The evidence for the accelerated expansion of the Universe and the time dependence of the fine-structure constant suggests the existence of at least one scalar field with a mass of order H 0 . If such a field exists, then it is generally assumed that its coupling to matter must be tuned to unnaturally small values in order to satisfy the tests of the equivalence principle ͑EP͒. In this paper, we present an alternative explanation which allows scalar fields to evolve cosmologically while having couplings to matter of order unity. In our scenario, the mass of the fields depends on the local matter density: the interaction range is typically of order 1 mm on Earth ͑where the density is high͒ and of order 10-10 4 AU in the solar system ͑where the density is low͒. All current bounds from tests of general relativity are satisfied. Nevertheless, we predict that near-future experiments that will test gravity in space will measure an effective Newton's constant different by order unity from that on Earth, as well as EP violations stronger than currently allowed by laboratory experiments. Such outcomes would constitute a smoking gun for our scenario.

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.

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After a decade and a half of research motivated by the accelerating universe, theory and experiment have a reached a certain level of maturity. The development of theoretical models beyond Λ or smooth dark energy, often called modified gravity, has led to broader insights into a path forward, and a host of observational and experimental tests have been developed. In this review we present the current state of the field and describe a framework for anticipating developments in the next decade. We identify the guiding principles for rigorous and consistent modifications of the standard model, and discuss the prospects for empirical tests.We begin by reviewing recent attempts to consistently modify Einstein gravity in the infrared, focusing on the notion that additional degrees of freedom introduced by the modification must "screen" themselves from local tests of gravity. We categorize screening mechanisms into three broad classes: mechanisms which become active in regions of high Newtonian potential, those in which first derivatives of the field become important, and those for which second derivatives of the field are important. Examples of the first class, such as f (R) gravity, employ the familiar chameleon or symmetron mechanisms, whereas examples of the last class are galileon and massive gravity theories, employing the Vainshtein mechanism. In each case, we describe the theories as effective theories and discuss prospects for completion in a more fundamental theory. We describe experimental tests of each class of theories, summarizing laboratory and solar system tests and describing in some detail astrophysical and cosmological tests. Finally, we discuss prospects for future tests which will be sensitive to different signatures of new physics in the gravitational sector.The review is structured so that those parts that are more relevant to theorists vs. observers/experimentalists are clearly indicated, in the hope that this will serve as a useful reference for both audiences, as well as helping those interested in bridging the gap between them.

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.Scalar fields are the simplest of fields. Light, gravitationally coupled scalars are generically predicted to exist by many theories of high energy physics. These scalars may play a crucial role in dark energy as quintessence fields, and generically arise in infrared-modified gravity theories [1][2][3][4][5][6][7]. Despite their apparent theoretical ubiquity, no sign of such a fundamental scalar field has ever been seen, despite many experimental tests designed to detect solar system effects or fifth forces that would naively be expected if such scalars existed [8,9].Several broad classes of theoretical mechanisms have been developed to explain why such light scalars, if they exist, may not be visible to experiments performed near the Earth. One such class, the chameleon mechanism [5,6], operates whenever the scalars are nonminimally coupled to matter in such a way that their effective mass depends on the local matter density. Deep in space, where the local mass density is low, the scalars would be light and would display their effects, but near the Earth, where experiments are performed, and where the local mass density is high, they would acquire a mass, making their effects short range and unobservable.Another such mechanism, the Vainshtein mechanism [10], operates when the scalar has derivative selfcouplings which become important near matter sources such as the Earth. The strong coupling near sources essentially cranks up the kinetic terms, which means, after canonical normalization, that the couplings to matter are weakened. Thus the scalar screens itself and becomes invisible to experiments. This mechanism is central to the phenomenological viability of brane-world modifications of gravity [1,2] and galileon scalar theories [3].In this Letter, we explore a third class of mechanisms for hiding a scalar. A similar framework was studied in [12,13] with different motivations, and some the results below overlap with these works. In this mechanism, the vacuum expectation value (VEV) of the scalar depends on the local mass density, becoming large in regions of low mass density, and small in regions of high mass density. In addition, the coupling of th...

We consider conditions under which a universe contracting towards a big crunch can make a transition to an expanding big bang universe. A promising example is 11-dimensional M-theory in which the eleventh dimension collapses, bounces, and re-expands. At the bounce, the model can reduce to a weakly coupled heterotic string theory and, we conjecture, it may be possible to follow the transition from contraction to expansion. The possibility opens the door to new classes of cosmological models. For example, we discuss how it suggests a major simplification and modification of the recently proposed ekpyrotic scenario.

We study the generation of density perturbations in the ekpyrotic scenario for the early universe, including gravitational backreaction. We expose interesting subtleties that apply to both inflationary and ekpyrotic models. Our analysis includes a detailed proposal of how the perturbations generated in a contracting phase may be matched across a 'bounce' to those in an expanding hot big bang phase. For the physical conditions relevant to the ekpyrotic scenario, we re-obtain our earlier result of a nearly scale-invariant spectrum of energy density perturbations. We find that the perturbation amplitude is typically small, as desired to match observation. Typeset using REVT E X 1We recently proposed a novel scenario for the early Universe in which the hot big bang is created by the collision between two M-theory branes 1 . The scenario assumes the Universe begins in an almost static, nearly BPS initial state consisting of empty, flat, parallel threebranes. In the effective 4d theory, the BPS state is homogeneous and has zero spatial curvature. Due to non-perturbative effects, however, a tiny force attracts the branes to one another. As the branes come together, quantum fluctuations create ripples in the brane surfaces that result in spatial variations in the time of collision. Consequently, some regions heat up and begin to cool before others, producing a spectrum of long wavelength density perturbations which can seed structure formation in the Universe.We estimated the perturbation spectrum using a 'time delay' formalism 2 , often used in simplified treatments of inflationary models. In that context, spatial variations in the time when inflation ends result in long wavelength density inhomogeneities. We applied the same formalism to variations in the time of collision in the ekpyrotic scenario. The equation for fluctuations in the scalar field φ describing the inter-brane separation in the ekpyrotic model is almost identical to that describing fluctuations in the inflaton during slow-roll inflation. Consequently, a nearly scale-invariant spectrum of fluctuations is found.The result is remarkable because it shows that the Harrison-Zel'dovich spectrum can be obtained without inflation in a space-time which is very nearly static Minkowski space.The time delay formalism is a crude approximation, and only quantitatively accurate for a small class of inflationary potentials 3 . Nevertheless, it often gives a good estimate of the spectral index for the power spectrum of perturbations. One of the goals of this paper is to investigate whether the same statement is true for the ekpyrotic model.In the case of the ekpyrotic model, there is the major complication that the perturbations are produced when the effective 4d scale factor is contracting. In order to have a viable scenario, a mechanism must be found to reverse from contraction to expansion. This issue has been addressed in a recent paper we have written with N. Seiberg 4 , where we argue that such a 'bounce' may be allowed in the context of M-theory, where i...

We show that the chameleon scalar field can drive the current phase of cosmic acceleration for a large class of scalar potentials that are also consistent with local tests of gravity. These provide explicit realizations of a quintessence model where the quintessence scalar field couples directly to baryons and dark matter with gravitational strength. We analyze the cosmological evolution of the chameleon field and show the existence of an attractor solution with the chameleon following the minimum of its effective potential. For a wide range of initial conditions, spanning many orders of magnitude in initial chameleon energy density, the attractor is reached before nucleosynthesis.Surprisingly, the range of allowed initial conditions leading to a successful cosmology is wider than in normal quintessence. We discuss applications to the cyclic model of the universe and show how the chameleon mechanism weakens some of the constraints on cyclic potentials. I. INTRODUCTIONA host of observations concord with the existence of a dark energy component with negative pressure, accounting for more than two thirds of the current energy budget. The evidence comes, for instance, from measurements of the cosmic microwave background temperature anisotropy [1] and Type Ia supernovae [2]. While the data is so far consistent with the dark fluid being a cosmological constant, it is nevertheless interesting to consider the possibility that near future observations will reveal that w differs from −1.Having w = −1 implies that a parameter of the effective Lagrangian, namely the vacuum energy, is time-dependent.It follows from general covariance and locality that it must also be a function of space; in other words, the vacuum energy is a field, assumed for simplicity to be a fundamental scalar φ. Scalar field models of dark energy generally come under the label of quintessence [3]. Of course, this argument assumes that gravity is described by General Relativity (GR) for all relevant scales, and it is conceivable that the observed acceleration could result from a break down of GR on large scales [4,5,6]. However, we focus on the former possibility.