arXiv subjects

Seokcheon Lee

Publications and source records attributed to Seokcheon Lee.

At least 55 records · Page 3Linked to original sources

arXiv2019

Application of Clustering Analysis for Investigation of Food Accessibility

Access to food assistance programs such as food pantries and food banks needs focus in order to mitigate food insecurity. Accessibility to the food assistance programs is impacted by demographics of the population and geography of the location. It hence becomes imperative to define and identify food assistance deserts (Under-served areas) within a given region to find out the ways to improve the accessibility of food. Food banks, the supplier of food to the food agencies serving the people, can manage its resources more efficiently by targeting the food assistance deserts and increase the food supply in those regions. This paper will examine the characteristics and structure of the food assistance network in the region of Ohio by presenting the possible reasons of food insecurity in this region and identify areas wherein food agencies are needed or may not be needed. Gaussian Mixture Model (GMM) clustering technique is employed to identify the possible reasons and address this problem of food accessibility.

stat.AP↗

arXiv2019

Gravitational Waves as a Probe of the Extra Dimension

We consider the Einstein-Hilbert action without cosmological constant in 5-dimensions and implement the Kaluza-Klein (KK) reduction by compactifying the fifth direction on a circle of small but finite radius. For non-zero compactification radius, the 4- dimensional spectrum contains massless and massive KK modes. For the massive KK modes, we retain four KK tensor and one KK scalar modes after a gauge fixing. We treat those massive KK modes as stochastic sources of gravitational wave (GW) with characteristic dependences of the frequencies on the size of the extra dimension. Using the observational bounds on the size of the extra dimension and on the characteristic strain, we make an order estimation on the frequencies and amplitudes of the massive KK modes that can contribute to the GW.

hep-th↗

arXiv2019

A solution to the initial condition problems of inflation : NATON

The recent astonishing realization of the negative absolute temperature (NAT) for motional degrees of freedom \cite{Braun_ea13} inspires its possible application to the early universe. The existence of the upper bound on the energy of the system is the sufficient requirement for a NAT and this might correspond to the Planck scale at the Big Bang model. It has been argued that standard inflation requires an initial patch that is smooth over distance scales a bit larger than the causal horizon at the onset of inflation. We show that this initial condition problem can be solved if the NAT fermion, dubbed ``NATON'', occupies the Universe before the standard inflation is ignited by providing mini inflation prior to the standard one. As long as NATON dominated era lasts until at least ten times older than the Planck time, it makes the Universe homogeneous enough to derive the successful standard inflation.

astro-ph.CO↗

arXiv2018

The Exact Amplitudes of Six Polarization Modes for Gravitational Waves

The formalism of the exact six polarization modes of gravitational waves is constructed in terms of both the small metric perturbations and the Newman-Penrose scalars. The obtained formulae are applicable to any metric-compatible gravity theories whose gravitational waves propagate along either the null or non-null geodesics. Once a gravity theory, specifically its linearized wave equation, is written, comparison to the observed data of the laser interferometer experiments is direct.

gr-qc↗

arXiv2018

Constraint on reconstructed f(R) gravity models from gravitational waves

The gravitational wave (GW) detection of a binary neutron star inspiral made by the Advanced LIGO and Advanced Virgo paves the unprecedented way for multi-messenger observations. The propagation speed of this GW can be scrutinized by comparing the arrival times between GW and neutrinos or photons. It provides the constraint on the mass of the graviton. f(R) gravity theories have the habitual non-zero mass gravitons in addition to usual massless ones. Previously, we show that the model independent f(R) gravity theories can be constructed from the both background evolution and the matter growth with one undetermined parameter. We show that this parameter can be constrained from the graviton mass bound obtained from GW detection. Thus, the GW detection provides the invaluable constraint on the validity of f(R) gravity theories.

gr-qc↗

arXiv2018

Modified Starobinsky Inflation

Starobinsky has suggested an inflation model which is obtained from the vacuum Einstein's equations modified by the one-loop corrections due to quantized matter fields. Although the one-loop gravitational action is not known for a general FRW background, it can be obtained in a de Sitter space to give $\Mp^2 R + αR^2 + βR^2 \ln (R/M^2)$. Thus, one needs to investigate the inflationary behavior of this model compared to the Starobinsky model (i.e. $β= 0$). The coefficient $α$ can be changed by varying the renormalization scale $M^2$ and $β$ is obtained from the quantum anomaly which is related to the numbers of quantum fields. It has been assumed that $α\gg β$. We investigate the viable values of $α$ and $β$ based on the CMB observation. We also scrutinize the reheating process in this model.

astro-ph.CO↗

arXiv2018

Reconstruction of real-space linear matter power spectrum from multipoles of BOSS DR12 results

Recently, the power spectrum (PS) multipoles using the Baryon Oscillation Spectroscopic Survey (BOSS) Data Release 12 (DR12) sample are analyzed \cite{160703150}. The based model for the analysis is the so-called TNS quasi-linear model and the analysis provides the multipoles up to the hexadecapole \cite{TNS}. Thus, one might be able to recover the real-space linear matter PS by using the combinations of multipoles to investigate the cosmology \cite{0407214}. We provide the analytic form of the ratio of quadrupole (hexadecapole) to monopole moments of the quasi-linear PS including the Fingers-of-God (FoG) effect to recover the real-space PS in the linear regime. One expects that observed values of the ratios of multipoles should be consistent with those of the linear theory at large scales. Thus, we compare the ratios of multipoles of the linear theory, including the FoG effect with the measured values. From these, we recover the linear matter power spectra in real-space. These recovered power spectra are consistent with the linear matter power spectra.

astro-ph.CO↗

arXiv2017

Conformal Frame dependence on Cosmological Observations in Scalar-Tensor Theories of Gravity

Cosmological observations provide more accurate values both for background evolution of the Universe and for the structure formation. These values are given by the so-called dark energy equation of state, $ω$ and the growth index parameter, $γ$. From these observed parameters, one can reconstruct the model functions in scalar tensor gravity theories. However, there is a long standing debate about the (in)equality between conformally transformed frames in scalar tensor gravity models. We show that cosmological observables are frame dependent when they are described by frame independent parameter, redshift. Thus, if the cosmological observables are interpreted in one frame, then all of the observables should also be interpreted in that frame. This explicitly shows the conformal inequality of cosmological observables. Also, our method provides the model independent analysis for STG models about various observables in both frames.

gr-qc↗

arXiv2017

Reconstruction of f(R) gravity models from observations

We investigate the method for the reconstruction of f(R) gravity models both from the background evolution observations and from the large scale structure measurements. Due to the lack of the first principles, one needs to rely on the observations to build the $f(R)$ gravity models. We show that the general $f(R)$ models can be specified if the 5\% accuracy level large scale structure formation observations will be available in the near future.

astro-ph.CO↗

arXiv2015

Cross correlation of Cosmic Microwave background and Weak Lensing

The integrated Sachs-Wolfe (ISW) effect and its non-linear extension Rees-Sciama (RS) effect provide us the information of the time evolution of gravitational potential. The cross-correlation between the cosmic microwave background (CMB) and the large scale structure (LSS) is known as a promising way to extract the ISW (RS) effect. It is known that the RS effect shows the unique behavior by changing the anti-correlated cross correlation between the CMB and the mass tracer into the positively correlated cross correlation compared to the linear ISW effect. We show that the dependence of this flipping scale of the cross-correlation between RS and weak lensing on dark energy models. However, there exists the degeneracy between DE and $Ω_{\rm{m}0}$ which might be broken by redshift dependent observables. The cross-correlation between the momentum field and the density field might be served as the better observable to be used for this purpose.

astro-ph.CO↗

arXiv2015

New Method for Probing Dark Energy using the Rees-Sciama Effect

The integrated Sachs-Wolfe (ISW) effect provides us the information of the time evolution of gravitational potential. The cross-correlation between the cosmic microwave background (CMB) and the large scale structure (LSS) is known as a promising way to extract the ISW effect. Compared to CMB, the matter fluctuation can grow non-linearly and this is represented in the gravitational potential. Compared to the linear ISW effect, this non-linear ISW effect known as the Rees-Sciama (RS) effect shows the unique behavior by changing the anti-correlated cross correlation between the CMB and the mass tracer into the positively correlated cross correlation. We show that the dependence of this flipping scale on dark energy models and it might be used as a new method to investigate dark energy models.

astro-ph.CO↗

arXiv2015

Cosmic Birefringence Fluctuations and Cosmic Microwave Background $B$-mode Polarization

Recently, BICEP2 measurements of the cosmic microwave background (CMB) $B$-mode polarization has indicated the presence of primordial gravitational waves at degree angular scales, inferring the tensor-to-scalar ratio of $r=0.2$ and a running scalar spectral index. In this {\em Letter}, we show that the existence of the fluctuations of cosmological birefringence can give rise to CMB $B$-mode polarization that fits BICEP2 data with $r<0.11$ and no running of the scalar spectral index. Thus, it might be too hasty to conclude that many inflation models with small $r$ are ruled out based on BICEP2 result.

astro-ph.CO↗

arXiv2014

Measuring bias from unbiased observable

Since Kaiser introduced galaxies as a biased tracer of the underlying total mass field, the linear galaxies bias, b(z) appears ubiquitously both in theoretical calculations and in observational measurements related to galaxy surveys. However, the generic approaches to the galaxy density is a non-local and stochastic function of the underlying dark matter density and it becomes difficult to make the analytic form of b(z). Due to this fact, b(z) is known as a nuisance parameter and the effort has been made to measure bias free observable quantities. We provide the exact and analytic function of b(z) which also can be measured from galaxy surveys using the redshift space distortions parameters, more accurately unbiased observable βσ_{\rm{gal}} = f σ_8. We also introduce approximate solutions for b(z) for different gravity theories. One can generalize these approximate solutions to be exact when one solves the exact evolutions for the dark matter density fluctuation of given gravity theories. These analytic solutions for b(z) make it advantage instead of nuisance.

astro-ph.CO↗

arXiv2014

Breaking CMB degeneracy in dark energy through LSS

The cosmic microwave background and large scale structure are complementary probes to investigate the early and late time universe. After the current accomplishment of the high accuracies of CMB measurements, accompanying precision cosmology from LSS data is emphasized. We investigate the dynamical dark energy models which can produce the same CMB angular power spectra as that of the LCDM model with less than a sub-percent level accuracy. If one adopts the dynamical DE models using the so-called Chevallier-Polarski-Linder (CPL) parametrization, w = w0 + wa(1-a), then one obtains models (w0,wa)= (-0.8,-0.767), (-0.9,-0.375), (-1.1,0.355), (-1.2,0.688) named as M8, M9, M11, and M12, respectively. The differences of the growth rate, f which is related to the redshift space distortions (RSD) between different DE models and the LCDM model are about 0.2% only at z=0. The difference of f between M8 (M9, M11, M12) and the LCDM model becomes maximum at z ~ 0.25 with -2.4 (-1.2, 1.2, 2.5)%. This is a scale-independent quantity. One can investigate the one-loop correction of the matter power spectrum of each model using the standard perturbation theory in order to probe the scale-dependent quantity in the quasi-linear regime ({\it i.e.} k < 0.4 h/Mpc). The differences in the matter power spectra including the one-loop correction between M8 (M9, M11, M12) and the LCDM model for k= 0.4 h/Mpc scale are 1.8 (0.9, 1.2, 3.0)% at z=0, 3.0 (1.6, 1.9, 4.2)% at z=0.5, and 3.2 (1.7, 2.0, 4.5)% at z=1.0. The bigger departure from -1 of w0, the larger the difference in the power spectrum. Thus, one should use both the RSD and the quasi-linear observable in order to discriminate a viable DE model among a slew of models which are degenerated in CMB. Also we obtain the lower limit on w0 > -1.5 from the CMB acoustic peaks and this will provide the useful limitation on phantom models.

astro-ph.CO↗

arXiv2014

Probing Dark Energy with Neutrino Number

From measurements of the cosmic microwave background (CMB), the effective number of neutrino is found to be close to the standard model value Neff = 3.046 for the \LambdaCDM cosmology. One can obtain the same CMB angular power spectrum as that of \LambdaCDM for the different value of Neff by using the different dark energy model (i.e. for the different value of w). This degeneracy between Neff and w in CMB can be broken from future galaxy survey using the matter power spectrum.

astro-ph.CO↗

arXiv2014

Lagrangian Perturbation Theory : Exact One-Loop Power Spectrum in General Dark Energy Models

Recently, we find that the correction for the EdS assumption on the one-loop matter power spectrum for general dark energy models using the standard perturbation theory is not negligible \cite{LPB}. Thus, we investigate the same problem by obtaining the exact displacement vector and kernels up to the third order for the general dark energy models in the Lagrangian perturbation theory (LPT). Using these exact solutions, we investigate the present one-loop matter power spectrum in the $Λ$CDM model with $Ω_{m0} = 0.25 (0.3)$ to obtain about 0.2 (0.18) \% error correction compared to that obtained from the EdS assumption for $k = 0.1 \rm{h\, Mpc}^{-1}$ mode. If we consider the total matter power spectrum, the correction is only 0.05 (0.03) \% for the same mode. It means that EdS assumption is a good approximation for $Λ$CDM model in LPT theory. However, one can use this method for general models where EdS assumption is improper.

astro-ph.CO↗

arXiv2014

Exact Third-Order Density Perturbation and One-Loop Power Spectrum in General Dark Energy Models

Under the standard perturbation theory (SPT), we obtain the fully consistent third-order density fluctuation and kernels for the general dark energy models without using the Einstein-de Sitter (EdS) universe assumption for the first time. We also show that even though the temporal and spatial components of the SPT solutions can not be separable, one can find the exact solutions to any order in general dark energy models. With these exact solutions, we obtain the less than \% error correction of one-loop matter power spectrum compared to that obtained from the EdS assumption for $k = 0.1 {\rm h\, Mpc}^{-1}$ mode at $z = 0$ (1, 1.5). Thus, the EdS assumption works very well at this scale. However, if one considers the correction for $P_{13}$, the error is about 6 (9, 11) \% for the same mode at $z = 0$ (1, 1.5). One absorbs $P_{13}$ into the linear power spectrum in the renormalized perturbation theory (RPT) and thus one should use the exact solution instead of the approximation one. The error on the resummed propagator $N$ of RPT is about 14 (8, 6) \% at $z =0$ (1, 1.5) for $k = 0.4 {\rm h\, Mpc}^{-1}$. For $k = 1 {\rm h\, Mpc}^{-1}$, the error correction of the total matter power spectrum is about 3.6 (4.6, 4.5) \% at $z = 0$ (1, 1.5). Upcoming observation is required to archive the sub-percent accuracy to provide the strong constraint on the dark energy and this consistent solution is prerequisite for the model comparison.

astro-ph.CO↗

arXiv2014

Imprint of Scalar Dark Energy on Cosmic Microwave Background Polarization

We study the imprint of a coupling of scalar dark energy to photon on the cosmic microwave background (CMB) polarization. The time-evolving field value as well as the perturbation of the scalar generically induce $B$-mode polarization. Future CMB data will find either a cosmic parity violation in temperature-polarization correlation due to the field value, or perturbation-induced $B$-mode polarization that is almost indistinguishable from that generated by gravitational lensing or primordial gravitational waves.

astro-ph.CO↗