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Observational Evidence for Extra Dimensions from Dark Matter

Observational Evidence for Extra Dimensions from Dark Matter. Bo Qin National Astronomical Observatories, China Bo Qin, Ue-Li Pen & Joseph Silk, PRL , submitted (astro-ph/0508572). Main results.

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Observational Evidence for Extra Dimensions from Dark Matter

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  1. Observational Evidence for Extra Dimensions from Dark Matter Bo Qin National Astronomical Observatories, China Bo Qin, Ue-Li Pen & Joseph Silk, PRL, submitted (astro-ph/0508572)

  2. Main results • Astronomical observations of systems of dark matter in recent years suggest that dark matter particles may have considerable self-interaction. • We find that the properties of this self-interaction are precisely the consequences of a gravity of the r-5 law at r<~1nm, corresponding to a world of 3 large extra dimensions of size R~1nm.

  3. String Theory: Extra Dimensions: --- Have never been tested • How to test? ---From gravitational behavior at small distance scales r<R • Size of extra dimensions: Planck scale ~10-33 cm • Large Extra Dimensions: 3 + n + m = 9 Arkani-Hamed, Dimopoulos & Dvali (ADD) 1998, Phys. Lett. B Gravity: F ~ r-(2+n)at r<R, R~10(30/n)-17 cm (for n=2, R~1mm) Opens New Window: Experimental test of string theory + Searches for extra dimensions, by precise measurement of gravity at submm scales

  4. Gravity has only been accurately measured at ~1cm and beyond But was extrapolated for 33 orders of magnitude down to ~10-33cm Does Newton’s Inverse Square Law still hold--- 1. At very small distance scales? 2. In very weak regimes? a ~10-8 cm s-2 Modified Newtonian Dynamics(MOND?) Milgrom 1983

  5. Experimental tests of Newton’s law at sub-mm scales & Searches for large extra dimensions e.g.: Long et al., Nature (2003) Hoyle et al., PRL (2001); PRD (2004) Chiaverini et al. PRL (2003) ( & e.g. hep-ph/0402168 for a review ) • No deviation from Newtonian has been found from ~1cm down to ~1μm

  6. Potential challenge to CDM model Collisionless Cold Dark Matter (CCDM) Very successful in explaining the origin and evolution of cosmic structure on large scales, but may have problems on galactic and sub-galactic scales Theory vs. Observation Conflict (Crisis?) Cuspy core problem of DM Solutions---Modify CCDM • Self-Interacting DM(Spergel & Steinhardt, 2000, PRL) • Warm Dark Matter (not favored by WMAP, early reionization)

  7. DM self-interaction cross section: xx/mx 810-(25-22) cm2/GeV (Spergel & Steinhardt 2000, PRL) Nature of this self-interaction is unknown: • Introduce a new interaction beyond the Standard Model? • SIMPs?(Strongly Interacting Massive Particles) Wandelt et al., astro-ph/0006344 Starkman et al., 1990, PRD Qin & Wu, 2001, PRL

  8. Further studies of SIDM • In galaxy clusters: ( X-ray / strong lensing / weak lensing )  xx is much smaller • In galaxies: still room to argue • xx may not be constant, but varies with mass of the system--- • more massive systems (clusters) have smaller xx, while less massive systems (like dwarf galaxies) have larger xx

  9. Velocity of DM particles in different systems • System Mass Typical velocity • Glaxy clusters 10^15 M_sun 1000 km/s • Milky Way 10^12 M_sun 200 km/s • Dwarf Galaxies 10^9 M_sun a few tens km/s

  10. Nature of SIDM xx Maybe velocity dependant Firmani et al. (2000): xx/mx 410-25(100kms-1/v) cm2/GeV v DM self-interaction--- m1 Long-range forces? m2

  11. Why Extra Dimensions? • DM self-interactionsome Variant of Gravity? • If n extra dimensions, gravity would be F~r-(2+n), greatly enhanced at r<R. May naturally provide the DM self-interaction. • Advantages: (of attributing DM self-interaction to extra dimensions) 1. Using the existing framework 2. Without introducing any new or fine-tuned interaction 3. Link string scenarios with observable/astronomical phenomena

  12. At small scales gravity takes the general form: where =Rn , from the boundary condition that at r=R, • The gravitational scattering cross section (1) • And the DM self-interaction cross section from observations: (2)

  13. The results Combining Eq (1) and (2):  The only solution isn=3 R~1nm mx~3×10-16GeV (axions?)

  14. Speculative? • What we have done? • Observations---Self-interacting Dark Matter (although debated) + • String scenarios (ADD)

  15. A very strange and « weired » picture? • Huge number density of CDM particles, 10^15 /cm^3 • Extremely small spacial seperation between the CDM particles 10^-5 cm • Large de Broglie wavelength of ~ km • But a mean free path of CDM collision ~ kpc! • Each DM particle have just ONE or several interactions with others, during the cosmic age. • Extremely low probability of scattering.

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