Antonio Padilla - Academia.edu (original) (raw)
Papers by Antonio Padilla
Physical Review D, 2015
ABSTRACT We investigate low energy limits of massive gauge theories that feature the Vainshtein m... more ABSTRACT We investigate low energy limits of massive gauge theories that feature the Vainshtein mechanism, focussing on the effects of the UV modes that are integrated out. It turns out that the Goldstone sectors are significantly influenced by the effects from such modes relative to the effective field theories where the irrelevant operators induced by heavy modes are simply cast aside. The effects of the consistently retained higher order corrections affect the strong coupling and show that the nature of the UV completion influences the low energy theory significantly. This casts doubts on the naively estimated environmental strong coupling scale, and on the effectiveness of the environmental enhancement of screening. The environmental effects by themselves might not suffice to cure the bad behavior of the theory beyond the vacuum cutoff.
Physical Review D, 2012
We have recently proposed a special class of scalar tensor theories known as the Fab Four. These ... more We have recently proposed a special class of scalar tensor theories known as the Fab Four. These arose from attempts to analyse the cosmological constant problem within the context of Horndeski's most general scalar tensor theory. The Fab Four together give rise to a model of self-tuning, with the relevant solutions evading Weinberg's no-go theorem by relaxing the condition of Poincaré invariance in the scalar sector. The Fab Four are made up of four geometric terms in the action with each term containing a free potential function of the scalar field. In this paper we rigorously derive this model from the general model of Horndeski, proving that the Fab Four represents the only classical scalar tensor theory of this type that has any hope of tackling the cosmological constant problem. We present the full equations of motion for this theory, and give an heuristic argument to suggest that one might be able to keep radiative corrections under control. We also give the Fab Four in terms of the potentials presented in Deffayet et al's version of Horndeski.
Physical Review D, 2014
Recently we suggested a reformulation of General Relativity which completely sequesters from grav... more Recently we suggested a reformulation of General Relativity which completely sequesters from gravity all of the vacuum energy from a protected matter sector, assumed to contain the Standard Model. Here we elaborate further on the mechanism, presenting additional details of how it cancels all loop corrections and renders all contributions from phase transitions automatically small. We also consider cosmological consequences in more detail and show that the mechanism is consistent with a variety of inflationary models that make a universe big and old. We discuss in detail the underlying assumptions behind the dynamics of our proposal, and elaborate on the relationship of the physical interpretation of divergent operators in quantum field theory and the apparent 'acausality' which our mechanism seems to entail, which we argue is completely harmless. It is merely a reflection of the fact that any UV sensitive quantity in quantum field theory cannot be calculated from first principles, but is an input whose numerical value must be measured. We also note that since the universe should be compact in spacetime, and so will collapse in the future, the current phase of acceleration with w DE ≈ −1 is just a transient. This could be tested by future cosmological observations.
Journal of High Energy Physics, 2011
We continue to introduce bi-galileon theory, the generalisation of the single galileon model intr... more We continue to introduce bi-galileon theory, the generalisation of the single galileon model introduced by Nicolis et al. The theory contains two coupled scalar fields and is described by a Lagrangian that is invariant under Galilean shifts in those fields. This paper is the second of two, and focuses on the phenomenology of the theory. We are particularly interesting in models that admit solutions that are asymptotically self accelerating or asymptotically self tuning. In contrast to the single galileon theories, we find examples of self accelerating models that are simultaneously free from ghosts, tachyons and tadpoles, able to pass solar system constraints through Vainshtein screening, and do not suffer from problems with superluminality, Cerenkov emission or strong coupling. We also find self tuning models and discuss how Weinberg's no go theorem is evaded by breaking Poincaré invariance in the scalar sector. Whereas the galileon description is valid all the way down to solar system scales for the self-accelerating models, unfortunately the same cannot be said for self tuning models owing to the scalars backreacting strongly on to the geometry.
Classical and Quantum Gravity - CLASS QUANTUM GRAVITY, 2003
We derive the gravitational Hamiltonian starting from the Gauss-Bonnet action, keeping track of a... more We derive the gravitational Hamiltonian starting from the Gauss-Bonnet action, keeping track of all surface terms. This is done using the language of orthonormal frames and forms to keep things as tidy as possible. The surface terms in the Hamiltonian give a remarkably simple expression for the total energy of a spacetime. This expression is consistent with energy expressions found in hep-th/0212292. However, we can apply our results whatever the choice of background and whatever the symmetries of the spacetime. * a.padilla1@physics.ox.ac.uk
Journal of High Energy Physics, 2006
We systematically explore the spectrum of gravitational perturbations in codimension-1 DGP branew... more We systematically explore the spectrum of gravitational perturbations in codimension-1 DGP braneworlds, and find a 4D ghost on the self-accelerating branch of solutions. The ghost appears for any value of the brane tension, although depending on the sign of the tension it is either the helicity-0 component of the lightest localized massive tensor of mass 0 < m 2 < 2H 2 for positive tension, the scalar 'radion' for negative tension, or their admixture for vanishing tension. Because the ghost is gravitationally coupled to the brane-localized matter, the self-accelerating solutions are not a reliable benchmark for cosmic acceleration driven by gravity modified in the IR. In contrast, the normal branch of solutions is ghost-free, and so these solutions are perturbatively safe at large distance scales. We further find that when the Z 2 orbifold symmetry is broken, new tachyonic instabilities, which are much milder than the ghosts, appear on the self-accelerating branch. Finally, using exact gravitational shock waves we analyze what happens if we relax boundary conditions at infinity. We find that non-normalizable bulk modes, if interpreted as 4D phenomena, may open the door to new ghost-like excitations.
Journal of High Energy Physics, 2009
By studying perturbations about the vacuum, we show that Hořava gravity suffers from two differen... more By studying perturbations about the vacuum, we show that Hořava gravity suffers from two different strong coupling problems, extending all the way into the deep infra-red. The first of these is associated with the principle of detailed balance and explains why solutions to General Relativity are typically not recovered in models that preserve this structure. The second of these occurs even without detailed balance and is associated with the breaking of diffeomorphism invariance, required for anisotropic scaling in the UV. Since there is a reduced symmetry group there are additional degrees of freedom, which need not decouple in the infra-red. Indeed, we use the Stuckelberg trick to show that one of these extra modes become strongly coupled as the parameters approach their desired infra-red fixed point. Whilst we can evade the first strong coupling problem by breaking detailed balance, we cannot avoid the second, whatever the form of the potential. Therefore the original Hořava model, and its "phenomenologically viable" extensions do not have a perturbative General Relativity limit at any scale. Experiments which confirm the perturbative gravitational wave prediction of General Relativity, such as the cumulative shift of the periastron time of binary pulsars, will presumably rule out the theory. * Electronic address: Christos.Charmousis@th.u-psud.fr † Electronic address: gustavo.niz@nottingham.ac.uk ‡ Electronic address: antonio.padilla@nottingham.ac.uk § Electronic address: paul.saffin@nottingham.ac.uk
Journal of High Energy Physics, 2013
ABSTRACT We consider the role of matter in the non-projectable version of Horava-Liftshitz gravit... more ABSTRACT We consider the role of matter in the non-projectable version of Horava-Liftshitz gravity at both a classical and a quantum level. At the classical level, we construct general forms of matter Lagrangians consistent with the reduced symmetry group and demonstrate that they must be reduced to their relativistic form if they are to avoid sourcing the gravitational Stuckelberg field. At the quantum level we consider one loop corrections to the propagator for a relativistic scalar minimally coupled to gravity at tree level. We find large corrections to the light cone at low energies arising from the strength of the coupling of the scalar graviton to matter. We also find evidence that higher order time derivatives may be generated, which is worrying if this is to be taken seriously as a UV complete theory.
Journal of High Energy Physics, 2008
Owing to the quadratic nature of the theory, Einstein-Gauss-Bonnet gravity generically permits tw... more Owing to the quadratic nature of the theory, Einstein-Gauss-Bonnet gravity generically permits two distinct vacuum solutions. One solution (the "Einstein" vacuum) has a well defined limit as the Gauss-Bonnet coupling goes to zero, whereas the other solution (the "stringy" vacuum) does not. There has been some debate regarding the stability of these vacua, most recently from Deser & Tekin who have argued that the corresponding black hole solutions have positive mass and as such both vacua are stable. Whilst the statement about the mass is correct, we argue that the stringy vacuum is still perturbatively unstable. Simply put, the stringy vacuum suffers from a ghostlike instability that is not excited by the spherically symmetric black hole, but would be excited by any source likely to emit gravitational waves, such as a binary system. This result is reliable except in the strongly coupled regime close to the Chern-Simons limit, when the two vacua are almost degenerate. In this regime, we study instanton transitions between branches via bubble nucleation, and calculate the nucleation probability. This demonstrates that there is large mixing between the vacua, so that neither of them can accurately describe the true quantum vacuum. We also present a new gravitational instanton describing black hole pair production in de Sitter space on the Einstein branch, which is preferred to the usual Nariai instantons and is not present in pure Einstein
Journal of High Energy Physics, 2010
We introduce bi-galileon theory, the generalisation of the single galileon model introduced by Ni... more We introduce bi-galileon theory, the generalisation of the single galileon model introduced by Nicolis et al. The theory contains two coupled scalar fields and is described by a Lagrangian that is invariant under Galilean shifts in those fields. This paper is the first of two, and focuses on the motivation and formulation of the theory. We show that the boundary effective theory of the cascading cosmology model corresponds to a bi-galileon theory in the decoupling limit, and argue that this is to be expected for co-dimension 2 braneworld models exhibiting infra-red modification of gravity. We then generalise this, by constructing the most general bi-galileon Lagrangian. By coupling one of the galileons to the energy-momentum tensor, we pitch this as a modified gravity theory in which the modifications to General Relativity are encoded in the dynamics of the two galileons. We initiate a study of phenomenology by looking at maximally symmetric vacua and their stability, developing elegant geometric techniques that trivially explain why some of the vacua have to be unstable in certain cases (eg DGP). A detailed study of phenomenology appears in our companion paper.
Journal of High Energy Physics, 2010
We consider the so-called "healthy" extension of Hořava gravity in the limit where the Stuckelber... more We consider the so-called "healthy" extension of Hořava gravity in the limit where the Stuckelberg field decouples from the graviton. We verify the alleged strong coupling problem in this limit, under the assumption that no large dimensionless parameters are put in by hand. This follows from the fact that the dispersion relation for the Stuckelberg field does not have the desired z = 3 anisotropic scaling in the UV. To get the desired scaling and avoid strong coupling one has to introduce a low scale of Lorentz violation and retain some coupling between the graviton and the Stuckelberg field. We also make use of the foliation preserving symmetry to show how the Stuckelberg field couples to some violation of energy conservation. We source the Stuckelberg field using a point particle with a slowly varying mass and show that two such particles feel a constant attractive force. In this particular example, we see no Vainshtein effect, and violations of the Equivalence Principle. The latter is probably generic to other types of source and could potentially be used to place lower bounds on the scale of Lorentz violation. * ppxik1@nottingham.ac.uk
Journal of High Energy Physics, 2012
We present a novel idea for screening the vacuum energy contribution to the overall value of the ... more We present a novel idea for screening the vacuum energy contribution to the overall value of the cosmological constant, thereby enabling us to choose the bare value of the vacuum curvature empirically, without any need to worry about the zero-point energy contributions of each particle. The trick is to couple matter to a metric that is really a composite of other fields, with the property that the square-root of its determinant is the integrand of a topological invariant, and/or a total derivative. This ensures that the vacuum energy contribution to the Lagrangian is non-dynamical. We then give an explicit example of a theory with this property that is free from Ostrogradski ghosts, and is consistent with solar system physics and cosmological tests.
Journal of High Energy Physics, 2012
We show how to generalize the classical duals found by Gabadadze et al to a very large class of s... more We show how to generalize the classical duals found by Gabadadze et al to a very large class of self-interacting theories. This enables one to adopt a perturbative description beyond the scale at which classical perturbation theory breaks down in the original theory. This is particularly relevant if we want to test modified gravity scenarios that exhibit Vainshtein screening on solar system scales. We recognise the duals as being related to the Legendre transform of the original Lagrangian, and present a practical method for finding the dual in general; our methods can also be applied to self-interacting theories with a hierarchy of strong coupling scales, and with multiple fields. We find the classical dual of the full quintic galileon theory as an example.
Journal of High Energy Physics, 2012
We compute the boundary terms and junction conditions for Horndeski's panoptic class of scalarten... more We compute the boundary terms and junction conditions for Horndeski's panoptic class of scalartensor theories, and write the bulk and boundary equations of motion in explicitly second order form. We consider a number of special subclasses, including galileon theories, and present the corresponding formulae. Our analysis opens up of the possibility of studying tunnelling between vacua in generalized scalar-tensor theories, and braneworld dynamics. The latter follows because our results are independent of spacetime dimension. * antonio.padilla@nottingham.ac.uk † ppxvs@nottingham.ac.uk 1 We thank Stanley Deser for enlightening us on the history of scalar-tensor theories. See for an historical overview. 2 In four dimensions, the Horndeski and DGSZ actions were shown to be equivalent , and given Horndeski's proof, we know this to be the most general scalar-tensor theory admitting second order field equations. In higher dimensions the DGSZ action is known to yield second order field equations, but it is not the most general theory.
Physical Review D, 2015
ABSTRACT We investigate low energy limits of massive gauge theories that feature the Vainshtein m... more ABSTRACT We investigate low energy limits of massive gauge theories that feature the Vainshtein mechanism, focussing on the effects of the UV modes that are integrated out. It turns out that the Goldstone sectors are significantly influenced by the effects from such modes relative to the effective field theories where the irrelevant operators induced by heavy modes are simply cast aside. The effects of the consistently retained higher order corrections affect the strong coupling and show that the nature of the UV completion influences the low energy theory significantly. This casts doubts on the naively estimated environmental strong coupling scale, and on the effectiveness of the environmental enhancement of screening. The environmental effects by themselves might not suffice to cure the bad behavior of the theory beyond the vacuum cutoff.
Physics Reports, 2012
In this review we present a thoroughly comprehensive survey of recent work on modified theories o... more In this review we present a thoroughly comprehensive survey of recent work on modified theories of gravity and their cosmological consequences. Amongst other things, we cover General Relativity, Scalar-Tensor, Einstein-Aether, and Bimetric theories, as well as TeVeS, f (R), general higher-order theories, Hořava-Lifschitz gravity, Galileons, Ghost Condensates, and models of extra dimensions including Kaluza-Klein, Randall-Sundrum, DGP, and higher co-dimension braneworlds. We also review attempts to construct a Parameterised Post-Friedmannian formalism, that can be used to constrain deviations from General Relativity in cosmology, and that is suitable for comparison with data on the largest scales. These subjects have been intensively studied over the past decade, largely motivated by rapid progress in the field of observational cosmology that now allows, for the first time, precision tests of fundamental physics on the scale of the observable Universe. The purpose of this review is to provide a reference tool for researchers and students in cosmology and gravitational physics, as well as a self-contained, comprehensive and up-to-date introduction to the subject as a whole.
Physical Review D, 2012
We have recently proposed a special class of scalar tensor theories known as the Fab Four. These ... more We have recently proposed a special class of scalar tensor theories known as the Fab Four. These arose from attempts to analyse the cosmological constant problem within the context of Horndeski's most general scalar tensor theory. The Fab Four together give rise to a model of self-tuning, with the relevant solutions evading Weinberg's no-go theorem by relaxing the condition of Poincaré invariance in the scalar sector. The Fab Four are made up of four geometric terms in the action with each term containing a free potential function of the scalar field. In this paper we rigorously derive this model from the general model of Horndeski, proving that the Fab Four represents the only classical scalar tensor theory of this type that has any hope of tackling the cosmological constant problem. We present the full equations of motion for this theory, and give an heuristic argument to suggest that one might be able to keep radiative corrections under control. We also give the Fab Four in terms of the potentials presented in Deffayet et al's version of Horndeski.
Physics Letters B, 2002
We examine the cosmological evolution equations of de Sitter, flat and anti-de Sitter braneworlds... more We examine the cosmological evolution equations of de Sitter, flat and anti-de Sitter braneworlds sandwiched in between two n dimensional AdS-Schwarzschild spacetimes. We are careful to use the correct form for the induced Newton's constant on the brane, and show that it would be naive to assume the energy of the bulk spacetime is just given by the sum of the black hole masses. By carefully calculating the energy of the bulk for large mass we show that the induced geometry of the braneworld is just a radiation dominated FRW universe with the radiation coming from a CFT that is dual to the AdS bulk.
Physical Review D, 2014
Recently we suggested a reformulation of General Relativity which completely sequesters from grav... more Recently we suggested a reformulation of General Relativity which completely sequesters from gravity all of the vacuum energy from a protected matter sector, assumed to contain the Standard Model. Here we elaborate further on the mechanism, presenting additional details of how it cancels all loop corrections and renders all contributions from phase transitions automatically small. We also consider cosmological consequences in more detail and show that the mechanism is consistent with a variety of inflationary models that make a universe big and old. We discuss in detail the underlying assumptions behind the dynamics of our proposal, and elaborate on the relationship of the physical interpretation of divergent operators in quantum field theory and the apparent 'acausality' which our mechanism seems to entail, which we argue is completely harmless. It is merely a reflection of the fact that any UV sensitive quantity in quantum field theory cannot be calculated from first principles, but is an input whose numerical value must be measured. We also note that since the universe should be compact in spacetime, and so will collapse in the future, the current phase of acceleration with w DE ≈ −1 is just a transient. This could be tested by future cosmological observations.
Physical Review D, 2015
ABSTRACT We investigate low energy limits of massive gauge theories that feature the Vainshtein m... more ABSTRACT We investigate low energy limits of massive gauge theories that feature the Vainshtein mechanism, focussing on the effects of the UV modes that are integrated out. It turns out that the Goldstone sectors are significantly influenced by the effects from such modes relative to the effective field theories where the irrelevant operators induced by heavy modes are simply cast aside. The effects of the consistently retained higher order corrections affect the strong coupling and show that the nature of the UV completion influences the low energy theory significantly. This casts doubts on the naively estimated environmental strong coupling scale, and on the effectiveness of the environmental enhancement of screening. The environmental effects by themselves might not suffice to cure the bad behavior of the theory beyond the vacuum cutoff.
Physical Review D, 2012
We have recently proposed a special class of scalar tensor theories known as the Fab Four. These ... more We have recently proposed a special class of scalar tensor theories known as the Fab Four. These arose from attempts to analyse the cosmological constant problem within the context of Horndeski's most general scalar tensor theory. The Fab Four together give rise to a model of self-tuning, with the relevant solutions evading Weinberg's no-go theorem by relaxing the condition of Poincaré invariance in the scalar sector. The Fab Four are made up of four geometric terms in the action with each term containing a free potential function of the scalar field. In this paper we rigorously derive this model from the general model of Horndeski, proving that the Fab Four represents the only classical scalar tensor theory of this type that has any hope of tackling the cosmological constant problem. We present the full equations of motion for this theory, and give an heuristic argument to suggest that one might be able to keep radiative corrections under control. We also give the Fab Four in terms of the potentials presented in Deffayet et al's version of Horndeski.
Physical Review D, 2014
Recently we suggested a reformulation of General Relativity which completely sequesters from grav... more Recently we suggested a reformulation of General Relativity which completely sequesters from gravity all of the vacuum energy from a protected matter sector, assumed to contain the Standard Model. Here we elaborate further on the mechanism, presenting additional details of how it cancels all loop corrections and renders all contributions from phase transitions automatically small. We also consider cosmological consequences in more detail and show that the mechanism is consistent with a variety of inflationary models that make a universe big and old. We discuss in detail the underlying assumptions behind the dynamics of our proposal, and elaborate on the relationship of the physical interpretation of divergent operators in quantum field theory and the apparent 'acausality' which our mechanism seems to entail, which we argue is completely harmless. It is merely a reflection of the fact that any UV sensitive quantity in quantum field theory cannot be calculated from first principles, but is an input whose numerical value must be measured. We also note that since the universe should be compact in spacetime, and so will collapse in the future, the current phase of acceleration with w DE ≈ −1 is just a transient. This could be tested by future cosmological observations.
Journal of High Energy Physics, 2011
We continue to introduce bi-galileon theory, the generalisation of the single galileon model intr... more We continue to introduce bi-galileon theory, the generalisation of the single galileon model introduced by Nicolis et al. The theory contains two coupled scalar fields and is described by a Lagrangian that is invariant under Galilean shifts in those fields. This paper is the second of two, and focuses on the phenomenology of the theory. We are particularly interesting in models that admit solutions that are asymptotically self accelerating or asymptotically self tuning. In contrast to the single galileon theories, we find examples of self accelerating models that are simultaneously free from ghosts, tachyons and tadpoles, able to pass solar system constraints through Vainshtein screening, and do not suffer from problems with superluminality, Cerenkov emission or strong coupling. We also find self tuning models and discuss how Weinberg's no go theorem is evaded by breaking Poincaré invariance in the scalar sector. Whereas the galileon description is valid all the way down to solar system scales for the self-accelerating models, unfortunately the same cannot be said for self tuning models owing to the scalars backreacting strongly on to the geometry.
Classical and Quantum Gravity - CLASS QUANTUM GRAVITY, 2003
We derive the gravitational Hamiltonian starting from the Gauss-Bonnet action, keeping track of a... more We derive the gravitational Hamiltonian starting from the Gauss-Bonnet action, keeping track of all surface terms. This is done using the language of orthonormal frames and forms to keep things as tidy as possible. The surface terms in the Hamiltonian give a remarkably simple expression for the total energy of a spacetime. This expression is consistent with energy expressions found in hep-th/0212292. However, we can apply our results whatever the choice of background and whatever the symmetries of the spacetime. * a.padilla1@physics.ox.ac.uk
Journal of High Energy Physics, 2006
We systematically explore the spectrum of gravitational perturbations in codimension-1 DGP branew... more We systematically explore the spectrum of gravitational perturbations in codimension-1 DGP braneworlds, and find a 4D ghost on the self-accelerating branch of solutions. The ghost appears for any value of the brane tension, although depending on the sign of the tension it is either the helicity-0 component of the lightest localized massive tensor of mass 0 < m 2 < 2H 2 for positive tension, the scalar 'radion' for negative tension, or their admixture for vanishing tension. Because the ghost is gravitationally coupled to the brane-localized matter, the self-accelerating solutions are not a reliable benchmark for cosmic acceleration driven by gravity modified in the IR. In contrast, the normal branch of solutions is ghost-free, and so these solutions are perturbatively safe at large distance scales. We further find that when the Z 2 orbifold symmetry is broken, new tachyonic instabilities, which are much milder than the ghosts, appear on the self-accelerating branch. Finally, using exact gravitational shock waves we analyze what happens if we relax boundary conditions at infinity. We find that non-normalizable bulk modes, if interpreted as 4D phenomena, may open the door to new ghost-like excitations.
Journal of High Energy Physics, 2009
By studying perturbations about the vacuum, we show that Hořava gravity suffers from two differen... more By studying perturbations about the vacuum, we show that Hořava gravity suffers from two different strong coupling problems, extending all the way into the deep infra-red. The first of these is associated with the principle of detailed balance and explains why solutions to General Relativity are typically not recovered in models that preserve this structure. The second of these occurs even without detailed balance and is associated with the breaking of diffeomorphism invariance, required for anisotropic scaling in the UV. Since there is a reduced symmetry group there are additional degrees of freedom, which need not decouple in the infra-red. Indeed, we use the Stuckelberg trick to show that one of these extra modes become strongly coupled as the parameters approach their desired infra-red fixed point. Whilst we can evade the first strong coupling problem by breaking detailed balance, we cannot avoid the second, whatever the form of the potential. Therefore the original Hořava model, and its "phenomenologically viable" extensions do not have a perturbative General Relativity limit at any scale. Experiments which confirm the perturbative gravitational wave prediction of General Relativity, such as the cumulative shift of the periastron time of binary pulsars, will presumably rule out the theory. * Electronic address: Christos.Charmousis@th.u-psud.fr † Electronic address: gustavo.niz@nottingham.ac.uk ‡ Electronic address: antonio.padilla@nottingham.ac.uk § Electronic address: paul.saffin@nottingham.ac.uk
Journal of High Energy Physics, 2013
ABSTRACT We consider the role of matter in the non-projectable version of Horava-Liftshitz gravit... more ABSTRACT We consider the role of matter in the non-projectable version of Horava-Liftshitz gravity at both a classical and a quantum level. At the classical level, we construct general forms of matter Lagrangians consistent with the reduced symmetry group and demonstrate that they must be reduced to their relativistic form if they are to avoid sourcing the gravitational Stuckelberg field. At the quantum level we consider one loop corrections to the propagator for a relativistic scalar minimally coupled to gravity at tree level. We find large corrections to the light cone at low energies arising from the strength of the coupling of the scalar graviton to matter. We also find evidence that higher order time derivatives may be generated, which is worrying if this is to be taken seriously as a UV complete theory.
Journal of High Energy Physics, 2008
Owing to the quadratic nature of the theory, Einstein-Gauss-Bonnet gravity generically permits tw... more Owing to the quadratic nature of the theory, Einstein-Gauss-Bonnet gravity generically permits two distinct vacuum solutions. One solution (the "Einstein" vacuum) has a well defined limit as the Gauss-Bonnet coupling goes to zero, whereas the other solution (the "stringy" vacuum) does not. There has been some debate regarding the stability of these vacua, most recently from Deser & Tekin who have argued that the corresponding black hole solutions have positive mass and as such both vacua are stable. Whilst the statement about the mass is correct, we argue that the stringy vacuum is still perturbatively unstable. Simply put, the stringy vacuum suffers from a ghostlike instability that is not excited by the spherically symmetric black hole, but would be excited by any source likely to emit gravitational waves, such as a binary system. This result is reliable except in the strongly coupled regime close to the Chern-Simons limit, when the two vacua are almost degenerate. In this regime, we study instanton transitions between branches via bubble nucleation, and calculate the nucleation probability. This demonstrates that there is large mixing between the vacua, so that neither of them can accurately describe the true quantum vacuum. We also present a new gravitational instanton describing black hole pair production in de Sitter space on the Einstein branch, which is preferred to the usual Nariai instantons and is not present in pure Einstein
Journal of High Energy Physics, 2010
We introduce bi-galileon theory, the generalisation of the single galileon model introduced by Ni... more We introduce bi-galileon theory, the generalisation of the single galileon model introduced by Nicolis et al. The theory contains two coupled scalar fields and is described by a Lagrangian that is invariant under Galilean shifts in those fields. This paper is the first of two, and focuses on the motivation and formulation of the theory. We show that the boundary effective theory of the cascading cosmology model corresponds to a bi-galileon theory in the decoupling limit, and argue that this is to be expected for co-dimension 2 braneworld models exhibiting infra-red modification of gravity. We then generalise this, by constructing the most general bi-galileon Lagrangian. By coupling one of the galileons to the energy-momentum tensor, we pitch this as a modified gravity theory in which the modifications to General Relativity are encoded in the dynamics of the two galileons. We initiate a study of phenomenology by looking at maximally symmetric vacua and their stability, developing elegant geometric techniques that trivially explain why some of the vacua have to be unstable in certain cases (eg DGP). A detailed study of phenomenology appears in our companion paper.
Journal of High Energy Physics, 2010
We consider the so-called "healthy" extension of Hořava gravity in the limit where the Stuckelber... more We consider the so-called "healthy" extension of Hořava gravity in the limit where the Stuckelberg field decouples from the graviton. We verify the alleged strong coupling problem in this limit, under the assumption that no large dimensionless parameters are put in by hand. This follows from the fact that the dispersion relation for the Stuckelberg field does not have the desired z = 3 anisotropic scaling in the UV. To get the desired scaling and avoid strong coupling one has to introduce a low scale of Lorentz violation and retain some coupling between the graviton and the Stuckelberg field. We also make use of the foliation preserving symmetry to show how the Stuckelberg field couples to some violation of energy conservation. We source the Stuckelberg field using a point particle with a slowly varying mass and show that two such particles feel a constant attractive force. In this particular example, we see no Vainshtein effect, and violations of the Equivalence Principle. The latter is probably generic to other types of source and could potentially be used to place lower bounds on the scale of Lorentz violation. * ppxik1@nottingham.ac.uk
Journal of High Energy Physics, 2012
We present a novel idea for screening the vacuum energy contribution to the overall value of the ... more We present a novel idea for screening the vacuum energy contribution to the overall value of the cosmological constant, thereby enabling us to choose the bare value of the vacuum curvature empirically, without any need to worry about the zero-point energy contributions of each particle. The trick is to couple matter to a metric that is really a composite of other fields, with the property that the square-root of its determinant is the integrand of a topological invariant, and/or a total derivative. This ensures that the vacuum energy contribution to the Lagrangian is non-dynamical. We then give an explicit example of a theory with this property that is free from Ostrogradski ghosts, and is consistent with solar system physics and cosmological tests.
Journal of High Energy Physics, 2012
We show how to generalize the classical duals found by Gabadadze et al to a very large class of s... more We show how to generalize the classical duals found by Gabadadze et al to a very large class of self-interacting theories. This enables one to adopt a perturbative description beyond the scale at which classical perturbation theory breaks down in the original theory. This is particularly relevant if we want to test modified gravity scenarios that exhibit Vainshtein screening on solar system scales. We recognise the duals as being related to the Legendre transform of the original Lagrangian, and present a practical method for finding the dual in general; our methods can also be applied to self-interacting theories with a hierarchy of strong coupling scales, and with multiple fields. We find the classical dual of the full quintic galileon theory as an example.
Journal of High Energy Physics, 2012
We compute the boundary terms and junction conditions for Horndeski's panoptic class of scalarten... more We compute the boundary terms and junction conditions for Horndeski's panoptic class of scalartensor theories, and write the bulk and boundary equations of motion in explicitly second order form. We consider a number of special subclasses, including galileon theories, and present the corresponding formulae. Our analysis opens up of the possibility of studying tunnelling between vacua in generalized scalar-tensor theories, and braneworld dynamics. The latter follows because our results are independent of spacetime dimension. * antonio.padilla@nottingham.ac.uk † ppxvs@nottingham.ac.uk 1 We thank Stanley Deser for enlightening us on the history of scalar-tensor theories. See for an historical overview. 2 In four dimensions, the Horndeski and DGSZ actions were shown to be equivalent , and given Horndeski's proof, we know this to be the most general scalar-tensor theory admitting second order field equations. In higher dimensions the DGSZ action is known to yield second order field equations, but it is not the most general theory.
Physical Review D, 2015
ABSTRACT We investigate low energy limits of massive gauge theories that feature the Vainshtein m... more ABSTRACT We investigate low energy limits of massive gauge theories that feature the Vainshtein mechanism, focussing on the effects of the UV modes that are integrated out. It turns out that the Goldstone sectors are significantly influenced by the effects from such modes relative to the effective field theories where the irrelevant operators induced by heavy modes are simply cast aside. The effects of the consistently retained higher order corrections affect the strong coupling and show that the nature of the UV completion influences the low energy theory significantly. This casts doubts on the naively estimated environmental strong coupling scale, and on the effectiveness of the environmental enhancement of screening. The environmental effects by themselves might not suffice to cure the bad behavior of the theory beyond the vacuum cutoff.
Physics Reports, 2012
In this review we present a thoroughly comprehensive survey of recent work on modified theories o... more In this review we present a thoroughly comprehensive survey of recent work on modified theories of gravity and their cosmological consequences. Amongst other things, we cover General Relativity, Scalar-Tensor, Einstein-Aether, and Bimetric theories, as well as TeVeS, f (R), general higher-order theories, Hořava-Lifschitz gravity, Galileons, Ghost Condensates, and models of extra dimensions including Kaluza-Klein, Randall-Sundrum, DGP, and higher co-dimension braneworlds. We also review attempts to construct a Parameterised Post-Friedmannian formalism, that can be used to constrain deviations from General Relativity in cosmology, and that is suitable for comparison with data on the largest scales. These subjects have been intensively studied over the past decade, largely motivated by rapid progress in the field of observational cosmology that now allows, for the first time, precision tests of fundamental physics on the scale of the observable Universe. The purpose of this review is to provide a reference tool for researchers and students in cosmology and gravitational physics, as well as a self-contained, comprehensive and up-to-date introduction to the subject as a whole.
Physical Review D, 2012
We have recently proposed a special class of scalar tensor theories known as the Fab Four. These ... more We have recently proposed a special class of scalar tensor theories known as the Fab Four. These arose from attempts to analyse the cosmological constant problem within the context of Horndeski's most general scalar tensor theory. The Fab Four together give rise to a model of self-tuning, with the relevant solutions evading Weinberg's no-go theorem by relaxing the condition of Poincaré invariance in the scalar sector. The Fab Four are made up of four geometric terms in the action with each term containing a free potential function of the scalar field. In this paper we rigorously derive this model from the general model of Horndeski, proving that the Fab Four represents the only classical scalar tensor theory of this type that has any hope of tackling the cosmological constant problem. We present the full equations of motion for this theory, and give an heuristic argument to suggest that one might be able to keep radiative corrections under control. We also give the Fab Four in terms of the potentials presented in Deffayet et al's version of Horndeski.
Physics Letters B, 2002
We examine the cosmological evolution equations of de Sitter, flat and anti-de Sitter braneworlds... more We examine the cosmological evolution equations of de Sitter, flat and anti-de Sitter braneworlds sandwiched in between two n dimensional AdS-Schwarzschild spacetimes. We are careful to use the correct form for the induced Newton's constant on the brane, and show that it would be naive to assume the energy of the bulk spacetime is just given by the sum of the black hole masses. By carefully calculating the energy of the bulk for large mass we show that the induced geometry of the braneworld is just a radiation dominated FRW universe with the radiation coming from a CFT that is dual to the AdS bulk.
Physical Review D, 2014
Recently we suggested a reformulation of General Relativity which completely sequesters from grav... more Recently we suggested a reformulation of General Relativity which completely sequesters from gravity all of the vacuum energy from a protected matter sector, assumed to contain the Standard Model. Here we elaborate further on the mechanism, presenting additional details of how it cancels all loop corrections and renders all contributions from phase transitions automatically small. We also consider cosmological consequences in more detail and show that the mechanism is consistent with a variety of inflationary models that make a universe big and old. We discuss in detail the underlying assumptions behind the dynamics of our proposal, and elaborate on the relationship of the physical interpretation of divergent operators in quantum field theory and the apparent 'acausality' which our mechanism seems to entail, which we argue is completely harmless. It is merely a reflection of the fact that any UV sensitive quantity in quantum field theory cannot be calculated from first principles, but is an input whose numerical value must be measured. We also note that since the universe should be compact in spacetime, and so will collapse in the future, the current phase of acceleration with w DE ≈ −1 is just a transient. This could be tested by future cosmological observations.