1,968 research outputs found
Extending Sibgatullin's ansatz for the Ernst potential to generate a richer family of axially symmetric solutions of Einstein's equations
The scope of this talk is to present some preliminary results on an effort,
currently in progress, to generate an exact solution of Einstein's equation,
suitable for describing spacetime around a rotating compact object.
Specifically, the form of the Ernst potential on the symmetry axis and its
connection with the multipole moments is discussed thoroughly. The way to
calculate the multipole moments of spacetime directly from the value of the
Ernst potential on the symmetry axis is presented. Finally, a mixed ansatz is
formed for the Ernst potential including parameters additional to the ones
dictated by Sibgatullin. Thus, we believe that this talk can also serve as a
comment on choosing the appropriate ansatz for the Ernst potential.Comment: Talk given in the 11th Conference on Recent Developments in Gravity,
2-5 June 2004, Lesbos, Greec
Curvature singularities, tidal forces and the viability of Palatini f(R) gravity
In a previous paper we showed that static spherically symmetric objects
which, in the vicinity of their surface, are well-described by a polytropic
equation of state with 3/2<Gamma<2 exhibit a curvature singularity in Palatini
f(R) gravity. We argued that this casts serious doubt on the validity of
Palatini f(R) gravity as a viable alternative to General Relativity. In the
present paper we further investigate this characteristic of Palatini f(R)
gravity in order to clarify its physical interpretation and consequences.Comment: 15 pages. CQG in press. Part of the material moved to an appendix,
discussion on the meV scale predictions of Palatini f(R) gravity adde
A comment on "The Cauchy problem of f(R)- gravity", Class. Quantum Grav., 24, 5667 (2007), arXiv:0709.4414
A critical comment on [N. Lanahan--Tremblay and V. Faraoni, 2007, {\it Class.
Quantum Grav.}, {\bf 24}, 5667, arXiv:0709.4414] is given discussing the
well-formulation of the Chauchy problem for -gravity in metric-affine
theories.Comment: 3 page
On the Viability of a Non-Analytical f(R)-Theory
In this paper, we show how a power-law correction to the Einstein-Hilbert
action provides a viable modified theory of gravity, passing the Solar-System
tests, when the exponent is between the values 2 and 3. Then, we implement this
paradigm on a cosmological setting outlining how the main phases of the
Universe thermal history are properly reproduced. As a result, we find two
distinct constraints on the characteristic length scale of the model, i.e., a
lower bound from the Solar-System test and an upper one by guaranteeing the
matter dominated Universe evolution.Comment: 9 pages, 2 figure
Open Schooling Matters: Student Effects in Science Motivation, Intrinsic Motivation and State Emotions
Open-schooling as an innovative management construct combines formal and informal stakeholders for joining forces for supporting schools. By highlighting this complex frame from a European vantage point, an experimental study of 316 open school environments with 1642 students was completed by applying the psychometric constructs of science motivation (SMOT), intrinsic motivation (IM) and state emotions (SE). SMOT contained five subscales: intrinsic motivation, career motivation, self-determination, self-efficacy, grade motivation; IM is using four subscales: interest, perceived competence, pressure and perceived choice; and SE contained three subscales: well-being, interest and boredom. In total, gender and selfdetermination produced a difference before participation as girls showed higher scores. This difference vanished later on which points to a positive catch-up for boys. IM scored high in all subscales while pressure and perceived choice competence was more present in boys. The same was true for state emotions of well-being and interest as well as low ones in boredom. In sum, SMOT subscales correlated positively for the state emotions of well-being and interest and negatively with boredom
Reply to `A comment on `The Cauchy problem of f(R) gravity''
We reply to a comment by Capozziello and Vignolo about the Cauchy problem of
Palatini f(R) gravity.Comment: 3 pages, late
Corrections and Comments on the Multipole Moments of Axisymmetric Electrovacuum Spacetimes
Following the method of Hoenselaers and Perj\'{e}s we present a new corrected
and dimensionally consistent set of multipole gravitational and electromagnetic
moments for stationary axisymmetric spacetimes. Furthermore, we use our results
to compute the multipole moments, both gravitational and electromagnetic, of a
Kerr-Newman black hole.Comment: This is a revised and corrected versio
On Modified Gravity
We consider some aspects of nonlocal modified gravity, where nonlocality is
of the type . In particular, using ansatz of the form
we find a few solutions for the spatially flat
FLRW metric. There are singular and nonsingular bounce solutions. For late
cosmic time, scalar curvature R(t) is in low regime and scale factor a(t) is
decelerated. R (t) = 0 satisfies all equations when k = -1.Comment: added references; made some clarifications; 8 page
Covariant conservation of energy momentum in modified gravities
An explicit proof of the vanishing of the covariant divergence of the
energy-momentum tensor in modified theories of gravity is presented. The
gravitational action is written in arbitrary dimensions and allowed to depend
nonlinearly on the curvature scalar and its couplings with a scalar field. Also
the case of a function of the curvature scalar multiplying a matter Lagrangian
is considered. The proof is given both in the metric and in the first-order
formalism, i.e. under the Palatini variational principle. It is found that the
covariant conservation of energy-momentum is built-in to the field equations.
This crucial result, called the generalized Bianchi identity, can also be
deduced directly from the covariance of the extended gravitational action.
Furthermore, we demonstrate that in all of these cases, the freely falling
world lines are determined by the field equations alone and turn out to be the
geodesics associated with the metric compatible connection. The independent
connection in the Palatini formulation of these generalized theories does not
have a similar direct physical interpretation. However, in the conformal
Einstein frame a certain bi-metricity emerges into the structure of these
theories. In the light of our interpretation of the independent connection as
an auxiliary variable we can also reconsider some criticisms of the Palatini
formulation originally raised by Buchdahl.Comment: 8 pages. v2: more discussio
Reply to "Can gravitational dynamics be obtained by diffeomorphism invariance of action?"
In a previous work we showed that, in a suitable setting, one can use
diffeomorphism invariance in order to derive gravitational field equations from
boundary terms of the gravitational action. Standing by our results we reply
here to a recent comment questioning their validity.Comment: Accepted for publication in PR
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