arXiv subjects

Seokcheon Lee

Publications and source records attributed to Seokcheon Lee.

82 records · Page 5Linked to original sources

arXiv2008

Stability of Palatini-f(R) cosmology

The evolution of linear cosmological perturbations in modified theories of gravity is investigated assuming the Palatini formalism. It has been discussed about the stability problem in this model based on the equivalence between f(R) gravity and the scalar tensor theory. However, we study this problem in the physical frame where the matter is minimally coupled. In general, the stability of the superhorizon metric evolution depends on models. We show that the deviation from the superhorizon metric evolution is null for a specific choice for the nonlinear Einstein-Hilbert action, $f(\hat{R}) \sim \hat{R}^{n}$, where $n \neq 0,2,3$. Thus the stability of metric fluctuation is guaranteed in these models. We also study the matter density fluctuation in the general gauge and show the differential equations in super and sub-horizon scales.

gr-qc↗

arXiv2008

Palatini f(R) Cosmology

We investigate the modified gravity theories in terms of the effective dark energy models. We compare the cosmic expansion history and the linear growth in different models. We also study the evolution of linear cosmological perturbations in modified theories of gravity assuming the Palatini formalism. We find the stability of the superhorizon metric evolution depends on models. We also study the matter density fluctuation in the general gauge and show the differential equations in super and sub-horizon scales.

gr-qc↗

arXiv2007

Can Strong Gravitational Lensing Constrain Dark Energy?

We discuss the ratio of the angular diameter distances from the source to the lens, $D_{ds}$, and to the observer at present, $D_{s}$, for various dark energy models. It is well known that the difference of $D_s$s between the models is apparent and this quantity is used for the analysis of Type Ia supernovae. However we investigate the difference between the ratio of the angular diameter distances for a cosmological constant, $(D_{ds}/D_{s})^Λ$ and that for other dark energy models, $(D_{ds}/D_{s})^{\rm{other}}$ in this paper. It has been known that there is lens model degeneracy in using strong gravitational lensing. Thus, we investigate the model independent observable quantity, Einstein radius ($θ_E$), which is proportional to both $D_{ds}/D_s$ and velocity dispersion squared, $σ_v^2$. $D_{ds}/D_s$ values depend on the parameters of each dark energy model individually. However, $(D_{ds}/D_s)^Λ - (D_{ds}/D_{s})^{\rm{other}}$ for the various dark energy models, is well within the error of $σ_v$ for most of the parameter spaces of the dark energy models. Thus, a single strong gravitational lensing by use of the Einstein radius may not be a proper method to investigate the property of dark energy. However, better understanding to the mass profile of clusters in the future or other methods related to arc statistics rather than the distances may be used for constraints on dark energy.

astro-ph↗

arXiv2007

Time Variation of Fine Structure Constant and Proton-Electron Mass Ratio with Quintessence

Recent astrophysical observations of quasar absorption systems indicate that the fine structure constant $α$ and the proton-electron mass ratio $μ$ may have evolved through the history of the universe. Motivated by these observations, we consider the cosmological evolution of a quintessence-like scalar field $ϕ$ coupled to gauge fields and matter which leads to effective modifications of the coupling constants and particle masses over time. We show that a class of models where the scalar field potential $V(ϕ)$ and the couplings to matter $B(ϕ)$ admit common extremum in $ϕ$ naturally explains constraints on variations of both the fine structure constant and the proton-electron mass ratio.

astro-ph↗

arXiv2006

Effect on cosmic microwave background polarization of coupling of quintessence to pseudoscalar formed from the electromagnetic field and its dual

We present the full set of power spectra of cosmic microwave background (CMB) temperature and polarization anisotropies due to the coupling between quintessence and pseudoscalar of electromagnetism. This coupling induces the rotation of the polarization plane of CMB photons, thus resulting in non-vanishing B mode and parity-violating TB and EB modes. Using the 2003 flight of BOOMERANG (B03) data, we give the most stringent constraint on the coupling strength. In some cases, the rotation-induced B mode can confuse the hunting for the gravitational lensing induced B mode.

astro-ph↗

arXiv2006

Coupled Quintessence and CMB

We revise the stability of the tracking solutions and briefly review the potentials of quintessence models. We discuss the evolution of linear perturbations for $V(ϕ) = V_{0} \exp(λϕ^2/2)$ potential in which the scalar field is non-minimally coupled to cold dark matter. We consider the effects of this coupling on both cosmic microwave background temperature anisotropies and matter perturbations. We find that the phenomenology of this model is consistent with current observations up to the coupling power $n_{c} \leq 0.01$ while adopting the current parameters measured by WMAP, $Ω_ϕ^{(0)}=0.76$, $Ω_{cdm}^{(0)}=0.191$, $Ω_{b}^{(0)}=0.049$, and $h=0.70$. Upcoming cosmic microwave background observations continuing to focus on resolving the higher peaks may put strong constraints on the strength of the coupling.

astro-ph↗

arXiv2006

Constraints on the coupled quintessence from cosmic microwave background anisotropy and matter power spectrum

We discuss the evolution of linear perturbations in a quintessence model in which the scalar field is non-minimally coupled to cold dark matter. We consider the effects of this coupling on both cosmic microwave background temperature anisotropies and matter perturbations. Due to the modification of the scale of cold dark matter as $ρ_{c} = ρ_{c}^{(0)} a^{-3 + ξ}$, we can shift the turnover in the matter power spectrum even without changing the present energy densities of matter and radiation. This can be used to constrain the strength of the coupling. We find that the phenomenology of this model is consistent with current observations up to the coupling power $n_{c} \leq 0.01$ while adopting the current parameters measured by WMAP. Upcoming cosmic microwave background observations continuing to focus on resolving the higher peaks may put strong constraints on the strength of the coupling.

astro-ph↗

arXiv2005

Constraints on the dark energy equation of state from the separation of CMB peaks and the evolution of alpha

We introduce a simple parametrization of the dark energy equation of state, ω, which is motivated from theory and recent experimental data. The theory is related to the tracker solution to alleviate the fine tuning problem. Recent experimental data indicates that the present value of ωis close to -1. We analyze the evolution of ωfrom the separation of CMB peaks and the time variation of the fine structure constant, α. We find that -1.00 \leq ω^{(0)} \leq -0.971_{-0.027}^{+0.017} and 1.76_{-0.42}^{+0.29} \times 10^{-4} \leq (d ω/ dz)_{z=0} \leq 0.041_{-0.037}^{+0.016} at 95% confidence level (CL).

astro-ph↗

arXiv2004

Quintessence Models and the Cosmological Evolution of alpha

The cosmological evolution of a quintessence-like scalar field, phi, coupled to matter and gauge fields leads to effective modifications of the coupling constants and particle masses over time. We analyze a class of models where the scalar field potential V(phi) and the couplings to matter B(phi) admit common extremum in phi, as in the Damour-Polyakov ansatz. We find that even for the simplest choices of potentials and B(phi), the observational constraints on delta alpha/alpha coming from quasar absorption spectra, the Oklo phenomenon and Big Bang nucleosynthesis provide complementary constraints on the parameters of the model. We show the evolutionary history of these models in some detail and describe the effects of a varying mass for dark matter.

astro-ph↗

arXiv2002

Stable, Time-Dependent, Exact Solutions for Brane Models with a Bulk Scalar Field

We derive two classes of brane-world solutions arising in the presence of a bulk scalar field. For static field configurations, we adopt a time-dependent, factorizable metric ansatz that allows for radion stabilization. The solutions are characterized by a non-trivial warping along the extra dimension, even in the case of a vanishing bulk cosmological constant, and lead to a variety of inflationary, time-dependent solutions of the 3D scale factor on the brane. We also derive the constraints necessary for the stability of these solutions under time-dependent perturbations of the radion field, and we demonstrate the existence of phenomenologically interesting, stable solutions with a positive cosmological constant on the brane.

hep-ph↗