29,499 research outputs found
Pion radiative weak decay from the instanton vacuum
We investigate the vector and axial-vector form factors for the pion
radiative weak decays and , based on the gauged effective chiral action from the instanton vacuum in
the large limit. The nonlocal contributions, which arise from the gauging
of the action, enhance the vector form factor by about , whereas the
axial-vector form factor is reduced by almost . Both the results for
the vector and axial-vector form factors at the zero momentum transfer are in
good agreement with the experimental data. The dependence of the form factors
on the momentum transfer is also studied. The slope parameters are computed and
compared with other works.Comment: 13 pages, 7 figures. Final version accepted for publication in
Phys.Lett.
Black Hole as a Wormhole Factory
On general grounds, one may argue that a black hole stops radiation at the
Planck mass, where the radiated energy is comparable to the black hole's mass.
And also, it has been argued that there would be a "wormhole-like" structure,
known as "space-time foam", due to large fluctuations below the Planck length.
In this paper, as an explicit example, we consider an exact classical solution
which represents nicely those two properties in a recently proposed quantum
gravity model based on different scaling dimensions between space and time
coordinates. The solution, called "Black Wormhole", consists of two different
states, depending on its mass M and an IR parameter omega: For the black hole
state, a non-traversable wormhole occupies the interior region of the black
hole around the singularity at the origin, whereas for the wormhole state, the
interior wormhole is exposed to an outside observer as the black hole horizon
is disappeared from evaporation. The black hole state becomes thermodynamically
stable as it approaches to the merge point where the interior wormhole throat
and the black hole horizon merges, and the Hawking temperature vanishes at the
exact merge point. This solution suggests the "Generalized Cosmic Censorship"
by the existence of a wormhole-like structure which protects the naked
singularity even after the black hole evaporation. One could understand the
would-be wormholes inside the black hole horizon as the results of microscopic
wormholes created by "negative" energy quanta which have entered the black hole
horizon in Hawking radiation processes: The quantum black hole could be a
wormhole factory. It is found that this speculative picture may be consistent
with the recent "ER=EPR" proposal for resolving the recent black hole
entanglement debates.Comment: Added some more words on (1) the transition between the black hole
phase and wormhole phase and (2) the notion of a wormhole "factory" in Fig.
5. Updated references, Accepted in PL
On a New Approach for Constructing Wormholes in Einstein-Born-Infeld Gravity
We study a new approach for the wormhole construction in Einstein-Born-Infeld
gravity, which does not require exotic matters in the Einstein equation. The
Born-Infeld field equation is not modified from "coordinate independent"
conditions of continuous metric tensor and its derivatives, even though the
Born-Infeld fields have discontinuities in their derivatives at the throat in
general. We study the relation of the newly introduced conditions with the
usual continuity equation for the energy-momentum tensor and the gravitational
Bianchi identity. We find that there is no violation of energy conditions for
the Born-Infeld fields contrary to the usual approaches. The exoticity of
energy-momentum tensor is not essential for sustaining wormholes. Some open
problems are discussed.Comment: Minor revision to improve the clarity, Corrected typos, Added
reference and footnot
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