arXiv subjects

Yun Soo Myung

Publications and source records attributed to Yun Soo Myung.

At least 109 records · Page 6Linked to original sources

arXiv2014

Massive gravitational waves in Chern-Simons modified gravity

We consider the nondynamical Chern-Simons (nCS) modified gravity, which is regarded as a parity-odd theory of massive gravity in four dimensions. We first find polarization modes of gravitational waves for $θ=x/μ$ in nCS modified gravity by using the Newman-Penrose formalism where the null complex tetrad is necessary to specify gravitational waves. We show that in the Newman-Penrose formalism, the number of polarization modes is one in addition to an unspecified $Ψ_4$, implying three degrees of freedom for $θ=x/μ$. This compares with two for a canonical embedding of $θ=t/μ$. Also, if one introduces the Ricci tensor formalism to describe a massive graviton arising from the nCS modified gravity, one finds one massive mode after making second-order wave equations, which is compared to five found from the parity-even Einstein-Weyl gravity.

gr-qc↗

arXiv2014

Primordial massive gravitational waves from Einstein-Chern-Simons-Weyl gravity

We investigate the evolution of cosmological perturbations during de Sitter inflation in the Einstein-Chern-Simons-Weyl gravity. Primordial massive gravitational waves are composed of one scalar, two vector and four tensor circularly polarized modes. We show that the vector power spectrum decays quickly like a transversely massive vector in the superhorizon limit $z\to 0$. In this limit, the power spectrum coming from massive tensor modes decays quickly, leading to the conventional tensor power spectrum. Also, we find that in the limit of $m^2 \to 0$ (keeping the Weyl-squared term only), the vector and tensor power spectra disappear. It implies that their power spectra are not gravitationally produced because they (vector and tensor) are decoupled from the expanding de Sitter background, as a result of conformal invariance.

gr-qc↗

arXiv2014

Primordial gravitational waves from conformal gravity

We investigate the evolution of cosmological perturbations generated during de Sitter inflation in the conformal gravity. Primordial gravitational waves are composed of vector and tensor modes. We obtain the constant vector and tensor power spectra which seems to be correct because the conformal gravity is invariant under conformal transformation like the Maxwell kinetic term.

gr-qc↗

arXiv2014

Quasinormal frequencies of a massless scalar in the hidden Kerr/CFT proposal

The hidden Kerr/CFT proposal implies that the massless scalar wave equation in the near-region and low-frequency limit respects a hidden SL(2,R)$\times$SL(2,R) invariance in the Kerr black hole spacetime. We may use this symmetry to determine quasinormal frequencies (QNFs) of the massless scalar wave propagating around the Kerr black hole algebraically. It is shown that QNFs obtained using the hidden conformal symmetry near the horizon correspond approximately to not only those of scalar perturbation around the near-horizon region of a nearly extremal Kerr (NEK) black hole, but also those of non-equatorial scalar modes around the NEK black hole. This indicates that the hidden Kerr/CFT proposal could determine quasinormal modes and frequencies of the massless scalar wave around the NEK (rapidly rotating) black hole approximately.

hep-th↗

arXiv2014

Black hole stability in Jordan and Einstein frames

We investigate the classical stability of Schwarzschild black hole in Jordan and Einstein frames which are related by the conformal transformations. For this purpose, we introduce two models of the Brans-Dicke theory and Brans-Dicke-Weyl gravity in Jordan frame and two corresponding models in the Einstein frame. The former model is suitable for studying the massless spin-2 graviton propagating around the Schwarzschild black hole, while the later is designed for the massive spin-2 graviton propagating around the black hole. It turns out that the black hole (in)stability is independent of the frame which shows that the two frames are equivalent to each other.

gr-qc↗

arXiv2014

Thermodynamic and classical instability of AdS black holes in fourth-order gravity

We study thermodynamic and classical instability of AdS black holes in fourth-order gravity. These include the BTZ black hole in new massive gravity, Schwarzschild-AdS black hole, and higher-dimensional AdS black holes in fourth-order gravity. All thermodynamic quantities which are computed using the Abbot-Deser-Tekin method are used to study thermodynamic instability of AdS black holes. On the other hand, we investigate the $s$-mode Gregory-Laflamme instability of the massive graviton propagating around the AdS black holes. We establish the connection between the thermodynamic instability and the GL instability of AdS black holes in fourth-order gravity. This may show the Gubser-Mitra conjecture which holds for AdS black holes found from fourth-order gravity.

hep-th↗

arXiv2014

New massive conformal gravity

We investigate the new massive conformal gravity which is not invariant under conformal transformations, in comparison to the massive conformal gravity. We find five polarization modes of gravitational waves propagating on the Minkowski spacetimes. The stability of Minkowski spacetimes is guaranteed if the mass squared is not negative and the linearized Ricci tenor is employed to describe a massive spin-2 graviton. However, the small Schwarzschild black hole is unstable against the $s$-mode massive graviton perturbations.

gr-qc↗

arXiv2014

Black holes in massive conformal gravity

We analyze the classical stability of Schwarzschild black hole in massive conformal gravity which was recently proposed for another massive gravity model. This model in the Jordan frame is conformally equivalent to the Einstein-Weyl gravity in the Einstein frame. The coupled linearized Einstein equation is decomposed into the traceless and trace equation when one chooses $6m^2φ=δR$. Solving the traceless equation exhibits unstable modes featuring the Gregory-Laflamme $s$-mode instability of five-dimensional black string, while we find no unstable modes when solving the trace equation. It is shown that the instability of the black hole in massive conformal gravity arises from the massiveness where the geometry of extra dimension trades for mass.

gr-qc↗

arXiv2013

Instability of Schwarzschild-AdS black hole in Einstein-Weyl gravity

We investigate the classical stability of Schwarzschild-AdS black hole in a massive theory of the Einstein-Weyl gravity. It turns out that the linearized Einstein tensor perturbations exhibit unstable modes featuring the Gregory-Laflamme instability of five-dimensional AdS black string, in contrast to the stable Schwarzschild-AdS black hole in the Einstein-Weyl gravity. We confirm that the instability of the black hole in the Einstein-Weyl gravity arises from the massiveness but not a feature of fourth-order derivative theory giving ghost states.

gr-qc↗

arXiv2013

Late-time acceleration in the coupled Cubic Galileon models

We investigate the linearly and quadratically coupled cubic Galileon models that include linear potentials. These models may explain the late-time acceleration. In these cases, we need two equations of state parameter named the native and effective equations of state to test whether the universe is accelerating or not because there is coupling between the cold dark matter and Galileon. It turns out that there is no transition from accelerating phase to phantom phase in the future.

gr-qc↗

arXiv2013

BRST quantization of a sixth-order derivative scalar field theory

We study a sixth order derivative scalar field model in Minkowski spacetime as a toy model of higher-derivative critical gravity theories. This model is consistently quantized when using the Becchi-Rouet-Stora-Tyutin (BRST) quantization scheme even though it does not show gauge symmetry manifestly. Imposing a BRST quartet generated by two scalars and ghosts, there remains a non-trivial subspace with positive norm. This might be interpreted as a Minkowskian dual version of the unitary truncation in the logarithmic conformal field theory.

hep-th↗

arXiv2013

Instability of a Kerr black hole in f(R) gravity

We study the stability of a rotating (Kerr) black hole in the viable $f(R)$ gravity. The linearized-Ricci scalar equation shows the superradiant instability, leading to the instability of the Kerr black hole in $f(R)$ gravity.

gr-qc↗

arXiv2013

Quantization of n coupled scalar field theory

We study a model of n coupled scalar fields in Minkowski spacetime where all masses degenerate, which is considered as a toy model of polycritical gravity on AdS spacetime. We quantize this model within the Becchi-Rouet-Stora-Tyutin (BRST) scheme by introducing n Faddeev-Popov (FP) ghost fields. Extending a BRST quartet generated by two scalars and two FP ghosts to n scalars and n FP ghosts, there remains a physical subspace with positive norm for odd n, but there exists only the vacuum for even n. This clearly shows a non-triviality of odd-higher order derivative scalar field theories. This is helpful to understand the truncation mechanism which is used to obtain a unitary conformal field theory dual to linearized polycritical gravity. It turns out that the truncation mechanism is nothing but a general quartet mechanism appeared when introducing the FP ghost action.

hep-th↗

arXiv2013

Gregory-Laflamme instability of BTZ black hole in new massive gravity

We find the Gregory-Laflamme $s$-mode instability of the non-rotating BTZ black hole in new massive gravity. This instability shows that the BTZ black hole could not exist as a stable static solution to the new massive gravity. For non-rotating BTZ black string in four dimensions, however, it is demonstrated that the BTZ black string can be stable against the metric perturbation.

hep-th↗

arXiv2013

Unstable Schwarzschild-Tangherlini black holes in fourth-order gravity

We study the stability of Schwarzschild-Tangherlini (ST) black holes in fourth-order gravity which provides a higher dimensional linearized massive equation. The linearized-Ricci tensor perturbation is employed to exhibit unstable modes featuring the Gregory-Laflamme (GL) instability of higher dimensional black strings, in comparison to the stable ST black holes in Einstein gravity. It turns out that the GL instability of the ST black holes in the fourth-order gravity originates from the massiveness, but not a nature of fourth-order derivative theories giving ghost states.

gr-qc↗

arXiv2013

Stability of the massive graviton around a BTZ black hole in three dimensions

We investigate the massive graviton stability of the BTZ black hole obtained from three dimensional massive gravities which are classified into the parity-even and parity-odd gravity theories. In the parity-even gravity theory, we perform the $s$-mode stability analysis by using the BTZ black string perturbations, which gives two Schrödinger equations with frequency-dependent potentials. The $s$-mode stability is consistent with the generalized Breitenlohner-Freedman bound for spin-2 field. It seems that for the parity-odd massive gravity theory, the BTZ black hole is stable when the imaginary part of quasinormal frequencies of massive graviton is positive. However, this condition is not consistent with the $s$-mode stability based on the second-order equation obtained after squaring the first-order equation. Finally we explore the black hole stability connection between the parity-odd and parity-even massive gravity theories.

hep-th↗

arXiv2013

Comment on Ricci dark energy in Chern-Simons modified gravity

We revisit Ricci dark energy in Chern-Simons modified gravity. As far as the cosmological evolution, this is nothing but the Ricci dark energy with a minimally coupled scalar without potential which means that the role of Chern-Simons term is suppressed. Using the equation of state parameter, this model is similar to the modified Chaplygin gas model only when two are around the de Sitter universe deriving by the cosmological constant in the future. However, two past evolutions are different.

gr-qc↗

arXiv2013

Stability of Schwarzschild black holes in fourth-order gravity revisited

We revisit the classical stability of Schwarzschild black hole in fourth-order theories of gravity. On the contrary of the stability of this black hole, it turns out that the linearized perturbations exhibit unstable modes featuring the Gregory-Laflamme instability of five-dimensional black string. This shows clearly an instability of black hole in fourth-order gravity with $α=-β/3$.

gr-qc↗