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

2. Outline Motivation String Cosmology: Why Bother? Branes and ‘late-Universe’ cosmology Some Dark (Energy) Thoughts String inflation A Sledgehammer for a Nutcracker? Outlook

3. Strings, Branes and Cosmology Why doesn’t string theory decouple from cosmology? Why are branes important for cosmology and particle physics?

4. Strings, Branes and Cosmology Why doesn’t string theory decouple from cosmology? Why are branes important for cosmology and particle physics? Science progresses because short- distance physics decouples from long distances.

5. Strings, Branes and Cosmology Why doesn’t string theory decouple from cosmology? Why are branes important for cosmology and particle physics? 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

6. Strings, Branes and Cosmology Why doesn’t string theory decouple from cosmology? Why are branes important for cosmology and particle physics? 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, etc) relies on finely-tuned properties of scalar potentials, which are extremely sensitive to short distances.

7. Strings, Branes and Cosmology Why doesn’t string theory decouple from cosmology? Why are branes important for cosmology and particle physics? 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, etc) relies on finely-tuned properties of scalar potentials, which are extremely sensitive to short distances. * Modifications to gravity (MOND, Bekenstein, DGP, etc) are very strongly constrained by UV consistency issues.

8. 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

9. 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

10. 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

11. Strings, Branes and Cosmology Why doesn’t string theory decouple from cosmology? Why are branes important for cosmology and particle physics? Identifies hidden assumptions which particle physicists and cosmologists have been making: eg: all interactions don’t see the same number of dimensions.

12. Branes and Naturalness Removal of such assumptions has allowed new insights into low- energy naturalness problems.

13. Branes and Naturalness Removal of such assumptions has allowed new insights into low- energy naturalness problems. * Shows that extra dimensions can be as large as microns; ADD

14. Branes and Naturalness Removal of such assumptions has allowed new insights into low- energy naturalness problems. * Shows that extra dimensions can be as large as microns; * Shows that the string scale could be as small as TeV Horava & Witten, Lykken, Antoniadis

15. Branes and Naturalness Removal of such assumptions has allowed new insights into low- energy naturalness problems. * Shows that extra dimensions can be as large as microns; * Shows that the string scale could be as small as TeV * Ordinary physics in extra dimensions (eg: warping) can have extraordinary implications for the low-energy 4D theory. Randall & Sundrum

16. Branes and Naturalness Removal of such assumptions has allowed new insights into low- energy naturalness problems. * Shows that extra dimensions can be as large as microns; * Shows that the string scale could be as small as TeV * Ordinary physics in extra dimensions (eg: warping) can have extraordinary implications for the low-energy 4D theory. * Shows that the vacuum energy need not be directly tied to the cosmological constant, as had been thought. ADKS, KSS

17. Branes and Naturalness Removal of such assumptions has allowed new insights into low- energy naturalness problems. * Shows that extra dimensions can be as large as microns; * Shows that the string scale could be as small as TeV * Shows that the vacuum energy is not as directly tied to the cosmological constant In 4D the cosmological constant problem arises because a vacuum energy is equivalent to a cosmological constant, and so also to a curved universe.

18. Branes and Naturalness Removal of such assumptions has allowed new insights into low- energy naturalness problems. * Shows that extra dimensions can be as large as microns; * Shows that the string scale could be as small as TeV * Shows that the vacuum energy is not as directly tied to the cosmological constant In higher D solutions exist having large 4D energy, but for which the 4D geometry is absolutely flat! CG, ABPQ

19. Branes and Naturalness Removal of such assumptions has allowed new insights into low- energy naturalness problems. * Shows that extra dimensions can be as large as microns; * Shows that the string scale could be as small as TeV * Shows that the vacuum energy is not as directly tied to the cosmological constant Are the choices required for 4D flatness stable against renormalization? With SUSY, quantum corrections are usually order M 2 /r 2 but can be as small as 1/r 4. BH

20. Branes and Naturalness Removal of such assumptions has allowed new insights into low- energy naturalness problems. * Shows that extra dimensions can be as large as microns; * Shows that the string scale could be as small as TeV * Shows that the vacuum energy is not as directly tied to the cosmological constant Are the choices required for 4D flatness stable against renormalization? With SUSY, quantum corrections are usually order M 2 /r 2 but can be as small as 1/r 4 ABPQ This can be small enough because 1/r can be as small as 10 -3 eV (since r ~  m is possible)!!!

21. Branes and Naturalness Removal of such assumptions has allowed new insights into low- energy naturalness problems. * Shows that extra dimensions can be as large as microns; * Shows that the string scale could be as small as TeV * Shows that the vacuum energy is not as directly tied to the cosmological constant Are the choices required for 4D flatness stable against renormalization? So far so good: quantum corrections are usually order M 2 /r 2 but can be as small as 1/r 4 BMQ,,ABB, BC Very predictive: time-dependent Dark Energy; tests of GR at both micron and astrophysical distances; implications for the LHC; etc

22. 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: Canonical Variables: SLED: Observational Consequences

23. 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

24. 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

25. 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

26. 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

27. 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?

28. 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

29. 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

30. 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

31. 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

32. 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

33. 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?

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

35. 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:

36. 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?)

37. 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

38. 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.

39. 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.

40. 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

41. 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.

42. 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.

43. 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.

44. 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

45. 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’

46. 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

47. 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

48. 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.

49. 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

50. 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’

51. 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

52. 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

53. 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

54. 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

55. 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

56. String Inflation ‘Kahler moduli’ inflation may be an important exception: slow roll relies largely on generic approximations. Why try to embed inflation into string theory? Why is it hard? What have we learned? - model comparisons - naturalness BCSQ, Conlon & Quevedo

57. 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

58. 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

59. 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

60. 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

61. Outlook Branes continue to provide a useful approach for naturalness problems. Dark Energy, Inflation,…possibly more. We are getting 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 Possibly even more novel physics can arise!

62. fin