arXiv subjects

Philippe Brax

Publications and source records attributed to Philippe Brax.

At least 145 records · Page 8Linked to original sources

arXiv2014

Explaining the Proton Radius Puzzle with Disformal Scalars

We analyse the consequences of a disformal interaction between a massless scalar and matter particles in the context of atomic physics. We focus on the displacement of the atomic energy levels that it induces, and in particular the change in the Lamb shift between the 2s and 2p states. We find that the correction to the Lamb shift depends on the mass of the fermion orbiting around the nucleus, implying a larger effect for muonic atoms. Taking the cut-off scale describing the effective scalar field theory close to the QCD scale, we find that the disformal interaction can account for the observed difference in the proton radius of muonic versus electronic Hydrogen. Explaining the proton radius puzzle is only possible when the scalar field is embedded in non-linear theories which alleviate constraints from collider and stellar physics. Short distance properties of the Galileon where non-perturbative effects in vacuum are present ensure that unitarity is preserved in high energy particle collisions. In matter, the chameleon mechanism alleviates the constraints on disformal interactions coming from the burning rates for stellar objects. We show how to combine these two properties in a single model which renders the proposed explanation of the proton radius puzzle viable.

hep-ph↗

arXiv2014

An Environmental Variation of Constants

Models of modified gravity, whereby local tests of gravity are evaded thanks to a screening mechanism of the chameleon or Damour-Polyakov types, lead to a spatial variation of the particle masses and the fine structure constant. This is triggered by the environmental dependence of the value of the scalar field whose presence modifies gravity. In dense media, the field settles at a density dependent value while in sparse environments it takes the background cosmological value. We estimate that the maximal deviation of constants from their present values is constrained by local tests of gravity, and must be less than $10^{-6}$.

astro-ph.CO↗

arXiv2014

Early Modified Gravity: Implications for Cosmology

We study the effects of modifications of gravity after Big Bang Nucleosynthesis (BBN) which would manifest themselves mainly before recombination. We consider their effects on the Cosmic Microwave Background (CMB) radiation and on the formation of large scale structure. The models that we introduce here represent all screened modifications of General Relativity (GR) which evade the local tests of gravity such as the violation of the strong equivalence principle as constrained by the Lunar Ranging experiment. We use the tomographic description of modified gravity which defines models with screening mechanisms of the chameleon or Damour-Polyakov types and allows one to relate the temporal evolution of the mass and the coupling to matter of a scalar field to its Lagrangian and also to cosmological perturbations. The models with early modifications of gravity all involve a coupling to matter which is stronger in the past leading to effects on perturbations before recombination while minimising deviations from Lambda-CDM structure formation at late times. We find that a new family of early transition models lead to discrepancies in the CMB spectrum which could reach a few percent and appear as both enhancements and reductions of power for different scales.

astro-ph.CO↗

arXiv2013

Chameleon Fragmentation

A scalar field dark energy candidate could couple to ordinary matter and photons, enabling its detection in laboratory experiments. Here we study the quantum properties of the chameleon field, one such dark energy candidate, in an "afterglow" experiment designed to produce, trap, and detect chameleon particles. In particular, we investigate the possible fragmentation of a beam of chameleon particles into multiple particle states due to the highly non-linear interaction terms in the chameleon Lagrangian. Fragmentation could weaken the constraints of an afterglow experiment by reducing the energy of the regenerated photons, but this energy reduction also provides a unique signature which could be detected by a properly-designed experiment. We show that constraints from the CHASE experiment are essentially unaffected by fragmentation for $ϕ^4$ and $1/ϕ$ potentials, but are weakened for steeper potentials, and we discuss possible future afterglow experiments.

hep-ph↗

arXiv2013

Impact on the power spectrum of Screening in Modified Gravity Scenarios

We study the effects of screened modified gravity of the $f(R)$, dilaton and symmetron types on structure formation, from the quasi-linear to the non-linear regime, using semi-analytical methods. For such models, where the range of the new scalar field is typically within the Mpc range and below in the cosmological context, non-linear techniques are required to understand the deviations of the power spectrum of the matter density contrast compared to the $Λ$-CDM template. This is nowadays commonly tackled using extensive N-body simulations. Here we present new results combining exact perturbation theory at the one loop level (and a partial resummation of the perturbative series) with a halo model. The former allows one to extend the linear perturbative analysis up to $k\lesssim 0.15{\rm h Mpc}^{-1}$ at the perturbative level while the latter leads to a reasonable, up to a few percent, agreement with numerical simulations for $k\lesssim 3{\rm h Mpc}^{-1}$ for large curvature $f(R)$ models, and $k\lesssim 1{\rm h Mpc}^{-1}$ for dilatons and symmetrons, at $z=0$. We also discuss how the behaviors of the perturbative expansions and of the spherical collapse differ for $f(R)$, dilaton, and symmetron models.

astro-ph.CO↗

arXiv2013

Dynamics of Supersymmetric Chameleons

We investigate the cosmological dynamics of a class of supersymmetric chameleon models coupled to cold dark matter fermions. Supergravity corrections ensure that these models are efficiently screened in all astrophysical objects of interest, however this does not preclude the enhancement of gravity on linear cosmological scales. We analyse the background cosmology and solve the modified equations for the growth of cold dark matter density perturbations in closed form. Using this, we go on to derive the modified linear power spectrum which is characterised by two scales, the horizon size at matter-radiation equality and at the redshift when the chameleon reaches the minimum of its effective potential. The model includes a cosmological constant in the form of a Fayet-Illiopolous term, which emerges at late times due to the coupling of the chameleon to two charged scalars. We examine the conditions under which this leads to viable background cosmology and go on to analyse the deviations from the LCDM predictions in the linear regime. We find that for reasonable values in the model's parameter space there is generically a region where the model's cosmology is viable and current measurements can be reproduced. A small discrepancy of the matter power spectrum from its LCDM counterpart can be obtained in a smaller subset of the parameter space.

astro-ph.CO↗

arXiv2013

Novel Probes of Gravity and Dark Energy

The discovery of cosmic acceleration has stimulated theorists to consider dark energy or modifications to Einstein's General Relativity as possible explanations. The last decade has seen advances in theories that go beyond smooth dark energy -- modified gravity and interactions of dark energy. While the theoretical terrain is being actively explored, the generic presence of fifth forces and dark sector couplings suggests a set of distinct observational signatures. This report focuses on observations that differ from the conventional probes that map the expansion history or large-scale structure. Examples of such novel probes are: detection of scalar fields via lab experiments, tests of modified gravity using stars and galaxies in the nearby universe, comparison of lensing and dynamical masses of galaxies and clusters, and the measurements of fundamental constants at high redshift. The observational expertise involved is very broad as it spans laboratory experiments, high resolution astronomical imaging and spectroscopy and radio observations. In the coming decade, searches for these effects have the potential for discovering fundamental new physics. We discuss how the searches can be carried out using experiments that are already under way or with modest adaptations of existing telescopes or planned experiments. The accompanying paper on the Growth of Cosmic Structure describes complementary tests of gravity with observations of large-scale structure.

astro-ph.CO↗

arXiv2013

Probing Strongly Coupled Chameleons with Slow Neutrons

We consider different methods to probe chameleons with slow neutrons. Chameleon modify the potential of bouncing neutrons over a flat mirror in the terrestrial gravitational field. This induces a shift in the energy levels of the neutrons which could be detected in current experiments like GRANIT. Chameleons between parallel plates have a field profile which is bubble-like and which would modify the phase of neutrons in interferometric experiments. We show that this new method of detection is competitive with the bouncing neutron one, hopefully providing an efficient probe of chameleons when strongly coupled to matter.

quant-ph↗

arXiv2013

Cosmological Tests of the Disformal Coupling to Radiation

Light scalar fields can naturally couple disformally to Standard Model fields without giving rise to the unacceptably large fifth forces usually associated with light scalars. We show that these scalar fields can still be studied and constrained through their interaction with photons, and focus particularly on changes to the Cosmic Microwave Background spectral distortions and violations of the distance duality relation. We then specialise our constraints to scalars which could play the role of axionic quintessence.

astro-ph.CO↗

arXiv2013

Systematic simulations of modified gravity: chameleon models

In this work we systematically study the linear and nonlinear structure formation in chameleon theories of modified gravity, using a generic parameterisation which describes a large class of models using only 4 parameters. For this we have modified the N-body simulation code ECOSMOG to perform a total of 65 simulations for different models and parameter values, including the default LCDM. These simulations enable us to explore a significant portion of the parameter space. We have studied the effects of modified gravity on the matter power spectrum and mass function, and found a rich and interesting phenomenology where the difference with the LCDM paradigm cannot be reproduced by a linear analysis even on scales as large as k~0.05h/Mpc, since the latter incorrectly assumes that the modification of gravity depends only on the background matter density. Our results show that the chameleon screening mechanism is significantly more efficient than other mechanisms such as the dilaton and symmetron, especially in high-density regions and at early times, and can serve as a guidance to determine the parts of the chameleon parameter space which are cosmologically interesting and thus merit further studies in the future.

astro-ph.CO↗

arXiv2012

SUPER-Screening

We present a framework for embedding scalar-tensor models of screened modifed gravity such as chameleons, symmetrons and environmental dilatons into global supersymmetry. This achieved by secluding the dark sector from both the observable and supersymmetry breaking sectors. We examine the resulting supersymmetric features in a model-independent manner and find that, when the theory follows from an underlying supergravity, the mediation of supersymmetry breaking to the dark sector induces a soft mass for the scalar of order the gravitino mass. This is enough to forbid the construction of supersymmetric symmetrons and ensures that when other screening mechanisms operate, no object in the universe is unscreened thereby precluding any observable signatures. In view of a possible origin of modifed gravity within fundamental physics, we find that no-scale models are the only ones that can circumvent these features. We also present a novel mechanism where the coupling of the scalar to two other scalars charged under U(1) can dynamically generate a small cosmological constant at late times in the form of a Fayet-Iliopoulos term.

hep-th↗

arXiv2012

Lectures on Screened Modified Gravity

The acceleration of the expansion of the Universe has led to the construction of Dark Energy models where a light scalar field may have a range reaching up to cosmological scales. Screening mechanisms allow these models to evade the tight gravitational tests in the solar system and the laboratory. I will briefly review some of the salient features of screened modified gravity models of the chameleon, dilaton or symmetron types using $f(R)$ gravity as a template.

hep-th↗

arXiv2012

Shining Light on Modifications of Gravity

Many modifications of gravity introduce new scalar degrees of freedom, and in such theories matter fields typically couple to an effective metric that depends on both the true metric of spacetime and on the scalar field and its derivatives. Scalar field contributions to the effective metric can be classified as conformal and disformal. Disformal terms introduce gradient couplings between scalar fields and the energy momentum tensor of other matter fields, and cannot be constrained by fifth force experiments because the effects of these terms are trivial around static non-relativistic sources. The use of high-precision, low-energy photon experiments to search for conformally coupled scalar fields, called axion-like particles, is well known. In this article we show that these experiments are also constraining for disformal scalar field theories, and are particularly important because of the difficulty of constraining these couplings with other laboratory experiments.

hep-th↗

arXiv2012

Systematic simulations of modified gravity: symmetron and dilaton models

We study the linear and nonlinear structure formation in the dilaton and symmetron models of modified gravity using a generic parameterisation which describes a large class of scenarios using only a few parameters, such as the coupling between the scalar field and the matter, and the range of the scalar force on very large scales. For this we have modified the N-body simulation code ECOSMOG, which is a variant of RAMSES working in modified gravity scenarios, to perform a set of 110 simulations for different models and parameter values, including the default LCDM. These simulations enable us to explore a large portion of the parameter space. We have studied the effects of modified gravity on the matter power spectrum and mass function, and found a rich and interesting phenomenology where the difference with the LCDM template cannot be reproduced by a linear analysis even on scales as large as k~0.05 h/Mpc. Our results show the full effect of screening on nonlinear structure formation and the associated deviation from LCDM. We also investigate how differences in the force mediated by the scalar field in modified gravity models lead to qualitatively different features for the nonlinear power spectrum and the halo mass function, and how varying the individual model parameters changes these observables. The differences are particularly large in the nonlinear power spectra whose shapes for f(R), dilaton and symmetron models vary greatly, and where the characteristic bump around 1 h/Mpc of f(R) models is preserved for symmetrons, whereas an increase on much smaller scales is particular to symmetrons. No bump is present for dilatons where a flattening of the power spectrum takes place on small scales. These deviations from LCDM and the differences between modified gravity models, such as dilatons and symmetrons, could be tested with future surveys.

astro-ph.CO↗

arXiv2012

Screening fifth forces in k-essence and DBI models

New fifth forces have not yet been detected in the laboratory or in the solar system, hence it is typically difficult to introduce new light scalar fields that would mediate such forces. In recent years it has been shown that a number of non-linear scalar field theories allow for a dynamical mechanism, such as the Vainshtein and chameleon ones, that suppresses the strength of the scalar fifth force in experimental environments. This is known as screening, however it is unclear how common screening is within non-linear scalar field theories. k-essence models are commonly studied examples of non-linear models, with DBI as the best motivated example, and so we ask whether these non-linearities are able to screen a scalar fifth force. We find that a Vainshtein-like screening mechanism exists for such models although with limited applicability. For instance, we cannot find a screening mechanism for DBI models. On the other hand, we construct a large class of k-essence models which lead to the acceleration of the Universe in the recent past for which the fifth force mediated by the scalar can be screened.

hep-th↗

arXiv2012

Structure Formation in Modified Gravity Scenarios

We study the growth of structures in modified gravity models where the Poisson equation and the relationship between the two Newtonian potentials are modified by explicit functions of space and time. This parameterisation applies to the $f(R)$ models and more generally to screened modified gravity models. We investigate the linear and weakly nonlinear regimes using the "standard" perturbative approach and a resummation technique, while we use the spherical dynamics to go beyond low-order results. This allows us to estimate the matter density power spectrum and bispectrum from linear to highly nonlinear scales, the full probability distribution of the density contrast on weakly nonlinear scales, and the halo mass function. We analyse the impact of modifications of gravity on these quantities for a few realistic models. In particular, we find that the standard one-loop perturbative approach is not sufficiently accurate to probe these effects on the power spectrum and it is necessary to use resummation methods even on weakly nonlinear scales which provide the best observational window for modified gravity as relative deviations from General Relativity do not grow significantly on smaller scales where theoretical predictions become increasingly difficult.

astro-ph.CO↗

arXiv2012

Modified Gravity Tomography

We consider the effect of a scalar field degree of freedom on the dynamics of gravity from small to large scales. We show that the effects of modified gravity can be completely captured by the time variations of the scalar field mass and its coupling to matter. This leads to a parameterisation of modified gravity where local constraints are easy to analyse and large scale scale structure effects apparent.

astro-ph.CO↗

arXiv2012

Signatures of Modified Gravity on the 21-cm Power Spectrum at Reionisation

Scalar modifications of gravity have an impact on the growth of structure. Baryon and Cold Dark Matter (CDM) perturbations grow anomalously for scales within the Compton wavelength of the scalar field. In the late time Universe when reionisation occurs, the spectrum of the 21cm brightness temperature is thus affected. We study this effect for chameleon-f(R) models, dilatons and symmetrons. Although the f(R) models are more tightly constrained by solar system bounds, and effects on dilaton models are negligible, we find that symmetrons where the phase transition occurs before z_* ~ 12 will be detectable for a scalar field range as low as 5 kpc. For all these models, the detection prospects of modified gravity effects are higher when considering modes parallel to the line of sight where very small scales can be probed. The study of the 21 cm spectrum thus offers a complementary approach to testing modified gravity with large scale structure surveys. Short scales, which would be highly non-linear in the very late time Universe when structure forms and where modified gravity effects are screened, appear in the linear spectrum of 21 cm physics, hence deviating from General Relativity in a maximal way.

astro-ph.CO↗