1. THE UNIVERSE AS A BRANE QG05 Sardegna Roy Maartens Portsmouth

2. general relativity + particle physics: expanding 4D universe model confirmed by observations

3. WMAP satellite and SDSS telescope

4. the model fits the high-precision data the model fits the high-precision data SDSS WMAP

5. 3 independent data sets intersect

6. successful successfulmodel but we know GR breaks down but we know GR breaks down quantum gravity will explain (pre-) inflation quantum gravity will explain (pre-) inflation GR + SM particle physics: inflationary LCDM model

7. inflation is not the only puzzle …. inflation is not the only puzzle …. dark energy?? dark energy?? maybe an IR modification to GR? (as well as UV in the early in the early universe) universe) LCDM is a framework seeking a fundamental theory LCDM is a framework seeking a fundamental theory more puzzles to come from observations? more puzzles to come from observations?

8. inflation is crucial – what underlies it? inflation is crucial – what underlies it? what is dark energy – matter or gravity? what is dark energy – matter or gravity? GR must break down in the UV GR must break down in the UV we need quantum gravity in the early universe +… maybe also in the late universe/ IR? candidate theories (Kiefer 2004) candidate theories (Kiefer 2004) quantum GR (Williams, Markopolou talks) string theory is high-precision cosmology probing the limits of GR?

9. removes the infinities of QFT removes the infinities of QFT and includes the graviton and includes the graviton but there is a price … but there is a price … 9 space dimensions!! string theory 6 compact 3

10. simple observation: we live in a large 3D world

11. why don’t we see the extra dimensions? conventional Kaluza-Klein idea: conventional Kaluza-Klein idea: extra dimensions too small to be seen extra dimensions too small to be seen discovery of D-brane discovery of D-brane matter fields restricted matter fields restricted to lower dimensional brane to lower dimensional brane external bulk felt only external bulk felt only through gravity through gravity extra dimensions can extra dimensions can be much larger 4D spacetime small extra dimension ~ L p large extra dimension >> L p gravity matter

12. (Tanaka)

13. 1+9 D universe has branes and strings some of the 6D may be small some of the 6D may be small 5 string theories + supergravity = limits of 1+10 D theory M THEORY (Bousso, Nicolai, Stelle, Zanelli talks)

14. string theory - the cosmologist’s wish-list * a natural mechanism for 3 dimensions to grow very large/ 6 dimensions to compactify * a natural mechanism for inflation * or … an alternative - eg brane collision * solve the dark energy problem …. then we test the answers COSMOLOGY AS A PROBE OF STRING THEORY

15. but … string theory is tough! string theory is tough! no convincing stringy inflation yet no convincing stringy inflation yet no convincing stringy dark energy yet no convincing stringy dark energy yet no convincing progress on perturbations no convincing progress on perturbations some progress in string brane inflation some progress in string brane inflation * D/ anti-D brane models * D/ anti-D brane models * higher-D cosmic strings can be produced * higher-D cosmic strings can be produced * … but still a long way to go * … but still a long way to go

16. the practical cosmologist’s approach while waiting for progress in string cosmology … develop phenomenology * it’s what we do with inflation * it’s what we do with inflation * and QG phenomenology (Liberati talk) * and QG phenomenology (Liberati talk) use string theory ideas in cosmology * in fact this can help string theory … * in fact this can help string theory … * brings high-precision data into the picture * brings high-precision data into the picture

17. experimental tests of gravity Cavendish Cavendish neutrons (Bertolami talk) neutrons (Bertolami talk) colliders (Cavaglia) colliders (Cavaglia) solar system solar system perihelion perihelion bending of light bending of light binary pulsars binary pulsars gravitational lensing gravitational lensing BHs and GWs BHs and GWs cosmology cosmology BBN, SNe BBN, SNe LSS, CMB LSS, CMB

18. cosmology probes gravity on vast scales (Tegmark)

19. eg - SDSS test of Newton ’ s law cccc (Shirata et al)

20. extra-dimensional gravity brings new features extra-dimensional gravity brings new features extra polarizations of the graviton (KK modes) extra polarizations of the graviton (KK modes) new fields (dilaton, moduli, radion, form fields) new fields (dilaton, moduli, radion, form fields) shadow matter shadow matter holography (AdS/CFT) (Bianchi talk) holography (AdS/CFT) (Bianchi talk) precision data constrains extra-dimensional gravity … hopefully rule out some models … hopefully rule out some models signature of extra dimensions in observations?

21. 5D gravitons – effective mass on brane (massive KK modes) polarizations: 5 = 2+2+1 graviton KK modes 5D spin-2 (5) = 4D spin-2 (2) + 4D spin-1 (2) + 4D spin-0 (1)

22. start with simple models start with simple models similar to inflation and dark energy models similar to inflation and dark energy models simplest models with predictive power simplest models with predictive power focus on one or two key aspects at a time focus on one or two key aspects at a time KK modes of graviton KK modes of graviton scalar fields – dilaton, radion, moduli scalar fields – dilaton, radion, moduli …. …. motivate by string theory 2 branes with large separation + small internal space (Horava-Witten) 2 branes with large separation + small internal space (Horava-Witten) AdS 5 X M models: 1 large dimension, warped AdS 5 X M models: 1 large dimension, warped …. ….

23. small extra dimensions (moduli stabilised) large extra dimension visible brane shadow brane L exp(-y/L) anti de Sitter 5D (infinite extra dimension) brane

24. all extra dimensions large (Antoniadis, Arkani-Hamed, Dimopoulos, Dvali 1998) identify brane n-torus (…. Antoniadis talk)

25. testing gravity in the laboratory testing gravity in the laboratory extra dimension changes Newton’s law extra dimension changes Newton’s law L torsion pendulum

26. warped geometry warped geometry extra dimension shrinks extra dimension shrinks toward infinity toward infinity finite volume bulk finite volume bulk curvature scale curvature scale 4D GR is recovered on large scales 5D solution of Einstein equations (Randall & Sundrum 1999) y

27. Discrete spectrum m=0: zero mode for tensors no zero-mode for scalars/ vectors Continuous spectrum m>0: massive KK modes of the graviton same massive modes for scalar/ vector/ tensor U(z)U(z) z  ～ z –2 KK modes of the graviton (Garriga, Tanaka) (de Risi talk)

28. cosmic expansion = movement of brane time 3D space

29. deviation from 4D GR in the early universe modified Friedmann equation modified Friedmann equation gravity leakage: gravity leakage: gravity becomes 5D gravity becomes 5D gravity leakage (Binetruy, Deffayet, Langlois) (Shiromizu, Maeda, Sasaki)

30. RS inflation modified Friedmann modified Friedmann boosts 4D perturbations boosts 4D perturbations but also creates 5D GW but also creates 5D GW * back-reaction of 5D * back-reaction of 5D metric perturbations * loss of power * loss of power large scales boosted large scales boosted but small scales? but small scales? 5D metric perturbation * V inf lowered * A S and A T boosted * but A T / A S suppressed (RM et al) (Langlois, RM, Wands) (Koyama,Mizuno,Wands)

31. modified background boosts 4D GW modified background boosts 4D GW but 5D GW emission damps 4D GW but 5D GW emission damps 4D GW competing effects – cancel! competing effects – cancel! recover GR result (Hiramatsu,Koyama,Taruya)

32. 5D BH induces “dark radiation” on the brane 5D BH induces “dark radiation” on the brane perturbation isocurvature perturbation perturbation isocurvature perturbation 5D anisotropy induces anisotropic stress 5D anisotropy induces anisotropic stress time bulk brane BH CMB anisotropies BH

33. simple model constraint on small BH mass in 5D bulk spacetime BH mass Spectral index probability 0 (Koyama)

34. is dark energy a gravitational effect? general relativity general relativity observations (nearly) flat, accelerating universe, with sub-critical matter observations (nearly) flat, accelerating universe, with sub-critical matter field equations imply dark energy field equations imply dark energy example from history Mercury perihelion – Newton + missing matter? no: modified gravity

35. modify Einstein’s equations? try try GR + light scalar field – same as DE field try ghosts try infinite extra degrees of freedom? try massive graviton (Pauli-Fierz) unstable theory

36. braneworld models with late-time acceleration Randall-Sundrum modifies GR in UV (early universe) - via gravity leakage off the brane at early times - via gravity leakage off the brane at early times recover GR in IR DGP model is an alternative (Dvali, Gabadadze, Porrati 2000) - gravity leakage off the brane at late times - gravity leakage off the brane at late times recover GR in UV L = crossover scale

37. L = crossover scale: L = crossover scale: 5D gravity dominates at late times 5D gravity dominates at late times no graviton zero-mode: no graviton zero-mode: 4D gravity recovered at high energy via lightest KK modes ‘metastable graviton’ 5D Minkowski bulk: infinite volume extra dimension gravity leakage brane

38. DGP cosmology Friedmann early universe – recover GR dynamics late universe – acceleration without dark energy * from leakage of gravity * but fine-tuned (Deffayet)

39. need to test this against SNe redshifts

40. comparison with SNe observations (Deffayet et al) (Sahni et al) (Song) (Bento talk)

41. structure formation degeneracy between DGP and GR+DE: both can fit SNe observations but this is broken by structure formation

42. how to distinguish DGP from GR+DE? structure formation is suppressed: * in GR – because DE dominates over matter * in DGP – because gravity weakens need to compute LSS and compare with SDSS L~1/H 0

43. CMB – the ISW effect difference in LSS formation will leave an imprint (Nichol)

44. ISW effect gravity leakage must be taken into account gravity leakage must be taken into account …. SNe+SDSS+CMB may rule out DGP …. SNe+SDSS+CMB may rule out DGP DGP (4D) LCDM Dark Energy DE curve: scalar field model with the same background evolution as DGP DGP(4D) curve: neglects 5D term (Takahashi)

45. brane cosmology brings new ideas brane cosmology brings new ideas * new effects in inflation, CMB, dark energy - even in the simplest models - even in the simplest models * RS + DGP models – extra-D gravity is central (unlike string brane inflation models) * probe stringy ideas via gravity – and test against observations * probe stringy ideas via gravity – and test against observations exciting challenges for theorists!

46. conclusion - the universe as a brane string theory demands extra dimensions this may explain the beginning of the universe this may explain what inflation is this may explain what the dark energy is ….. but not yet - brane models can help