We show that Solar System tests can place very strong constraints on K-mouflage models of gravity... more We show that Solar System tests can place very strong constraints on K-mouflage models of gravity, which are coupled scalar field models with nontrivial kinetic terms that screen the fifth force in regions of large gravitational acceleration. In particular, the bounds on the anomalous perihelion of the Moon imposes stringent restrictions on the K-mouflage Lagrangian density, which can be met when the contributions of higher order operators in the static regime are sufficiently small. The bound on the rate of change of the gravitational strength in the Solar System constrains the coupling strength β to be smaller than 0.1. These two bounds impose tighter constraints than the results from the Cassini satellite and Big Bang Nucleosynthesis. Despite the Solar System restrictions, we show that it is possible to construct viable models with interesting cosmological predictions. In particular, relative to Λ-CDM, such models predict percent level deviations for the clustering of matter and the number density of dark matter haloes. This makes these models predictive and testable by forthcoming observational missions.
ABSTRACT Disformally coupled, light scalar fields arise in many of the theories of dark energy an... more ABSTRACT Disformally coupled, light scalar fields arise in many of the theories of dark energy and modified gravity that attempt to explain the accelerated expansion of the universe. They have proved difficult to constrain with precision tests of gravity because they do not give rise to fifth forces around static non-relativistic sources. However, because the scalar field couples derivatively to standard model matter, measurements at high energy particle colliders offer an effective way to constrain and potentially detect a disformally coupled scalar field. Here we derive new constraints on the strength of the disformal coupling from LHC run 1 data and provide a forecast for the improvement of these constraints from run 2. We additionally comment on the running of disformal and standard model couplings in this scenario under the renormalisation group flow.
We study the cosmology of K-mouflage theories at the background level. We show that the effects o... more We study the cosmology of K-mouflage theories at the background level. We show that the effects of the scalar field are suppressed at high matter density in the early Universe and only play a role in the late time Universe where the deviations of the Hubble rate from its $\Lambda$-CDM counterpart can be of order five percent for redshifts $1 \lesssim z \lesssim 5$. Similarly, we find that the equation of state can cross the phantom divide in the recent past and even diverge when the effective scalar energy density goes negative and subdominant compared to matter, preserving the positivity of the squared Hubble rate. These features are present in models for which Big Bang Nucleosynthesis is not affected. We analyse the fate of K-mouflage when the nonlinear kinetic terms give rise to ghosts, particle excitations with negative energy. In this case, we find that the K-mouflage theories can only be considered as an effective description of the Universe at low energy below $1$ keV. In the...
Journal of Cosmology and Astroparticle Physics, 2015
We investigate the cosmological properties of Galileon models with positive kinetic terms. We inc... more We investigate the cosmological properties of Galileon models with positive kinetic terms. We include both conformal and disformal couplings to matter and focus on constraints on the theory that arise because of these couplings. The disformal coupling to baryonic matter is extremely constrained by astrophysical and particle physics effects. The disformal coupling to photons induces a cosmological variation of the speed of light and therefore distorsions of the Cosmic Microwave Background spectrum which are known to be very small. The conformal coupling to baryons leads to a variation of particle masses since Big Bang Nucleosynthesis which is also tightly constrained. We consider the background cosmology of Galileon models coupled to Cold Dark Matter (CDM), photons and baryons and impose that the speed of light and particle masses respect the observational bounds on cosmological time scales. We find that requiring that the equation of state for the Galileon models must be close to -1 now restricts severely their parameter space and can only be achieved with a combination of the conformal and disformal couplings. This leads to large variations of particle masses and the speed of light which are not compatible with observations. As a result, we find that cosmological Galileon models are viable dark energy theories coupled to dark matter but their couplings, both disformal and conformal, to baryons and photons must be heavily suppressed making them only sensitive to CDM.
Journal of Cosmology and Astroparticle Physics, 2014
We formulate new conformal models of inflation and dark energy which generalise the Higgs-Dilaton... more We formulate new conformal models of inflation and dark energy which generalise the Higgs-Dilaton scenario. We embed these models in unimodular gravity whose effect is to break scale invariance in the late time Universe. In the early Universe, inflation occurs close to a maximum of both the scalar potential and the scalar coupling to the Ricci scalar in the Jordan frame. At late times, the dilaton, which decouples from the dynamics during inflation, receives a potential term from unimodular gravity and leads to the acceleration of the Universe. We address two central issues in this scenario. First we show that the Damour-Polyalov mechanism, when non-relativistic matter is present prior to the start of inflation, sets the initial conditions for inflation at the maximum of the scalar potential. We then show that conformal invariance implies that matter particles are not coupled to the dilaton in the late Universe at the classical level. When fermions acquire masses at low energy, scale invariance is broken and quantum corrections induce a coupling between the dilaton and matter which is still small enough to evade the gravitational constraints in the solar system.
We show that the chameleon scalar field can drive the current phase of cosmic acceleration for a ... more 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.
We calculate the shift in the atomic energy levels induced by the presence of a scalar field whic... more We calculate the shift in the atomic energy levels induced by the presence of a scalar field which couples to matter and photons. We find that a combination of atomic measurements can be used to probe both these couplings independently. A new and stringent bound on the matter coupling springs from the precise measurement of the 1s to 2s energy level difference in the hydrogen atom, while the coupling to photons is essentially constrained by the Lamb shift. Combining these constraints with current particle physics bounds we find that the contribution of a scalar field to the recently claimed discrepancy in the proton radius measured using electronic and muonic atoms is negligible.
We study the effects of modifications of gravity after Big Bang Nucleosynthesis (BBN) which would... more 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 ΛCDM 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.
We consider massive half-maximal supergravity in (d + 3) dimensions and compactify it on a symmet... more We consider massive half-maximal supergravity in (d + 3) dimensions and compactify it on a symmetric three-space. We find that the static configurations of Minkowski d × S 3 obtained by balancing the positive scalar potential for the dilaton and the flux of a threeform through the three-sphere are unstable. The resulting cosmological evolution breaks supersymmetry and leads to an accelerated expansion in d dimensions.
We consider models where moduli fields are not stabilized and play the role of quintessence. In o... more We consider models where moduli fields are not stabilized and play the role of quintessence. In order to evade gravitational tests, we investigate the possibility that moduli behave as chameleon fields. We find that, for realistic moduli superpotentials, the chameleon effect is not strong enough, implying that moduli quintessence models are gravitationally ruled out. More generally, we state a no-go theorem for quintessence in supergravity whereby models either behave like a pure cosmological constant or violate gravitational tests.
We consider a warped brane world scenario with two branes, Gauss-Bonnet gravity in the bulk, and ... more We consider a warped brane world scenario with two branes, Gauss-Bonnet gravity in the bulk, and brane localised curvature terms. When matter is present on both branes, we investigate the linear equations of motion and distinguish three regimes. At very high energy and for an observer on the positive tension brane, gravity is four dimensional and coupled to the brane bending mode in a Brans-Dicke fashion. The coupling to matter and brane bending on the negative tension brane is exponentially suppressed. In an intermediate regime, gravity appears to be five dimensional while the brane bending mode remains four dimensional. At low energy, matter on both branes couple to gravity for an observer on the positive tension brane, with a Brans-Dicke description similar to the 2-brane Randall-Sundrum setup. We also consider the zero mode truncation at low energy and show that the moduli approximation fails to reproduce the low energy action.
Chameleon fields are scalar fields whose mass depends on the ambient matter density. We investiga... more Chameleon fields are scalar fields whose mass depends on the ambient matter density. We investigate the effects of these fields on the growth of density perturbations on sub-galactic scales and the formation of the first dark matter halos. Density perturbations on comoving scales R < 1pc go non-linear and collapse to form structure much earlier than in standard ΛCDM cosmology. The resulting mini-halos are hence more dense and resilient to disruption. We therefore expect (provided that the density perturbations on these scales have not been erased by damping processes) that the dark matter distribution on small scales would be more clumpy in chameleon cosmology than in the ΛCDM model.
We consider open supermembranes in eleven dimensions in the presence of closed M-Theory five-bran... more We consider open supermembranes in eleven dimensions in the presence of closed M-Theory five-branes. It has been shown that, in a flat space-time, the worldvolume action is kappa invariant and preserves a fraction of the eleven dimensional supersymmetries if the boundaries of the membranes lie on the five-branes. We calculate the reparametrisation anomalies due to the chiral fermions on the boundaries of the membrane and examine their cancellation mechanism. We show that these anomalies cancel with the aid of a classical term in the world-volume action, provided that the tensions of the five-brane and the membrane are related to the eleven dimensional gravitational constant in a way already noticed in M-Theory. 1
In the context of quintessence, the concept of tracking solutions allows to address the fine-tuni... more In the context of quintessence, the concept of tracking solutions allows to address the fine-tuning and coincidence problems. When the field is on tracks today, one has Q ≈ m Pl demonstrating that, generically, any realistic model of quintessence must be based on supergravity. We construct the most simple model for which the scalar potential is positive. The scalar potential deduced from the supergravity model has the form V (Q) = Λ 4+α Q α e κ 2 Q 2 . We show that despite the appearence of positive powers of the field, the coincidence problem is still solved. If α ≥ 11, the fine-tuning problem can be overcome. Moreover, due to the presence of the exponential term, the value of the equation of state, ω Q , is pushed towards the value −1 in contrast to the usual case for which it is difficult to go beyond ω Q ≈ −0.7. For Ω m ≈ 0.3, the model presented here predicts ω Q ≈ −0.82. Finally, we establish the Ω m − ω Q relation for this model. PACS numbers: 95.35+d, 98.80.Cq 1
We propose a stringy mechanism whereby a large hierarchy between symmetry breaking scales is gene... more We propose a stringy mechanism whereby a large hierarchy between symmetry breaking scales is generated. This mechanism is based upon the existence of a fth dimension compacti ed on a segment. We focus on a simple supersymmetric model with one massless Higgs eld in the 3 of SU(3) and another one in the 3 on each four dimensional end point of the fth dimension. Along supersymmetric at directions the gauge symmetry is broken down to SU(2) due to the vacuum expectation value of the Higgs elds on one of the end points. The remaining massless mode on the other end point acquires a potential due to a massive ve dimensional state propagating between the end points. This potential breaks the SU(2) symmetry at an exponentially suppressed scale compared to the SU(3) breaking scale. The suppression factor depends exponentially on the mass M of the massive state and the length R of the fth dimension . For reasonably large values of the length scale R one can achieve a factor of order M W =M GUT .
We study the transformation properties of a scalar-tensor theory, coupled to fermions, under the ... more We study the transformation properties of a scalar-tensor theory, coupled to fermions, under the Weyl rescaling associated with a transition from the Jordan to the Einstein frame. We give a simple derivation of the corresponding modification to the gauge couplings. After changing frames, this gives rise to a direct coupling between the scalar and the gauge fields.
We analyse the PVLAS results using a chameleon field whose properties depend on the environment. ... more We analyse the PVLAS results using a chameleon field whose properties depend on the environment. We find that, assuming a runaway bare potential V (φ) and a universal coupling to matter, the chameleon potential is such that the scalar field can act as dark energy. Moreover the chameleon field model is compatible with the CAST results, fifth force experiments and cosmology.
The best laboratory constraints on strongly coupled chameleon fields come not from tests of gravi... more The best laboratory constraints on strongly coupled chameleon fields come not from tests of gravity per se but from precision measurements of the Casimir force. The chameleonic force between two nearby bodies is more akin to a Casimir-like force than a gravitational one: The chameleon force behaves as an inverse power of the distance of separation between the surfaces of two bodies, just as the Casimir force does. Additionally, experimental tests of gravity often employ a thin metallic sheet to shield electrostatic forces, however this sheet mask any detectable signal due to the presence of a strongly coupled chameleon field. As a result of this shielding, experiments that are designed to specifically test the behaviour of gravity are often unable to place any constraint on chameleon fields with a strong coupling to matter. Casimir force measurements do not employ a physical electrostatic shield and as such are able to put tighter constraints on the properties of chameleons fields with a strong matter coupling than tests of gravity. Motivated by this, we perform a full investigation on the possibility of testing chameleon model with both present and future Casimir experiments. We find that present days measurements are not able to detect the chameleon. However, future experiments have a strong possibility of detecting or rule out a whole class of chameleon models.
It was recently argued that the observed PVLAS anomaly can be explained by chameleon field theori... more It was recently argued that the observed PVLAS anomaly can be explained by chameleon field theories in which large deviations from Newton's law can be avoided. Here we present the predictions for the dichroism and the birefringence induced in the vacuum by a magnetic field in these models. We show that chameleon particles behave very differently from standard axion-like particles (ALPs). We find that, unlike ALPs, the chameleon particles are confined within the experimental set-up. As a consequence, the birefringence is always bigger than the dichroism in PVLAS-type experiments.
In models of coupled dark energy, in which a dark energy scalar field couples to other matter com... more In models of coupled dark energy, in which a dark energy scalar field couples to other matter components, it is natural to expect a coupling to the inflaton as well. We explore the consequences of such a coupling in the context of single field slow-roll inflation. Assuming an exponential potential for the quintessence field we show that the coupling to the inflaton causes the quintessence field to be attracted towards the minimum of the effective potential. If the coupling is large enough, the field is heavy and is located at the minimum. We show how this affects the expansion rate and the slow-roll of the inflaton field, and therefore the primordial perturbations generated during inflation. We further show that the coupling has an important impact on the processes of reheating and preheating.
We show that Solar System tests can place very strong constraints on K-mouflage models of gravity... more We show that Solar System tests can place very strong constraints on K-mouflage models of gravity, which are coupled scalar field models with nontrivial kinetic terms that screen the fifth force in regions of large gravitational acceleration. In particular, the bounds on the anomalous perihelion of the Moon imposes stringent restrictions on the K-mouflage Lagrangian density, which can be met when the contributions of higher order operators in the static regime are sufficiently small. The bound on the rate of change of the gravitational strength in the Solar System constrains the coupling strength β to be smaller than 0.1. These two bounds impose tighter constraints than the results from the Cassini satellite and Big Bang Nucleosynthesis. Despite the Solar System restrictions, we show that it is possible to construct viable models with interesting cosmological predictions. In particular, relative to Λ-CDM, such models predict percent level deviations for the clustering of matter and the number density of dark matter haloes. This makes these models predictive and testable by forthcoming observational missions.
ABSTRACT Disformally coupled, light scalar fields arise in many of the theories of dark energy an... more ABSTRACT Disformally coupled, light scalar fields arise in many of the theories of dark energy and modified gravity that attempt to explain the accelerated expansion of the universe. They have proved difficult to constrain with precision tests of gravity because they do not give rise to fifth forces around static non-relativistic sources. However, because the scalar field couples derivatively to standard model matter, measurements at high energy particle colliders offer an effective way to constrain and potentially detect a disformally coupled scalar field. Here we derive new constraints on the strength of the disformal coupling from LHC run 1 data and provide a forecast for the improvement of these constraints from run 2. We additionally comment on the running of disformal and standard model couplings in this scenario under the renormalisation group flow.
We study the cosmology of K-mouflage theories at the background level. We show that the effects o... more We study the cosmology of K-mouflage theories at the background level. We show that the effects of the scalar field are suppressed at high matter density in the early Universe and only play a role in the late time Universe where the deviations of the Hubble rate from its $\Lambda$-CDM counterpart can be of order five percent for redshifts $1 \lesssim z \lesssim 5$. Similarly, we find that the equation of state can cross the phantom divide in the recent past and even diverge when the effective scalar energy density goes negative and subdominant compared to matter, preserving the positivity of the squared Hubble rate. These features are present in models for which Big Bang Nucleosynthesis is not affected. We analyse the fate of K-mouflage when the nonlinear kinetic terms give rise to ghosts, particle excitations with negative energy. In this case, we find that the K-mouflage theories can only be considered as an effective description of the Universe at low energy below $1$ keV. In the...
Journal of Cosmology and Astroparticle Physics, 2015
We investigate the cosmological properties of Galileon models with positive kinetic terms. We inc... more We investigate the cosmological properties of Galileon models with positive kinetic terms. We include both conformal and disformal couplings to matter and focus on constraints on the theory that arise because of these couplings. The disformal coupling to baryonic matter is extremely constrained by astrophysical and particle physics effects. The disformal coupling to photons induces a cosmological variation of the speed of light and therefore distorsions of the Cosmic Microwave Background spectrum which are known to be very small. The conformal coupling to baryons leads to a variation of particle masses since Big Bang Nucleosynthesis which is also tightly constrained. We consider the background cosmology of Galileon models coupled to Cold Dark Matter (CDM), photons and baryons and impose that the speed of light and particle masses respect the observational bounds on cosmological time scales. We find that requiring that the equation of state for the Galileon models must be close to -1 now restricts severely their parameter space and can only be achieved with a combination of the conformal and disformal couplings. This leads to large variations of particle masses and the speed of light which are not compatible with observations. As a result, we find that cosmological Galileon models are viable dark energy theories coupled to dark matter but their couplings, both disformal and conformal, to baryons and photons must be heavily suppressed making them only sensitive to CDM.
Journal of Cosmology and Astroparticle Physics, 2014
We formulate new conformal models of inflation and dark energy which generalise the Higgs-Dilaton... more We formulate new conformal models of inflation and dark energy which generalise the Higgs-Dilaton scenario. We embed these models in unimodular gravity whose effect is to break scale invariance in the late time Universe. In the early Universe, inflation occurs close to a maximum of both the scalar potential and the scalar coupling to the Ricci scalar in the Jordan frame. At late times, the dilaton, which decouples from the dynamics during inflation, receives a potential term from unimodular gravity and leads to the acceleration of the Universe. We address two central issues in this scenario. First we show that the Damour-Polyalov mechanism, when non-relativistic matter is present prior to the start of inflation, sets the initial conditions for inflation at the maximum of the scalar potential. We then show that conformal invariance implies that matter particles are not coupled to the dilaton in the late Universe at the classical level. When fermions acquire masses at low energy, scale invariance is broken and quantum corrections induce a coupling between the dilaton and matter which is still small enough to evade the gravitational constraints in the solar system.
We show that the chameleon scalar field can drive the current phase of cosmic acceleration for a ... more 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.
We calculate the shift in the atomic energy levels induced by the presence of a scalar field whic... more We calculate the shift in the atomic energy levels induced by the presence of a scalar field which couples to matter and photons. We find that a combination of atomic measurements can be used to probe both these couplings independently. A new and stringent bound on the matter coupling springs from the precise measurement of the 1s to 2s energy level difference in the hydrogen atom, while the coupling to photons is essentially constrained by the Lamb shift. Combining these constraints with current particle physics bounds we find that the contribution of a scalar field to the recently claimed discrepancy in the proton radius measured using electronic and muonic atoms is negligible.
We study the effects of modifications of gravity after Big Bang Nucleosynthesis (BBN) which would... more 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 ΛCDM 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.
We consider massive half-maximal supergravity in (d + 3) dimensions and compactify it on a symmet... more We consider massive half-maximal supergravity in (d + 3) dimensions and compactify it on a symmetric three-space. We find that the static configurations of Minkowski d × S 3 obtained by balancing the positive scalar potential for the dilaton and the flux of a threeform through the three-sphere are unstable. The resulting cosmological evolution breaks supersymmetry and leads to an accelerated expansion in d dimensions.
We consider models where moduli fields are not stabilized and play the role of quintessence. In o... more We consider models where moduli fields are not stabilized and play the role of quintessence. In order to evade gravitational tests, we investigate the possibility that moduli behave as chameleon fields. We find that, for realistic moduli superpotentials, the chameleon effect is not strong enough, implying that moduli quintessence models are gravitationally ruled out. More generally, we state a no-go theorem for quintessence in supergravity whereby models either behave like a pure cosmological constant or violate gravitational tests.
We consider a warped brane world scenario with two branes, Gauss-Bonnet gravity in the bulk, and ... more We consider a warped brane world scenario with two branes, Gauss-Bonnet gravity in the bulk, and brane localised curvature terms. When matter is present on both branes, we investigate the linear equations of motion and distinguish three regimes. At very high energy and for an observer on the positive tension brane, gravity is four dimensional and coupled to the brane bending mode in a Brans-Dicke fashion. The coupling to matter and brane bending on the negative tension brane is exponentially suppressed. In an intermediate regime, gravity appears to be five dimensional while the brane bending mode remains four dimensional. At low energy, matter on both branes couple to gravity for an observer on the positive tension brane, with a Brans-Dicke description similar to the 2-brane Randall-Sundrum setup. We also consider the zero mode truncation at low energy and show that the moduli approximation fails to reproduce the low energy action.
Chameleon fields are scalar fields whose mass depends on the ambient matter density. We investiga... more Chameleon fields are scalar fields whose mass depends on the ambient matter density. We investigate the effects of these fields on the growth of density perturbations on sub-galactic scales and the formation of the first dark matter halos. Density perturbations on comoving scales R < 1pc go non-linear and collapse to form structure much earlier than in standard ΛCDM cosmology. The resulting mini-halos are hence more dense and resilient to disruption. We therefore expect (provided that the density perturbations on these scales have not been erased by damping processes) that the dark matter distribution on small scales would be more clumpy in chameleon cosmology than in the ΛCDM model.
We consider open supermembranes in eleven dimensions in the presence of closed M-Theory five-bran... more We consider open supermembranes in eleven dimensions in the presence of closed M-Theory five-branes. It has been shown that, in a flat space-time, the worldvolume action is kappa invariant and preserves a fraction of the eleven dimensional supersymmetries if the boundaries of the membranes lie on the five-branes. We calculate the reparametrisation anomalies due to the chiral fermions on the boundaries of the membrane and examine their cancellation mechanism. We show that these anomalies cancel with the aid of a classical term in the world-volume action, provided that the tensions of the five-brane and the membrane are related to the eleven dimensional gravitational constant in a way already noticed in M-Theory. 1
In the context of quintessence, the concept of tracking solutions allows to address the fine-tuni... more In the context of quintessence, the concept of tracking solutions allows to address the fine-tuning and coincidence problems. When the field is on tracks today, one has Q ≈ m Pl demonstrating that, generically, any realistic model of quintessence must be based on supergravity. We construct the most simple model for which the scalar potential is positive. The scalar potential deduced from the supergravity model has the form V (Q) = Λ 4+α Q α e κ 2 Q 2 . We show that despite the appearence of positive powers of the field, the coincidence problem is still solved. If α ≥ 11, the fine-tuning problem can be overcome. Moreover, due to the presence of the exponential term, the value of the equation of state, ω Q , is pushed towards the value −1 in contrast to the usual case for which it is difficult to go beyond ω Q ≈ −0.7. For Ω m ≈ 0.3, the model presented here predicts ω Q ≈ −0.82. Finally, we establish the Ω m − ω Q relation for this model. PACS numbers: 95.35+d, 98.80.Cq 1
We propose a stringy mechanism whereby a large hierarchy between symmetry breaking scales is gene... more We propose a stringy mechanism whereby a large hierarchy between symmetry breaking scales is generated. This mechanism is based upon the existence of a fth dimension compacti ed on a segment. We focus on a simple supersymmetric model with one massless Higgs eld in the 3 of SU(3) and another one in the 3 on each four dimensional end point of the fth dimension. Along supersymmetric at directions the gauge symmetry is broken down to SU(2) due to the vacuum expectation value of the Higgs elds on one of the end points. The remaining massless mode on the other end point acquires a potential due to a massive ve dimensional state propagating between the end points. This potential breaks the SU(2) symmetry at an exponentially suppressed scale compared to the SU(3) breaking scale. The suppression factor depends exponentially on the mass M of the massive state and the length R of the fth dimension . For reasonably large values of the length scale R one can achieve a factor of order M W =M GUT .
We study the transformation properties of a scalar-tensor theory, coupled to fermions, under the ... more We study the transformation properties of a scalar-tensor theory, coupled to fermions, under the Weyl rescaling associated with a transition from the Jordan to the Einstein frame. We give a simple derivation of the corresponding modification to the gauge couplings. After changing frames, this gives rise to a direct coupling between the scalar and the gauge fields.
We analyse the PVLAS results using a chameleon field whose properties depend on the environment. ... more We analyse the PVLAS results using a chameleon field whose properties depend on the environment. We find that, assuming a runaway bare potential V (φ) and a universal coupling to matter, the chameleon potential is such that the scalar field can act as dark energy. Moreover the chameleon field model is compatible with the CAST results, fifth force experiments and cosmology.
The best laboratory constraints on strongly coupled chameleon fields come not from tests of gravi... more The best laboratory constraints on strongly coupled chameleon fields come not from tests of gravity per se but from precision measurements of the Casimir force. The chameleonic force between two nearby bodies is more akin to a Casimir-like force than a gravitational one: The chameleon force behaves as an inverse power of the distance of separation between the surfaces of two bodies, just as the Casimir force does. Additionally, experimental tests of gravity often employ a thin metallic sheet to shield electrostatic forces, however this sheet mask any detectable signal due to the presence of a strongly coupled chameleon field. As a result of this shielding, experiments that are designed to specifically test the behaviour of gravity are often unable to place any constraint on chameleon fields with a strong coupling to matter. Casimir force measurements do not employ a physical electrostatic shield and as such are able to put tighter constraints on the properties of chameleons fields with a strong matter coupling than tests of gravity. Motivated by this, we perform a full investigation on the possibility of testing chameleon model with both present and future Casimir experiments. We find that present days measurements are not able to detect the chameleon. However, future experiments have a strong possibility of detecting or rule out a whole class of chameleon models.
It was recently argued that the observed PVLAS anomaly can be explained by chameleon field theori... more It was recently argued that the observed PVLAS anomaly can be explained by chameleon field theories in which large deviations from Newton's law can be avoided. Here we present the predictions for the dichroism and the birefringence induced in the vacuum by a magnetic field in these models. We show that chameleon particles behave very differently from standard axion-like particles (ALPs). We find that, unlike ALPs, the chameleon particles are confined within the experimental set-up. As a consequence, the birefringence is always bigger than the dichroism in PVLAS-type experiments.
In models of coupled dark energy, in which a dark energy scalar field couples to other matter com... more In models of coupled dark energy, in which a dark energy scalar field couples to other matter components, it is natural to expect a coupling to the inflaton as well. We explore the consequences of such a coupling in the context of single field slow-roll inflation. Assuming an exponential potential for the quintessence field we show that the coupling to the inflaton causes the quintessence field to be attracted towards the minimum of the effective potential. If the coupling is large enough, the field is heavy and is located at the minimum. We show how this affects the expansion rate and the slow-roll of the inflaton field, and therefore the primordial perturbations generated during inflation. We further show that the coupling has an important impact on the processes of reheating and preheating.
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Papers by Philippe Brax