• The Bulletin of The Korean Astronomical Society
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• v.32 no.1
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• pp.47.2-47.2
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• 2007

• The Bulletin of The Korean Astronomical Society
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• v.46 no.2
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• pp.43.3-43.3
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• 2021

The peculiar motion of the observer, if not (or only imperfectly) accounted for, is bound to induce a well-defined clustering signal in the distribution of galaxies. This spurious signal is related to the Kaiser rocket effect. We examined the amplitude of this effect and discuss possible implications for analysis and interpretation of future cosmological surveys. We found that it can in principle bias very significantly the inference of cosmological parameters, especially for primordial non-Gaussianity.

• The Bulletin of The Korean Astronomical Society
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• v.45 no.1
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• pp.47.3-48
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• 2020

I will present the Sejong Suite, an extensive collection of state-of-the-art high-resolution cosmological hydrodynamical simulations spanning a variety of cosmological and astrophysical parameters, primarily developed for modeling the Lyman-Alpha forest. Adopting a particle-based implementation, we follow the evolution of gas, dark matter (cold and warm), massive neutrinos, and dark radiation, and consider several combinations of box sizes and number of particles. Noticeably, for the first time, we simulate extended mixed scenarios describing the combined effects of warm dark matter, neutrinos, and dark radiation, modeled consistently by taking into account the neutrino mass splitting. Along the way, I will also highlight some new results focused on the matter and flux statistics.

• The Bulletin of The Korean Astronomical Society
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• v.39 no.2
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• pp.42.1-42.1
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• 2014

Neutrino science has received a boost of attention quite recently in cosmology, since the outstanding discovery in particle physics over the last decade that neutrinos are massive: pinpointing the neutrino masses is one of the greatest challenges in science today, at the cross-road between particle-physics, astrophysics, and cosmology. Cosmology offers a unique 'laboratory' with the best sensitivity to the neutrino mass, as primordial massive neutrinos comprise a small portion of the dark matter and are known to significantly alter structure formation. I will first introduce a new suite of state-of-the-art hydrodynamical simulations with cold dark matter, baryons and massive neutrinos, specifically targeted for modeling the low-density regions of the intergalactic medium as probed by the Lyman-Alpha forest at high-redshift. I will then present and discuss how these simulations are used to constrain the parameters of the LCDM cosmological model in presence of massive neutrinos, in combination with BOSS data and other cosmological probes, leading to the strongest bound to date on the total neutrino mass.

• The Bulletin of The Korean Astronomical Society
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• v.39 no.2
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• pp.41.1-41.1
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• 2014

Clusters of galaxies provide unique laboratories to study astrophysical processes on large scales, and are also important probes for cosmology. X-ray observations are still the best way to find and characterise clusters. The extended ROSAT-ESO flux-limited X-ray (REFLEX II) galaxy clusters form currently the largest well-defined and tested X-ray galaxy cluster sample, providing a census of the large-scale structure of the Universe out to redshifts of z-0.4. I will describe the properties of the survey and the X-ray luminosity function, which led to our recent cosmological constraints on omegaM-sigma8. They tighten the previous constraints from other X-ray experiments, showing good agreements with those from the Planck clusters, but some tension exists with the Planck CMB constraints. The second part of my talk will concern the structure of the local Universe, and the study of the first X-ray superclusters. The density of the clusters reveals an under-dense region in the nearby Universe, which has an interesting implication for the cosmological parameters. Using the X-ray superclusters, that are constructed with a physically motivated procedure, I will show environmental aspects that X-ray superclusters provide, and compare to cosmological N-body simulations.

• The Bulletin of The Korean Astronomical Society
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• v.25 no.2
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• pp.104-104
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• 2000

• Bulletin of the Korean Space Science Society
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• 2000.10a
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• pp.104-104
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• 2000

No Abstract, See Full Text

• The Bulletin of The Korean Astronomical Society
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• v.45 no.1
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• pp.49.3-49.3
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• 2020

I will give an overview of the recent work in deriving cosmological constraints from deep learning methods applied to the large scale distribution of galaxies. I will specifically highlight the success of convolutional neural networks in linking the morphology of the large scale matter distribution to dark energy parameters and modified gravity scenarios.

• Publications of The Korean Astronomical Society
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• v.30 no.2
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• pp.309-313
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• 2015

If the present universe is slightly open then pre-inflation curvature would appear as a cosmic dark-flow component of the CMB dipole moment. We summarize current cosmological constraints on this cosmic dark flow and analyze the possible constraints on parameters characterizing the pre-inflating universe in an inflation model with a present-day very slightly open ${\Lambda}CDM$ cosmology. We employ an analytic model to show that for a broad class of inflation-generating effective potentials, the simple requirement that the observed dipole moment represents the pre-inflation curvature as it enters the horizon allows one to set upper and lower limits on the magnitude and wavelength scale of pre-inflation fluctuations in the inflaton field and the curvature parameter of the pre-inflation universe, as a function of the fraction of the total initial energy density in the inflaton field. We estimate that if the current CMB dipole is a universal dark flow (or if it is near the upper limit set by the Planck Collaboration) then the present constraints on ${\Lambda}CDM$ cosmological parameters imply rather small curvature ${\Omega}_k{\sim}0.1$ for the pre-inflating universe for a broad range of the fraction of the total energy in the inflaton field at the onset of inflation. Such small pre-inflation curvature might be indicative of open-inflation models in which there are two epochs of inflation.

• The Bulletin of The Korean Astronomical Society
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• v.38 no.1
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• pp.29.1-29.1
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• 2013

We investigate the direct determination of expansion history using redshift distortions without plugging into detailed cosmological parameters. The observed spectra in redshift space include a mixture of information: fluctuations of density-density and velocity-velocity spectra, and distance measures of perpendicular and parallel components to the line of sight. Unfortunately it is hard to measure all the components simultaneously without any specific prior assumption. The degeneracy breaking, between the effect of cosmic distances and redshift distortions for example, depends on the prior we assume. An alternative approach is to utilize the cosmological principle inscribed in the heart of the Friedmann-Lematre-Robertson-Walker (hereafter FLRW) universe, that is, the specific relation between the angular diameter distance and the Hubble parameter, in this degeneracy breaking.