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Takashi Tamaki

Publications and source records attributed to Takashi Tamaki.

At least 19 recordsLinked to original sources
arXiv2016

Holographic bound in covariant loop quantum gravity

We investigate puncture statistics based on the covariant area spectrum in loop quantum gravity. First, we consider Maxwell-Boltzmann statistics with a Gibbs factor for punctures. We establish formulae which relate physical quantities such as horizon area to the parameter characterizing holographic degrees of freedom. We also perform numerical calculations and obtain consistency with these formulae. These results tell us that the holographic bound is satisfied in the large area limit and correction term of the entropy-area law can be proportional to the logarithm of the horizon area. Second, we also consider Bose-Einstein statistics and show that the above formulae are also useful in this case. By applying the formulae, we can understand intrinsic features of Bose-Einstein condensate which corresponds to the case when the horizon area almost consists of punctures in the ground state. When this phenomena occurs, the area is approximately constant against the parameter characterizing the temperature. When this phenomena is broken, the area shows rapid increase which suggests the phase transition from quantum to classical area.

hep-th↗
arXiv2014

Gravastar Shadows

Direct observation of black holes is one of the grand challenges in astronomy. If there are super-compact objects which possess unstable circular orbits of photons, however, it may be difficult to distinguish them from black holes by observing photons. As a model of super-compact objects, we consider a gravastar (gravitational-vacuum-star) which was originally proposed by Mazur and Mottola. For definiteness, we adopt a spherical thin-shell model of a gravastar developed by Visser and Wiltshire, which connects interior de-Sitter geometry and exterior Schwarzschild geometry. We find that unstable circular orbits of photons can appear around the gravastar. Then, we investigate the optical images of the gravastar possessing unstable circular orbits, with assuming the optically transparent surface of it and two types of optical sources behind the gravastar: (i) an infinite optical plane and (ii) a companion star. The main feature of the image of (i) is that a bright disk and a dark thick ring surrounding the disk appear in the center of the region which would be completely dark if the compact object was not the gravastar but Schwarzschild black hole. Also in the case (ii), a small disk and arcs around the disk appear in the region which would be completely dark for the lensing image by Schwarzschild black hole. Because characteristic images appear inside the gravastar in both cases, we could tell the difference between a black hole and a gravastar with high-resolution VLBI observations near future.

gr-qc↗
arXiv2014

Gauged Q-balls in the Affleck-Dine mechanism

We consider gauged Q-balls in the gravity-mediation-type model in the Affleck-Dine mechanism, which is described by the potential $V_{\rm grav.}(ϕ):=(m_{\rm grav.}^2/2)ϕ^2\left[1+K\ln(ϕ/M)^2\right]$ with $K<0$. In many models of gauged Q-balls, which were studied in the literature, there are upper limits for charge and size of Q-balls due to repulsive Coulomb force. In the present model, by contrast, our numerical calculation strongly suggests that stable solutions with any amount of charge and size exist. As the electric charge $Q$ increases, the field configuration of the scalar field becomes shell-like; because the charge is concentrated on the surface, the Coulomb force does not destroy the Q-ball configuration. These properties are analogous to those in the V-shaped model, which was studied by Arodź and Lis. We also find that for each $K$ there is another sequence of unstable solutions, which is separated from the other sequence of the stable solutions. As $|K|$ increases, the two sequences approach; eventually at some point in $-1.07<K<-1.06$, the "recombination" of the two sequences takes place.

hep-th↗
arXiv2012

Unified pictures of Q-balls and Q-tubes

While Q-balls have been investigated intensively for many years, another type of nontopological solutions, Q-tubes, have not been understood very well. In this paper we make a comparative study of Q-balls and Q-tubes. First, we investigate their equilibrium solutions for four types of potentials. We find, for example, that in some models the charge-energy relation is similar between Q-balls and Q-tubes while in other models the relation is quite different between them. To understand what determines the charge-energy relation, which is a key of stability of the equilibrium solutions, we establish an analytical method to obtain the two limit values of the energy and the charge. Our prescription indicates how the existent domain of solutions and their stability depends on their shape as well as potentials, which would also be useful for a future study of Q-objects in higher-dimensional spacetime.

gr-qc↗
arXiv2012

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.

gr-qc↗
arXiv2011

What are universal features of gravitating Q-balls?

We investigate how gravity affects Q-balls by exemplifying the case of the Affleck-Dine potential $V(ϕ):=m^4 \ln (1+\frac{ϕ^2}{m^2})$. Surprisingly, stable Q-balls with arbitrarily small charge exist, no matter how weak gravity is, contrary to the case of flat spacetime. We also show analytically that this feature holds true for general models as long as the leading order term of the potential is a positive mass term in its Maclaurin series.

gr-qc↗
arXiv2011

Gravitating Q-balls in the Affleck-Dine mechanism

We investigate how gravity affects "Q-balls" with the Affleck-Dine potential $V_{AD}(ϕ):=\frac{m^2}{2}ϕ^2[ 1+K\ln (\fracϕ{M})^2]$. Contrary to the flat case, in which equilibrium solutions exist only if $K<0$, we find three types of gravitating solutions as follows. In the case that $K<0$, ordinary Q-ball solutions exist; there is an upper bound of the charge due to gravity. In the case that K=0, equilibrium solutions called (mini-)boson stars appear due to gravity; there is an upper bound of the charge, too. In the case that $K>0$, equilibrium solutions appear, too. In this case, these solutions are not asymptotically flat but surrounded by Q-matter. These solutions might be important in considering a dark matter scenario in the Affleck-Dine mechanism.

gr-qc↗
arXiv2011

How does gravity save or kill Q-balls?

We explore stability of gravitating Q-balls with potential $V_4(ϕ)={m^2\over2}ϕ^2-λϕ^4+\frac{ϕ^6}{M^2}$ via catastrophe theory, as an extension of our previous work on Q-balls with potential $V_3(ϕ)={m^2\over2}ϕ^2-μϕ^3+λϕ^4$. 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.

gr-qc↗
arXiv2011

Unified picture of Q-balls and boson stars via catastrophe theory

We make an analysis of Q-balls and boson stars using catastrophe theory, as an extension of the previous work on Q-balls in flat spacetime. We adopt the potential $V_3(ϕ)={m^2\over2}ϕ^2-μϕ^3+λϕ^4$ for Q-balls and that with $μ=0$ for boson stars. For solutions with $|g^{rr}-1|\sim 1$ at its peak, stability of Q-balls has been lost regardless of the potential parameters. As a result, phase relations, such as a Q-ball charge versus a total Hamiltonian energy, approach those of boson stars, which tell us an unified picture of Q-balls and boson stars.

gr-qc↗
arXiv2010

Discretisation parameter and operator ordering in loop quantum cosmology with the cosmological constant

In loop quantum cosmology, the Hamiltonian reduces to a finite difference operator. We study the initial singularity and the large volume limit against the ambiguities in the discretisation and the operator ordering within a homogeneous, isotropic and spatially flat model with the cosmological constant. We find that the absence of the singularity strongly depends on the choice of the operator ordering and the requirement for the absence singles out a very small class of orderings. Moreover we find a general ordering rule required for the absence of the singularity. We also find that the large volume limit naturally recovers a smooth wave function in the discretisation where each step corresponds to a fixed volume increment but not in the one where each step corresponds to a fixed area increment. If loop quantum cosmology is to be a phenomenological realisation of full loop quantum gravity, these results are important to fix the theoretical ambiguities.

gr-qc↗
arXiv2009

Chameleon scalar fields in relativistic gravitational backgrounds

We study the field profile of a scalar field $ϕ$ that couples to a matter fluid (dubbed a chameleon field) in the relativistic gravitational background of a spherically symmetric spacetime. Employing a linear expansion in terms of the gravitational potential $Φ_c$ at the surface of a compact object with a constant density, we derive the thin-shell field profile both inside and outside the object, as well as the resulting effective coupling with matter, analytically. We also carry out numerical simulations for the class of inverse power-law potentials $V(ϕ)=M^{4+n} ϕ^{-n}$ by employing the information provided by our analytical solutions to set the boundary conditions around the centre of the object and show that thin-shell solutions in fact exist if the gravitational potential $Φ_c$ is smaller than 0.3, which marginally covers the case of neutron stars. Thus the chameleon mechanism is present in the relativistic gravitational backgrounds, capable of reducing the effective coupling. Since thin-shell solutions are sensitive to the choice of boundary conditions, our analytic field profile is very helpful to provide appropriate boundary conditions for $Φ_c \lesssim O(0.1)$.

gr-qc↗
arXiv2009

Black hole entropy for the general area spectrum

We consider the possibility that the horizon area is expressed by the general area spectrum in loop quantum gravity and calculate the black hole entropy by counting the degrees of freedom in spin-network states related to its area. Although the general area spectrum has a complex expression, we succeeded in obtaining the result that the black hole entropy is proportional to its area as in previous works where the simplified area formula has been used. This gives new values for the Barbero-Immirzi parameter ($γ=0.5802... \mathrm{or} 0.7847...$) which are larger than that of previous works.

hep-th↗
arXiv2008

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

We derive analytic solutions of a chameleon scalar field $ϕ$ 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 $ϕ$ 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 $ϕ_A$ at the extremum of an effective potential induced by the matter coupling.

gr-qc↗
arXiv2008

Post-Newtonian parameters in the tensor-vector-scalar theory

We investigate post-Newtonian parameters in the tensor-vector-scalar (TeVeS) theory in a general setting while previous researches have been restricted to spherically symmetric cases. Based on the assumption that both the physical and Einstein metrics have Minkowski metric at the zeroth order, we show $γ=1$ as in the previous researches. We find two remarkable things for other parameters. The first is the value $β=1$ while it has been reported that $β\neq 1$ for the case when the vector field is not purely timelike. This discrepancy occurs from the above assumption which is natural as a starting point. The second is the result that the Newtonian potential must be static to be consistent with the vector equation. As a result, we cannot determine $α_{1}$ and $α_{2}$. We consider that it is related to the instability against linear perturbation and occurrence of caustic singularities for various initial perturbations which have been reported recently.

gr-qc↗
arXiv2007

Generic features of Einstein-Aether black holes

We reconsider spherically symmetric black hole solutions in Einstein-Aether theory with the condition that this theory has identical PPN parameters as those for general relativity, which is the main difference from the previous research. In contrast with previous study, we allow superluminal propagation of a spin-0 Aether-gravity wave mode. As a result, we obtain black holes having a spin-0 "horizon" inside an event horizon. We allow a singularity at a spin-0 "horizon" since it is concealed by the event horizon. If we allow such a configuration, the kinetic term of the Aether field can be large enough for black holes to be significantly different from Schwarzschild black holes with respect to ADM mass, innermost stable circular orbit, Hawking temperature, and so on. We also discuss whether or not the above features can be seen in more generic vector-tensor theories.

gr-qc↗
arXiv2005

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.

hep-th↗
arXiv2005

Continuous area spectrum in regular black hole

We investigate highly damped quasinormal modes of regular black hole coupled to nonlinear electrodynamics. Using the WKB approximation combined with complex-integration technique, we show that the real part of the frequency disappears in the highly damped limit. If we use the Bohr's correspondence principle, the area spectrum of this black hole is continuous. We discuss its implication in the loop quantum gravity.

hep-th↗