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Gravity
Presents the basic principles of gravity, a history of how the theory was developed, the revisions made by Albert Einstein, and descriptions of how gravity works in space, with examples from America's space program.
Book
The Palatini formalism of the Formula omitted theory of gravity
We present the first formulation of the recently proposed [Formula omitted] theory of gravity within the Palatini formalism, a well-known alternative variational approach where the metric and connection are treated as independent variables. By applying this formalism, we derive a new set of field equations that exhibit, as expected, distinct properties compared to their metric formalism counterparts. We particularly present the Newtonian limit of this formalism, as well as the resulting Friedmann-like equations. We highlight that potential observational signatures may distinguish between the metric and Palatini frameworks. Our results open new pathways for exploring the phenomenology of modified gravity theories and their testability with observational data.
Journal Article
Gravity
\"What keeps objects from floating out of your hand? What if your feet drifted away from the ground? What stops everything from floating into space? Gravity ... Jason Chin has taken a complex subject and made it brilliantly accessible to young readers in this unusual, innovative, and very beautiful book.
Book
Impact of star pressure on Formula omitted in modified gravity beyond post-Newtonian approach
We offer a concrete example exhibiting marked departure from the Parametrized Post-Newtonian (PPN) approximation in a modified theory of gravity. Specifically, we derive the exact formula for the Robertson parameter [Formula omitted] in Brans-Dicke gravity for spherical compact stars, explicitly incorporating the pressure content of the stars. We achieve this by exploiting the integrability of the 00-component of the Brans-Dicke field equation. In place of the conventional PPN result [Formula omitted], we obtain the analytical expression [Formula omitted] where [Formula omitted] is the ratio of the total pressure [Formula omitted] and total energy [Formula omitted] contained within the star. The dimensionless quantity [Formula omitted] participates in [Formula omitted] due to the scalar degree of freedom of Brans-Dicke gravity. Our non-perturbative formula is valid for all field strengths and types of matter comprising the star. In addition, we establish two new mathematical identities linking the active gravitational mass, the ADM (Arnowitt-Deser-Misner) mass, and the Tolman mass, applicable for Brans-Dicke gravity. We draw four key conclusions:(1) The usual [Formula omitted] formula is violated for high-pressure mass sources, such as neutron stars, viz. when [Formula omitted], revealing a limitation of the PPN approximation in Brans-Dicke gravity. (2) The PPN result mainly stems from the assumption of pressureless matter. Even in the weak-field star case, non-zero pressure leads to a violation of the PPN formula for [Formula omitted]. Conversely, the PPN result is a good approximation for low-pressure matter, i.e. when [Formula omitted], for all field strengths. (3) Observational constraints on [Formula omitted] set joint bounds on [Formula omitted] and [Formula omitted], with the latter representing a global characteristic of a mass source. If the equation of state of matter comprising the mass source approaches the ultra-relativistic form, entailing [Formula omitted], [Formula omitted] converges to 1 irrespective of [Formula omitted]. More generally, regardless of [Formula omitted], ultra-relativistic matter tends to suppress the scalar degree of freedom in the exterior vacuum of Brans-Dicke stars, reducing the vacuum to the Schwarzschild solution. (4) In a broader context, by exposing a limitation of the PPN approximation in Brans-Dicke gravity, our findings indicate the significance of considering the interior structure of stars in observational astronomy when testing candidate theories of gravitation that involve additional degrees of freedom besides the metric tensor.
Journal Article
The trouble with gravity : solving the mystery beneath our feet
\"[A] ... science writer traces our millennia-long effort to understand the phenomenon of gravity--the greatest mystery in physics, and a force that has shaped our universe and our minds in ways we have never fully understood until now\"-- Provided by publisher.
Book
Gravity : investigating the force, mass, and attraction of physical bodies
A look at various theories throughout history about gravity.
Book
Rapidly rotating neutron stars in Formula omitted gravity
In this work, we study the influence of f(R, T) gravity on rapidly rotating neutron stars. First we discuss the main aspects of this modified theory of gravity where the gravitational Lagrangian is an arbitrary function of the Ricci scalar R and of the trace of the energy-momentum tensor T. Then we present the basic equations for neutron stars including the equations of state used in the present work to describe the hadronic matter. Some physical quantities of interest are calculated such as mass-radius relations, moments of inertia, angular momentum, and compactness. By considering four different rotation regimes, we obtain results that indicate substantial modifications in the physical properties of neutron stars in f(R, T) gravity when compared to those in the context of general relativity. In particular, the mass-radius relation for sequences of stars indicates that f(R, T) gravity increases the mass and the equatorial radius of the neutron stars for stars rotating with an angular velocity smaller than Kepler limit.
Journal Article
