arXiv subjects

Junsup Shim

Publications and source records attributed to Junsup Shim.

15 recordsLinked to original sources

arXiv2026

Enhancing the Detection Sensitivity of Primordial Parity Violation using Galaxy Spins

It has been recently demonstrated that the signature of primordial parity violation could be imprinted in halo spins, indicating its potential detectability through the late-time galaxy spin field (Shim et al. 2025). In this study, we develop an optimized halo selection strategy to enhance the detection significance of such a signal, focusing on halo mass and local density. Using N-body simulations with parity-asymmetric initial conditions, we show that the optimized halo sample allows for a higher detection sensitivity than the full halo sample, despite its reduced sample size. Finally, we assess the observational feasibility of our strategy and show that future spectroscopic surveys can provide sufficient data to realize this enhanced sensitivity.

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arXiv2026

Which filaments matter: the relative scalings of anisotropic infall

Dark-matter haloes do not form in isolation but within the surrounding cosmic web. By the time a halo begins to collapse, its larger-scale environment has typically collapsed along two axes, forming filaments that channel anisotropic infall toward the halo. In this work, we derive from first principles the characteristic Lagrangian scale ratio at which such an anisotropic tidal field most strongly influences halo formation. Specifically, we identify the inflection point of the conditional probability that the tidal field, smoothed on a scale Rsd, undergoes two-dimensional compression, given the presence of a density peak of rarity nu on a smaller scale Rpk. For a standard LambdaCDM cosmology, we find (Rsd/Rpk)infl = 2.2 + (nu-2.5) for Rpk corresponding to a tophat filter of 8Mpc/h. This result implies that the anisotropic tidal influence on a collapsing halo typically extends to 2-3 times the size of its Lagrangian patch. Recast as a function of formation redshift z, the characteristic filament scale around 2.5 sigma peaks can be approximated by Rsd(z) = 31 /(2+(1+z)**2)Mpc/h. We provide practical scaling laws for selecting dynamically relevant smoothing scales in large-scale surveys and for setting initial patch sizes in high-resolution zoom simulations.

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arXiv2024

Improving cosmological constraints via galaxy intrinsic alignment in full-shape analysis

The intrinsic alignment (IA) of galaxy shapes probes the underlying gravitational tidal field, thus offering cosmological information complementary to galaxy clustering. In this paper, we perform a Fisher forecast to assess the benefit of IA in improving cosmological parameter constraints, for the first time, leveraging the full-shape (FS) information of IA statistics. Our forecast is based on PFS-like and Euclid-like surveys as examples of deep and wide galaxy surveys, respectively. We explore various cosmological models, with the most comprehensive one simultaneously including dynamical dark energy, curvature, massive neutrinos, and modified gravity (MG). We find that adding FS IA information significantly tightens cosmological constraints relative to the FS clustering-only cases, particularly for dynamical dark energy and nonflat-MG models. For a deep galaxy survey, the Figure-of-Merit for the dark energy equation of state parameters is improved by at least more than $40\%$ in all dynamical dark energy models investigated. For nonflat-MG models, parameter constraints are tightened by $6-28\%$, except for the dark matter density and spectral index parameters. For a wide galaxy survey, improvements with IA become milder, although its joint constraints are tighter than those from the deep survey. Our findings highlight the efficacy of the galaxy IA as a complementary cosmological probe to galaxy clustering.

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arXiv2024

Squeezing Full-Shape Dynamical Dark Energy Constraints with Galaxy Alignments

Recent $2-4\sigma$ deviations from the Cosmological Constant $\Lambda$ suggest that dark energy (DE) may be dynamical, based on baryon acoustic oscillations and full-shape galaxy clustering (FS GC) analyses. This calls for even tighter DE constraints to narrow down its true nature. In this Letter, we explore how galaxy intrinsic alignments (IA) can enhance the FS GC-based DE constraints, using Fisher forecasts on various extensions of dynamical DE models, including scenarios with curvature, massive neutrinos, and modified gravity. Incorporating IA improves the DE Figure-of-Merit by $42-57\%$ and tightens the primordial power spectrum amplitude constraints by $17-19\%$. Our findings highlight IA's potential as a valuable cosmological probe complementary to GC.

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arXiv2024

Probing vector chirality in the early Universe

We explore the potential of using late-time galaxy spins to test the parity symmetry of primordial vector fossils. Using $N$-body simulations, we analyze halo spins as a reliable proxy for galaxy spins to investigate the detectability of this effect. We develop a novel approach to generate initial conditions (ICs) that have substantial parity asymmetry but do not alter the initial matter power spectrum. We construct the initial spin fields from the parity broken ICs and halo spin fields using late-time halos evolved from such ICs. Focusing on the helicity of these vector fields, we detect substantial asymmetry in the initial spin field. In addition, we find that over $50\%$ of the initial spin field's asymmetry remains in the late-time halo spin field on a range of scales. Based on mock galaxy spin fields derived from the halo spin fields, we forecast that a maximum detection at $13\sigma$ is possible with the final DESI BGS for the model considered in this analysis. Our findings demonstrate that primordial vectorial parity violation survives nonlinear gravitational evolution, and thus, can be effectively probed with galaxy spins at late times.

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arXiv2023

Probing cosmology via the clustering of critical points

Exclusion zones in the cross-correlations between critical points (peak-void, peak-wall, filament-wall, filament-void) of the density field define quasi-standard rulers that can be used to constrain dark matter and dark energy cosmological parameters. The average size of the exclusion zone is found to scale linearly with the typical distance between extrema. The latter changes as a function of the matter content of the universe in a predictable manner, but its comoving size remains essentially constant in the linear regime of structure growth on large scales, unless the incorrect cosmology is assumed in the redshift-distance relation. This can be used to constrain the dark energy parameters when considering a survey that scans a range of redshifts. The precision of the parameter estimation is assessed using a set of cosmological simulations, and is found to be a 4$σ$ detection of a change in matter content of 5%, or about 3.8$σ$ detection of 50% shift in the dark energy parameter using a full sky survey up to redshift 0.5.

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arXiv2023

Cluster-counterpart Voids: Void Identification from Galaxy Density Field

We identify cosmic voids from galaxy density fields under the theory of void-cluster correspondence. We extend the previous novel void-identification method developed for the matter density field to the galaxy density field for practical applications. From cosmological N-body simulations, we construct galaxy number- and mass-weighted density fields to identify cosmic voids that are counterparts of galaxy clusters of specific mass. The parameters for the cluster-counterpart void identification such as Gaussian smoothing scale, density threshold, and core volume fraction are found for galaxy density fields. We achieve about $60$--$67\%$ of completeness and reliability for identifying the voids of corresponding cluster mass above $3\times10^{14}h^{-1}M_{\odot}$ from a galaxy sample with the mean number density, $\bar{n}=4.4\times10^{-3} (h^{-1}{\rm Mpc})^{-3}$. When the mean density is increased to $\bar{n}=10^{-2} (h^{-1}{\rm Mpc})^{-3}$, the detection rate is enhanced by $\sim2$--$7\%$ depending on the `mass scale' of voids. We find that the detectability is insensitive to the density weighting scheme applied to generate the density field. Our result demonstrates that we can apply this method to the galaxy redshift survey data to identify cosmic voids corresponding statistically to the galaxy clusters in a given mass range.

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arXiv2022

Forecasts for WEAVE-QSO: 3D clustering and connectivity of critical points with Lyman-$α$ tomography

The upcoming WEAVE-QSO survey will target a high density of quasars over a large area, enabling the reconstruction of the 3D density field through Lyman-$α$ tomography over unprecedented volumes smoothed on intermediate scales ($\approx$ 16 Mpc/$h$). We produce mocks of the Lyman-$α$ forest using LyMAS, and reconstruct the 3D density field between sightlines through Wiener filtering in a configuration compatible with the future WEAVE-QSO observations. The fidelity of the reconstruction is assessed by measuring one- and two-point statistics from the distribution of critical points in the cosmic web. In addition, initial Lagrangian statistics are predicted from first principles, and measurements of the connectivity of the cosmic web are performed. The reconstruction captures well the expected features in the auto- and cross-correlations of the critical points. This remains true after a realistic noise is added to the synthetic spectra, even though sparsity of sightlines introduces systematics, especially in the cross-correlations of points with mixed signature. Specifically, for walls and filaments, the most striking clustering features could be measured with up to 4 sigma of significance with a WEAVE-QSO-like survey. Moreover, the connectivity of each peak identified in the reconstructed field is globally consistent with its counterpart in the original field, indicating that the reconstruction preserves the geometry of the density field not only statistically, but also locally. Hence the critical points relative positions within the tomographic reconstruction could be used as standard rulers for dark energy by WEAVE-QSO and similar surveys.

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arXiv2021

Identification of Cosmic Voids as Massive Cluster Counterparts

We develop a method to identify cosmic voids from the matter density field by adopting a physically-motivated concept that voids are the counterpart of massive clusters. To prove the concept we use a pair of $Λ$CDM simulations, a reference and its initial density-inverted mirror simulation, and study the relation between the effective size of voids and the mass of corresponding clusters. Galaxy cluster-scale dark matter halos are identified in the Mirror simulation at $z=0$ by linking dark matter particles. The void corresponding to each cluster is defined in the Reference simulation as the region occupied by the member particles of the cluster. We study the voids corresponding to the halos more massive than $10^{13}h^{-1}M_{\odot}$. We find a power-law scaling relation between the void size and the corresponding cluster mass. Voids with corresponding cluster mass above $10^{15}h^{-1}M_{\odot}$ occupy $\sim1\%$ of the total simulated volume, whereas this fraction increases to $\sim54\%$ for voids with corresponding cluster mass above $10^{13}h^{-1}M_{\odot}$. It is also found that the density profile of the identified voids follows a universal functional form. Based on these findings, we propose a method to identify cluster-counterpart voids directly from the matter density field without their mirror information by utilizing three parameters such as the smoothing scale, density threshold, and minimum core fraction. We recover voids corresponding to clusters more massive than $3\times10^{14}h^{-1}M_{\odot}$ at 70--74 \% level of completeness and reliability. Our results suggest that we are able to identify voids in a way to associate them with clusters of a particular mass-scale.

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arXiv2020

The clustering of critical points in the evolving cosmic web

Focusing on both small separations and Baryonic Acoustic Oscillation scales, the cosmic evolution of the clustering properties of peak, void, wall, and filament-type critical points is measured using two-point correlation functions in $Λ$CDM dark matter simulations as a function of their relative rarity. A qualitative comparison to the corresponding theory for Gaussian Random fields allows us to understand the following observed features: i) the appearance of an exclusion zone at small separation, whose size depends both on rarity and on the signature (\ie the number of negative eigenvalues) of the critical points involved; ii) the amplification of the Baryonic Acoustic Oscillation bump with rarity and its reversal for cross-correlations involving negatively biased critical points; iii) the orientation-dependent small-separation divergence of the cross-correlations of peaks and filaments (voids and walls) which reflects the relative loci of such points in the filament's (wall's) eigenframe. The most significant features of the correlations are tabulated. The (cross-) correlations involving the most non-linear critical points (peaks, voids) display significant variation with redshift, while those involving less non-linear critical points seem mostly insensitive to redshift evolution, which should prove advantageous to model. The relative distances to the maxima of the peak-to-wall and peak-to-void over that of the peak-to-filament cross-correlation are in ratios of $\sim\sqrt{2}$ and $\sim\sqrt{3}$, respectively which could be interpreted as an indication of the cosmic crystal being on average close to a cubic lattice. The insensitivity to redshift evolution suggests that the absolute and relative clustering of critical points could become a topologically robust alternative to standard clustering techniques when analyzing upcoming large scale surveys such as Euclid or LSST.

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arXiv2018

Dependence of the Substructure Abundance on the Orientation Coherence of the Halo Tidal Field

A numerical evidence for the dependence of the substructure abundance of cluster halos on the orientation coherence of the surrounding tidal fields is presented. Applying the adapted minimal spanning tree (MST) algorithm to the cluster halos with $M\ge 10^{14}\,h^{-1}\,M_{\odot}$ from the Big MultiDark-Planck Simulations, we identify primary MST stems composed of multiple nodes as the filaments and measure their specific sizes (spatial extents per node), which quantify the orientation coherence of the tidal field on the cluster scales. Classifying the cluster halos into five samples by the specific sizes of their host filaments and tallying the mass distributions of the five samples, we investigate if and how the substructure abundance of the cluster halos differs among the five samples. It is found that the cluster halos embedded in the filaments with larger specific sizes tend to possess less substructures. This anti-correlation is also shown robust against narrowing down the formation epochs of the cluster halos as well as against fixing the node number of the filaments. Given that the filaments with larger specific sizes form at the regions where the surrounding tidal fields are coherent over large scales in the orientations of their principal axes, we suggest that in the filaments with larger specific sizes the satellite infall and matter accretion onto the cluster halos should be obstructed due to the development of the tangential velocities in the plane perpendicular to the elongated axes of the filaments.

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arXiv2015

An observational detection of the bridge effect of void filaments

The bridge effect of void filaments is a phrase coined by Park & Lee (2009b) to explain the correlations found in a numerical experiment between the luminosity of the void galaxies and the degree of the straightness of their host filaments. Their numerical finding implies that a straight void filament provides a narrow channel for the efficient transportation of gas and matter particles from the surroundings into the void galaxies. Analyzing the Sloan void catalog constructed by Pan et al (2012), we identify the filamentary structures in void regions and determine the specific size of each void filament as a measure of its straightness. To avoid possible spurious signals caused by the Malmquist bias, we consider only those void filaments whose redshifts are in the range of 0=< z <= 0.02 and find a clear tendency that the void galaxies located in the more straight filaments are on average more luminous, which is in qualitative agreement with the numerical prediction. It is also shown that the strength of correlation increases with the number of the member galaxies of the void filaments, which can be physically understood on the grounds that the more stretched filaments can connect the dense surroundings even to the galaxies located deep in the central parts of the voids. This observational evidence may provide a key clue to the puzzling issue of why the void galaxies have higher specific star formation rates and bluer colors than their wall counterparts.

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arXiv2014

Breaking the Cosmic Degeneracy between Modified Gravity and Massive Neutrinos with the Cosmic Web

In a recent work, Baldi et al. highlighted the issue of cosmic degeneracies, consisting in the fact that the standard statistics of the large-scale structure might not be sufficient to conclusively test cosmological models beyond $Λ$CDM when multiple extensions of the standard scenario coexist in nature. In particular, it was shown that the characteristic features of an $f(R)$ Modified Gravity theory and of massive neutrinos with an appreciable total mass $Σ_{i}m_{ν_{i}}$ are suppressed in most of the basic large-scale structure observables for a specific combination of the main parameters of the two non-standard models. In the present work, we explore the possibility that the mean specific size of the supercluster spines -- which was recently proposed as a non-standard statistics by Shim and Lee to probe gravity at large scales -- can help to break this cosmic degeneracy. By analyzing the halo samples from N-body simulations featuring various combinations of $f(R)$ and $Σ_{i}m_{ν_{i}}$ we find that -- at the present epoch -- the value of $Σ_{i}m_{ν_{i}}$ required to maximally suppress the effects of $f(R)$ gravity on the specific sizes of the superclusters spines is different from that found for the other standard statistics. Furthermore, it is also shown that at higher redshifts ($z\ge 0.3$) the deviations of the mean specific sizes of the supercluster spines for all of the four considered combinations from its value for the standard $Λ$CDM case are statistically significant.

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arXiv2014

Massive Gravity wrapped in the Cosmic Web

We study how the filamentary pattern of the cosmic web changes if the true gravity deviates from the general relativity (GR) on the large scale. The f(R) gravity whose strength is controlled to satisfy the current observational constraints on the cluster scale is adopted as our fiducial model and a large N-body simulation of high-resolution is utilized for this study. By applying the minimal spanning tree algorithm to the halo catalogs from the simulation at various epochs, we identify the main stems of the rich superclusters located in the most prominent filamentary section of the cosmic web and determine their spatial extents per member cluster as the degree of their straightness. It is found that the $f(R)$ gravity has an effect of significantly bending the superclusters and that the effect becomes stronger as the universe evolves. Even in the case where the deviation from GR is too small to be detectable by any other observables, the degree of the supercluster straightness exhibits conspicuous difference between the $f(R)$ and the GR models. Our results also imply that the supercluster straightness could be a useful discriminator of f(R) gravity from the coupled dark energy since it is shown to evolve differently between the two models. As a final conclusion, the degree of the straightness of the rich superclusters should provide a powerful cosmological test of large scale gravity.

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arXiv2013

Dark Sector Coupling Bends the Superclusters

The galaxy clusters exhibit noticeably anisotropic pattern in their clustering, which is vividly manifested by the presence of rich filament-like superclusters. The more anisotropic the clustering of galaxy clusters is, the more straight the rich filament-like superclusters become. Given that the degree of the anisotropy in the largest-scale clustering depends sensitively on the nature of dark energy, the supercluster straightness may play a complimentary role in testing dynamic dark energy models. Here we focus on the coupled dark energy (cDE) models which assume the existence of dark sector coupling between scalar field dark energy and nonbaryonic dark matter. Determining the spines of the superclusters identified in the publicly available group catalogs from the CODECS (COupled Dark Energy Cosmological Simulations) for four different cDE models as well as for the LCDM model, we quantify the straightness of each supercluster as the spatial extent of its spine per member cluster where a supercluster spine represents the main stem of the minimal spanning tree constructed out of the member clusters. It is shown that the dark sector coupling plays a role in making the supercluster less straight relative to the LCDM case and that in a cDE model with supergravity potential the superclusters are least straight. We also find that the difference in the degree of the supercluster straightness between the cDE and the LCDM cases increases with redshifts. A physical interpretation of our result as well as its cosmological implication are discussed.

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