Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

The Giant Arc -- Filament or Figment?

Till Sawala, Meri Teeriaho

TL;DR

The Giant Arc paper interrogates whether a large MgII absorber pattern signifies a breakdown of cosmic homogeneity or is a statistical artifact. By applying the same FoF linking-length to both Lambda-CDM simulations and uniform random controls and by testing proposed inhomogeneity corrections using Gaussian random-field backgrounds, the authors show Giant Arc–like structures are common in homogeneous distributions and can be amplified by background fluctuations. They argue that the originally claimed fixes for linking length are unnecessary and often counterproductive, highlighting the risk of look-elsewhere effects and parameter-tuning. The work reinforces the interpretation that observed large patterns can arise in a statistically homogeneous universe and underscores the need for rigorous null controls when making claims about fundamental cosmology.

Abstract

The so-called "Giant Arc" is a sparse pattern of MgII absorbers spanning approximately 740 comoving Mpc, whose discovery has been claimed to contradict the large-scale homogeneity inherent to the standard cosmological model. We previously showed that, with the same algorithm and parameters used for its discovery, very similar patterns are abundant in uniform random distributions, and among equivalent halo samples in a cosmological simulation of the standard model. In a response, the original discoverers of the "Giant Arc" have argued that these parameters were only appropriate for their specific observational data, but that a smaller linking length should be used for control studies, in which case far fewer patterns are detected. We briefly review and disprove these arguments, and demonstrate that large patterns like the "Giant Arc" are indeed ubiquitous in a statistically homogeneous universe.

The Giant Arc -- Filament or Figment?

TL;DR

The Giant Arc paper interrogates whether a large MgII absorber pattern signifies a breakdown of cosmic homogeneity or is a statistical artifact. By applying the same FoF linking-length to both Lambda-CDM simulations and uniform random controls and by testing proposed inhomogeneity corrections using Gaussian random-field backgrounds, the authors show Giant Arc–like structures are common in homogeneous distributions and can be amplified by background fluctuations. They argue that the originally claimed fixes for linking length are unnecessary and often counterproductive, highlighting the risk of look-elsewhere effects and parameter-tuning. The work reinforces the interpretation that observed large patterns can arise in a statistically homogeneous universe and underscores the need for rigorous null controls when making claims about fundamental cosmology.

Abstract

The so-called "Giant Arc" is a sparse pattern of MgII absorbers spanning approximately 740 comoving Mpc, whose discovery has been claimed to contradict the large-scale homogeneity inherent to the standard cosmological model. We previously showed that, with the same algorithm and parameters used for its discovery, very similar patterns are abundant in uniform random distributions, and among equivalent halo samples in a cosmological simulation of the standard model. In a response, the original discoverers of the "Giant Arc" have argued that these parameters were only appropriate for their specific observational data, but that a smaller linking length should be used for control studies, in which case far fewer patterns are detected. We briefly review and disprove these arguments, and demonstrate that large patterns like the "Giant Arc" are indeed ubiquitous in a statistically homogeneous universe.
Paper Structure (4 sections, 1 equation, 2 figures)

This paper contains 4 sections, 1 equation, 2 figures.

Table of Contents

  1. Introduction
  2. Arguments for different linking lengths
  3. The actual effect of background inhomogeneity and errors
  4. Conclusion

Figures (2)

  • Figure 1: Realisations of random fields of covariance lengths, $\mathrm{L_{cov}}$, ranging from 0 (homogeneous) to 400 $c$Mpc, representing increasingly inhomogeneous background densities, coloured here in terms of quasar surface density, $\Sigma_q$. $\sigma_{65}$ denotes the standard deviation in quasar number counts in cells of area 65$^2$$c$Mpc$^2$ averaged over 1000 realisations.
  • Figure 2: Probability density (left) and cumulative distribution (right) for the length ,$\mathrm{L_{max}}$, of the longest anisotropic ($b/a < 0.4$) pattern per slice of $2800\times2800\times338$$c$Mpc$^3$ under different assumptions for the inhomogeneity of the background density. Dotted vertical lines denote 742 $c$Mpc, the extent of the "Giant Arc". Foreground patterns as long or longer than the "Giant Arc" are common for all backgrounds, and slightly more common for more inhomogeneous backgrounds.