681 research outputs found

### Ambiguity of black hole entropy in loop quantum gravity

We reexmine some proposals of black hole entropy in loop quantum gravity
(LQG) and consider a new possible choice of the Immirzi parameter which has not
been pointed out so far. We also discuss that a new idea is inevitable if we
regard the relation between the area spectrum in LQG and that in the
quasinormal mode analysis seriously.Comment: 4 pages, 1 figure, error corrected, PRD published versio

### Revisiting chameleon gravity - thin-shells and no-shells with appropriate boundary conditions

We derive analytic solutions of a chameleon scalar field $\phi$ that couples
to a non-relativistic matter in the weak gravitational background of a
spherically symmetric body, paying particular attention to a field mass $m_A$
inside of the body. The standard thin-shell field profile is recovered by
taking the limit $m_A*r_c \to \infty$, where $r_c$ is a radius of the body. We
show the existence of "no-shell" solutions where the field is nearly frozen in
the whole interior of the body, which does not necessarily correspond to the
"zero-shell" limit of thin-shell solutions. In the no-shell case, under the
condition $m_A*r_c \gg 1$, the effective coupling of $\phi$ with matter takes
the same asymptotic form as that in the thin-shell case. We study experimental
bounds coming from the violation of equivalence principle as well as
solar-system tests for a number of models including $f(R)$ gravity and find
that the field is in either the thin-shell or the no-shell regime under such
constraints, depending on the shape of scalar-field potentials. We also show
that, for the consistency with local gravity constraints, the field at the
center of the body needs to be extremely close to the value $\phi_A$ at the
extremum of an effective potential induced by the matter coupling.Comment: 14 pages, no figure

### What happens to Q-balls if $Q$ is so large?

In the system of a gravitating Q-ball, there is a maximum charge $Q_{{\rm
max}}$ inevitably, while in flat spacetime there is no upper bound on $Q$ in
typical models such as the Affleck-Dine model. Theoretically the charge $Q$ is
a free parameter, and phenomenologically it could increase by charge
accumulation. We address a question of what happens to Q-balls if $Q$ is close
to $Q_{{\rm max}}$. First, without specifying a model, we show analytically
that inflation cannot take place in the core of a Q-ball, contrary to the claim
of previous work. Next, for the Affleck-Dine model, we analyze perturbation of
equilibrium solutions with $Q\approx Q_{{\rm max}}$ by numerical analysis of
dynamical field equations. We find that the extremal solution with $Q=Q_{{\rm
max}}$ and unstable solutions around it are "critical solutions", which means
the threshold of black-hole formation.Comment: 9 pages, 10 figures, results for large $\kappa$ added, to appear in
PR

### The universal area spectrum in single-horizon black holes

We investigate highly damped quasinormal mode of single-horizon black holes
motivated by its relation to the loop quantum gravity. Using the WKB
approximation, we show that the real part of the frequency approaches the value
$T_{\rm H}\ln 3$ for dilatonic black hole as conjectured by Medved et al. and
Padmanabhan. It is surprising since the area specrtum of the black hole
determined by the Bohr's correspondence principle completely agrees with that
of Schwarzschild black hole for any values of the electromagnetic charge or the
dilaton coupling. We discuss its generality for single-horizon black holes and
the meaning in the loop quantum gravity.Comment: 5 pages, 1 figure, references and comments adde

### How does gravity save or kill Q-balls?

We explore stability of gravitating Q-balls with potential
$V_4(\phi)={m^2\over2}\phi^2-\lambda\phi^4+\frac{\phi^6}{M^2}$ via catastrophe
theory, as an extension of our previous work on Q-balls with potential
$V_3(\phi)={m^2\over2}\phi^2-\mu\phi^3+\lambda\phi^4$. In flat spacetime
Q-balls with $V_4$ in the thick-wall limit are unstable and there is a minimum
charge $Q_{{\rm min}}$, where Q-balls with $Q<Q_{{\rm min}}$ are nonexistent.
If we take self-gravity into account, on the other hand, there exist stable
Q-balls with arbitrarily small charge, no matter how weak gravity is. That is,
gravity saves Q-balls with small charge. We also show how stability of Q-balls
changes as gravity becomes strong.Comment: 10 pages, 10 figure

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