Can you detect dark energy in the laboratory? Clare Burrage University of Nottingham

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1 Can you detect dark energy in the laboratory? Clare Burrage University of Nottingham

2 2 (Planck 2013 Results. XVI. Cosmological Parameters)

3 The Universe today 3 Credit: ESA/Planck

4 The cosmological constant problem The Einstein equations: The energy density of the vacuum looks like a cosmological constant Observed value: Expected value: 4

5 Possible Solutions Can a dynamical mechanism explain the smallness of the cosmological constant? If Λ = 0 is the acceleration caused by a new kind of matter? Does gravity work differently on the largest scales? 5

6 Possible Solutions Can a dynamical mechanism explain the smallness of the cosmological constant? New fields If Λ = 0 is the acceleration caused by a new kind of matter? New fields Does gravity work differently on the largest scales? Can indirectly introduce extra degrees of freedom 6

7 Modified gravity A massive graviton has more degrees of freedom than a massless one At low energies one of these behaves like an additional scalar mode f(r) theories contain an extra scalar degree of freedom because of the presence of higher derivatives Many brane world models have an extra scalar mode corresponding to the position of the brane in the extra dimension 7

8 New fields are light The cause of the acceleration of the expansion of the universe today is (approximately) coherent across the observable universe This corresponds to very light masses 8

9 DARK ENERGY INTERACTIONS 9

10 What s the problem with scalar fields? Expect a new scalar field to couple to matter fields with at least gravitational strength (Planck suppressed couplings) Bosons that couple to matter mediate forces In the non-relativistic limit: 10

11 11 Results of the Eöt-Wash experiment at the University of Washington

12 Dark energy interactions Can we forbid interactions that give rise to fifth forces? Yes, quintessential axion / pseudo-goldstone boson models Can we dynamically suppress the fifth forces? Yes, chameleon-like models Are there other observational signatures we could look for? 12

13 PSEUDO-GOLDSTONE BOSONS AS DARK ENERGY 13

14 Pseudo-Goldstone bosons If the field is a pseudo-goldstone boson then it posses an approximate global symmetry Forbids couplings to the Standard Model Lagrangian of the form These are the couplings that give fifth forces Shift symmetry also keeps scalar potential flat 14

15 A brief history of axionic quintessence Carroll, A pseudo-goldstone boson dark energy would forbid fifth forces. Kim, Nilles, Quintessence could be the model independent axion of string theory. Arvanitaki, Dimopoulos, Dubovsky, Kaloper, March-Russell, String Axiverse simultaneous presence of many light axions. Panda, Sumitomo, Trivedi, Axion monodromy applied to quintessence. (McAllister, et al Wrapped branes in string compactifications implies closed string axions with approximate shift symmetries.) Kim, Nilles, Pseudo-Goldstone boson from discrete symmetries in the UV 15

16 Allowed couplings The scalar field can couple to a total derivative This makes it an axion-like particle Many laboratory and astrophysics based searches, based on oscillations between scalar and photon in a magnetic field 16

17 17 (Baker et al. 2013)

18 Dark Energy modifies electromagnetism The Earth s gravitational field will source a profile for the scalar field Maxwell s equations become 18 Romalis, Caldwell. 2013

19 Dark Energy modifies electromagnetism A magnetic field can give rise to an anomalous electric field Voltage measured 19 Romalis, Caldwell. 2013

20 Disformal couplings We can also have couplings to the Standard Model of the form Higher order operator so might expect effects to be negligible Counter example: Massive gravity where disformal coupling strength Axion-like coupling strength 20

21 Disformal couplings Fifth forces return through loop corrections Giving a force law Fifth force experiments (over ~1 mm) constrain 21 Kaloper. 2003

22 Disformal couplings Disformal couplings produce distinctive signatures in laboratory searches for axions Changes the probability of a photon converting into dark energy in the presence of a magnetic field (CB, Brax, Davis, 2012) Changes the propagation of light through the Universe, and induces CMB spectral distortions Modifications to distance duality relations (CB, Brax, David, Gubitosi, 2013) 22

23 CHAMELEONIC DARK ENERGY 23

24 24 Common chameleon. Photo credit: Nanosanchez

25 The chameleon A scalar field theory With self interactions Which couples to matter Evading fifth force constraints requires.. The chameleon mechanism is relevant if. f(r) modifications of gravity correspond to Khoury, Weltman. 2004

26 The effective potential High density, no pressure Low density, no pressure The mass of the chameleon changes depending on its environment

27 Small Objects The object only causes a small perturbation of the scalar field Find a gravity like form for the scalar potential The force is the gradient of the potential 27

28 Large Objects Inside Outside 28

29 Suppressing the fifth force The increased mass makes it hard for the chameleon field to adjust its value Chameleon Newtonian potential The chameleon potential well around massive objects is shallower than for standard light scalar fields

30 Chameleon fluctuations The chameleon Lagrangian is non-linear This makes the mass of chameleon fluctuations depend on the background configuration 30

31 Screening mechanisms Start with a non-linear scalar field theory Solve the equations of motion for the background The Lagrangian for fluctuations (to second order): 31 Large Z makes it hard for the scalar to propagate - Galileons - Massive gravity - Vainshtein mechanism Large m means the scalar only propagates over shorter distances - Chameleon Small β makes the interaction with matter fields weaker - Symmetron

32 TESTING DARK ENERGY WITH SCREENING MECHANISMS 32

33 Searches for dark energy interactions Interactions with dark energy strongest in diffuse environments Constraints from high precision experiments performed in near vacuum Can exploit: Particle physics experiments High precision photon measurements Atomic structure measurements (Astrophysical observations) 33

34 Scalar Bremsstrahlung Contribution to the width of Z decay Decay rate: Prediction from the Standard Model: Measurement at LEP: Dark Energy correction negligible if 34 Brax, CB, Davis, Seery, Weltman. 2009

35 Atomic precision measurements In an atom a scalar field profile is sourced by the: Nuclear electric field Density of the atomic nucleus Leads to a perturbed Schrodinger equation 35 Brax, CB. 2010

36 Atomic precision measurements Perturbed atomic energy levels: 1 sigma uncertainty on the 1s - 2s transition in hydrogen is 10-9 ev. Requires: Constrains: Schwob et al

37 Chameleon After-glow Chameleons in a vacuum chamber are light, to pass through the walls they would need to become heavier. If the chameleons are not energetic enough this is forbidden and they remain trapped in the vacuum chamber 37 Gies, Mota, Shaw Ahlers, Lindner, Ringwald, Schrempp, Weniger Diagram from Upadhye, Steffen, Chou

38 GammeV-CHASE Results from the Gammev Chameleon Afterglow Search at Fermilab 38 Steffen et al. 2010

39 Local EP violation In the Lagrangian the chameleon couples to all particle species in the same way At the macroscopic level the chameleon force behaves differently for screened (large) and unscreened (small) objects Looks like a violation of the equivalence principle Hui, Nicolis, Stubbs The parameter which controls the suppression of the dark energy force The chameleon charge of an object 39

40 Proposed EP Violation Tests The gas in dwarf galaxies may be unscreened but the stars are screened They will fall differently towards local over densities Jain, VanderPlas Can use diffuse clouds of cold atoms to measure gravity in the laboratory Atoms are unscreened over a much larger region of parameter space than macroscopic objects CB, Copeland, Hinds. (to appear) 40

41 Dark energy and BECs Coherent waves in Bose-Einstein condensates can be used for interferometry 41 Credit: Centre for Cold Matter, Imperial

42 Dark energy and BECs Interference of waves in condensates at different heights has already detected gravitational effects Dimopoulos, Geraci Baumgärtner et al A measurement of G or g would be sensitive to the equivalence principle violating effects of dark energy Can also look for chameleon effects directly by putting the condensate in different environments 42

43 Conclusions Attempts to explain the current acceleration of the expansion of the Universe commonly introduce new fields These new fields should couple to the Standard Model and therefore we expect fifth forces Understanding why we haven t seen these forces leads to two types of dark energy theory Pseudo-Goldstone boson Chameleonic Possible to search for both kinds of field in the laboratory 43

44 PROBLEMS WITH SCREENING MECHANISMS 44

45 Strong coupling A simple example Perturb around a background The Lagrangian is Where Luty, Porrati, 45 Rattazzi 2003

46 Strong coupling A simple example Self interactions of the canonically normalised scalar are suppressed by Around a static, spherically symmetric source of mass M When the graviton mass is ~ H 0, the rescaled strong coupling scale (at the surface of the Earth) corresponds to distances ~ 1 cm 46

47 Strong coupling in massive gravity There are additional higher order terms and interactions with the metric Perturb around the scalar and metric field configuration due to the Earth Then canonically normalise the scalar The interactions for fluctuations in the Lagrangian become

48 Strong coupling problems for massive gravity The theory becomes non-perturbative at the lowest energy scale controlling these interactions The lowest scale at the surface of the Earth is For gravity to be valid on the distance scales probed by laboratory experiments requires

49 Chameleon cosmology During radiation domination the field is initially frozen due to Hubble friction Decoupling of Standard Model particles kicks the chameleon towards smaller values Brax, van de Bruck, Davis, Khoury, Weltman. 2004

50 Chameleon cosmology The kicks drive the chameleon towards the steep part of the potential with high velocity Classically the field rapidly climbs up the potential and falls back down

51 A chameleon catastrophe Rapid (non-adiabatic) changes in the mass of the field excite quantum fluctuations Very high energy fluctuations are excited With small occupation numbers

52 The death of the chameleon? Production of high energy particles leads to a break down of calculability This can only be evaded for weak couplings and very fine tuned initial conditions This fine tuning requires knowledge of the full particle content of the Universe Picture credit: Karen Watson