[Paper Review] Constraints on large scale voids from WMAP-5 and SDSS

Paul M. Hunt, S. Sarkar|arXiv (Cornell University)|Jul 28, 2008

Cosmology and Gravitation Theories1 references15 citations

TL;DR

This paper investigates whether a large underdense void near Earth could explain cosmic acceleration without dark energy, using WMAP-5 CMB data and SDSS galaxy clustering. It finds that while such voids can fit the data under non-standard assumptions, Gaussian primordial fluctuations make them statistically implausible, challenging the concordance ΛCDM model despite observed voids and high peculiar velocities.

ABSTRACT

Measurements of the SNe Ia Hubble diagram which suggest that the universe is accelerating due to the effect of dark energy may be biased because we are located in a 200-300 Mpc underdense which is expanding 20-30% faster than the average rate. With the smaller global Hubble parameter, the WMAP-5 data on cosmic microwave background anisotropies can be fitted without requiring dark energy if there is some excess power in the spectrum of primordial perturbations on 100 Mpc scales. The SDSS data on galaxy clustering can also be fitted if there is a small component of hot dark matter in the form of 0.5 eV mass neutrinos. We show however that if the primordial fluctuations are gaussian, the expected variance of the Hubble parameter and the matter density are far too small to allow such a large local void. Nevertheless many such large voids have been identified in the SDSS LRG survey in a search for the late-ISW effect due to dark energy. The observed CMB temperature decrements imply that they are nearly empty, thus these real voids too are in gross conflict with the concordance LCDM model. The recently observed high peculiar velocity flow presents another challenge for the model. Therefore whether a large local void actually exists must be tested through observations and cannot be dismissed a priori.

Motivation & Objective

• To test whether a large underdense void near Earth could account for the observed cosmic acceleration without invoking dark energy.
• To assess whether WMAP-5 CMB anisotropy data and SDSS galaxy clustering data can be fitted under the void hypothesis with modified primordial power spectra and hot dark matter.
• To evaluate the statistical plausibility of such a void under the assumption of Gaussian primordial fluctuations.
• To reconcile observed large-scale voids and high peculiar velocity flows with the standard ΛCDM model.

Proposed method

• Analyzes WMAP-5 cosmic microwave background (CMB) anisotropy data to test fits without dark energy, assuming excess power in primordial perturbations on 100 Mpc scales.
• Uses SDSS luminous red galaxy (LRG) clustering data to test consistency with a void model including a 0.5 eV mass neutrino component as hot dark matter.
• Evaluates the expected variance of the Hubble parameter and matter density in a Gaussian random field to assess the likelihood of a 200–300 Mpc underdense void.
• Compares theoretical predictions of void-induced Hubble parameter variations with observational constraints from CMB temperature decrements and peculiar velocity flows.
• Applies statistical tests to determine whether the observed voids in the SDSS LRG survey are consistent with Gaussian initial conditions.

Experimental results

Research questions

• RQ1 Can the observed cosmic acceleration be explained by a large local void without requiring dark energy, using WMAP-5 CMB data?
• RQ2 To what extent can SDSS galaxy clustering data be fitted by a void model with a small hot dark matter component?
• RQ3 How likely is a 200–300 Mpc underdense void under the assumption of Gaussian primordial fluctuations?
• RQ4 Are the observed CMB temperature decrements associated with voids consistent with the predictions of the ΛCDM model?
• RQ5 Do high peculiar velocity flows challenge the standard ΛCDM cosmology, and can they be explained by a local void?

Key findings

• The WMAP-5 CMB data can be fitted without dark energy if there is excess power in primordial perturbations on 100 Mpc scales.
• SDSS galaxy clustering data can be fitted with a small component of hot dark matter, specifically 0.5 eV mass neutrinos, in the void model.
• Under Gaussian primordial fluctuations, the expected variance in the Hubble parameter and matter density is too small to allow a 200–300 Mpc void with 20–30% faster expansion.
• The observed large voids in the SDSS LRG survey, which are nearly empty and cause CMB temperature decrements, are in gross conflict with the ΛCDM model.
• High peculiar velocity flows observed in the universe further challenge the standard model, suggesting that the existence of a large local void must be tested observationally rather than ruled out a priori.