1. Testing Chameleon Dark Energy Amanda Weltman University of Cambridge Portsmouth June 2008 University of Cape Town

2. 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

3. Observations Dark Energy p < 0 Cosmological Constant,  Dynamical e.o.s w  -1 Accelerated expansion of the Universe  light scalar field Quintessence  Need light scalar field

4. Chameleon Effect 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 ~ H 0 Field may be a candidate for acc of universe astro-ph/0309300 PRL J. Khoury and A.W astro-ph/0309411 PRD J. Khoury and A.W

5. Ingredients Matter Fields Reduced Planck Mass Einstein Frame Metric Conformally Coupled Potential is of the runaway form Coupling to photons astro-ph/0408415 PRD P. Brax, C. van de Bruck, J.Khoury, A. Davis and A.W

6. Effective Potential Equation of motion : Dynamics governed by Effective potential : Energy density in the i th form of matter

7. Predictions for Tests in Space Different behaviour in space STEP  ~ 10 -18 GG  ~ 10 -17 MICROSCOPE  ~ 10 -15 Tests for UFF Near- future experiments in space : We predict New Feature !! SEE Capsule Corrections of O(1) to Newton’s Constant Eöt-Wash Bound  < 10 -13  R E /R E < 10 -7 10 -15 <

8. Cosmological Evolution What do we need? attractor solution If field starts at min, will follow the min  Slow rolls along the attractor  must join attractor before current epoch    Variation in m  is constrained to be less than ~ 10%. Constrains  BBN  the initial energy density of the field. Weaker bound than usual quintessence astro-ph/0408415 PRD P. Brax, C. van de Bruck, J.Khoury, A. Davis and A.W

9. Strong Coupling  Thin shell suppression  Where :  Effective coupling is independent of  !!  independent If an object satisfies thin shell condition - the  force is  independent Mota and Shaw Strong coupling not ruled out by local experiments!   >> 1  thin shell more likely  suppresses space signal Lab Lab tests on earth can probe a range of parameter complementary space that is complementary to space tests. Opportunity? Loophole!

10. 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 F 2 term

11. To explain unexpected birefringence and dichroism results requiresand Conflicts with astrophysical bounds e.g. CAST (solar cooling)  Chameleons - naturally evade CAST bounds and explain PVLAS Brax, Davis, van de Bruck (g = 1/M) (Polarizzazione del Vuoto con LASer) PVLAS and CAST  Too heavy to produce  CAST bounds easily satisfied But + (CERN Axion Solar Telescope)

12. Particles Trapped in a Jar See also - Gies et. Al. + Ahlers et. Al. (DESY) Alps at DESY, LIPSS at JLab, OSQAR at CERN, BMV A. Chou et. Al. 0806.2438 [hep-ex] “ [Photon]-[dilaton-like chameleon particle] regeneration using a "particle trapped in a jar" technique “ Idea : Send a laser through a magnetic field Photons turn into chameleons via F 2 coupling Turn of the laser Chameleons turn back into photons Observe the afterglow Failing which - rule out chunks of parameter space! http://gammev.fnal.gov

13. GammeV a) Chameleon production phase: photons propagating through a region of magnetic field oscillate into chameleons Nd:YAG laser at 532nm, 5ns wide pulses, power 160mJ, rep rate 20Hz Tevatron dipole magnet at 5T PMT with single photon sensitivity Glass window b) Afterglow phase: chameleons in chamber gradually decay back into photons and are detected by a PMT Photons travel through the glass Chameleons see the glass as a wall - trapped A. Uphadye http://gammev.fnal.gov Schematic

14. GammeV

15. Afterglow Observing window Stronger coupling decays too fast

16. Excluded Region Pseudoscalar Scalar Fast afterglow decay rates prevent excluding large coupling Excluded regions Using minimum afterglow predictions, the sensitivity at low coupling is determined by the PMT noise rate.

17. Results Fixing  = 2.3 meV,  m = 10 13 V(  ) =  4 exp(  n /  n ) Testable Ruled out

18. Results Fixing   =5e11  m must be in this region for the   constraint to be valid

19. Complications Not longitudinal motion - chameleons and photons bounce absorption of photons by the walls reflections don’t 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 P gas 10 -3 Torr Turbo molecular pump decreases to 10 -7 Torr but removes gas volume. I.e. can remove chameleons.

20. Parameter Space Estimates PVLAS PRELIMINARY PRELIMINARY

21. Conclusions/Outlook Complementary Complementary tools of probing fundamental physics Space Space tests of gravity Dark Energy Dark Energy candidate Lab Lab tests can probe a range of parameter space that complementary is complementary to space tests ( qm vacuum and casimir ) Chameleon fields Chameleon fields: Concrete, testable predictions AstrophysicsCosmology New bounds from Astrophysics and Cosmology First results now out Potential to dramatically improve these constraints in next generation experiment Chameleonsf(R) models Chameleons weaken bounds on f(R) models (Hu and Sawicki 2007)