Chameleons Galore Philippe Brax (IPhT CEA-Saclay) IHES ¡ Collaboration with C. Burrage, C. vandeBruck, A. C. Davis, J. Khoury, D. Bures-‑sur-‑Yve*e ¡ ¡January ¡2010 ¡ Mota, J. Martin, D. Seery, D. Shaw, A. Weltman.
Outline 1-Scalars and Cosmic Acceleration? 2-Chameleons and Thin Shell effect 3-The Casimir Effect 4- Chameleon Optics 5-Modifying gravity at low redshift.
Scalars and Dark Energy
Dark Energy V( � ! Planck scale now � Field rolling down a runaway potential, reaching large values now (Planck scale) Extremely flat potential for an almost decoupled field
How Flat? Energy density and pressure: Runaway fields can be classified according to very fast roll slow roll ( inflation) gentle roll (dark energy ) strong gravitational constraints
Gravitational Tests Dark energy theories suffer from the potential presence of a fifth force mediated by the scalar field. Alternatives: Non-existent if the scalar field has a mass greater than : If not, strong bound from Cassini experiments on the gravitational coupling:
Scalar-Tensor Effective Theory Effective field theories with gravity and scalars: Scalars differ from axions (pseudo-scalars) inasmuch as they can couple to matter with non-derivative interactions. All the physics is captured by the function A( ϕ ). In the Einstein frame, masses become conformally related to the bare mass.
Gravitational Constraints o Deviations from Newton’s law are tested on macroscopic objects. The gravitational coupling is: o The deviation is essentially given by:
An Example: the radion The distance between branes in the Randall-Sundrum model: where Gravitational coupling: close branes: constant coupling constant
Example: Moduli coupled to the standard model The Standard Model fermion masses become moduli dependent Scalar-tensor theory Yukawa Kahler n=1 dilaton, n=3 volume modulus
Gravitational Problems o Deviations from Newton’s law are tested on macroscopic objects. The gravitational coupling is: o For moduli fields: Too Large !
f(R) gravity The simplest modification of General Relativity is f(R) gravity: o The function f(R) must be close to R, so f(R)= R+ h(R), h<< R in the o solar system. f(R) gravity addresses the dark energy issue for certain choices of o h(R).
f(R) vs Scalar-Tensor Theories f(R) totally equivalent to an effective field theory with gravity and scalars The potential V is directly related to f(R). Same problems as dark energy: coincidence problem, cosmological constant value etc…
A Few Examples A large class of models is such that h(R) C for large curvatures. This mimics a cosmological constant for large value of Another class of models leads to a quintessence like behaviour: V( � ! � Ratra-Peebles ! n=-(p+1)/p
Chameleons
Chameleons Chameleon field: field with a matter dependent mass A way to reconcile gravity tests and cosmology: Nearly massless field on cosmological Massive field in the laboratory scales
The effect of the environment When coupled to matter, scalar fields have a matter dependent effective potential Environment dependent V ( � ! eff minimum � exp( � " � #""""""""! M Pl V( � ! �
An Example: Ratra-Peebles potential Constant coupling to matter V V eff eff for f(R) theories � � Large Small � �
What is dense enough? The environment dependent mass is enough to hide the fifth force in dense o media such as the atmosphere, hence no effect on Galileo’s Pisa tower experiment! It is not enough to explain why we see no deviations from Newtonian gravity o in the lunar ranging experiment It is not enough to explain no deviation in laboratory tests of gravity o carried in “vacuum”
The Thin Shell Effect I The force mediated by the chameleon is: o The force due to a compact body of radius R is generated by the gradient o of the chameleon field outside the body. The field outside a compact body of radius R interpolates between the o minimum inside and outside the body Inside the solution is nearly constant up to the boundary of the object and o jumps over a thin shell Outside the field is given by: o
100 80 99.9 60 99.8 !! � ! [M] ! � [M] 40 99.7 20 99.6 99.5 0 0 5 10 15 0 40 80 120 160 -1 ] -1 ] r[M r [M No shell Thin shell
The Thin Shell Effect II The force on a test particle outside a spherical body is shielded: o When the shell is thin, the deviation from Newtonian gravity is small. o The size of the thin-shell is: o Small for large bodies (sun etc..) when Newton’s potential at the surface o of the body is large enough.
Laboratory tests In a typical experiment, one measures the force between two test objects o and compare to Newton’s law (this is very crude, more about the Eot-wash experiment later…). The test objects are taken to be small and spherical. They are placed in a vacuum chamber of size L. In a vacuum chamber, the chameleon “resonates” and the field value adjusts o itself according to: The vacuum is not dense enough to lead to a large chameleon mass, hence o the need for a thin shell. Typically for masses of order 40 g and radius 1 cm, the thin shell requires o for the Ratra-Peebles case: We will be more precise later…. o
The Casimir Effect
Casimir Force Experiments • Measure force between • Two parallel plates • A plate and a sphere
The Casimir Force The inter-plate force is in fact the contribution from a chameleon to the o Casimir effect. The acceleration due to a chameleon is: The attractive force per unit surface area is then: o where is the change of the boundary value of the scalar field due to the presence of the second plate.
The Casimir Force We focus on the plate-plate interaction in the range: o Mass in the Mass in the plates cavity The force is algebraic: o The dark energy scale sets a typical scale: o
Behaviour of Chameleonic Pressure for V = � 4 0 (1+ � n / � n ); n = 1 0 10 Constant 0 ) � 1 F � /A force � 2 behaviour 10 Chameleonic Pressure: (V( � c ) � � 4 Exponential behaviour Power � law behaviour � 4 10 � 6 10 � 8 10 � 1 � 1 d = m c d = m b � 10 10 � 2 0 2 4 6 8 10 10 10 10 10 10 Separation of plates: m c d
Detectability The Casimir forces is also an algebraic law implying: o This can be a few percent when d=10µm and would be 100% for o d=30 µm
Eot Wash Experiment Measurement of the torque between o two plaques with holes (no effect for Newtonian forces) The potential energy of the system due o to a chameleon force between the plates is The force per unit surface area can be o approximated by the force between two plates, the torque becomes:
Power Law Example Power law: o Constraints on Power � Law f(R) theories: f(R) = R+h(R) � 10 10 Energy scale in h(R): � 0 (GeV) � 15 10 Integrating the field equations o between the plates: Excluded Region � 20 10 Allowed Region We find constraints on the scale: o � 25 10 Eot � Wash bound (thin shells assumed) Cosmological thin shell bound m c D p >> 1 � 30 10 � 1 � 0.8 � 0.6 � 0.4 � 0.2 0 Slope of h(R): p
Chameleon Optics
Induced Coupling When the coupling to matter is universal, and heavy fermions are integrated out, a photon coupling is induced.
Experimental Setup
Chameleons Coupled to Photons Chameleons may couple to electromagnetism: o Cavity experiments in the presence of a constant magnetic field may reveal o the existence of chameleons. The chameleon mixes with the polarisation orthogonal to the magnetic field and oscillations occur (like neutrino oscillations) The coherence length o depends on the mass in the optical cavity and therefore becomes pressure and magnetic field dependent: The mixing angle between chameleons and photons is: o
Ellipticity and Rotation Photons remain N passes in the cavity. The perpendicular photon o polarisation after N passes and taking into account the chameleon mixing becomes: The phase shifts and attenuations are given by: o identified with the phase shift and attenuation after one pass of length nL. At the end of the cavity z=L, this can be easily identified for o commensurate cavities whose lengths corresponds to P coherence lengths Rotation ellipticity
Realistic Chameleon Optics o Must take other effects Very fast (step � like) change in the Chameleon Mass into account. m c o Chameleons never leave the cavity (outside mass too m � large, no tunnelling) o Chameleons do not reflect m b simultaneously with 0 distance from surface of mirror photons . More realistic m � ~ O(1)/d change in Chameleon Mass m c m � o Chameleons propagate slower in the no-field zone within the cavity m b 0 distance from surface of mirror
Rotation predictions: n = 1 & � = 2.3 ! 10 � 3 eV � 10 10 PVLAS 07 @ 2.3T upper bound | Rotation | (rad / pass) � 15 10 PVLAS @ 5.5T PVLAS @ 2.3T BMV @ 11.5T � 20 10 � 25 10 4 5 6 7 8 9 10 10 10 10 10 10 10 10 Chameleon to matter coupling: M (GeV)
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