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Using machine learning to optimise chameleon fifth force experiments (2024)
Journal Article
Briddon, C., Burrage, C., Moss, A., & Tamosiunas, A. (2024). Using machine learning to optimise chameleon fifth force experiments. Journal of Cosmology and Astroparticle Physics, 2024(2), Article 11. https://doi.org/10.1088/1475-7516/2024/02/011

The chameleon is a theorised scalar field that couples to matter and possess a screening mechanism, which weakens observational constraints from experiments performed in regions of higher matter density. One consequence of this screening mechanism is... Read More about Using machine learning to optimise chameleon fifth force experiments.

Higgs-induced screening mechanisms in scalar-tensor theories (2023)
Journal Article
Burrage, C., & Millington, P. (2024). Higgs-induced screening mechanisms in scalar-tensor theories. Annals of the New York Academy of Sciences, 1531(1), 95-103. https://doi.org/10.1111/nyas.15092

We consider the theory of a light conformally coupled scalar field, that is, one that is coupled directly to the Ricci scalar of the gravitational sector. This theory can be written equivalently as one of a light scalar that is coupled to the Standar... Read More about Higgs-induced screening mechanisms in scalar-tensor theories.

Spinning black holes with axion hair (2023)
Journal Article
Burrage, C., Fernandes, P. G. S., Brito, R., & Cardoso, V. (2023). Spinning black holes with axion hair. Classical and Quantum Gravity, 40(20), Article 205021. https://doi.org/10.1088/1361-6382/acf9d6

In this work we construct and analyze non-perturbative stationary and axially-symmetric black hole solutions in general relativity coupled to an electromagnetic and an axion field. The axion field is coupled to the electromagnetic field, which leads... Read More about Spinning black holes with axion hair.

Invisible dilaton (2023)
Journal Article
Brax, P., Burrage, C., Cembranos, J. A., Valageas, P., & Cembranos, J. A. (2023). Invisible dilaton. Physical Review D, 107(9), 095015. https://doi.org/10.1103/PhysRevD.107.095015

We analyze the dynamics of a light scalar field responsible for the μ term of the Higgs potential and coupled to matter via the Higgs-portal mechanism. We find that this dilaton model is stable under radiative corrections induced by the standard mode... Read More about Invisible dilaton.

Chameleon screening in cosmic voids (2022)
Journal Article
Tamosiunas, A., Briddon, C., Burrage, C., Cutforth, A., Moss, A., & Vincent, T. (2022). Chameleon screening in cosmic voids. Journal of Cosmology and Astroparticle Physics, 2022(11), Article 056. https://doi.org/10.1088/1475-7516/2022/11/056

A key goal in cosmology in the upcoming decade will be to form a better understanding of the accelerated expansion of the Universe. Upcoming surveys, such as the Vera C. Rubin Observatory's 10-year Legacy Survey of Space and Time (LSST), Euclid and t... Read More about Chameleon screening in cosmic voids.

Dark sector domain walls could explain the observed planes of satellites (2022)
Journal Article
Naik, A. P., & Burrage, C. (2022). Dark sector domain walls could explain the observed planes of satellites. Journal of Cosmology and Astroparticle Physics, 2022(08), Article 020. https://doi.org/10.1088/1475-7516/2022/08/020

The observed 'planes of satellites' around the Milky Way and other nearby galaxies are notoriously difficult to explain under the ΛCDM paradigm. Here, we propose an alternative solution: domain walls arising in theories with symmetry-breaking scalar... Read More about Dark sector domain walls could explain the observed planes of satellites.

Chameleon screening depends on the shape and structure of NFW halos (2022)
Journal Article
Tamosiunas, A., Briddon, C., Burrage, C., Cui, W., & Moss, A. (2022). Chameleon screening depends on the shape and structure of NFW halos. Journal of Cosmology and Astroparticle Physics, 2022(4), Article 047. https://doi.org/10.1088/1475-7516/2022/04/047

Chameleon gravity is an example of a model that gives rise to interesting phenomenology on cosmological scales while simultaneously possessing a screening mechanism, allowing it to avoid solar system constraints. Such models result in non-linear fiel... Read More about Chameleon screening depends on the shape and structure of NFW halos.

Charting galactic accelerations - II: How to ‘learn’ accelerations in the solar neighbourhood (2022)
Journal Article
Naik, A. P., An, J., Burrage, C., & Evans, N. W. (2022). Charting galactic accelerations - II: How to ‘learn’ accelerations in the solar neighbourhood. Monthly Notices of the Royal Astronomical Society, 511(2), 1609-1621. https://doi.org/10.1093/mnras/stac153

Gravitational acceleration fields can be deduced from the collisionless Boltzmann equation, once the distribution function is known. This can be constructed via the method of normalizing flows from data sets of the positions and velocities of stars.... Read More about Charting galactic accelerations - II: How to ‘learn’ accelerations in the solar neighbourhood.

SELCIE: a tool for investigating the chameleon field of arbitrary sources (2021)
Journal Article
Briddon, C., Burrage, C., Moss, A., & Tamosiunas, A. (2021). SELCIE: a tool for investigating the chameleon field of arbitrary sources. Journal of Cosmology and Astroparticle Physics, 2021(12), 1-24. https://doi.org/10.1088/1475-7516/2021/12/043

The chameleon model is a modified gravity theory that introduces an additional scalar field that couples to matter through a conformal coupling. This `chameleon field' possesses a screening mechanism through a nonlinear self-interaction term which al... Read More about SELCIE: a tool for investigating the chameleon field of arbitrary sources.

Fifth-force screening around extremely compact sources (2021)
Journal Article
Burrage, C., Elder, B., Millington, P., Saadeh, D., & Thrussell, B. (2021). Fifth-force screening around extremely compact sources. Journal of Cosmology and Astroparticle Physics, 2021(August 2021), Article JCAP08(2021)052. https://doi.org/10.1088/1475-7516/2021/08/052

Many non-linear scalar field theories possess a screening mechanism that can suppress any associated fifth force in dense environments. As a result, these theories can evade local experimental tests of new forces. Chameleon-like screening, which occu... Read More about Fifth-force screening around extremely compact sources.

Charting galactic accelerations: when and how to extract a unique potential from the distribution function (2021)
Journal Article
An, J., Naik, A. P., Evans, N. W., & Burrage, C. (2021). Charting galactic accelerations: when and how to extract a unique potential from the distribution function. Monthly Notices of the Royal Astronomical Society, 506(4), 5721-5730. https://doi.org/10.1093/mnras/stab2049

The advent of data sets of stars in the Milky Way with 6D phase-space information makes it possible to construct empirically the distribution function (DF). Here, we show that the accelerations can be uniquely determined from the DF using the collisi... Read More about Charting galactic accelerations: when and how to extract a unique potential from the distribution function.

φenics: Vainshtein screening with the finite element method (2021)
Journal Article
Braden, J., Burrage, C., Elder, B., & Saadeh, D. (2021). φenics: Vainshtein screening with the finite element method. Journal of Cosmology and Astroparticle Physics, 2021(03), Article 010. https://doi.org/10.1088/1475-7516/2021/03/010

Within the landscape of modified theories of gravity, progress in understanding the behaviour of, and developing tests for, screening mechanisms has been hindered by the complexity of the field equations involved, which are nonlinear in nature and ch... Read More about φenics: Vainshtein screening with the finite element method.

Massive Galileons and Vainshtein screening (2021)
Journal Article
Burrage, C., Coltman, B., Padilla, A., Saadeh, D., & Wilson, T. (2021). Massive Galileons and Vainshtein screening. Journal of Cosmology and Astroparticle Physics, 2021(02), Article 050. https://doi.org/10.1088/1475-7516/2021/02/050

The Vainshtein screening mechanism relies on nonlinear interaction terms becoming dominant close to a compact source. However, theories displaying this mechanism are generally understood to be low-energy theories: it is unclear that operators emergin... Read More about Massive Galileons and Vainshtein screening.

Prospects for fundamental physics with LISA (2020)
Journal Article
Barausse, E., Berti, E., Hertog, T., Hughes, S. A., Jetzer, P., Pani, P., …Zumalacarregui, M. (2020). Prospects for fundamental physics with LISA. General Relativity and Gravitation, 52(8), Article 81. https://doi.org/10.1007/s10714-020-02691-1

In this paper, which is of programmatic rather than quantitative nature, we aim to further delineate and sharpen the future potential of the LISA mission in the area of fundamental physics. Given the very broad range of topics that might be relevant... Read More about Prospects for fundamental physics with LISA.

Open quantum dynamics induced by light scalar fields (2019)
Journal Article
Burrage, C., Käding, C., Millington, P., & Minár, J. (2019). Open quantum dynamics induced by light scalar fields. Physical Review D, 100(7), Article 076003. https://doi.org/10.1103/PhysRevD.100.076003

We consider the impact of a weakly coupled environment comprising a light scalar field on the open dynamics of a quantum test mass, resulting in a master equation for the test mass that features corrections to the coherent dynamics, as well as decohe... Read More about Open quantum dynamics induced by light scalar fields.

Influence functionals, decoherence and conformally coupled scalars (2019)
Journal Article
Burrage, C., Käding, C., Millington, P., & Minář, J. (2019). Influence functionals, decoherence and conformally coupled scalars. Journal of Physics: Conference Series, 1275(1), Article 012041. https://doi.org/10.1088/1742-6596/1275/1/012041

Some of the simplest modifications to general relativity involve the coupling of additional scalar fields to the scalar curvature. By making a Weyl rescaling of the metric, these theories can be mapped to Einstein gravity with the additional scalar f... Read More about Influence functionals, decoherence and conformally coupled scalars.

Experiment to Detect Dark Energy Forces Using Atom Interferometry (2019)
Journal Article
Sabulsky, D., Dutta, I., Hinds, E., Elder, B., Burrage, C., & Copeland, E. J. (2019). Experiment to Detect Dark Energy Forces Using Atom Interferometry. Physical Review Letters, 123(6), 1-6. https://doi.org/10.1103/PhysRevLett.123.061102

The accelerated expansion of the universe motivates a wide class of scalar field theories that modify General Relativity (GR) on large scales. Such theories require a screening mechanism to suppress the new force in regions where the weak field limit... Read More about Experiment to Detect Dark Energy Forces Using Atom Interferometry.

Black holes, gravitational waves and fundamental physics: A roadmap (2019)
Journal Article
Askar, A., Bertone, G., Bon, E., Blas, D., Brito, R., Burrage, C., …Zilhão, M. (2019). Black holes, gravitational waves and fundamental physics: A roadmap. Classical and Quantum Gravity, 36(14), Article 143001. https://doi.org/10.1088/1361-6382/ab0587

© 2019 IOP Publishing Ltd. The grand challenges of contemporary fundamental physics-dark matter, dark energy, vacuum energy, inflation and early universe cosmology, singularities and the hierarchy problem-all involve gravity as a key component. And o... Read More about Black holes, gravitational waves and fundamental physics: A roadmap.

Symmetron scalar fields: modified gravity, dark matter, or both? (2019)
Journal Article
Burrage, C., Copeland, E. J., Käding, C., & Millington, P. (2019). Symmetron scalar fields: modified gravity, dark matter, or both?. Physical Review D, 99(4), Article 043539. https://doi.org/10.1103/physrevd.99.043539

Scalar fields coupled to gravity through the Ricci scalar have been considered both as dark matter candidates and as a possible modified gravity explanation for galactic dynamics. It has recently been demonstrated that the dynamics of baryonic matter... Read More about Symmetron scalar fields: modified gravity, dark matter, or both?.

Particle level screening of scalar forces in 1+1 dimensions (2019)
Journal Article
Burrage, C., Elder, B., & Millington, P. (2019). Particle level screening of scalar forces in 1+1 dimensions. Physical Review D, 99(2), Article 024045. https://doi.org/10.1103/physrevd.99.024045

We investigate how nonlinear scalar field theories respond to point sources. Taking the symmetron as a specific example of such a theory, we solve the nonlinear equation of motion in one spatial dimension for (i) an isolated point source and (ii) two... Read More about Particle level screening of scalar forces in 1+1 dimensions.

Fifth forces, Higgs portals and broken scale invariance (2018)
Journal Article
Burrage, C., Copeland, E. J., Millington, P., & Spannowsky, M. (2018). Fifth forces, Higgs portals and broken scale invariance. Journal of Cosmology and Astroparticle Physics, 2018(11), 1-31. https://doi.org/10.1088/1475-7516/2018/11/036

© 2018 IOP Publishing Ltd and Sissa Medialab. We study the relationship between the strength of fifth forces and the origin of scale breaking in the Standard Model (SM) of particle physics. We start with a light scalar field that is conformally coupl... Read More about Fifth forces, Higgs portals and broken scale invariance.

Stellar kinematics from the symmetron fifth force in the Milky Way disk (2018)
Journal Article
O’Hare, C. A., & Burrage, C. (2018). Stellar kinematics from the symmetron fifth force in the Milky Way disk. Physical Review D, 98(6), Article 064019. https://doi.org/10.1103/PhysRevD.98.064019

It has been shown that the presence of nonminimally coupled scalar fields giving rise to a fifth force can noticeably alter dynamics on galactic scales. Such a fifth force must be screened in the Solar System but if unscreened it can have similar obs... Read More about Stellar kinematics from the symmetron fifth force in the Milky Way disk.

Laboratory constraints (2018)
Journal Article
Brax, P., Burrage, C., & Davis, A. (2018). Laboratory constraints. International Journal of Modern Physics D, (1848009). doi:10.1142/s0218271818480097. ISSN 0218-2718

We review laboratory constraints on theories of modified gravity and show that they are complementary to cosmological and astrophysical tests. We particularly focus on the environmentally dependent dilaton, as a worked example to show how such constr... Read More about Laboratory constraints.

The shape dependence of chameleon screening (2018)
Journal Article
Burrage, C., Copeland, E. J., Moss, A., & Stevenson, J. A. (2018). The shape dependence of chameleon screening. Journal of Cosmology and Astroparticle Physics, 2018(1), https://doi.org/10.1088/1475-7516/2018/01/056

Chameleon scalar fields can screen their associated fifth forces from detection by changing their mass with the local density. These models are an archetypal example of a screening mechanism, and have become an important target for both cosmological... Read More about The shape dependence of chameleon screening.

Beyond the growth rate of cosmic structure: Testing modified gravity models with an extra degree of freedom (2017)
Journal Article
Burrage, C., Parkinson, D., & Seery, D. (2017). Beyond the growth rate of cosmic structure: Testing modified gravity models with an extra degree of freedom. Physical Review D, 96(4), Article 043509. https://doi.org/10.1103/PhysRevD.96.043509

© 2017 American Physical Society. In "modified" gravity the observed acceleration of the universe is explained by changing the gravitational force law or the number of degrees of freedom in the gravitational sector. Both possibilities can be tested b... Read More about Beyond the growth rate of cosmic structure: Testing modified gravity models with an extra degree of freedom.

Radial acceleration relation from symmetron fifth forces (2017)
Journal Article
Burrage, C., Copeland, E. J., & Millington, P. (2017). Radial acceleration relation from symmetron fifth forces. Physical Review D, D95(6), Article 064050. https://doi.org/10.1103/PhysRevD.95.064050

We show that the radial acceleration relation for rotationally supported galaxies may be explained, in the absence of cold dark matter, by a nonminimally coupled scalar field, whose fifth forces are partially screened on galactic scales by the symmet... Read More about Radial acceleration relation from symmetron fifth forces.

Constraining symmetron fields with atom interferometry (2016)
Journal Article
Burrage, C., Kuribayashi-Coleman, A., Stevenson, J., & Thrussell, B. (2016). Constraining symmetron fields with atom interferometry. Journal of Cosmology and Astroparticle Physics, 2016, Article 041. https://doi.org/10.1088/1475-7516/2016/12/041

We apply the new constraints from atom-interferometry searches for screening mechanisms to the symmetron model, finding that these experiments exclude a previously unexplored region of the parameter space. We discuss the possibility of networks of do... Read More about Constraining symmetron fields with atom interferometry.

A compendium of chameleon constraints (2016)
Journal Article
Burrage, C., & Sakstein, J. (2016). A compendium of chameleon constraints. Journal of Cosmology and Astroparticle Physics, 2016(11), Article 045. https://doi.org/10.1088/1475-7516/2016/11/045

The chameleon model is a scalar field theory with a screening mechanism that explains how a cosmologically relevant light scalar can avoid the constraints of intra-solar-system searches for fifth-forces. The chameleon is a popular dark energy candida... Read More about A compendium of chameleon constraints.

Radiative screening of fifth forces (2016)
Journal Article
Burrage, C., Copeland, E. J., & Millington, P. (2016). Radiative screening of fifth forces. Physical Review Letters, 117(21), Article 211102. https://doi.org/10.1103/PhysRevLett.117.211102

© 2016 American Physical Society. We describe a symmetron model in which the screening of fifth forces arises at the one-loop level through the Coleman-Weinberg mechanism of spontaneous symmetry breaking. We show that such a theory can avoid current... Read More about Radiative screening of fifth forces.

LHC signatures of scalar dark energy (2016)
Journal Article
Burrage, C., Brax, P., Englert, C., & Spannowsky, M. (2016). LHC signatures of scalar dark energy. Physical Review D - Particles, Fields, Gravitation and Cosmology, 94, Article 084054. https://doi.org/10.1103/PhysRevD.94.084054

Scalar dark energy fields that couple to the standard model can give rise to observable signatures at the LHC. In this work, we show that tt¯+missing energy and monojet searches are suitable probes in the limit where the dark energy scalar is stable... Read More about LHC signatures of scalar dark energy.

A proposed experimental search for chameleons using asymmetric parallel plates (2016)
Journal Article
Burrage, C., Copeland, E. J., & Stevenson, J. A. (2016). A proposed experimental search for chameleons using asymmetric parallel plates. Journal of Cosmology and Astroparticle Physics, 2016(8), https://doi.org/10.1088/1475-7516/2016/08/070

Light scalar fields coupled to matter are a common consequence of theories of dark energy and attempts to solve the cosmological constant problem. The chameleon screening mechanism is commonly invoked in order to suppress the fifth forces mediated by... Read More about A proposed experimental search for chameleons using asymmetric parallel plates.

Disformal transformations on the CMB (2016)
Journal Article
Burrage, C., Cespedes, S., & Davis, A. (2016). Disformal transformations on the CMB. Journal of Cosmology and Astroparticle Physics, 2016, https://doi.org/10.1088/1475-7516/2016/08/024

In this work we study the role of disformal transformation on cosmological backgrounds and its relation to the speed of sound for tensor modes. A speed different from one for tensor modes can arise in several contexts, such as Galileons theories or m... Read More about Disformal transformations on the CMB.

Vacuum Cherenkov radiation and bremsstrahlung from disformal couplings (2016)
Journal Article
van de Bruck, C., Burrage, C., & Morrice, J. (2016). Vacuum Cherenkov radiation and bremsstrahlung from disformal couplings. Journal of Cosmology and Astroparticle Physics, 2016(8), Article 003. https://doi.org/10.1088/1475-7516/2016/08/003

The simplest way to modify gravity is to extend the gravitational sector to include an additional scalar degree of freedom. The most general metric that can be built in such a theory includes disformal terms, so that standard model fields move on a m... Read More about Vacuum Cherenkov radiation and bremsstrahlung from disformal couplings.

The speed of Galileon gravity (2016)
Journal Article
Brax, P., Burrage, C., & Davis, A. (2016). The speed of Galileon gravity. Journal of Cosmology and Astroparticle Physics, 2016(3), Article 004. https://doi.org/10.1088/1475-7516/2016/03/004

We analyse the speed of gravitational waves in coupled Galileon models with an equation of state ?phgr=?1 now and a ghost-free Minkowski limit. We find that the gravitational waves propagate much faster than the speed of light unless these models are... Read More about The speed of Galileon gravity.

Disformal dark energy at colliders (2015)
Journal Article
Brax, P., Burrage, C., & Englert, C. (2015). Disformal dark energy at colliders. Physical Review D, D92(4), Article 044036. https://doi.org/10.1103/PhysRevD.92.044036

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 beca... Read More about Disformal dark energy at colliders.

Using atom interferometry to detect dark energy (2015)
Journal Article
Burrage, C., & Copeland, E. J. (in press). Using atom interferometry to detect dark energy. Contemporary Physics, 57(2), https://doi.org/10.1080/00107514.2015.1060058

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 f... Read More about Using atom interferometry to detect dark energy.

Shape dependence of Vainshtein screening (2015)
Journal Article
Bloomfield, J. K., Burrage, C., & Davis, A. (2015). Shape dependence of Vainshtein screening. Physical Review D, D91(8), Article 083510. https://doi.org/10.1103/PhysRevD.91.083510

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 nonlinearities. The resulting equations of motion for the scalar are highly n... Read More about Shape dependence of Vainshtein screening.

Ellipticity weakens chameleon screening (2015)
Journal Article
Burrage, C., Copeland, E. J., & Stevenson, J. (2015). Ellipticity weakens chameleon screening. Physical Review D, D91(6), Article 065030. https://doi.org/10.1103/PhysRevD.91.065030

The chameleon mechanism enables a long-range fifth force to be screened in dense environments when nontrivial self-interactions of the field cause its mass to increase with the local density. To date, chameleon fifth forces have mainly been studied f... Read More about Ellipticity weakens chameleon screening.

Probing dark energy with atom interferometry (2015)
Journal Article
Burrage, C., Copeland, E. J., & Hinds, E. A. (2015). Probing dark energy with atom interferometry. Journal of Cosmology and Astroparticle Physics, 2015(3), Article 042. https://doi.org/10.1088/1475-7516/2015/03/042

© 2015 IOP Publishing Ltd and Sissa Medialab srl. 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... Read More about Probing dark energy with atom interferometry.

Cosmological tests of coupled Galileons (2015)
Journal Article
Brax, P., Burrage, C., Davis, A., & Gubitosi, G. (2015). Cosmological tests of coupled Galileons. Journal of Cosmology and Astroparticle Physics, 15(3), Article 028. https://doi.org/10.1088/1475-7516/2015/03/028

We investigate the cosmological properties of Galileon models which admit Minkowski space as a stable solution in vacuum. This is motivated by stable, positive tension brane world constructions that give rise to Galileons. We include both conformal a... Read More about Cosmological tests of coupled Galileons.

Explaining the proton radius puzzle with disformal scalars (2015)
Journal Article
Brax, P., & Burrage, C. (2015). Explaining the proton radius puzzle with disformal scalars. Physical Review D, D91(4), Article 043515. https://doi.org/10.1103/PhysRevD.91.043515

We analyze 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... Read More about Explaining the proton radius puzzle with disformal scalars.

Constraining disformally coupled scalar fields (2014)
Journal Article
Brax, P., & Burrage, C. (2014). Constraining disformally coupled scalar fields. Physical Review D, D90(10), Article 104009. https://doi.org/10.1103/PhysRevD.90.104009

Light scalar fields can naturally couple disformally to matter fields. Static, nonrelativistic 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 f... Read More about Constraining disformally coupled scalar fields.

Screening of scalar fields in Dirac-Born-Infeld theory (2014)
Journal Article
Burrage, C., & Khoury, J. (2014). Screening of scalar fields in Dirac-Born-Infeld theory. Physical Review D, D90(2), Article 024001. https://doi.org/10.1103/PhysRevD.90.024001

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, nonlinearities in the scalar domina... Read More about Screening of scalar fields in Dirac-Born-Infeld theory.

Chameleons in the early universe: kicks, rebounds, and particle production (2014)
Journal Article
Erickcek, A. L., Barnaby, N., Burrage, C., & Huang, Z. (2014). Chameleons in the early universe: kicks, rebounds, and particle production. Physical Review D, D89(8), Article 084074. https://doi.org/10.1103/PhysRevD.89.084074

Chameleon gravity is a scalar-tensor theory that includes a nonminimal 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... Read More about Chameleons in the early universe: kicks, rebounds, and particle production.