into string theory? Why is it hard? What have we learned? String theory has many scalars having very flat potentials. BUT their potentials are usually very difficult to calculate. A convincing case for inflation requires knowing the potential for all of the scalars.

141. Cosmo 07 String Inflation Why try to embed inflation into string theory? Why is it hard? What have we learned? For Type IIB strings it is now known how to compute the potentials for some of the low- energy string scalars. GKP

142. Cosmo 07 String Inflation Why try to embed inflation into string theory? Why is it hard? What have we learned? Branes want to squeeze extra dimensions while the fluxes they source want the extra dimensions to grow. The competition stabilizes many of the ‘moduli’

143. Cosmo 07 String Inflation Why try to embed inflation into string theory? Why is it hard? What have we learned? Branes want to squeeze extra dimensions while the fluxes they source want the extra dimensions to grow. The competition stabilizes many of the ‘moduli’

144. Cosmo 07 String Inflation Why try to embed inflation into string theory? Why is it hard? What have we learned? The moduli which remain after this stabilization can also acquire a potential due to nonperturbative effects. Plausibly estimated… KKLT models KKLT, KKLMMT

145. Cosmo 07 String Inflation Why try to embed inflation into string theory? Why is it hard? What have we learned? The moduli which remain after this stabilization can also acquire a potential due to nonperturbative effects. Improved for P 4 [11169] ‘The Better Racetrack’ Douglas & Denef

146. Cosmo 07 String Inflation Why try to embed inflation into string theory? Why is it hard? What have we learned? The inflaton in these models can describe the relative positions of branes; or the volume or shape of the extra dimensions.

147. Cosmo 07 String Inflation Why try to embed inflation into string theory? Why is it hard? What have we learned? The motion of several complex fields must generically be followed through a complicated landscape: many possible trajectories for each vacuum

148. Cosmo 07 String Inflation Why try to embed inflation into string theory? Why is it hard? What have we learned? The motion of several complex fields must generically be followed through a complicated landscape: many possible trajectories for each vacuum

149. Cosmo 07 String Inflation Why try to embed inflation into string theory? Why is it hard? What have we learned? The potential can inflate, e.g. for some choices for the properties of P 4 [11169] – giving rise to realistic inflationary fluctuations The ‘Racetrack Eight’

150. Cosmo 07 String Inflation CMB measurements begin to distinguish different inflationary models Why try to embed inflation into string theory? Why is it hard? What have we learned? Barger et al hep-ph/0302150 - model comparisons

151. Cosmo 07 String Inflation CMB measurements begin to distinguish different inflationary models Why try to embed inflation into string theory? Why is it hard? What have we learned? WMAP preferred - model comparisons

152. Cosmo 07 String Inflation Trajectories through string landscape predict same regions as do their low-energy effective theories. Why try to embed inflation into string theory? Why is it hard? What have we learned? brane-antibrane racetrack - model comparisons

153. Cosmo 07 String Inflation The measurements can already distinguish amongst some stringy inflationary models. Why try to embed inflation into string theory? Why is it hard? What have we learned? KKLMMT* P 4 [11169] WMAP preferred - model comparisons KKLMMT, BCSQ, Racetrack 8

154. Cosmo 07 String Inflation Most inflationary trajectories require fine tuning as do their field theory counterparts… Why try to embed inflation into string theory? Why is it hard? What have we learned? - model comparisons - naturalness KKLMMT, BCSQ, Racetrack 8

155. Cosmo 07 String Inflation Two possible exceptions: DBI Inflation: relativistic brane motion where H changes slowly. Kahler moduli inflation: slow roll from ‘generic’ approximations. Why try to embed inflation into string theory? Why is it hard? What have we learned? - model comparisons - naturalness Silverstein & Tong BCSQ, Conlon & Quevedo

156. Cosmo 07 String Inflation Although robust against most stringy details, predictions for CMB can be sensitive to specific kinds of physics near horizon exit Why try to embed inflation into string theory? Why is it hard? What have we learned? - model comparisons - naturalness - robustness

157. Cosmo 07 String Inflation Although robust against most stringy details, predictions for CMB can be sensitive to specific kinds of physics near horizon exit Why try to embed inflation into string theory? Why is it hard? What have we learned? - model comparisons - naturalness - robustness

158. Cosmo 07 String Inflation Although robust against most stringy details, predictions for CMB can be sensitive to specific kinds of physics near horizon exit Why try to embed inflation into string theory? Why is it hard? What have we learned? - model comparisons - naturalness - robustness

159. Cosmo 07 String Inflation Although robust against most stringy details, predictions for CMB can be sensitive to specific kinds of physics near horizon exit Why try to embed inflation into string theory? Why is it hard? What have we learned? - model comparisons - naturalness - robustness

160. Cosmo 07 String Inflation Although robust against most stringy details, predictions for CMB can be sensitive to specific kinds of physics near horizon exit Why try to embed inflation into string theory? Why is it hard? What have we learned? - model comparisons - naturalness - robustness

161. Cosmo 07 Outlook Branes continue to provide a useful approach for naturalness problems. Dark Energy, Hierarchy Problem, Inflation… more? We are very close to finding inflation in explicit controlled string calculations Possible progress on fine-tunings; New insights on reheating (eg cosmic strings); Signals largely robust, except near horizon exit Small tensor perturbations? Possibly even more novel physics can arise!

162. Cosmo 07 fin