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Atom interferometry experiments are searching for evidence of chameleon scalar fields with ever-increasing precision. As experiments become more precise, so too must theoretical predictions. Previous work has made numerous approximations to simplify the calculation, which in general requires solving a 3-dimensional nonlinear partial differential equation (PDE). In this paper, we introduce a new technique for calculating the chameleonic force, using a numerical relaxation scheme on a uniform grid. This technique is more general than previous work, which assumed spherical symmetry to reduce the PDE to a 1-dimensional ordinary differential equation (ODE). We examine the effects of approximations made in previous efforts on this subject, and calculate the chameleonic force in a set-up that closely mimics the recent experiment of Hamilton et al. Specifically, we simulate the vacuum chamber as a cylinder with dimensions matching those of the experiment, taking into account the backreaction of the source mass, its offset from the center, and the effects of the chamber walls. Remarkably, the acceleration on a test atomic particle is found to differ by only 20% from the approximate analytical treatment. These results allow us to place rigorous constraints on the parameter space of chameleon field theories, although ultimately the constraint we find is the same as the one we reported in Hamilton et al. because we had slightly underestimated the size of the vacuum chamber. This new computational technique will continue to be useful as experiments become even more precise, and will also be a valuable tool in optimizing future searches for chameleon fields and related theories.

The accelerated expansion of the universe motivates a wide class of scalar field theories that modify gravity on large scales. In regions where the weak field limit of General Relativity has been confirmed by experiment, such theories need a screening mechanism to suppress the new force. We have measured the acceleration of an atom toward a macroscopic test mass inside a high vacuum chamber, where the new force is unscreened in some theories. Our measurement, made using atom interferometry, shows that the attraction between atoms and the test mass does not differ appreciably from Newtonian gravity. This result places stringent limits on the free parameters in chameleon and symmetron theories of modified gravity.

We construct a theory which admits a time-dependent solution smoothly interpolating between a null energy condition (NEC)-satisfying phase at early times and a NEC-violating phase at late times. We first review earlier attempts to violate the NEC and an argument of Rubakov, presented in 1305.2614, which forbids the existence of such interpolating solutions in a single-field dilation-invariant theory. We then construct a theory which, in addition to possessing a Poincaré-invariant vacuum, does admit such a solution. For a wide range of parameters, perturbations around this solution are at all times stable, comfortably subluminal and weakly-coupled. The theory requires us to explicitly break dilation-invariance, so it is unlikely that the theory is fully stable under quantum corrections, but we argue that the existence of a healthy interpolating solution is quantum-mechanically robust. arXiv:1311.5889v2 [hep-th] 22 Jan 2014 pathologies [63], including ghosts, gradient instabilities, superluminality, absence of a Lorentzinvariant vacuum, etc. 1 Progress has been made in avoiding some of these shortcomings [53,54,[59][60][61]65], as reviewed below (see Table 1), though a fully satisfactory example remains elusive. It is important to push this program further, to sharpen the connection between the NEC and the standard assumptions of quantum field theory.The DBI Genesis scenario [61], based on the DBI conformal galileons [66], is the closest any theory has come to achieving NEC violation while satisfying the standard properties of a local quantum field theory. Specifically, as shown in [61], the coefficients of the five DBI galileon terms can be chosen such that:1. The theory admits a stable, Poincaré-invariant vacuum. Further, the Lorentz-invariant Smatrix about this vacuum obeys the simplest dispersion relations for 2 → 2 scattering coming from analyticity constraints.2. The theory admits a time-dependent, homogeneous and isotropic solution which violates the NEC in a stable manner. In fact, this NEC-violating background is an exact solution of the effective theory, including all possible higher-dimensional operators consistent with the assumed symmetries.3. Perturbations around the NEC-violating background, and around small deformations thereof, propagate subluminally.4. This solution is stable against radiative corrections and the effective theory for perturbations about this solution is well-defined.This represents a significant improvement over ghost condensation [67] (which fails to satisfy 1) and the ordinary conformal galileons [54,68] (which fail to satisfy 1 and 3). 2 Additionally, consistency with black hole thermodynamics is desirable [69]. This remains an open issue which deserves further study. It is worth pointing out that the non-minimal couplings to gravity inherent in the theory will modify the usual link between NEC violation and the black hole area law.Unfortunately, the DBI Genesis theory itself suffers from two drawbacks. Similar to the conformal galileons, one can find weak deformations of the ...

We compute the dynamical friction on a small perturber moving through an inviscid fluid, i.e., a superfluid. Crucially, we account for the tachyonic gravitational mass for sound waves, reminiscent of the Jeans instability of the fluid, which results in non-zero dynamical friction even for subsonic velocities. Moreover, we illustrate that the standard leading order effective theory in the derivative expansion is in general inadequate for analysing supersonic processes. We show this in two ways: (i) with a fluid treatment, where we solve the linearized hydrodynamical equations coupled to Newtonian gravity; and (ii) with a quasiparticle description, where we study the energy dissipation of a moving perturber due to phonon radiation. Ordinarily a subsonic perturber moving through a superfluid is kinematically prohibited from losing energy, however the Jeans instability modifies the dispersion relation of the fluid which can result in a small but non-vanishing dynamical friction force. We also analyse the soft phonon bremsstrahlung by a subsonic perturber scattered off an external field.