Testing Chameleon Dark Energy

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Testing Chameleon Dark Energy
Amanda Weltman
Portsmouth June 2008

University of Cambridge
University of Cape Town
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Motivation

• Massless scalar fields are abundant in String
  and SUGRA
• theories

• Massless fields generally couple directly to
  matter with
• gravitational strength

• Unacceptably large Equivalence Principle
  violations
• Coupling constants can vary
• Masses of elementary particles can vary

Gravitational strength coupling
Light scalar field
?
Tension between theory and observations
Opportunity! - Connect to Cosmology
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Observations
Accelerated expansion of the Universe

• Dark Energy p lt 0
• Cosmological Constant, ?
• Dynamical e.o.s w ? -1

Quintessence ? Need light scalar field
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Chameleon Effect
astro-ph/0309300 PRL J. Khoury and A.W
astro-ph/0309411 PRD J. Khoury and A.W
Mass of scalar field depends on local matter
density
In region of high density ? mass is large ? EP
viol suppressed
In solar system ? density much lower ? fields
essentially free
On cosmological scales ? density very low ? m
H0
Field may be a candidate for acc of universe
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Ingredients
astro-ph/0408415 PRD P. Brax, C. van de Bruck,
J.Khoury, A. Davis and A.W
Reduced Planck Mass
Coupling to photons
Matter Fields
Einstein Frame Metric
Conformally Coupled
Potential is of the runaway form
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Effective Potential
Energy density in the ith form of matter
Equation of motion
Dynamics governed by Effective potential
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Predictions for Tests in Space
Different behaviour in space
New Feature !!
Eöt-Wash Bound ? lt 10-13
Tests for UFF
Near- future experiments in space
STEP ? 10-18 GG
? 10-17 MICROSCOPE ? 10-15
We predict
SEE Capsule
lt 10-7
?RE/RE
10-15 lt
Corrections of O(1) to Newtons Constant
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Cosmological Evolution
astro-ph/0408415 PRD P. Brax, C. van de Bruck,
J.Khoury, A. Davis and A.W
What do we need?

• attractor solution

?
If field starts at min, will follow the min
?

• ? must join attractor before current epoch

?

• ? Slow rolls along the attractor

• Variation in m ? is constrained to be less than
  10.
• Constrains ?BBN ? the initial energy density
  of the field.

Weaker bound than usual quintessence
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Strong Coupling
Strong coupling not ruled out by local
experiments!
Mota and Shaw
Thin shell suppression ?
Where
?
Effective coupling is independent of ?!!
If an object satisfies thin shell condition - the
? force is ? independent
? gtgt 1 ? thin shell more likely ? suppresses
space signal
Opportunity?
Loophole!
Lab tests on earth can probe a range of parameter
space that is complementary to space tests.
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Coupling to Photons
Remember
Introduces a new mass scale
Effective potential
We can probe this term in quantum vacuum
experiments

• Use a magnetic field to disturb the vacuum
• Probe the disturbance with photons

Test the F2 term
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PVLAS and CAST
(Polarizzazione del Vuoto con LASer)
(CERN Axion Solar Telescope)
To explain unexpected birefringence and dichroism
results
requires
and
(g 1/M)
Conflicts with astrophysical bounds e.g. CAST
(solar cooling)
?

But
Too heavy to produce ? CAST bounds easily
satisfied
Chameleons - naturally evade CAST bounds and
explain PVLAS
Brax, Davis, van de Bruck
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Particles Trapped in a Jar
Photon-dilaton-like chameleon particle
regeneration using a "particle trapped in a
jar" technique
A. Chou et. Al. 0806.2438 hep-ex
http//gammev.fnal.gov
See also - Gies et. Al. Ahlers et. Al. (DESY)
Alps at DESY, LIPSS at JLab, OSQAR at CERN, BMV

• Send a laser through a magnetic field

Idea

• Photons turn into chameleons via F2 coupling

• Turn of the laser

• Chameleons turn back into photons

• Observe the afterglow

Failing which - rule out chunks of parameter
space!
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GammeV
http//gammev.fnal.gov
NdYAG laser at 532nm, 5ns wide pulses, power
160mJ, rep rate 20Hz
Glass window
Tevatron dipole magnet at 5T
PMT with single photon sensitivity
Schematic
A. Uphadye

• Chameleon production phase photons propagating
  through a region
• of magnetic field oscillate into chameleons

• Photons travel through the glass

• Chameleons see the glass as a wall - trapped

b) Afterglow phase chameleons in chamber
gradually decay back into photons and are
detected by a PMT
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GammeV
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Afterglow
Stronger coupling decays too fast
Observing window
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Excluded Region
Fast afterglow decay rates prevent excluding
large coupling
Pseudoscalar Scalar
Excluded regions
Using minimum afterglow predictions, the
sensitivity at low coupling is determined by the
PMT noise rate.
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Results
Fixing ? 2.3 meV, ?m 1013
V(?) ?4 exp(?n/?n)
Ruled out
Testable
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Results
Fixing ??5e11
?m must be in this region for the ?? constraint
to be valid
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Complications

• Not longitudinal motion - chameleons and photons
  bounce
• absorption of photons by the walls
• reflections dont occur at same place

• Photon penetrates into wall by skin depth
• Chameleon bounces before it reaches the wall

Phase difference at each reflection. V dependent

• Other loss modes. Chameleon could decay to other
  fields?
• Fragmentation? ?? ? ????

• Vacuum design is ineficient for constraining
  models

• Roughing pump decreases Pgas 10-3 Torr
• Turbo molecular pump decreases to 10-7 Torr but
  removes
• gas volume. I.e. can remove chameleons.

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Parameter Space Estimates
PRELIMINARY
PVLAS
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Conclusions/Outlook

• Chameleon fields Concrete, testable predictions

• Space tests of gravity

• Dark Energy candidate

• Lab tests can probe a range of parameter space
  that
• is complementary to space tests (qm vacuum and
  casimir)

• First results now out
• Potential to dramatically improve these
  constraints in
• next generation experiment

• New bounds from Astrophysics and Cosmology

• Chameleons weaken bounds on f(R) models

(Hu and Sawicki 2007)
Complementary tools of probing fundamental
physics
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