Residual Hubble-bubble effects on supernova cosmology
  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 {\Omega}{\Lambda} 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 {\Omega}{\Lambda} 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.
