Observational Evidence for Extra Dimensions from Dark Matter

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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)

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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 rlt1nm,
  corresponding to a world of 3 large extra
  dimensions of size R1nm.

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• String Theory
  Extra Dimensions ---
  Have never been tested
• How to test?
• ---From gravitational behavior at small
  distance scales
• 
  rltR
• 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-(2n) at
  rltR,
• R10(30/n)-17 cm
  (for n2, R1mm)
• Opens New Window
• Experimental test of string theory Searches
  for extra dimensions,
• by precise measurement of gravity at submm
  scales

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• Gravity has only been accurately measured
• at 1cm and beyond
• But was extrapolated for 33 orders of
• magnitude down to 10-33cm
• Does Newtons 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

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Experimental tests of Newtons 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

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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)

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• 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

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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

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Velocity of DM particles in different systems

• System Mass Typical
  velocity
• Glaxy clusters 1015 M_sun 1000 km/s
• Milky Way 1012 M_sun 200
  km/s
• Dwarf Galaxies 109 M_sun a few tens
  km/s

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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

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Why Extra Dimensions?

• DM self-interaction ? some Variant of Gravity?
• If n extra dimensions, gravity would be
  Fr-(2n),
• greatly
  enhanced at rltR.
• 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

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• At small scales gravity takes the general
  form
• where ?Rn , from the boundary condition
  that at rR,
• The gravitational scattering cross section
• 
  (1)
• And the DM self-interaction cross section from
  observations
• 
  (2)

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The results

• Combining Eq (1) and (2)
• ? The only solution is n3
• R1nm
• 
  mx310-16GeV
• 
  (axions?)

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Speculative?

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

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A very strange and  weired  picture?

• Huge number density of CDM particles, 1015
  /cm3
• 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.