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Modern cosmology: a challenge for fundamental physics

Modern cosmology: a challenge for fundamental physics. Diederik Roest (University of Groningen, The Netherlands) November 6, 2009 – UAM, Ciudad de México. Size matters!. Why is there any relation at all between cosmology and string theory?. Outline. Modern Cosmology Fundamental Physics

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Modern cosmology: a challenge for fundamental physics

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  1. Modern cosmology: a challenge for fundamental physics Diederik Roest (University of Groningen, The Netherlands) November 6, 2009 – UAM, Ciudad de México

  2. Size matters! Why is there any relation at all between cosmology and string theory?

  3. Outline • Modern Cosmology • Fundamental Physics • How to Realise Cosmic Acceleration in String Theory

  4. 1. Modern Cosmology

  5. Cosmological principle Universe has no structure at large scales: stars -> galaxies -> clusters -> superclusters -> … No preferred points or directions: homogeneous and isotropic.

  6. Cosmological principle General Relativity simplifies to: • Space-time described by • scale factor a(t) • curvature k • Matter described by ‘perfect fluids’ with • energy density ρ(t) • equation of state parameter w Fractions of critical energy density: Ω(t) = ρ(t) / ρcrit(t)

  7. Table of content? What are the ingredients of the universe? Dominant components: • w=0 - non-relativistic matter M (attractive) • w=-1 - cosmological constant Λ(repulsive)

  8. History of CC Who ordered Λ? • First introduced by Einstein to counterbalance matter • Overtaken by expansion of universe Convoluted history through the 20th century.

  9. Modern cosmology

  10. Supernovae • Explosions of fixed brightness • Standard candles • Luminosity vs. redshift plot • SNe at high redshift (z~0.75) appear dimmer • Sensitive to ΩM- ΩΛ [Riess et al (Supernova Search Team Collaboration) ’98][Perlmutter et al (Supernova Cosmology Project Collaboration) ’98]

  11. Cosmic Microwave Background • Primordial radiation from recombination era • Blackbody spectrum of T=2.7 K • Anisotropies of 1 in 105 • Power spectrum of correlation in δT • Location of first peak is sensitive to ΩM +ΩΛ [Bennett et al (WMAP collaboration) ’03]

  12. Baryon acoustic oscillations • Anisotropies in CMB are the seeds for structure formation. • Acoustic peak also seen in large scale surveys around z=0.35 • Sensitive to ΩM [Eisenstein et al (SDSS collaboration) ’05] [Cole et al (2dFGRS collaboration) ’05]

  13. Putting it all together

  14. Concordance Model Nearly flat Universe, 13.7 billion years old. Present ingredients: • 73% dark energy • 23% dark matter • 4% SM baryons

  15. Concordance Model Open questions: • What are dark components made of? • CC unnaturally small: 30 orders below Planck mass! • Fine-tuning mechanism? • Anthropic reasoning? • Cosmic coincidence problem

  16. Inflation Period of accelerated expansion in very early universe (~10-36 sec) to explain: • Cosmological principle • Why universe is flat • Absence of magnetic monopoles Bonus: quantum fluctuations during inflation can become source for structure formation ( CMB). Probes physics of very high energies (GUT scale ~ 1016 GeV).

  17. The future is bright! • Many models of inflation are possible • Inflationary properties are now being measured • Planck satellite: • Tensor modes? • Constraints on inflation? … three, two, one, and TAKE-OFF! [May 14, 2009]

  18. 2. Fundamental Physics

  19. Matter consists of fermions: three generattions of quarks and leptons. Forces are mediated by bosons: belonging to SU(3) x SU(2) x U(1). Unprecedented experimental verification!  Elementary particle physics • Standard Model (1970 - )

  20. Where is Higgs particle? Why three generations? Any questions? Include gravity? Effective description of fundamental theory?

  21. Experimental input?

  22. Strings • Quantum gravity • No point particles, but small strings • Unique theory • Bonus: gauge forces Unification of four forces of Nature?

  23. …and then some! String theory has many implications: How can one extract 4D physics from this?

  24. Compactifications

  25. energy simple comp. with fluxes and branes Scalar field Stable compactifications • Simple compactifications yield massless scalar fields, so-called moduli, in 4D. • Would give rise to a new type of force, in addition to gravity and gauge forces. Has not been observed! • Need to give mass terms to these scalar fields (moduli stabilisation). • Extra ingredients of string theory, such as branes and fluxes, are crucial!

  26. 3. How to Realise Cosmic Accelerationin String Theory

  27. Cosmic acceleration Two periods of accelerated expansion: • inflation in very early universe • present-time acceleration No microscopic understanding Cosmic challenges for fundamental physics!

  28. 0 0 0 V V 2 2 2 1 ¡ ¢ M M 1 1 ¿ ¿ ² ´ = = P P 2 ; : V V Cosmic acceleration Modelled by scalar field with non-trivial scalar potential V Slow-roll parameters:

  29. Cosmic acceleration in string theory String theory also gives rise to scalar potentials! Idea: use string theory potentials to model cosmic acceleration. Can provide information about e.g. possible inflationary scenarios at very high energies. Extreme case ε=0 corresponds to positive CC with w=-1. Leads to De Sitter space-time. Benchmark solution for string theory.

  30. Top-down approach • Generically string compactifications lead to Anti-De Sitter space-times • Is it even possible to get De Sitter from string theory? • A number of working models: • Start with moduli stabilisation in AdS using gauge fluxes and non-perturbative effects • Uplift scalar potential using • Anti-D3-branes [1] • D7-brane fluxes [2] • … [1: Kallosh, Kachru, Linde, Trivedi ’03][2: Burgess, Kallosh, Quevedo ’03]

  31. Bottom-up approach First understand 4D part and then connect to 10D. Effective description in 4D: supergravity theories. Field theories with local supersymmetry, which include gravity and gauge forces. Specified by number of supersymmetries N.

  32. Bottom-up approach Analysis of De Sitter in different supergravity theories: • N=4,8: unstable solutions with η= O(1) [1] • N=2: stable solutions [2] • Recent no-go theorems for stable solutions in various N=1,2 theories [3,4] • Requirements for De Sitter similar to those for slow-roll inflation [4] Tension between supersymmetry and cosmic acceleration! [1: Kallosh, Linde, Prokushkin, Shmakova ’02][2: Fre, Trigiante, Van Proeyen ’02][3: Gomez-Reino, (Louis), Scrucca ’06, ’07, ’08] [4: Covi, Gomez-Reino, Gross, Louis, Palma, Scrucca ’08]

  33. Building a bridge Connecting bottom-up and top-down approaches? How can 4D supergravity results be embedded in string theory? [1: D.R. ’09][2: Dibitetto, Linares, D.R. – in progress]

  34. 4. Conclusions

  35. Conclusions • Modern cosmological paradigm involves inflation and dark energy • Link with fundamental physics • Can one stabilise the moduli of string theory in a De Sitter vacuum? • What about inflation? • Many interesting (future) developments!

  36. Thanks for your attention! Diederik Roest (University of Groningen, The Netherlands) November 6, 2009 – UAM, Ciudad de México

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