We show that, as a result of nonlinear self-interactions, it is feasible, at least in light the bounds coming from terrestrial tests gravity, measurements Casimir force and those constraints imposed by physics compact objects, big-bang nucleosynthesis cosmic microwave background, for there to exist, our Universe, one or more scalar fields that couple matter much strongly than gravity does. These behave like chameleons: changing their properties fit surroundings. As these can be not only very...

We analyze $f(R)$ modifications of Einstein's gravity as dark energy models in the light their connection with chameleon theories. Formulated scalar-tensor theories, theories imply existence a strong coupling scalar field to matter. This would violate all experimental gravitational tests on deviations from Newton's law. Fortunately, matter dependent mass and thin-shell effect allows one alleviate these constraints. The condition also implies restrictions cosmological dynamics As consequence,...

We consider the dilaton in strong string-coupling limit and elaborate on original idea of Damour Polyakov whereby coupling to matter is minimized vanishes at a finite value field. Combining this type with an exponential potential, effective potential becomes density dependent. study background cosmology, showing that can play role dark energy. also analyze constraints imposed by absence violation equivalence principle. Imposing these assuming plays energy, we consequences large scale...

We show that, as a result of nonlinear self-interactions, scalar field theories that couple to matter much more strongly than gravity are not only viable but could well be detected by number future experiments provided they properly designed do so.

The best laboratory constraints on strongly coupled chameleon fields come not from tests of gravity per se but precision measurements the Casimir force. chameleonic force between two nearby bodies is more akin to a Casimir-like than gravitational one: behaves as an inverse power distance separation surfaces bodies, just does. Additionally, experimental often employ thin metallic sheet shield electrostatic forces; however, this masks any detectable signal due presence field. As result...

We introduce and carefully define an entire class of field theories based on non-standard spinors. Their dominant interaction is via the gravitational which makes them naturally dark; we refer to as Dark Spinors. provide a critical analysis previous proposals for dark spinors noting that they violate Lorentz invariance. As working assumption restrict our preserve invariance, whilst being non-local explicitly construct such theory. complete energy-momentum tensor derive its components by...

We investigate the effect of modifed gravity with screening mechanisms, such as chameleon or symmetron models, upon structure main sequence stars. find that unscreened stars can be significantly more luminous and ephemeral than their screened doppelgangers. By embedding these into dwarf galaxies, which for values parameters not yet ruled out observationally, we show cumulative increased luminosity enhance total galactic luminosity. estimate this enhancement it considerable given model are...

We have studied the nonlinear structure formation of environmentally dependent dilaton model using $N$-body simulations. find that mechanism suppressing scalar fifth force in high-density regions works very well. Within parameter space allowed by solar system tests, predicts small deviations matter power spectrum and mass function from their $\Lambda$CDM counterparts. The importance taking full account nonlinearity is also emphasized.

We show that a coupling between chameleonlike scalar fields and photons induces linear circular polarization in the light from astrophysical sources. In this context include those of Olive-Pospelov (OP) model, which describes varying fine structure constant. determine form numerically give analytic expressions two useful limits. By comparing predicted signal with current observations we are able to improve constraints on chameleon-photon OP model by over 2 orders magnitude. It is argued...

It was recently argued that the observed PVLAS anomaly can be explained by chameleon field theories in which large deviations from Newton's law avoided. Here we present predictions for dichroism and birefringence induced vacuum a magnetic these models. We show particles behave very differently standard axionlike (ALPs). find that, unlike ALPs, are confined within experimental setup. As consequence, is always bigger than PVLAS-type experiments.

We extend the usual gravitational action principle by promoting bare cosmological constant (CC) from a parameter to field which can take many possible values. Variation leads new integral constraint equation determines classical value of effective CC that dominates wave function universe. In realistic model, expected CC, is calculated measurable quantities be O(t_U), as observed, where t_U present age universe in Planck units,. Any application our model produces falsifiable prediction for...

We demonstrate that the scatter in luminosity relations of astrophysical objects can be used to search for axionlike particles. This analysis is applied observations active galactic nuclei, where we find evidence highly suggestive existence a very light particle.

We show that a direct coupling between chameleonlike scalar field and photons can give rise to modified Sunyaev-Zel'dovich (SZ) effect in the cosmic microwave background (CMB). The induces mixing chameleon particles CMB when they pass through magnetic of galaxy cluster. Both intensity polarization radiation are modified. degree modification depends strongly on properties cluster such as strength electron number density. Existing SZ measurements Coma enable us place constraints...

We use covariant and first-order formalism techniques to study the properties of general relativistic cosmology in three dimensions.The approach provides an irreducible decomposition equations, which allows for a mathematically compact physically transparent description 3-dimensional spacetimes.Using this information we review features homogeneous isotropic 3-d cosmologies, provide number new solutions gauge invariant perturbations around them.The is then used detailed most spacetimes...

We propose an afterglow phenomenon as a unique trace of chameleon fields in optical experiments. The vacuum interaction laser pulse with magnetic field can lead to production and subsequent trapping chameleons the chamber, owing their mass dependence on ambient matter density. Magnetically induced reconversion trapped into photons creates over macroscopic timescales that conveniently be searched for by current show parameter range accessible available laboratory technology is comparable...

We show that, within modified gravity, the nonlinear nature of field equations implies that usual na\"{\i}ve averaging procedure (replacing microscopic energy-momentum by its cosmological average) is invalid. discuss then how should be performed correctly and as a consequence, at classical level physical masses geodesics particles, cosmology, astrophysics in Palatini gravity theories are all indistinguishable from results general relativity plus constant. is, however, different theory...

We have calculated the chameleon pressure between two parallel plates in presence of an intervening medium that affects mass field. As intuitively expected, gas gap weakens interaction mechanism with a screening effect increases plate separation and density medium. This phenomenon might open up new directions search particles future long-range Casimir force experiments.

We present a new solution to the cosmological constant (CC) and coincidence problems in which observed value of CC, $\ensuremath{\Lambda}$, is linked other observable properties Universe. This achieved by promoting CC from parameter that must be specified, field can take many possible values. The $\ensuremath{\Lambda}\ensuremath{\approx}(9.3\text{ }\text{ }\mathrm{Gyrs}{)}^{\ensuremath{-}2}$ [$\ensuremath{\approx}{10}^{\ensuremath{-}120}$ Planck units] determined constraint equation follows...

A 17.5 kDa protein was isolated from porcine whey by reverse phase HPLC and identified as a putative acidic (WAP) homologue sequencing 35 40 amino acid residues of the amino- carboxy-terminus respectively. Degenerate oligonucleotides to both these sequences were designed used in transcriptase PCR with RNA lactating mammary gland template. 162 bp fragment detected sequenced. Compilation deduced determined sequence revealed 111 acids, which had approximately 75, 50, 35% similarity at level...

We investigate the clustering of dark energy within matter overdensities and voids. In particular, we derive an analytical expression for density perturbations, which is valid both in linear, quasi-linear fully non-linear regime structure formation. also possibility detecting such through ISW effect. case uncoupled quintessence models, if mass field order Hubble scale today or smaller, fluctuations are always small compared to contrast. Even when perturbations enter regime, remain linear....

In the symmetron mechanism, fifth force mediated by a coupled scalar field (the symmetron) is suppressed in high-density regions due to restoration of symmetry potential. this paper we study background cosmology and large-scale structure formation linear perturbation regime model. Analytic solutions cosmological are found, which agree well with numerical results. We discuss effect on growth matter perturbation, particular, implications brief period tachyonic instability caused negative...

We apply the method of matched asymptotic expansions to analyze whether cosmological variations in physical ``constants'' and scalar fields are detectable, locally, on surface local gravitationally bound systems such as planets stars, or inside virialized like galaxies clusters. assume spherical symmetry derive a sufficient condition for time variation that drive varying constants track one. calculate number specific examples detail by matching Schwarzschild spacetime spherically symmetric...