Residual Hubble-bubble effects on supernova cosmology

Abstract

Even in a universe that is homogeneous on large scales, local density fluctuations can imprint a systematic signature on the cosmological inferences we make from distant sources. One example is the effect of a local underdensity on supernova cosmology. Also known as a Hubble-bubble, it has been suggested that a large enough underdensity could account for the supernova magnitude-redshift relation without the need for dark energy or acceleration. Although the size and depth of the underdensity required for such an extreme result is extremely unlikely to be a random fluctuation in an on-average homogeneous universe, even a small underdensity can leave residual effects on our cosmological inferences. It is these small underdensities that we consider here. In this paper, we show that there remain systematic shifts in our cosmological parameter measurements, even after excluding local supernovae that are likely to be within any small Hubble-bubble. We study theoretically the low-redshift cutoff typically imposed by supernova cosmology analyses and show that a low-redshift cut of z0 ∼ 0.02 may be too low based on the observed inhomogeneity in our local universe. Neglecting to impose any low-redshift cutoff can have a significant effect on the cosmological parameters derived from supernova data. A slight local underdensity, just 30% underdense with scale 70 h-1 Mpc, causes an error in the inferred cosmological constant density Omega; Λof ∼4%. Imposing a low-redshift cutoff reduces this systematic error but does not remove it entirely. A residual systematic shift of 0.99% remains in the inferred value Ω Λ even when neglecting all data within the currently preferred low-redshift cutoff of 0.02. Given current measurement uncertainties, this shift is not negligible and will need to be accounted for when future measurements yield higher precision. © 2010. The American Astronomical Society. All rights reserved.