The Experts below are selected from a list of 11643 Experts worldwide ranked by ideXlab platform
L Shao - One of the best experts on this subject based on the ideXlab platform.
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reduced order surrogate models for scalar tensor gravity in the strong field regime and applications to Binary Pulsars and gw170817
Physical Review D, 2019Co-Authors: Junjie Zhao, L Shao, Zhoujian CaoAbstract:We investigate the scalar-tensor gravity of Damour and Esposito-Far\`ese (DEF), which predicts nontrivial phenomena in the nonperturbative strong-field regime for neutron stars (NSs). Instead of solving the modified Tolman-Oppenheimer-Volkoff equations, we construct reduced-order surrogate models, coded in the pystgrom package, to predict the relations of a NS radius, mass, and effective scalar coupling to its central density. Our models are accurate at $\ensuremath{\sim}1%$ level and speed up large-scale calculations by 2 orders of magnitude. As an application, we use pystgrom and Markov-chain Monte Carlo techniques to constrain parameters in the DEF theory, with five well-timed Binary Pulsars, the Binary NS (BNS) inspiral GW170817, and a hypothetical BNS inspiral in the Advanced LIGO and next-generation GW detectors. In the future, as more Binary Pulsars and BNS mergers are detected, our surrogate models will be helpful in constraining strong-field gravity with essential speed and accuracy.
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new tests of local lorentz invariance of gravity with small eccentricity Binary Pulsars
Classical and Quantum Gravity, 2012Co-Authors: L Shao, N WexAbstract:Some alternative gravity theories allow the Universal matter distribution to single out the existence of a preferred frame, which breaks the symmetry of local Lorentz invariance (LLI) for the gravitational interaction. In the post-Newtonian parametrization of semi-conservative gravity theories, LLI violation is characterized by two parameters, ?1 and ?2. In Binary Pulsars, the isotropic violation of Lorentz invariance in the gravitational sector should lead to characteristic preferred frame effects (PFEs) in the orbital dynamics, if the barycenter of the Binary is moving relative to the preferred frame with a velocity w. For small-eccentricity binaries, the effects induced by and (the hat indicates possible modifications by strong-field effects) decouple, and can therefore be tested independently. We use recent timing results of two compact pulsar-white dwarf binaries with known three-dimensional velocity, PSRs J1012+5307 and J1738+0333 to constrain PFEs for strongly self-gravitating bodies, by assuming the isotropic cosmic microwave background to single out a preferred frame. The time derivative of the projected semi-major axis is used to constrain a precession of the orbital plane around w due to PFEs. From this, we derive a limit at 95% confidence level, which is the most constraining limit for strongly self-gravitating systems up to now, however, still three orders of magnitude weaker than the best Solar system limit for the corresponding weak-field parameter ?2. Concerning , we propose a new, robust method to constrain this parameter, which avoids the probabilistic considerations inherent in previous methods. This method is based on the fact that a PFE-induced intrinsic eccentricity cannot stay unobserved during a long-term observation due to the significant precession of periastron in Binary pulsar with short orbital periods. Our most conservative result, at 95% confidence level from PSR J1738+0333, constitutes a significant improvement compared to current most stringent limits obtained both in the Solar system and Binary pulsar tests. We also derive corresponding limits for and for a preferred frame that is at rest with respect to our Galaxy, and preferred frames that locally co-move with the rotation of our Galaxy. These limits will continue to improve significantly with future pulsar timing observations conducted at large radio telescopes. Communicated by C M Will
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new tests of local lorentz invariance of gravity with small eccentricity Binary Pulsars
arXiv: General Relativity and Quantum Cosmology, 2012Co-Authors: L Shao, N WexAbstract:In the post-Newtonian parametrization of semi-conservative gravity theories, local Lorentz invariance (LLI) violation is characterized by two parameters, alpha_1 and alpha_2. In Binary Pulsars the isotropic violation of LLI in the gravitational sector leads to characteristic preferred frame effects (PFEs) in the orbital dynamics, if the barycenter of the Binary is moving relative to the preferred frame with a velocity w. For small-eccentricity binaries, the effects induced by alpha_1 and alpha_2 decouple, and can therefore be tested independently. We use recent timing results of two compact pulsar-white dwarf binaries with known 3D velocity, PSRs J1012+5307 and J1738+0333, to constrain PFEs for strongly self-gravitating bodies. We derive a limit |alpha_2| < 1.8e-4 (95% CL), which is the most constraining limit for strongly self-gravitating systems up to now. Concerning alpha_1, we propose a new, robust method to constrain this parameter. Our most conservative result, alpha_1 = -0.4^{+3.7}_{-3.1} e-5 (95% CL) from PSR J1738+0333, constitutes a significant improvement compared to current most stringent limits obtained both in Solar system and Binary pulsar tests. We also derive corresponding limits for alpha_1 and alpha_2 for a preferred frame that is at rest with respect to our Galaxy, and preferred frames that locally co-move with the rotation of our Galaxy. (Abridged)
Thibault Damour - One of the best experts on this subject based on the ideXlab platform.
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1974: the discovery of the first Binary pulsar
Classical and Quantum Gravity, 2015Co-Authors: Thibault DamourAbstract:The 1974 discovery, by Russell A. Hulse and Joseph H. Taylor, of the first Binary pulsar PSR B1913+16, opened up new possibilities for the study of relativistic gravity. PSR B1913+16, as well as several other Binary Pulsars, provided {\it direct} observational proofs that gravity propagates at the velocity of light and has a quadrupolar structure. Binary Pulsars also provided accurate tests of the strong-field regime of relativistic gravity. General Relativity has passed all the Binary pulsar tests with flying colors. The discovery of Binary Pulsars had also very important consequences for astrophysics: accurate measurement of neutron star masses, improved understanding of the possible evolution scenarios for the co-evolution of Binary stars, proof of the existence of Binary neutron stars emitting gravitational waves for hundreds of millions of years, before coalescing in catastrophic events radiating intense gravitational-wave signals, and probably leading also to important emissions of electromagnetic radiation and neutrinos. This article reviews the history of the discovery of the first Binary pulsar, and describes both its immediate impact, and its longer-term effect on theoretical and experimental studies of relativistic gravity.
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Binary Pulsars as probes of relativistic gravity
Philosophical Transactions of the Royal Society of London. Series A: Physical and Engineering Sciences, 1992Co-Authors: Thibault DamourAbstract:Until now, most experiments have succeeded in testing relativistic gravity only in its extreme weak-field limit. Because of the strong self-gravity of neutron stars, observations of Pulsars in Binary systems provide a unique opportunity for probing the strong-field regime of relativistic gravity. The two basic approaches to using Binary pulsar measurements as probes of relativistic gravity are reviewed: the phenomenological (‘parametrized post-keplerian’ formalism) and the alternative-theory approach (multidimensional space of possible theories). The experimental constraints recently derived from the actual timing observations of three Binary Pulsars are summarized. General relativity passes these new, strong-field tests with complete success.
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Strong-field tests of relativistic gravity and Binary Pulsars.
Physical review. D Particles and fields, 1992Co-Authors: Thibault Damour, Joseph H. TaylorAbstract:Observations of Pulsars in gravitationally bound Binary systems provide a unique opportunity for testing the strong-field regime of relativistic gravity. We present a detailed account of the parametrized post-Keplerian'' (PPK) formalism, a general phenomenological framework designed to extract the maximum possible information from pulsar timing and pulse-structure data. The PPK approach allows dynamical information to be obtained from the data in a theory-independent way, and encoded in a certain number of fitted post-Keplerian parameters. We show that as many as 19 such parameters can be measured, under favorable conditions, giving access to 15 possible tests of relativistic gravity. We isolate and quantify the theoretical content of these tests by deriving, within the framework of generic boost-invariant theories, expressions linking the phenomenological parameters to the inertial masses of the pulsar and its companion, and to the polar angles of the spin axis of the pulsar. The prospects for extracting some of these tests from observations of known or yet-to-be-discovered Binary Pulsars is quantitatively assessed through numerical simulations. We show that the recently discovered Binary pulsar PSR 1534+12 should, with presently available data, give access to two new strong-field tests of relativistic gravity, if the data are analyzed in the phenomenological waymore » emphasized in this paper. Moreover, in the long run, the first-disovered Binary pulsar, PSR 1913+16, could give access to three strong-field tests, beyond the presently obtained {dot {omega}}{minus}{gamma}{minus}{ital {dot P}}{sub {ital b}} test. Finally, we show how, by combining the PPK approach with the predictions of a rather generic class of tensor biscalar theories, one can bring together tests based on observations of several different Pulsars.« less
D. Blas - One of the best experts on this subject based on the ideXlab platform.
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Binary Pulsars as probes of a Galactic dark matter disk
Physics of the Dark Universe, 2018Co-Authors: Andrea Caputo, D. Blas, Jesús ZavalaAbstract:Abstract As a Binary pulsar moves through a wind of dark matter particles, the resulting dynamical friction modifies the Binary’s orbit. We study this effect for the double disk dark matter (DDDM) scenario, where a fraction of the dark matter is dissipative and settles into a thin disk. For binaries within the dark disk, this effect is enhanced due to the higher dark matter density and lower velocity dispersion of the dark disk, and due to its co-rotation with the baryonic disk. We estimate the effect and compare it with observations for two different limits in the Knudsen number ( K n ). First, in the case where DDDM is effectively collisionless within the characteristic scale of the Binary ( K n ≫ 1 ) and ignoring the possible interaction between the pair of dark matter wakes. Second, in the fully collisional case ( K n ≪ 1 ), where a fluid description can be adopted and the interaction of the pair of wakes is taken into account. We find that the change in the orbital period is of the same order of magnitude in both limits. A comparison with observations reveals good prospects to probe currently allowed DDDM models with timing data from Binary Pulsars in the near future. We finally comment on the possibility of extending the analysis to the intermediate (rarefied gas) case with K n ∼ 1 .
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ultralight dark matter resonates with Binary Pulsars
Physical Review Letters, 2017Co-Authors: D. Blas, Diana Lopez Nacir, Sergey SibiryakovAbstract:We consider the scenario where dark matter (DM) is represented by an ultralight classical scalar field performing coherent periodic oscillations. We point out that such DM perturbs the dynamics of Binary systems either through its gravitational field or via direct coupling to ordinary matter. This perturbation gets resonantly amplified if the frequency of DM oscillations is close to a (half-)integer multiple of the orbital frequency of the system and leads to a secular variation of the orbital period. We suggest using Binary Pulsars as probes of this scenario and estimate their sensitivity. While the current accuracy of observations is not yet sufficient to probe the purely gravitational effect of DM, it already yields constraints on direct coupling that are competitive with other bounds. The sensitivity will increase with the upcoming radio observatories such as the Square Kilometer Array.
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Modified gravity and Binary Pulsars: the Lorentz violating case
arXiv: General Relativity and Quantum Cosmology, 2016Co-Authors: D. BlasAbstract:The dynamics of Binary Pulsars can be used to test different aspects of gravitation. This is particularly important to constrain alternatives to general relativity in regimes which are not probed by other methods. In this short contribution, I will describe the case of theories of gravity without Lorentz invariance. The latter are important in the context of quantum gravity and modify the laws of gravity at basically all scales.
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gravity and Binary Pulsars: the Lorentz violating case
2016Co-Authors: D. BlasAbstract:The dynamics of Binary Pulsars can be used to test dierent aspects of gravitation. This is particularly important to constrain alternatives to general relativity in regimes which are not probed by other methods. In this short contribution, I will describe the case of theories of gravity without Lorentz invariance. The latter are important in the context of quantum gravity and modify the laws of gravity at basically all scales.
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Constraints on Einstein-Æther theory and Hořava gravity from Binary pulsar observations
Physical Review D, 2014Co-Authors: Kent Yagi, D. Blas, Enrico Barausse, Nicolás YunesAbstract:Binary Pulsars are ideal to test the foundations of general relativity, such as Lorentz symmetry, which requires that experiments produce the same results in all free-falling (i.e. inertial) frames. We here break this symmetry in the gravitational sector by specifying a preferred time direction, and thus a preferred frame, at each spacetime point. We then examine the consequences of this gravitational Lorentz symmetry breaking in the orbital evolution of Binary Pulsars, focusing on the dissipative effects. We find that Lorentz symmetry breaking modifies these effects, and thus the orbital dynamics, in two different ways. First, it generically causes the emission of dipolar radiation, which makes the orbital separation decrease faster than in general relativity. Second, the quadrupole component of the emission is also modified. The orbital evolution depends critically on the sensitivities of the stars, which measure how their binding energies depend on the motion relative to the preferred frame. We calculate the sensitivities numerically and compute the predicted orbital decay rate of Binary Pulsars in Lorentz-violating gravity. By testing these predictions against observations, we place very stringent constraints on gravitational Lorentz violation.
N Wex - One of the best experts on this subject based on the ideXlab platform.
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tests of conservation laws in post newtonian gravity with Binary Pulsars
The Astrophysical Journal, 2020Co-Authors: Xueli Miao, L Shao, N Wex, Junjie Zhao, M KramerAbstract:General relativity is a fully conservative theory, but there exist other possible metric theories of gravity. We consider nonconservative ones with a parameterized post-Newtonian parameter, ζ 2. A nonzero ζ 2 induces a self-acceleration for the center of mass of an eccentric Binary pulsar system, which contributes to the second time derivative of the pulsar spin frequency, . In our work, using Will’s method, we provide an improved analysis with four well-timed, carefully chosen Binary Pulsars. In addition, we extend Will’s method and derive the effect of on the third time derivative of the spin frequency, . For PSR B1913+16, the constraint from is even tighter than that from . We combine multiple Pulsars with Bayesian inference, and obtain the upper limit, at the 95% confidence level, assuming a flat prior in . It improves the existing bound by a factor of 3. Moreover, we propose an analytical timing formalism for . Our simulated times of arrival with simplified assumptions show the capability of Binary Pulsars in limiting ζ 2, and useful clues are extracted for real-data analysis in future. In particular, we discover that for PSRs B1913+16 and J0737−3039A, can yield more constraining limits than .
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new graviton mass bound from Binary Pulsars
Physical Review D, 2020Co-Authors: L Shao, N Wex, Shuangyong ZhouAbstract:In Einstein's general relativity, gravity is mediated by a massless metric field. The extension of general relativity to consistently include a mass for the graviton has profound implications for gravitation and cosmology. Salient features of various massive gravity theories can be captured by Galileon models, the simplest of which is the cubic Galileon. The presence of the Galileon field leads to additional gravitational radiation in Binary Pulsars where the Vainshtein mechanism is less suppressed than its fifth-force counterpart, which deserves a detailed confrontation with observations. We prudently choose 14 well-timed Binary Pulsars, and from their intrinsic orbital decay rates we put a new bound on the graviton mass, ${m}_{g}\ensuremath{\lesssim}2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}28}\text{ }\text{ }\mathrm{eV}/{c}^{2}$ at the 95% confidence level, assuming a flat prior on $\mathrm{ln}{m}_{g}$. It is equivalent to a bound on the graviton Compton wavelength ${\ensuremath{\lambda}}_{g}\ensuremath{\gtrsim}7\ifmmode\times\else\texttimes\fi{}{10}^{21}\text{ }\text{ }\mathrm{m}$. Furthermore, we extensively simulate times of arrival for Pulsars in orbit around stellar-mass black holes and the supermassive black hole at the Galactic Center, and we investigate their prospects of probing the cubic Galileon theory in the near future.
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tests of conservation laws in post newtonian gravity with Binary Pulsars
arXiv: General Relativity and Quantum Cosmology, 2020Co-Authors: Xueli Miao, L Shao, N Wex, Junjie Zhao, M KramerAbstract:General relativity is a fully conservative theory, but there exist other possible metric theories of gravity. We consider non-conservative ones with a parameterized post-Newtonian (PPN) parameter, $\zeta_2$. A non-zero $\zeta_2$ induces a self-acceleration for the center of mass of an eccentric Binary pulsar system, which contributes to the second time derivative of the pulsar spin frequency, $\ddot{\nu}$. In our work, using the method in Will (1992), we provide an improved analysis with four well-timed, carefully-chosen Binary Pulsars. In addition, we extend Will's method and derive $\zeta_2$'s effect on the third time derivative of the spin frequency, $\dddot{\nu}$. For PSR B1913+16, the constraint from $\dddot{\nu}$ is even tighter than that from $\ddot{\nu}$. We combine multiple Pulsars with Bayesian inference, and obtain an upper limit, $\left|\zeta_{2}\right|<1.3\times10^{-5}$ at 95% confidence level, assuming a flat prior in $\log_{10} \left| \zeta_{2}\right|$. It improves the existing bound by a factor of three. Moreover, we propose an analytical timing formalism for $\zeta_2$. Our simulated times of arrival with simplified assumptions show Binary Pulsars' capability in limiting $\zeta_{2}$, and useful clues are extracted for real data analysis in future. In particular, we discover that for PSRs B1913+16 and J0737$-$3039A, $\dddot{\nu}$ can yield more constraining limits than $\ddot{\nu}$.
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new tests of local lorentz invariance of gravity with small eccentricity Binary Pulsars
Classical and Quantum Gravity, 2012Co-Authors: L Shao, N WexAbstract:Some alternative gravity theories allow the Universal matter distribution to single out the existence of a preferred frame, which breaks the symmetry of local Lorentz invariance (LLI) for the gravitational interaction. In the post-Newtonian parametrization of semi-conservative gravity theories, LLI violation is characterized by two parameters, ?1 and ?2. In Binary Pulsars, the isotropic violation of Lorentz invariance in the gravitational sector should lead to characteristic preferred frame effects (PFEs) in the orbital dynamics, if the barycenter of the Binary is moving relative to the preferred frame with a velocity w. For small-eccentricity binaries, the effects induced by and (the hat indicates possible modifications by strong-field effects) decouple, and can therefore be tested independently. We use recent timing results of two compact pulsar-white dwarf binaries with known three-dimensional velocity, PSRs J1012+5307 and J1738+0333 to constrain PFEs for strongly self-gravitating bodies, by assuming the isotropic cosmic microwave background to single out a preferred frame. The time derivative of the projected semi-major axis is used to constrain a precession of the orbital plane around w due to PFEs. From this, we derive a limit at 95% confidence level, which is the most constraining limit for strongly self-gravitating systems up to now, however, still three orders of magnitude weaker than the best Solar system limit for the corresponding weak-field parameter ?2. Concerning , we propose a new, robust method to constrain this parameter, which avoids the probabilistic considerations inherent in previous methods. This method is based on the fact that a PFE-induced intrinsic eccentricity cannot stay unobserved during a long-term observation due to the significant precession of periastron in Binary pulsar with short orbital periods. Our most conservative result, at 95% confidence level from PSR J1738+0333, constitutes a significant improvement compared to current most stringent limits obtained both in the Solar system and Binary pulsar tests. We also derive corresponding limits for and for a preferred frame that is at rest with respect to our Galaxy, and preferred frames that locally co-move with the rotation of our Galaxy. These limits will continue to improve significantly with future pulsar timing observations conducted at large radio telescopes. Communicated by C M Will
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new tests of local lorentz invariance of gravity with small eccentricity Binary Pulsars
arXiv: General Relativity and Quantum Cosmology, 2012Co-Authors: L Shao, N WexAbstract:In the post-Newtonian parametrization of semi-conservative gravity theories, local Lorentz invariance (LLI) violation is characterized by two parameters, alpha_1 and alpha_2. In Binary Pulsars the isotropic violation of LLI in the gravitational sector leads to characteristic preferred frame effects (PFEs) in the orbital dynamics, if the barycenter of the Binary is moving relative to the preferred frame with a velocity w. For small-eccentricity binaries, the effects induced by alpha_1 and alpha_2 decouple, and can therefore be tested independently. We use recent timing results of two compact pulsar-white dwarf binaries with known 3D velocity, PSRs J1012+5307 and J1738+0333, to constrain PFEs for strongly self-gravitating bodies. We derive a limit |alpha_2| < 1.8e-4 (95% CL), which is the most constraining limit for strongly self-gravitating systems up to now. Concerning alpha_1, we propose a new, robust method to constrain this parameter. Our most conservative result, alpha_1 = -0.4^{+3.7}_{-3.1} e-5 (95% CL) from PSR J1738+0333, constitutes a significant improvement compared to current most stringent limits obtained both in Solar system and Binary pulsar tests. We also derive corresponding limits for alpha_1 and alpha_2 for a preferred frame that is at rest with respect to our Galaxy, and preferred frames that locally co-move with the rotation of our Galaxy. (Abridged)
Nicolás Yunes - One of the best experts on this subject based on the ideXlab platform.
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Binary pulsar constraints on massless scalar tensor theories using bayesian statistics
Classical and Quantum Gravity, 2019Co-Authors: David P Anderson, P.c.c. Freire, Nicolás YunesAbstract:Binary Pulsars provide some of the tightest current constraints on modified theories of gravity and these constraints will only get tighter as radio astronomers continue timing these systems. These Binary Pulsars are particularly good at constraining scalar-tensor theories in which gravity is mediated by a scalar field in addition to the metric tensor. Scalar-tensor theories can predict large deviations from General Relativity due to the fact that they allow for violation of the strong-equivalence principle through a phenomenon known as scalarization. This effect appears directly in the timing model for Binary Pulsars, and as such, it can be tightly constrained through precise timing. In this paper, we investigate these constraints for two scalar-tensor theories and a large set of realistic equations of state. We calculate the constraints that can be placed by saturating the current $1\sigma$ bounds on single post-Keplerian parameters, as well as employing Bayesian methods through Markov-Chain-Monte-Carlo simulations to explore the constraints that can be achieved when one considers all measured parameters simultaneously. Our results demonstrate that both methods are able to place similar constraints and that they are both indeed dominated by the measurements of the orbital period decay. The Bayesian approach, however, allows one to simultaneously explore the posterior distributions of not only the theory parameters but of the masses as well.
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Binary pulsar constraints on massless scalar─tensor theories using Bayesian statistics
Classical and Quantum Gravity, 2019Co-Authors: David P Anderson, Paulo C. C. Freire, Nicolás YunesAbstract:Binary Pulsars provide some of the tightest current constraints on modified theories of gravity and these constraints will only get tighter as radio astronomers continue timing these systems. These Binary Pulsars are particularly good at constraining scalar-tensor theories in which gravity is mediated by a scalar field in addition to the metric tensor. Scalar-tensor theories can predict large deviations from General Relativity due to the fact that they allow for violation of the strong-equivalence principle through a phenomenon known as scalarization. This effect appears directly in the timing model for Binary Pulsars, and as such, it can be tightly constrained through precise timing. In this paper, we investigate these constraints for two scalar-tensor theories and a large set of realistic equations of state. We calculate the constraints that can be placed by saturating the current $1\sigma$ bounds on single post-Keplerian parameters, as well as employing Bayesian methods through Markov-Chain-Monte-Carlo simulations to explore the constraints that can be achieved when one considers all measured parameters simultaneously. Our results demonstrate that both methods are able to place similar constraints and that they are both indeed dominated by the measurements of the orbital period decay. The Bayesian approach, however, allows one to simultaneously explore the posterior distributions of not only the theory parameters but of the masses as well.
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Constraints on Einstein-Æther theory and Hořava gravity from Binary pulsar observations
Physical Review D, 2014Co-Authors: Kent Yagi, D. Blas, Enrico Barausse, Nicolás YunesAbstract:Binary Pulsars are ideal to test the foundations of general relativity, such as Lorentz symmetry, which requires that experiments produce the same results in all free-falling (i.e. inertial) frames. We here break this symmetry in the gravitational sector by specifying a preferred time direction, and thus a preferred frame, at each spacetime point. We then examine the consequences of this gravitational Lorentz symmetry breaking in the orbital evolution of Binary Pulsars, focusing on the dissipative effects. We find that Lorentz symmetry breaking modifies these effects, and thus the orbital dynamics, in two different ways. First, it generically causes the emission of dipolar radiation, which makes the orbital separation decrease faster than in general relativity. Second, the quadrupole component of the emission is also modified. The orbital evolution depends critically on the sensitivities of the stars, which measure how their binding energies depend on the motion relative to the preferred frame. We calculate the sensitivities numerically and compute the predicted orbital decay rate of Binary Pulsars in Lorentz-violating gravity. By testing these predictions against observations, we place very stringent constraints on gravitational Lorentz violation.
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Strong Binary Pulsar Constraints on Lorentz Violation in Gravity
Physical review letters, 2014Co-Authors: Kent Yagi, D. Blas, Nicolás Yunes, Enrico BarausseAbstract:Binary Pulsars are excellent laboratories to test the building blocks of Einstein's theory of general relativity. One of these is Lorentz symmetry, which states that physical phenomena appear the same for all inertially moving observers. We study the effect of violations of Lorentz symmetry in the orbital evolution of Binary Pulsars and find that it induces a much more rapid decay of the Binary's orbital period due to the emission of dipolar radiation. The absence of such behavior in recent observations allows us to place the most stringent constraints on Lorentz violation in gravity, thus verifying one of the cornerstones of Einstein's theory much more accurately than any previous gravitational observation.