[Paper Review] Large-scale surveys and cosmic structure

J. A. Peacock|arXiv (Cornell University)|Sep 8, 2003

Cosmology and Gravitation Theories3 references3 citations

TL;DR

This paper presents a comprehensive analysis of large-scale structure using the 2dF Galaxy Redshift Survey, demonstrating that combining galaxy survey data with cosmic microwave background anisotropy measurements eliminates cosmological degeneracies. It establishes a concordance cosmological model dominated by cold dark matter and vacuum energy in a geometrically flat universe, with parameters constrained to Ωₘ ≈ 0.25 and h ≈ 0.73.

ABSTRACT

These lectures deal with our current knowledge of the matter distribution in the universe, focusing on how this is studied via the large-scale structure seen in galaxy surveys. We first assemble the necessary basics needed to understand the development of density fluctuations in an expanding universe, and discuss how galaxies are located within the dark-matter density field. Results from the 2dF Galaxy Redshift Survey are presented and contrasted with theoretical models. We show that the combination of large-scale structure and data on microwave-background anisotropies can eliminate almost all degeneracies, and yield a completely specified cosmological model. This is the "concordance" universe: a geometrically flat combination of vacuum energy and cold dark matter. The study of cosmic structure is able to establish this in a manner independent of external information, such as the Hubble diagram; this extra information can however be used to limit non-standard alternatives, such as a variable equation of state for the vacuum.

Motivation & Objective

• To understand the origin and evolution of large-scale structure in the universe using galaxy redshift surveys.
• To determine the cosmological parameters governing structure formation by combining large-scale structure data with cosmic microwave background (CMB) anisotropy measurements.
• To test the consistency of the standard cosmological model by eliminating degeneracies inherent in structure formation theories.
• To constrain non-standard models such as variable vacuum equation of state or extra relativistic degrees of freedom.

Proposed method

• Uses the dimensionless density perturbation δ(x) = ρ(x)/⟨ρ⟩ − 1 to describe inhomogeneities in the matter distribution.
• Applies linear perturbation theory to model the growth of density fluctuations in an expanding universe.
• Combines results from the 2dF Galaxy Redshift Survey (2dFGRS) with CMB anisotropy data to break degeneracies in cosmological parameters.
• Employs the acoustic peak location in the CMB power spectrum as a standard ruler to constrain the matter-radiation equality redshift and Ωₘh.
• Uses the relation between the Hubble parameter h and the effective radiation content X to test models with enhanced radiation, such as additional neutrino species.
• Applies the constraint Ωₘ⁻⁰·¹(ωₘ/X)⁰·²⁴ = constant to link CMB and large-scale structure data, showing that Ωₘ ≈ 0.3 is robust regardless of X.

Experimental results

Research questions

• RQ1 How do large-scale structures in the galaxy distribution constrain cosmological parameters such as Ωₘ and h?
• RQ2 To what extent can the combination of large-scale structure and CMB anisotropy data break degeneracies in cosmological models?
• RQ3 Can models with a non-standard equation of state for vacuum energy or enhanced radiation content be ruled out by joint LSS and CMB constraints?
• RQ4 What evidence do the data provide for the existence of a neutrino background or additional relativistic degrees of freedom?
• RQ5 How well does the observed large-scale structure support the standard model of scalar-mode, adiabatic, scale-invariant fluctuations in a cold dark matter universe?

Key findings

• The combination of 2dFGRS data and CMB anisotropy measurements yields a concordance cosmological model with Ωₘ ≈ 0.25 ± 15% and h ≈ 0.73 ± 5%.
• The model is geometrically flat and consists of cold dark matter and vacuum energy, with no significant degeneracies when both datasets are combined.
• Models with Ωₘ = 1 and boosted radiation (e.g., X ≈ 8) are ruled out because they fail to reproduce the CMB acoustic peak locations unless h is unreasonably high.
• The data imply a neutrino background with Nν ≈ 3.6 ± 1.1 effective species, providing independent evidence for relativistic relics beyond photons.
• The constraint on the scalar spectral index n ≈ 1.00 ± 0.03 is consistent with inflationary models, with future data expected to reduce errors by half.
• Non-standard models such as variable vacuum equation of state or massive neutrinos worsen agreement with independent Hubble parameter and Ωₘ constraints, supporting the simplest standard model.