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تسجيل مستخدم جديدIs there Evidence for a Hubble bubble? The Nature of Type Ia Supernova Colors and Dust in External Galaxies
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We examine recent evidence from the luminosity-redshift relation of Type Ia Supernovae (SNe Ia) for the $sim 3 sigma$ detection of a ``Hubble bubble -- a departure of the local value of the Hubble constant from its globally averaged value citep{Jha:07}. By comparing the MLCS2k2 fits used in that study to the results from other light-curve fitters applied to the same data, we demonstrate that this is related to the interpretation of SN color excesses (after correction for a light-curve shape-color relation) and the presence of a color gradient across the local sample. If the slope of the linear relation ($beta$) between SN color excess and luminosity is fit empirically, then the bubble disappears. If, on the other hand, the color excess arises purely from Milky Way-like dust, then SN data clearly favors a Hubble bubble. We demonstrate that SN data give $beta simeq 2$, instead of the $beta simeq 4$ one would expect from purely Milky-Way-like dust. This suggests that either SN intrinsic colors are more complicated than can be described with a single light-curve shape parameter, or that dust around SN is unusual. Disentangling these possibilities is both a challenge and an opportunity for large-survey SN Ia cosmology.
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