The Filament Rift: $Λ$CDM's Structural Challenge Against Observation

TL;DR

This study tests the reliability of $\Lambda$CDM predictions for the Cosmic Web by comparing observed SDSS DR10 filaments ($z<0.05$) with those from the IllustrisTNG300-1 simulation ($z=0$) using the GrAviPaSt filament finder. It builds macro- and micro-filament catalogs and analyzes their lengths, mass-density contrasts, and the properties of filament galaxies, including $GG$ (group-group) and $CC$ (cluster-cluster) subcategories. The results reveal that observed macro-filaments are denser yet shorter than their simulated counterparts, while micro-filaments in $\Lambda$CDM generally show higher density contrasts at fixed geometry; unexpectedly, the $GG$ and $CC$ trends invert between data and model. These tensions indicate potential gaps in the environmental processes modeled in simulations and motivate further multi-method, multi-simulation cross-checks to robustly test large-scale structure formation and galaxy evolution in filaments.

Abstract

This study presents the first extended comparison of cosmic filaments identified in SDSS DR10 observations ($z < 0.05$) and the IllustrisTNG300-1 $Λ$CDM simulation ($z = 0$), utilizing the novel GrAviPaSt filament-finder method. The analyses are performed on both macro- and micro-filaments, each characterized by their length, thickness, and contrast in mass density. In addition to total sample comparisons, two subcategories of micro-filaments, GG (linking galaxy groups) and CC (linking galaxy clusters), are introduced to further analyze discrepancies between the $Λ$CDM model and observation. While $Λ$CDM produces extended macro-filaments, such structures are largely absent in SDSS, and where present, they exhibit higher densities than their simulated counterparts. Micro-filaments also show notable density discrepancies: at fixed length and thickness, observational filaments are significantly denser than those in the simulation. Employing radial density profiles reveal that micro-filaments in the $Λ$CDM simulation exhibit higher contrasts in mass density relative to the background compared to their observational counterparts. Notably, CC type micro-filaments displayed enhanced density contrasts over GG types in the simulation, while observational data showed the opposite trend. Furthermore, SDSS galaxies in both GG and CC micro-filaments exhibit lower specific star formation rates (sSFR) and older stellar populations, while TNG300-1 micro-filaments host more actively star-forming galaxies within the intermediate stellar mass range. These results reveal persistent discrepancies between observational data and the $Λ$CDM reconstruction of cosmic filaments, pointing to possible tensions in our current understanding of large-scale structures and their environmental effects on galaxy evolution.

Figures (5)

• Figure 1: Median trends of macro-filament mass density contrast ($\delta_{M_{\odot}/h}$) are shown with respect to filament length for both observational and simulation datasets, with $1\sigma$ scatter represented as error bars. Histograms of the distributions are displayed adjacent to the central plot to highlight the comparative structure of the two samples.
• Figure 2: Median trends of micro-filament mass density contrast ($\delta_{M_{\odot}/h}$) with respect to length and thickness are shown for the $Total$, $GG$, and $CC$ categories, arranged from top to bottom. Error bars represent the $1\sigma$ scatter. Dashed lines and darker colors correspond to the $\Lambda$CDM simulation dataset, while solid lines and lighter colors denote the observational data.
• Figure 3: Radial (left) and longitudinal (right) mass density contrast profiles for micro-filament samples are shown for both the $\Lambda$CDM simulation (top) and the observational dataset (bottom). Each panel presents three micro-filament categories, $Total$, $CC$, and $GG$, distinguished by different colors. Dashed lines represent the fitted curves, based on the Plummer (radial) and Hyperbolic-Secant (longitudinal) profiles.
• Figure 4: Median trends for distances to micro-filament skeletons are shown relative to the normalized longitudinal position ($\Gamma$), along with corresponding 1$\sigma$ errors, for $CC$ and $GG$ micro-filaments. Results are presented for both the $\Lambda$CDM simulation and observational datasets, with dashed lines indicating simulation-based medians and solid lines representing observational values.
• Figure 5: Median trends and associated $1\sigma$ errors of the specific star formation rate ($sSFR$) of filament galaxies with respect to stellar mass ($top$) and color-magnitude diagram ($bottom$), shown for both $\Lambda$CDM simulation (dashed lines) and observational (solid lines) samples. The $CC$ and $GG$ filament categories are represented using dark and light colors, respectively.