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We present early-time photometric and spectroscopic observations of the TypeIa Supernova (SN Ia) 2021aefx. The early time u-band light curve shows anexcess flux when compared to normal SNe Ia. We suggest that the early-excessblue flux may be due to a rapid change in spectral velocity in the first fewdays post explosion, produced by the emission of the Ca II H&K feature passingfrom the u to the B bands on the time scale of a few days. This effect could bedominant for all SNe Ia which have broad absorption features and early-timevelocities over 25,000 km/s. It is likely to be one of the main causes ofearly-excess u-band flux in SNe Ia which have early-time high-velocities. Thiseffect may also be dominant in the UV filters, as well as in places where theSN spectral energy distribution is quickly rising to longer wavelengths. Therapid change in velocity can only produce a monotonic change (in flux-space) inthe u-band. For objects which explode at lower velocities, and have a morestructured shape in the early-excess emission, there must also be an additionalparameter producing the early-time diversity. More early time observations, inparticular early spectra, are required to determine how prominent this effectis within SNe Ia.

A Speed Bump: SN 2021aefx Shows that Doppler Shift Alone can Explain Early-Excess Blue Flux in Some Type Ia Supernovae