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 under-density on supernova cosmology. Also known as a Hubble-bubble, it has been suggested that a large enough under-density could account for the supernova magnitude- redshift relation without the need for dark energy or acceleration. Although the size and depth of under-density required for such an extreme result is extremely unlikely to be a random fluctuation in an on-average homogeneous universe, even a small under-density can leave residual effects on our cosmological inferences. In this paper we show that there remain systematic shifts in our cosmological parameter measure- ments, 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 \sim 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 pa- rameters derived from supernova data. A slight local under-density, just 30% under-dense with scale 70h^{-1} Mpc, causes an error in the inferred cosmological constant density ΩΛ of \sim 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 ΩΛ even when neglecting all data within the currently pre- ferred 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.

12 pages, 11 figures. Accepted for publication in The Astrophysical Journal

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