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Yun Soo Myung

Publications and source records attributed to Yun Soo Myung.

At least 91 records · Page 5Linked to original sources
arXiv2017

The origin of regular Newtonian potential in infinite derivative gravity

It turns out that the infinite derivative gravity (IDG) is ghost-free and renormalizable when one chooses the exponential of an entire function. For this IDG case, the corresponding Newtonian potential generated from the delta function is non-singular at the origin. However, we will explicitly show that the source generating this non-singular potential is given not by the delta-function due to the point-like source of mass, but by the Gaussian mass distribution. This explains clearly why the IDG with the exponential of an entire function yields the finite potential at the origin.

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arXiv2017

Renormalizability, vDVZ discontinuity and Newtonian singularity in higher-derivative gravity

It was proposed that if a higher-derivative gravity is renormalizable, it implies necessarily a finite Newtonian potential at the origin, but the reverse of this statement is not true. Here we show that the reverse is true when taking into account the vDVZ discontinuity which states that the theory obtained from massive one by taking zero mass limit is not equivalent to the theory obtained in the zero mass case. The surviving degree of freedom in the zero mass limit is an extra scalar which does not affect the light bending angle, but affects the Newtonian potential. This asserts that in order to make the singularity cancellation, the number of massive ghost and healthy tensors matches with that of massive ghost and healthy scalars.

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arXiv2017

Entropy of black hole in the infinite-derivative gravity

We compute the Wald entropy of the Schwarzschild black hole in the ghost-free, infinite derivative gravity that is quadratic in curvature. This is not given purely by the area law but includes an additional contribution depending on the power of d'Alembertian operator, when requiring that the massless graviton be the only propagating mode in the Minkowski sapcetime.

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arXiv2016

Propagating degrees of freedom in f(R) gravity

We have computed the number of polarization modes of gravitational waves propagating in the Minkowski background in $f(R)$ gravity. This is three of two from transverse-traceless tensor modes and one from a massive trace mode, which confirms the results found in the literature. There is no massless breathing mode and the massive trace mode corresponds to the Ricci scalar. A newly defined metric tensor in $f(R)$ gravity satisfies the transverse-traceless (TT) condition as well as the TT wave equation.

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arXiv2016

Preheating in the nonminimal derivative coupling to gravity

We revisit the reheating mechanism after the end of inflation in the non-minimal derivative coupling (NDC) to gravity with quadratic potential. This is because the inflaton of the NDC should describe the slow-roll inflation as well as the preheating stage after the end of inflation. We point out that the non-periodic inflaton solution implies the absence of parametric resonance, compared to the periodic oscillating inflaton for the canonical coupling (CC) to gravity. Furthermore, it is demonstrated that narrow and broad parametric resonances do not appear after the end of inflation in the NDC model by solving the differential equation numerically for the quantum field, which differs from the case of the the CC model obtained by solving Mathieu equation.

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arXiv2016

Inflaton decay and reheating in nonminimal derivative coupling

We investigate the inflaton decay and reheating period after the end of inflation in the non-minimal derivative coupling (NDC) model with chaotic potential. In general, this model is known to provide an enhanced slow-roll inflation caused by gravitationally enhanced friction. We find violent oscillations of Hubble parameter which induces oscillations of the sound speed squared, implying the Lagrangian instability of curvature perturbation $ζ$ under the comoving gauge $φ=0$. Also, it is shown that the curvature perturbation blows up at $\dotϕ=0$, leading to the breakdown of the comoving gauge at $\dotϕ=0$. Therefore, we use the Newtonian gauge to perform the perturbation analysis where the Newtonian potential is employed as a physical variable. The curvature perturbation is not considered as a physical variable which describes a relevant perturbation during reheating.

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arXiv2016

Einstein-singleton theory and its power spectra in de Sitter inflation

We study the Einstein-singleton theory during de Sitter inflation since it provides a way of degenerate fourth-order scalar theory. We obtain an exact solution expressed in terms of the exponential-integral function by solving the degenerate fourth-order scalar equation in de Sitter spacetime. Furthermore, we find that its power spectrum blows negatively up in the superhorizon limit, while it is negatively scale-invariant in the subhorizon limit. This suggests that the Einstein-singleton theory contains the ghost-instability and thus, it is not suitable for developing a slow-roll inflation model.

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arXiv2016

Comment on Quantum Massive Conformal Gravity

In a recent paper in EPJC March 2016, Faria has shown that quantum massive conformal gravity is renormalizable but has ghost states. We comment this paper on the aspect of renormalizability.

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arXiv2016

Scale-invariant power spectra from a Weyl-invariant scalar-tensor theory

We obtain scale-invariant scalar and tensor power spectra from a Weyl-invariant scalar-tensor theory in de Sitter spacetime. This implies that the Weyl-invariance guarantees to implement the scale-invariance of power spectrum in de Sitter spacetime. We establish a deep connection between the Weyl-invariance of the action and scale-invariance of power spectrum in de Sitter spacetime.

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arXiv2015

Phase transitions of the BTZ black hole in new massive gravity

We investigate thermodynamics of the BTZ black hole in new massive gravity explicitly. For $m^2\ell^2>1/2$ with $m^2$ the mass parameter of fourth-order terms and $\ell^2$ AdS$_3$ curvature radius, the Hawking-Page phase transition occurs between the BTZ black hole and AdS (thermal) soliton. For $m^2\ell^2<1/2$, however, this transition unlikely occurs but a phase transition between the BTZ black hole and the massless BTZ black hole is possible to occur. We may call the latter as the inverse Hawking-Page phase transition and this transition is favored in the new massive gravity.

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arXiv2015

Revisiting slow-roll inflation in nonminimal derivative coupling with potentials

We investigate the slow-roll inflation in the nonminimal derivative coupling (NDC) model with exponential, quadric, and quartic potentials. It was known that this model provides an enhanced slow-roll inflation induced by gravitationally enhanced friction even for a steep exponential potential. In the phase portrait, the inflationary attractor is described by the slow-roll equation. Introducing the autonomous form, the inflation is regarded as an emergence from the saddle point and it leaves this fixed point along the slow-roll equation. We show explicitly that if one uses the NDC with potentials, the slow-roll inflation is easier to be implemented than the canonical coupling with the same potentials.

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arXiv2015

Scale-invariant scalar spectrum from the nonminimal derivative coupling with fourth-order term

An exactly scale-invariant spectrum of scalar perturbation generated during de Sitter spacetime is found from the gravity model of the nonminimal derivative coupling with fourth-order term. The nonminimal derivative coupling term generates a healthy (ghost-free) fourth-order derivative term, while the fourth-order term provides an unhealthy (ghost) fourth-order derivative term. The Harrison-Zel'dovich spectrum obtained from Fourier transforming the fourth-order propagator in de Sitter space is recovered by computing the power spectrum in its momentum space directly. It shows that this model provides a truly scale-invariant spectrum, in addition to the Lee-Wick scalar theory.

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arXiv2015

Stability of black holes in f(R) gravity

We investigate the stability of black holes in the viable model of $f(R)=R+R^2$ gravity which was known to be the best fit for inflation. These include Schwarzschild and Kerr black holes. Instead of studying the fourth-order linearized equation around the black hole background, we use the corresponding tensor-scalar theory of the Starobinsky model to perform their stability. The Schwarzschild black hole is stable, while the Kerr black hole is unstable because of superradiant instability.

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arXiv2015

Cosmological three-coupled scalar theory for the dS/LCFT correspondence

We investigate cosmological perturbations generated during de Sitter inflation in the three-coupled scalar theory. This theory is composed of three coupled scalars ($ϕ_p,p=1,2,3$) to give a sixth-order derivative scalar theory for $ϕ_3$, in addition to tensor. Recovering the power spectra between scalars from the LCFT correlators in momentum space indicates that the de Sitter/logarithmic conformal field theory (dS/LCFT) correspondence works in the superhorizon limit. We use LCFT correlators derived from the dS/LCFT differentiate dictionary to compare cosmological correlators (power spectra) and find also LCFT correlators by making use of extrapolate dictionary. This is because the former approach is more conventional than the latter. A bulk version dual to the truncation process to find a unitary CFT in the LCFT corresponds to selecting a physical field $ϕ_2$ with positive norm propagating on the dS spacetime.

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arXiv2014

Scale-invariant spectrum of Lee-Wick model in de Sitter spacetime

We obtain a scale-invariant spectrum from the Lee-Wick model in de Sitter spacetime. This model is a fourth-order scalar theory whose mass parameter is determined by $M^2=2H^2$. The Harrison-Zel'dovich scale-invariant spectrum is obtained by Fourier transforming the propagator in position space as well as by computing the power spectrum directly. It shows clearly that the LW scalar theory provides a truly scale-invariant spectrum in whole de Sitter, while the massless scalar propagation in de Sitter shows a scale-invariant spectrum in the superhorizon region only.

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arXiv2014

Cosmological singleton gravity theory and dS/LCFT correspondence

We study the evolution of cosmological perturbations generated during de Sitter inflation in the singleton gravity theory. This theory is composed of a dipole ghost pair in addition to tensor. We obtain the singleton power spectra which show that the de Sitter/logarithmic conformal field theory (dS/LCFT) correspondence works for computing the power spectra in the superhorizon limit. Also we compute the spectral indices for light singleton which contains a logarithmic correction.

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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.

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