石山 智明

Observations favor cosmological models with a time-varying dark energy component. But how does dynamical dark energy (DDE) influence the growth of structure in an expanding Universe? We investigate this question using high-resolution $N$-body simulations based on a DDE cosmology constrained by first-year DESI data (DESIY1$+$DDE), characterized by a 4% lower Hubble constant ($H_0$) and 10% higher matter density ($\Omega_0$) than the Planck-2018 $\Lambda$CDM model. We examine the impact on the matter power spectrum, halo abundances, clustering, and Baryonic Acoustic Oscillations (BAO). We find that DESIY1$+$DDE exhibits a 10% excess in power at small scales and a 15% suppression at large scales, driven primarily by its higher $\Omega_0$. This trend is reflected in the halo mass function: DESIY1$+$DDE predicts up to 70% more massive halos at $z = 2$ and a 40% excess at $z = 0.3$. Clustering analysis reveals a 3.71% shift of the BAO peak towards smaller scales in DESIY1$+$DDE, consistent with its reduced sound horizon compared to Planck18 Measurements of the BAO dilation parameter $\alpha$, using halo samples with DESI-like tracer number densities across $0 < z < 1.5$, agree with the expected DESIY1$+$DDE-to-Planck18 sound horizon ratio. After accounting for cosmology-dependent distances, the simulation-based observational dilation parameter closely matches DESI Y1 data. We find that the impact of DDE is severely limited by current observational constraints, which strongly favor cosmological models -- whether including DDE or not -- with a tightly constrained parameter $\Omega_0h^2\approx 0.143$, within 1-2% uncertainty. Indeed, our results demonstrate that variations in cosmological parameters, particularly $\Omega_0$, have a greater influence on structure formation than the DDE component alone.

Abstract We present the Baryon Pasted (BP) X-ray and thermal Sunyaev–Zel’dovich (tSZ) maps derived from the half-sky Uchuu light-cone simulation. These BP-Uchuu maps are constructed using more than 75 million dark matter halos with masses M _500c ≥ 10¹³ M _⊙ within the redshift range 0 ≤ z ≤ 2. A distinctive feature of our BP-Uchuu light-cone maps is their capability to assess the influence of both extrinsic and intrinsic scatter caused by triaxial gaseous halos and internal gas characteristics, respectively, at the map level. We show that triaxial gas drives substantial scatter in X-ray luminosities of clusters and groups, accounting for nearly half of the total scatter in core-excised measurements. Additionally, scatter in the thermal pressure and gas density profiles of halos enhances the X-ray and SZ power spectra, leading to biases in cosmological parameter estimates. These findings are statistically robust due to the extensive sky coverage and large halo sample in the BP-Uchuu maps. The BP-Uchuu maps are publicly available online via Globus (https://app.globus.org/file-manager?origin_id=cf8dadb7-b6e9-4e2c-abc1-0813877efc13).

We develop a new semi-analytic framework of Population (Pop) III and subsequent galaxy formation designed to run on dark matter halo merger trees. In our framework, we consider the effect of the Lyman-Werner flux from Pop III and II stars and the dark matter baryon streaming velocity on the critical minihalo mass for the Pop III formation. Our model incorporates the Lyman-Werner feedback in a self-consistent way, therefore, the spatial variation of Lyman-Werner feedback naturally emerges. The Pop III mass depends on the properties of a minihalo as reproducing radiative hydrodynamical simulation results. We perform statistical studies of Pop III stars by applying this framework to high-resolution cosmological N-body simulations with a maximum box size of 16 Mpc/h and enough mass resolution to resolve Pop III-forming minihalos. A top-heavy initial mass function emerges and two peaks corresponding to the H$_2$ ($20 \lesssim z \lesssim 25$) and atomic cooling halos ($z \lesssim 15$) exist in the distribution. Supermassive stars can be formed in the atomic cooling halos, and the fractions of such supermassive stars increase with the value of streaming velocity. At least an 8 Mpc/h simulation box and the self-consistent model for the Lyman-Werner feedback are necessary to correctly model the Pop III formation in the atomic cooling halos. Our model predicts one supermassive star per halo with several $10^9$ Msun at z=7.5, which is enough to reproduce a high redshift quasar.

We aim to constrain the amplitude of the linear spectrum of density fluctuations (σ_8), the matter density parameter (Ω_ m), the Hubble constant (H_0) c h, and S_8 from Sloan Digital Sky Survey Data Release 7 (SDSS DR7) by studying the abundance of large voids in the large-scale structure of galaxies. Voids are identified as maximal non-overlapping spheres within SDSS DR7 galaxies with redshifts of $0.02&lt;z&lt;0.132$ and absolute magnitudes of M_ r &lt;-20.5. We used the theoretical framework developed in previous works and recalibrated the data using halo simulations to constrain σ_8, Ω_ m, and H_0 from the sample of SDSS galaxies mentioned above using a Bayesian analysis and Markov chain Monte Carlo (MCMC) technique. This method has also been validated using simulated halo boxes and galaxy lightcones. We have proven that the theoretical framework recovers σ_8, Ω_ m, and H_0 values from the halo simulation boxes for different values of σ_8 within 1σ ($2σ$) in real (redshift) space. The theoretical framework void statistics from mock lightcones shows significant potential: we have studied the marginalised posteriors in each plane and checked that we were able to recover Planck values for the all the parameters. The results we obtained from the SDSS sample are: Ω_ m H_ Γ=0.270^ and S_ . Combining these constraints with the Kilo Degree Survey (KiDS-1000) and the Dark Energy Survey (DESY3) yields σ_ Ω_ m H_ and S_ . The combined uncertainties of σ_8 and Ω_ m have been reduced by a factor of 2-3, compared to KiDS-100+DESY3 alone, due to the nearly orthogonal marginalised posteriors of SDSS voids and weak lensing in the -Ω_ m plane.

MPI-Rockstar is a massively parallel halo finder based on the Rockstar phase-space temporal halo finder code, which is one of the most extensively used halo finding codes. Compared to the original code, parallelized by a primitive socket communication library, we parallelized it in a hybrid way using MPI and OpenMP, which is suitable for analysis on the hybrid shared and distributed memory environments of modern supercomputers. This implementation can easily handle the analysis of more than a trillion particles on more than 100,000 parallel processes, enabling the production of a huge dataset for the next generation of cosmological surveys. As new functions to the original Rockstar code, MPI-Rockstar supports HDF5 as an output format and can output additional halo properties such as the inertia tensor.