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- 1. Strings, Extra Dimensions and Cosmology Ishwaree Neupane Dept of Physics & Astronomy University of Canterbury
- 2. <ul><li>Quantum Mechanics is the framework for </li></ul><ul><li>studying the physics of very short distances . </li></ul>Special Relativity: A novel framework for describing the physics of objects moving at speeds close to the speed of light. Theory of General Relativity is the extension of this theory to include gravity. <ul><li>Synthesis of Special Relativity + Quantum Mechanics = Quantum Field Theory </li></ul><ul><li>Amazing Unification of Space-time & QM doubled the world : </li></ul>11/11/11 Strings, Extra Dimensions and Cosmology
- 3. 72.6±1.5% 22.8±1.3% 13.73 ± 0.1 Gyr A preposterous universe! Our present understanding of cosmology is grounded in General Relativity and Quantum Field Theory, which have individually passed a remarkable panoply of experimental tests; we are still not able to account for what seems to be 73% of the energy budget of the universe and hence its mystic name Dark Energy Accelerating Universe! 2011 Nobel prize in physics 11/11/11 Strings, Extra Dimensions and Cosmology
- 4. Einstein (1917) Dark Energy is explained by Einstein’s theory with a Cosmological Constant This is consistent with the present size and age of the Universe 11/11/11 Strings, Extra Dimensions and Cosmology
- 5. “ Electro-weak length” size of universe solar system galaxy universe people DNA atom quarks LHC ? Two options 11/11/11 Strings, Extra Dimensions and Cosmology
- 6. <ul><li>Today theoretical physics deals basically with two options: </li></ul><ul><li>(1) Grand desert stretching from ∼ 250 GeV (10 -18 cm)∼ 10 19 GeV (10 -33 cm) with no new physics inside ( highly unlikely ); </li></ul><ul><li>(2) Theory of Extra Dimensions – a paradigm in which one expects a new physics at a much lower energy scale </li></ul><ul><li>The aim is to bring the scale of new fundamental physics from inaccessible 10 19 GeV down to 1-100 TeV or so </li></ul>What are those two options? 11/11/11 Strings, Extra Dimensions and Cosmology
- 7. “ weak length ” size of universe “ Planck length ” Quantum-Gravity Emergent SpaceTime String Theory + Beyond Hierarchy Problem Taming the vacuum (cosmological constant) dark energy problem The hope is that the ultimate theory of Nature contains a fundamental and simple, concise relation between close-by and far-away, and past and future 11/11/11 Strings, Extra Dimensions and Cosmology
- 8. Taming the Vacuum Even vacuum (empty space) has energy! bigger fluctuation at smaller distances Estimate: But energy curves spacetime! curvature The universe would double in size every An interesting experimental discovery of last 20 years -- the Universe is accelerating , but doubles in size every paradox 11/11/11 Strings, Extra Dimensions and Cosmology
- 9. Theoretical Physics Thrives on CRISES & PARADOXES 11/11/11 Strings, Extra Dimensions and Cosmology
- 10. In string theory Inflation is likely to be linked to moduli fields, e.g., size and shape of extra (hidden) dimensions Λ – could depend on fundamental (UV) structure of theory: internal space geometry, branes, fluxes, etc Λ >0 Λ <0 Λ =0 Prospects of describing the observed universe in string theory 11/11/11 Strings, Extra Dimensions and Cosmology
- 11. 11/11/11 Strings, Extra Dimensions and Cosmology
- 12. String Theory Pre `95 points 10 dimensions 6 “curled up” to yield 4 Our 4D ? 11/11/11 Strings, Extra Dimensions and Cosmology
- 13. String Theory Post `95 Not a theory of strings! <ul><li>Strings + extra dimensions can have any size </li></ul><ul><li>Particle string unification </li></ul>HOLOGRAPHY Apparently, the best understanding of string theory is in terms of a particle theory via the incredible idea of points strings photon photino Ordinary dimension Quantum dimensions Gravity lives everywhere 3-d Membrane floating in extra dimensions 11/11/11 Strings, Extra Dimensions and Cosmology
- 14. Holographic Universe 4D particle theory (no gravity) static, highly curved space 10D theory of strings, gravity + everything Presumably, most important theoretical discovery of last 20 years ! 11/11/11 Strings, Extra Dimensions and Cosmology
- 15. AdS/CFT (Maldacena, 1997) Boundary R 4 AdS 5 z = 0 z N = 4 SU(N) Yang-Mills theory AdS 5 x S 5 String theory on String theory has led to an impressive and growing array of insights on diverse problems in theoretical physics but there has been comparatively little progress in understanding whether string theory actually describes our universe. 11/11/11 Strings, Extra Dimensions and Cosmology
- 16. Quantum gravity effect cosmological perturbations The power-law index of the primordial power spectrum n s = 0.968 ± 0.012 (68% CL) 11/11/11 Strings, Extra Dimensions and Cosmology Observations give us important new windows into the Properties of the very early universe
- 17. Extra dimensions: motivation <ul><li>very early universe – general relativity breaks down </li></ul><ul><li>key idea: precision cosmology </li></ul><ul><li>= test of Quantum Gravity </li></ul><ul><li>superstring theories M theory </li></ul><ul><li> 1 time +10 space dimensions </li></ul><ul><li>new view of dimensional reduction </li></ul><ul><li> 1+4 1+3+1 1+3+(1) </li></ul><ul><li>5D Brane-world gravity (Randall-Sundrum’99) </li></ul><ul><li>matter on brane / gravity in bulk </li></ul>matter gravity An important features of our late-time low-energy world: Why it appears to be four-dimensional? 11/11/11 Strings, Extra Dimensions and Cosmology
- 18. Can an embedding of a four-dimensional spacetime in dS 5 explain the observed cosmic acceleration? dS 4 dS 5 Effective Planck mass: This is different from the result obtainable in a static AdS 5 background IPN, PRD83, 2011 (arXiv:1011.6357) IJMP D19, 2010 (arXiv:1004.0254) zero-mode solution is normalizable 11/11/11 Strings, Extra Dimensions and Cosmology
- 19. Compactification of D=10 string theory The Klebanov-Strassler geometry in D=10 has some common features with warped Randall-Sundrum 5D braneworld model Does it work in higher dimensions? 11/11/11 Strings, Extra Dimensions and Cosmology
- 20. IPN: Phys. Rev. Lett: 98 (2007) [hep-th/0609086] The first explicit realisation of a cosmological compactification of 10D Einstein theory for which all extra dimensions except one can be stabilised by introducing time-dependent metric moduli or Weyl scalars leading to a “4+1+compact space” Essentially, all of our knowledge of string theory in the cosmological setting is founded on approximations 11/11/11 Strings, Extra Dimensions and Cosmology
- 21. Freezing the time-dependent metric moduli IPN: CQG,28 (2011) 1006.4495 [hep-th] The discovery of a new class of simple warped vacuum solutions with a 4D de sitter part and non-compact internal space with still a finite 4D Planck mass and normalizable zero-mode graviton wave-function IPN:Nucl. Phys. B847 (2011) 1011.5007 [hep-th] 11/11/11 Strings, Extra Dimensions and Cosmology
- 22. A disturbing realisation (?) String theory is “unique” – no adjustable parameters. But it has zillions of (possible) solutions! Pre ‘95 : zillions ~ millions Post `95 : zillions ~ 10 500 ! All part of same theory, but different looking at long distances (in Infrared limit) “Multiverse” or “String Landscape” – A Controversial Subject! There are hints that inflationary cosmological model can be naturally obtained from string theory other than that string theory is a mathematically consistent quantum gravity theory Testing string theory itself by any means is extremely difficult. 11/11/11 Strings, Extra Dimensions and Cosmology
- 23. <ul><li>Summary: </li></ul><ul><li>Inflation and Dark Energy both require new microphysics, solutions must rely on the intimate connection between GR and QFT. </li></ul><ul><li>Most examples worked out as string cosmology invoke string theory, branes, or extra dimensions, and may be identified as quantitative approximations to solutions in string theory. A reliably anchoring cosmology in string theory is hard, and success is rare. </li></ul><ul><li>A successful grounding of cosmology in string theory remains a worthy and realistic goal. </li></ul><ul><li>If Dark Energy is vacuum energy, cosmological observations won't tell us anything; we'll have to understand fundamental physics, probably through experiments – LHC is a great hope. </li></ul>11/11/11 Strings, Extra Dimensions and Cosmology
- 24. Signatures for New Physics, Extra dimensions, Supersymmetry Higgs scalar, String Theory, … Large Hadron Collider 11/11/11 Strings, Extra Dimensions and Cosmology

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