4,323 research outputs found
Center of Mass and spin for isolated sources of gravitational radiation
We define the center of mass and spin of an isolated system in General
Relativity. The resulting relationships between these variables and the total
linear and angular momentum of the gravitational system are remarkably similar
to their Newtonian counterparts, though only variables at the null boundary of
an asymptotically flat spacetime are used for their definition. We also derive
equations of motion linking their time evolution to the emitted gravitational
radiation. The results are then compared to other approaches. In particular one
obtains unexpected similarities as well as some differences with results
obtained in the Post Newtonian literature . These equations of motion should be
useful when describing the radiation emitted by compact sources such as
coalescing binaries capable of producing gravitational kicks, supernovas, or
scattering of compact objects.Comment: 16 pages. Accepted for publication in Phys. Rev.
Little Rip, CDM and singular dark energy cosmology from Born-Infeld- gravity
We study late-time cosmic accelerating dynamics from Born-Infeld-
gravity in a simplified conformal approach. We find that a variety of cosmic
efects such as Little Rip, CDM universe and dark energy cosmology with
finite-time future singularities may occur. Unlike the convenient Born-Infeld
gravity where in the absence of matter only de Sitter expansion may emerge,
apparentlly any FRW cosmology maybe reconstructed from this conformal version
of the Born-Infeld- theory. Despite the fact that the explicit form of
is fixed by the conformal ansatz, the relation between the two metrics
in this approach may be changed so as to bring out any desired FRW cosmology.Comment: 7 pages, 5 figures. This version accepted in Phys. Lett.
Semiclassical geons as solitonic black hole remnants
We find that the end state of black hole evaporation could be represented by
non-singular and without event horizon stable solitonic remnants with masses of
the order the Planck scale and up to 16 units of charge. Though these objects
are locally indistinguishable from spherically symmetric, massive electric (or
magnetic) charges, they turn out to be sourceless geons containing a wormhole
generated by the electromagnetic field. Our results are obtained by
interpreting semiclassical corrections to Einstein's theory in the first-order
(Palatini) formalism, which yields second-order equations and avoids the
instabilities of the usual (metric) formulation of quadratic gravity. We also
discuss the potential relevance of these solutions for primordial black holes
and the dark matter problem.Comment: 9 pages, 1 figur
Microscopic wormholes and the geometry of entanglement
It has recently been suggested that Einstein-Rosen (ER) bridges can be
interpreted as maximally entangled states of two black holes that form a
complex Einstein-Podolsky-Rosen (EPR) pair. This relationship has been dubbed
as the ER = EPR correlation. In this work, we consider the latter conjecture in
the context of quadratic Palatini theory. An important result, which stems from
the underlying assumptions about the geometry on which the theory is
constructed, is the fact that all the charged solutions of the quadratic
Palatini theory possess a wormhole structure. Our results show that spacetime
may have a foam-like microstructure with wormholes generated by fluctuations of
the quantum vacuum. This involves the spontaneous creation/annihilation of
entangled particle-antiparticle pairs, existing in a maximally entangled state
connected by a non-traversable wormhole. Since the particles are produced from
the vacuum and therefore exist in a singlet state, they are necessarily
entangled with one another. This gives further support to the ER=EPR claim.Comment: 5 pages. V2: minor changes and references adde
Hybrid modified gravity unifying local tests, galactic dynamics and late-time cosmic acceleration
The non-equivalence between the metric and Palatini formalisms of
gravity is an intriguing feature of these theories. However, in the recently
proposed hybrid metric-Palatini gravity, consisting of the superposition of the
metric Einstein-Hilbert Lagrangian with an term constructed \`{a}
la Palatini, the "true" gravitational field is described by the interpolation
of these two non-equivalent approaches. The theory predicts the existence of a
light long-range scalar field, which passes the local constraints and affects
the galactic and cosmological dynamics. Thus, the theory opens new
possibilities for a unified approach, in the same theoretical framework, to the
problems of dark energy and dark matter, without distinguishing a priori matter
and geometric sources, but taking their dynamics into account under the same
standard.Comment: 8 pages. Received an Honorable Mention in the Gravity Research
Foundation Essay Contest 2013. V2: references added; version to appear in the
International Journal of Modern Physics
Tunable Charge and Spin Seebeck Effects in Magnetic Molecular Junctions
We study the charge and spin Seebeck effects in a spin-1 molecular junction
as a function of temperature (T), applied magnetic field (H), and magnetic
anisotropy (D) using Wilson's numerical renormalization group. A hard-axis
magnetic anisotropy produces a large enhancement of the charge Seebeck
coefficient Sc (\sim k_B/|e|) whose value only depends on the residual
interaction between quasiparticles in the low temperature Fermi-liquid regime.
In the underscreened spin-1 Kondo regime, the high sensitivity of the system to
magnetic fields makes it possible to observe a sizable value for the spin
Seebeck coefficient even for magnetic fields much smaller than the Kondo
temperature. Similar effects can be obtain in C60 junctions where the control
parameter is the gap between a singlet and a triplet molecular state.Comment: 5 pages, 4 figure
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