Showing papers by "Jeremy Sakstein published in 2016"
University of Oslo1, Heidelberg University2, University of Geneva3, University of Oxford4, University of Barcelona5, Aix-Marseille University6, National Institute of Geophysics and Volcanology7, University of Manchester8, RWTH Aachen University9, King's College London10, Stockholm University11, Imperial College London12, ETH Zurich13, Leiden University14, Durham University15, University of Portsmouth16, Lund University17, Case Western Reserve University18, Universidade Estadual de Londrina19, Federal University of Rio de Janeiro20, University of Stavanger21, University of Szczecin22, University of Cambridge23, University of Pennsylvania24, University of Paris25, University of Lisbon26, Radcliffe Institute for Advanced Study27, Institut d'Astrophysique de Paris28
TL;DR: There is a persistent interest in extending cosmology beyond the standard model, ΛCDM, motivated by a range of apparently serious theoretical issues, involving such questions as the cosmological constant problem, the particle nature of dark matter, the validity of general relativity on large scales, the existence of anomalies in the CMB and on small scales, and the predictivity and testability of the inflationary paradigm as mentioned in this paper.
378 citations
TL;DR: 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 as mentioned in this paper.
Abstract: 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 candidate and also arises in f(R) theories of gravity. Whilst the chameleon is designed to avoid historical searches for fifth-forces it is not unobservable and much effort has gone into identifying the best observables and experiments to detect it. These results are not always presented for the same models or in the same language, a particular problem when comparing astrophysical and laboratory searches making it difficult to understand what regions of parameter space remain. Here we present combined constraints on the chameleon model from astrophysical and laboratory searches for the first time and identify the remaining windows of parameter space. We discuss the implications for cosmological chameleon searches and future small-scale probes.
105 citations
TL;DR: In this paper, the authors study relativistic stars in beyond Horndeski scalar-tensor theories that exhibit a breaking of the Vainshtein mechanism inside matter.
Abstract: This work studies relativistic stars in beyond Horndeski scalar–tensor theories that exhibit a breaking of the Vainshtein mechanism inside matter, focusing on a model based on the quartic beyond Horndeski Lagrangian. We self-consistently derive the scalar field profile for static spherically symmetric objects in asymptotically de Sitter space–time and show that the Vainshtein breaking branch of the solutions is the physical branch thereby resolving several ambiguities with non-relativistic frameworks. The geometry outside the star is shown to be exactly Schwarzschild-de Sitter and therefore the parameterised post-Newtonian parameter ${\beta }_{{\rm{PPN}}}=1$, confirming that the external screening works at the post-Newtonian level. The Tolman–Oppenheimer–Volkoff (TOV) equations are derived and a new lower bound on the Vainshtein breaking parameter ${{\rm{\Upsilon }}}_{1}\gt -4/9$ is found by requiring the existence of static spherically symmetric stars. Focusing on the unconstrained case where ${{\rm{\Upsilon }}}_{1}\lt 0$, we numerically solve the TOV equations for polytropic and realistic equations of state and find stars with larger radii at fixed mass. Furthermore, the maximum mass can increase dramatically and stars with masses in excess of $3{M}_{\odot }$ can be found for relatively small values of the Vainshtein breaking parameter. We re-examine white dwarf stars and show that post-Newtonian corrections are important in beyond Horndeski theories and therefore the bounds coming from previous analyses should be revisited.
103 citations
TL;DR: In this article, a multi-parameter Markov chain Monte Carlo analysis was used to fit the X-ray and lensing profiles of galaxy clusters and obtain new constraints on the parameters governing deviations from Newtons law ϒ1 = -0.11+0.93-0.67 and light bending ϒ2 = −0.22+1.22-1.19.
Abstract: The Beyond Horndeski class of alternative gravity theories allow for Self-accelerating de-Sitter cosmologies with no need for a cosmological constant. This makes them viable alternatives to LambdaCDM and so testing their small-scale predictions against General Relativity is of paramount importance. These theories generically predict deviations in both the Newtonian force law and the gravitational lensing of light inside extended objects. Therefore, by simultaneously fitting the X-ray and lensing profiles of galaxy clusters new constraints can be obtained. In this work, we apply this methodology to the stacked profiles of 58 high-redshift (0.1< z <1.2) clusters using X-ray surface brightness profiles from the XMM Cluster Survey and weak lensing profiles from CFHTLenS. By performing a multi-parameter Markov chain Monte Carlo analysis, we are able to place new constraints on the parameters governing deviations from Newtons law ϒ1 = -0.11+0.93-0.67 and light bending Υ2 = -0.22+1.22-1.19. Both constraints are consistent with General Relativity, for which Υ1 = Υ2 = 0. We present here the first observational constraints on ϒ2, as well as the first extragalactic measurement of both parameters.
82 citations
TL;DR: In this paper, the authors study relativistic stars in beyond Horndeski scalar-tensor theories that exhibit a breaking of the Vainshtein mechanism inside matter.
Abstract: This work studies relativistic stars in beyond Horndeski scalar-tensor theories that exhibit a breaking of the Vainshtein mechanism inside matter, focusing on a model based on the quartic beyond Horndeski Lagrangian. We self-consistently derive the scalar field profile for static spherically symmetric objects in asymptotically de Sitter space-time and show that the Vainshtein breaking branch of the solutions is the physical branch thereby resolving several ambiguities with non-relativistic frameworks. The geometry outside the star is shown to be exactly Schwarzschild-de Sitter and therefore the PPN parameter $\beta_{\rm PPN}=1$, confirming that the external screening works at the post-Newtonian level. The Tolman-Oppenheimer-Volkoff (TOV) equations are derived and a new lower bound on the Vainshtein breaking parameter $\Upsilon_1>-4/9$ is found by requiring the existence of static spherically symmetric stars. Focusing on the unconstrained case where $\Upsilon_1<0$, we numerically solve the TOV equations for polytropic and realistic equations of state and find stars with larger radii at fixed mass. Furthermore, the maximum mass can increase dramatically and stars with masses in excess of $3M_\odot$ can be found for relatively small values of the Vainshtein breaking parameter. We re-examine white dwarf stars and show that post-Newtonian corrections are important in beyond Horndeski theories and therefore the bounds coming from previous analyses should be revisited.
76 citations
TL;DR: In this article, the authors study the pulsations of stars by deriving and solving the wave equation governing linear adiabatic oscillations to find the modified period of pulsation, and perform a preliminary survey of the stellar zoo in an attempt to identify the best candidate objects for testing the theory.
Abstract: Theories of gravity in the beyond Horndeski class recover the predictions of general relativity in the solar system whilst admitting novel cosmologies, including late-time de Sitter solutions in the absence of a cosmological constant. Deviations from Newton's law are predicted inside astrophysical bodies, which allow for falsifiable, smoking-gun tests of the theory. In this work we study the pulsations of stars by deriving and solving the wave equation governing linear adiabatic oscillations to find the modified period of pulsation. Using both semi-analytic and numerical models, we perform a preliminary survey of the stellar zoo in an attempt to identify the best candidate objects for testing the theory. Brown dwarfs and Cepheid stars are found to be particularly sensitive objects and we discuss the possibility of using both to test the theory.
20 citations
TL;DR: In this paper, a multi-parameter Markov chain Monte Carlo analysis was performed on the stacked profiles of 58 high-redshift galaxy clusters using X-ray surface brightness profiles from the XMM Cluster Survey and weak lensing profiles from CFHTLenS.
Abstract: The Beyond Horndeski class of alternative gravity theories allow for Self-accelerating de-Sitter cosmologies with no need for a cosmological constant. This makes them viable alternatives to $\Lambda$CDM and so testing their small-scale predictions against General Relativity is of paramount importance. These theories generically predict deviations in both the Newtonian force law and the gravitational lensing of light inside extended objects. Therefore, by simultaneously fitting the X-ray and lensing profiles of galaxy clusters new constraints can be obtained. In this work, we apply this methodology to the stacked profiles of 58 high-redshift ($ 0.1
19 citations
TL;DR: 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 as mentioned in this paper.
Abstract: 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 candidate and also arises in $f(R)$ theories of gravity. Whilst the chameleon is designed to avoid historical searches for fifth-forces it is not unobservable and much effort has gone into identifying the best observables and experiments to detect it. These results are not always presented for the same models or in the same language, a particular problem when comparing astrophysical and laboratory searches making it difficult to understand what regions of parameter space remain. Here we present combined constraints on the chameleon model from astrophysical and laboratory searches for the first time and identify the remaining windows of parameter space. We discuss the implications for cosmological chameleon searches and future small-scale probes.
19 citations