Testing Chameleon Dark Energy

1. Testing Chameleon Dark Energy Amanda Weltman Portsmouth June 2008 University of Cambridge 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 + Light scalar field Gravitational strength coupling  Tension between theory and observations Opportunity! - Connect to Cosmology

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

4. 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

5. 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

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

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

8. 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

9. 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  >> 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.

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 F2 term

11. 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

12. 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!

13. GammeV http://gammev.fnal.gov Nd:YAG 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

14. GammeV

15. Afterglow Stronger coupling decays too fast Observing window

16. 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.

17. Results V() = 4 exp(n/n) Fixing  = 2.3 meV, m = 1013 Ruled out Testable

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 Pgas10-3 Torr • Turbo molecular pump decreases to 10-7 Torr but removes • gas volume. I.e. can remove chameleons.

20. Parameter Space Estimates • PRELIMINARY PVLAS

21. 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