Measuring gravitational attraction with a lattice atom interferometer.

Despite being the dominant force of nature on large scales, gravity remains relatively elusive to precision laboratory experiments. Atom interferometers are powerful tools for investigating, for example, Earth's gravity1, the gravitational constant2, deviations from Newtonian gravity3-6 and general relativity7. However, using atoms in free fall limits measurement time to a few seconds8, and much less when measuring interactions with a small source mass2,5,6,9. Recently, interferometers with atoms suspended for 70 s in an optical-lattice mode filtered by an optical cavity have been demonstrated10-14. However, the optical lattice must balance Earth's gravity by applying forces that are a billionfold stronger than the putative signals, so even tiny imperfections may generate complex systematic effects. Thus, lattice interferometers have yet to be used for precision tests of gravity. Here we optimize the gravitational sensitivity of a lattice interferometer and use a system of signal inversions to suppress and quantify systematic effects. We measure the attraction of a miniature source mass to be amass = 33.3 ± 5.6stat ± 2.7syst nm s-2, consistent with Newtonian gravity, ruling out 'screened fifth force' theories3,15,16 over their natural parameter space. The overall accuracy of 6.2 nm s-2 surpasses by more than a factor of four the best similar measurements with atoms in free fall5,6. Improved atom cooling and tilt-noise suppression may further increase sensitivity for investigating forces at sub-millimetre ranges17,18, compact gravimetry19-22, measuring the gravitational Aharonov-Bohm effect9,23 and the gravitational constant2, and testing whether the gravitational field has quantum properties24.

Dirac-Born-Infeld genesis: an improved violation of the null energy condition.

We show that the Dirac-Born-Infeld conformal galileons, derived from the world-volume theory of a 3-brane moving in an anti-de Sitter bulk, admit a background, stable under quantum corrections, which violates the null energy condition. The perturbations around this background are stable and propagate subluminally. Unlike other known examples of null energy condition violation, such as ghost condensation and conformal galileons, this theory also admits a stable, Poincaré-invariant vacuum. The 2 → 2 amplitude satisfies standard analyticity conditions. The full S matrix is likely not analytic, however, since perturbations around deformations of the Poincaré invariant vacuum propagate superluminally.

Quantum stability of chameleon field theories.

Chameleon scalar fields are dark-energy candidates which suppress fifth forces in high density regions of the Universe by becoming massive. We consider chameleon models as effective field theories and estimate quantum corrections to their potentials. Requiring that quantum corrections be small, so as to allow reliable predictions of fifth forces, leads to an upper bound m<0.0073(ρ/10 g cm(-3))(1/3) eV for gravitational-strength coupling whereas fifth force experiments place a lower bound of m>0.0042 eV. An improvement of less than a factor of two in the range of fifth force experiments could test all classical chameleon field theories whose quantum corrections are well controlled and couple to matter with nearly gravitational strength regardless of the specific form of the chameleon potential.

No-go theorems for generalized chameleon field theories.

The chameleon, or generalizations thereof, is a light scalar that couples to matter with gravitational strength, but whose manifestation depends on the ambient matter density. A key feature is that the screening mechanism suppressing its effects in high-density environments is determined by the local scalar field value. Under very general conditions, we prove two theorems limiting its cosmological impact: (i) the Compton wavelength of such a scalar can be at most ~/= 1 MPc at the present cosmic density, which restricts its impact to nonlinear scales; and (ii) the conformal factor relating Einstein- and Jordan-frame scale factors is essentially constant over the last Hubble time, which precludes the possibility of self-acceleration. These results imply that chameleonlike scalar fields have a negligible effect on the linear-scale growth history; theories that invoke a chameleonlike scalar to explain cosmic acceleration rely on a form of dark energy rather than a genuine modified gravity effect. Our analysis applies to a broad class of chameleon, symmetron, and dilaton theories.

Screening long-range forces through local symmetry restoration.

We present a screening mechanism that allows a scalar field to mediate a long-range (∼Mpc) force of gravitational strength in the cosmos while satisfying local tests of gravity. The mechanism hinges on local symmetry restoration in the presence of matter. In regions of sufficiently high matter density, the field is drawn towards ϕ = 0 where its coupling to matter vanishes and the ϕ → -ϕ symmetry is restored. In regions of low density, however, the symmetry is spontaneously broken, and the field couples to matter with gravitational strength. We predict deviations from general relativity in the solar system that are within reach of next-generation experiments, as well as astrophysically observable violations of the equivalence principle. The model can be distinguished experimentally from Brans-Dicke gravity, chameleon theories and brane-world modifications of gravity.

Adiabatic ekpyrosis: scale-invariant curvature perturbations from a single scalar field in a contracting universe.

The Universe can be made flat and smooth by undergoing a phase of ultraslow (ekpyrotic) contraction, a condition achievable with a single, canonical scalar field and conventional general relativity. It has been argued, though, that generating scale-invariant density perturbations requires at least two scalar fields and a two-step process that first produces entropy fluctuations and then converts them to curvature perturbations. In this Letter we identify a loophole in the argument and introduce an ekpyrotic model based on a single, canonical scalar field that generates nearly scale-invariant curvature fluctuations through a purely "adiabatic mechanism" in which the background evolution is a dynamical attractor. The resulting spectrum can be slightly red with distinctive non-Gaussian fluctuations.

Flat 3-brane with tension in cascading gravity.

In the cascading gravity brane-world scenario, our 3-brane lies within a succession of lower-codimension branes, each with their own induced gravity term, embedded into each other in a higher-dimensional space-time. In the (6+1)-dimensional version of this scenario, we show that a 3-brane with tension remains flat, at least for sufficiently small tension that the weak-field approximation is valid. The bulk solution is singular nowhere and remains in the perturbative regime everywhere.

Cascading gravity: extending the Dvali-Gabadadze-Porrati model to higher dimension.

We present a generalization of the Dvali-Gabadadze-Porrati scenario to higher codimensions which, unlike previous attempts, is free of ghost instabilities. The 4D propagator is made regular by embedding our visible 3-brane within a 4-brane, each with their own induced gravity terms, in a flat 6D bulk. The model is ghost-free if the tension on the 3-brane is larger than a certain critical value, while the induced metric remains flat. The gravitational force law "cascades" from a 6D behavior at the largest distances followed by a 5D and finally a 4D regime at the shortest scales.

Non-Gaussianities in New Ekpyrotic Cosmology.

The new ekpyrotic model is an alternative scenario of the early Universe which relies on a phase of slow contraction before the big bang. We calculate the 3-point and 4-point correlation functions of primordial density perturbations and find a generically large non-Gaussian signal, just below the current sensitivity level of cosmic microwave background experiments. This is in contrast with slow-roll inflation, which predicts negligible non-Gaussianity. The model is also distinguishable from alternative inflationary scenarios that can yield large non-Gaussianity, such as Dirac-Born-Infeld inflation and the simplest curvatonlike models, through the shape dependence of the correlation functions. Non-Gaussianity therefore provides a distinguishing and testable prediction of New Ekpyrotic Cosmology.

Weighing neutrinos with galaxy cluster surveys.

Large future galaxy cluster surveys, combined with cosmic microwave background observations, can achieve a high sensitivity to the masses of cosmologically important neutrinos. We show that a weak lensing selected sample of > or approximately 100,000 clusters could tighten the current upper bound on the sum of masses of neutrino species by an order of magnitude, to a level of 0.03 eV. Since this statistical sensitivity is below the best existing lower limit on the mass of at least one neutrino species, a future detection is likely, provided that systematic errors can be controlled to a similar level.

Chameleon fields: awaiting surprises for tests of gravity in space.

We present a novel scenario where a scalar field acquires a mass which depends on the local matter density: the field is massive on Earth, where the density is high, but is essentially free in the solar system, where the density is low. All existing tests of gravity are satisfied. We predict that near-future satellite experiments could measure an effective Newton's constant in space different from that on Earth, as well as violations of the equivalence principle stronger than currently allowed by laboratory experiments.

Designing cyclic universe models.

The phenomenological constraints on the scalar field potential in cyclic models of the Universe are presented. We show that cyclic models require a comparable degree of tuning to that needed for inflationary models. The constraints are reduced to a set of simple design rules including "fast-roll" parameters analogous to the "slow-roll" parameters in inflation.

Inflation versus cyclic predictions for spectral tilt.

We present a nearly model-independent estimate that yields the predictions of a class of simple inflationary and ekpyrotic or cyclic models for the spectral tilt of the primordial density inhomogeneities that enables us to compare the two scenarios. Remarkably, we find that the two produce an identical result, n(s) approximately 0.95. For inflation, the same estimate predicts a ratio of tensor to scalar contributions to the low l multipoles of the microwave background anisotropy of T/S approximately 20%; the tensor contribution is negligible for ekpyrotic or cyclic models, as shown in earlier papers.

The Friedmann equation in brane-world scenarios.

The Friedmann law on the brane generically depends quadratically on the brane energy density and involves a "dark radiation" term due to the bulk Weyl tensor. Despite its unfamiliar form, we show how it can be derived from a standard four-dimensional Brans-Dicke theory at low energy. In particular, the dark radiation term is found to depend linearly on the brane energy densities. For any equation of state on the branes, the radion evolves such as to generate radiation-dominated cosmology. The radiation-dominated era is conventional and consistent with nucleosynthesis.