Gravitational chameleons'

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Gravitational chameleons.
Karel Van Acoleyen Durham University, IPPP
Cosmo-UK 2006, Ambleside 29-31 August 2006

• Work done in collaboration with Ignacio Navarro.
• gr-qc/0506096 (Phys. Lett. B 622, 2005)
• gr-qc/0511045 (JCAP 03, 2006 )
• gr-qc/0512109
• In preparation

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Late time acceleration from something different
than ?

• New light degrees of freedom

• Avoiding the fifth-force constraints
• Make the couplings to (ordinary) matter very
  small.
• Have some chameleon mechanism the properties of
  the new propagating degrees of freedom depend on
  the background.

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Chamelonlike models.

• Example 1 The chameleon fields the mass of the
  extra scalar depends on the background matter
  density. Thin shell effect. (Khoury
  and Weltman PRL 93 2004)
• Example 2 The DGP model the extra degrees of
  freedom decouple when approaching a mass source.
  (Dvali, Gabadadze and Porrati Phys. Lett. B485
  2000)

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The chameleonmechanism implies a low cut off of
the linearization
Example 1 the chameleonfields.

Linearization in the solar system breaks down
when
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The chameleonmechanism implies a low cut off of
the linearization
Example 2 the DGP model.

Linearization on Minkowski space breaks down when
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Example 3
with
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Prime motivation alternatives for Dark Energy

• With crossover scale

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Propagating degrees of freedom
(on deSitter
space)

• massless spin 2 graviton
• extra massive scalar
• no ghosts (but see De
  Felice et al astro-ph/0604154 )
  

Effective Planck mass and the mass of the scalar
depend on the backgroundcurvature !!
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Schwarzschild solution at large distances

• On vacuum, the mass of the scalar is very light.
• problem for the Solar
  System tests?

BUT, the perturbation series breaks down at
short distances Corrections go like
with
This distance is huge 10 kpc for the Sun,
1 Mpc for the Milky Way )
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Schwarzschild solution at short distances

• The curvature in the Solar System is huge. So we
  dont expect much modification.
• What about the extra scalar?
• Corrections for
• Mass depends on the background
• 
  
• Explicitly

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To summarize
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Could the non-perturbative region play the role
of the dark matter halo in galaxies?

• Yes! (maybe) in the limit n 0
• ? Logarithmic actions

Departure from Newtonian gravity, at a critical
acceleration set by the same crossover scale.
MOND?
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MOdified Newtonian Dynamics

• A modification of Newtons law that explains the
  rotation curves without DM (Milgrom, 1984)
• 
  
  

Works pretty well for all sorts of (spiral)
galaxies. One particular prediction is the
Tully-Fischer law Explaining this phenomenology
from DM seems quite hard. Structure
formation/gastronomy is a stochastic process.
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MOdified Newtonian Dynamics
(R. Sanders, S.McGaugh astro-ph/0204521)
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The Bullet for MOND?
astro-ph/0608407 D. Clowe et al
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NO

• The bullet cluster is an observation at the
  cluster galactic scale. We knew already that
• MOND needs dark matter to explain the clusters.
  (Saunders astro-ph/0212293 )
• With Bekensteins TeVeS, you can get the CMB
  right but you still need massive 2eV neutrinos
  (Skordis et al PRL 96, 2006)
• MOND can only be ruled out by observations at the
  galactic scale.
• Does the bulletcluster prove dark matter? Not
  necessarily. But gut reaction is yes.

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Conclusions

• Viable long distance modifications are
  characterized by a chameleonlike behavior of the
  degrees of freedom.
• In our case the modification is characterized by
  
• 1. an extra scalar degree of freedom,
  with a mass that runs with the backgroundcurvature
  ( )
• 2. a running Newtons constant
• MOND is not ruled out yet (which is surprising).
  And one can get MONDlike modifications from
  logarithmic actions.