arXiv subjects

Clare Burrage

Publications and source records attributed to Clare Burrage.

At least 73 records · Page 4Linked to original sources

arXiv2015

The Shape Dependence of Vainshtein Screening

Scalar field theories that possess a Vainshtein mechanism are able to dynamically suppress the associated fifth forces in the presence of massive sources through derivative non-linearities. The resulting equations of motion for the scalar are highly non-linear and therefore very few analytic solutions are known. Here we present a brief investigation of the structure of Vainshtein screening in symmetrical configurations, focusing in particular on the spherical, cylindrical and planar solutions that are relevant for observations of the cosmic web. We consider Vainshtein screening in both the Galileon model, where the non-linear terms involve second derivatives of the scalar, and a k-essence theory, where the non-linear terms involve only first derivatives of the scalar. We find that screening, and consequently the suppression of the scalar force, is most efficient around spherical sources, weaker around cylindrical sources and can be absent altogether around planar sources.

gr-qc↗

arXiv2015

Using Atom Interferometry to Detect Dark Energy

We review the tantalising prospect that the first evidence for the dark energy driving the observed acceleration of the Universe on giga-parsec scales may be found through metre scale laboratory based atom interferometry experiments. To do that, we first introduce the idea that scalar fields could be responsible for dark energy and show that in order to be compatible with fifth force constraints these fields must have a screening mechanism which hides their effects from us within the solar system. Particular emphasis is placed on one such screening mechanism known as the chameleon effect where the field's mass becomes dependent on the environment. The way the field behaves in the presence of a spherical source is determined and we then go on to show how in the presence of the kind of high vacuum associated with atom interferometry experiments, and when the test particle is an atom, it is possible to use the associated interference pattern to place constraints on the acceleration due to the fifth force of the chameleon field - this has already been used to rule out large regions of the chameleon parameter space and maybe one day will be able to detect the force due to the dark energy field in the laboratory.

astro-ph.CO↗

arXiv2015

Disformal dark energy at colliders

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.

hep-ph↗

arXiv2015

Probing Dark Energy with Atom Interferometry

Theories of dark energy require a screening mechanism to explain why the associated scalar fields do not mediate observable long range fifth forces. The archetype of this is the chameleon field. Here we show that individual atoms are too small to screen the chameleon field inside a large high-vacuum chamber, and therefore can detect the field with high sensitivity. We derive new limits on the chameleon parameters from existing experiments, and show that most of the remaining chameleon parameter space is readily accessible using atom interferometry.

astro-ph.CO↗

arXiv2014

Ellipticity Weakens Chameleon Screening

The chameleon mechanism enables a long range fifth force to be screened in dense environments when non-trivial self interactions of the field cause its mass to increase with the local density. To date, chameleon fifth forces have mainly been studied for spherically symmetric sources, however the non-linear self interactions mean that the chameleon responds to changes in the shape of the source differently to gravity. In this work we focus on ellipsoidal departures from spherical symmetry and compute the full form of the chameleon force, comparing it's shape dependence to that of gravity. Enhancement of the chameleon force by up to 40% is possible when deforming a sphere to an ellipsoid of the same mass, with an ellipticity $\simeq 0.99$.

hep-th↗

arXiv2014

Cosmological Tests of Coupled Galileons

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 distortions 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.

astro-ph.CO↗

arXiv2014

Constraining Disformally Coupled Scalar Fields

Light scalar fields can naturally couple disformally to matter fields. Static, non-relativistic sources do not generate a classical field profile for a disformally coupled scalar, and so such scalars are free from the constraints on the existence of fifth forces that are so restrictive for conformally coupled scalars. In this work we show that disformally coupled scalars can still be studied and constrained through their microscopic interactions with fermions and photons, both in terrestrial laboratories and from observations of stars. The strongest constraint on the coupling scale comes from mono-photon searches at the LHC and requires $M \gtrsim 10^2 \mbox{ GeV}$.

astro-ph.CO↗

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

Chameleons in the Early Universe: Kicks, Rebounds, and Particle Production

Chameleon gravity is a scalar-tensor theory that includes a non-minimal coupling between the scalar field and the matter fields and yet mimics general relativity in the Solar System. The scalar degree of freedom is hidden in high-density environments because the effective mass of the chameleon scalar depends on the trace of the stress-energy tensor. In the early Universe, when the trace of the matter stress-energy tensor is nearly zero, the chameleon is very light, and Hubble friction prevents it from reaching the minimum of its effective potential. Whenever a particle species becomes non-relativistic, however, the trace of the stress-energy tensor is temporarily nonzero, and the chameleon begins to roll. We show that these "kicks" to the chameleon field have catastrophic consequences for chameleon gravity. The velocity imparted to the chameleon by the kick is sufficiently large that the chameleon's mass changes rapidly as it slides past its potential minimum. This nonadiabatic evolution shatters the chameleon field by generating extremely high-energy perturbations through quantum particle production. If the chameleon's coupling to matter is slightly stronger than gravitational, the excited modes have trans-Planckian momenta. The production of modes with momenta exceeding 1e7 GeV can only be avoided for small couplings and finely tuned initial conditions. These quantum effects also significantly alter the background evolution of the chameleon field, and we develop new analytic and numerical techniques to treat quantum particle production in the regime of strong dissipation. This analysis demonstrates that chameleon gravity cannot be treated as a classical field theory at the time of Big Bang Nucleosynthesis and casts doubt on chameleon gravity's viability as an alternative to general relativity.

astro-ph.CO↗

arXiv2014

D-BIonic Screening of Scalar Fields

We study a new screening mechanism which is present in Dirac-Born-Infeld (DBI)-like theories. A scalar field with a DBI-like Lagrangian is minimally coupled to matter. In the vicinity of sufficiently dense sources, non-linearities in the scalar dominate and result in an approximately constant acceleration on a test particle, thereby suppressing the scalar force relative to gravity. Unlike generic P(X) theories, screening happens within the regime of validity of the effective field theory, thanks to the DBI symmetry. This symmetry also allows the removal of a constant field gradient, like in galileons. Not surprisingly, perturbations around the spherically-symmetry background propagate superluminally, but we argue for a chronology protection analogous to galileons. We derive constraints on the theory parameters from tests of gravity and discuss various extensions.

hep-th↗

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

Strong Coupling and Bounds on the Graviton Mass in Massive Gravity

The theory of a single massive graviton has a cutoff much below its Planck scale, because the extra modes from the graviton multiplet involve higher derivative self-interactions, controlled by a scale convoluted from the small graviton mass. Generically, these correct the propagator by environmental effects. The resulting effective cutoff depends on the environmental parameters and the graviton mass. Requiring the theory to be perturbative down to ${\cal O}(1) mm$, we derive bounds on the graviton mass, corresponding to $\gtrsim {\cal O}(1) meV$ for the generic case, and somewhat weaker bounds in cases of fine-tuning. In all cases the mass is required to be much too large for the theory to conform with GR at cosmological distances. Similar results also hold in quartic and quintic Galileon theory.

hep-th↗

arXiv2013

Catastrophic Consequences of Kicking the Chameleon

The physics of the "dark energy" that drives the current cosmological acceleration remains mysterious, and the dark sector may involve new light dynamical fields. If these light scalars couple to matter, a screening mechanism must prevent them from mediating an unacceptably strong fifth force locally. Here we consider a concrete example: the chameleon mechanism. We show that the same coupling between the chameleon field and matter employed by the screening mechanism also has catastrophic consequences for the chameleon during the Universe's first minutes. The chameleon couples to the trace of the stress-energy tensor, which is temporarily non-zero in a radiation-dominated universe whenever a particle species becomes non-relativistic. These "kicks" impart a significant velocity to the chameleon field, causing its effective mass to vary non-adiabatically and resulting in the copious production of quantum fluctuations. Dissipative effects strongly modify the background evolution of the chameleon field, invalidating all previous classical treatments of chameleon cosmology. Moreover, the resulting fluctuations have extremely high characteristic energies, which casts serious doubt on the validity of the effective theory. Our results demonstrate that quantum particle production can profoundly affect scalar-tensor gravity, a possibility not previously considered. Working in this new context, we also develop the theory and numerics of particle production in the regime of strong dissipation.

astro-ph.CO↗

arXiv2013

Shining Light on Dark Energy and Modifications of Gravity

Many theories of dark energy and modified gravity give rise to scalar fields that couple derivatively to the energy momentum tensor of matter. This is known as disformal coupling. I will show that laboratory searches for axions are ideally suited to search for and constrain disformal scalar fields.

hep-ph↗

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

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↗

arXiv2011

Chronology Protection in Galileon Models and Massive Gravity

Galileon models are a class of effective field theories that have recently received much attention. They arise in the decoupling limit of theories of massive gravity, and in some cases they have been treated in their own right as scalar field theories with a specific nonlinearly realized global symmetry (Galilean transformation). It is well known that in the presence of a source, these Galileon theories admit superluminal propagating solutions, implying that as quantum field theories they must admit a different notion of causality than standard local Lorentz invariant theories. We show that in these theories it is easy to construct closed timelike curves (CTCs) within the {\it naive} regime of validity of the effective field theory. However, on closer inspection we see that the CTCs could never arise since the Galileon inevitably becomes infinitely strongly coupled at the onset of the formation of a CTC. This implies an infinite amount of backreaction, first on the background for the Galileon field, signaling the break down of the effective field theory, and subsequently on the spacetime geometry, forbidding the formation of the CTC. Furthermore the background solution required to create CTCs becomes unstable with an arbitrarily fast decay time. Thus Galileon theories satisfy a direct analogue of Hawking's chronology protection conjecture.

hep-th↗

arXiv2011

Laboratory Tests of the Galileon

The Galileon model is a ghost free scalar effective field theory containing higher derivative terms that are protected by the Galileon symmetry. The presence of a Vainshtein screening mechanism allows the scalar field to couple to matter without mediating unacceptably large fifth forces in the solar system. We describe how laboratory measurements of the Casimir effect and possible deviations from Newtonian gravity can be used to search for Galileon scalar fields. Current experimental measurements are used to bound a previously unconstrained combination of Galileon parameters.

hep-ph↗