  The non-detection of companion stars in Type Ia supernova (SN) progenitor
systems lends support to the notion of double-degenerate (DD) systems and
explosions triggered by the merging of two white dwarfs. This very asymmetric
process should lead to a conspicuous polarimetric signature. By contrast,
observations consistently find very low continuum polarization as the
signatures from the explosion process largely dominate over the pre-explosion
configuration within several days. Critical information about the interaction
of the ejecta with a companion and any circumstellar matter is encoded in the
early polarization spectra. In this study, we obtain spectropolarimetry of
SN\,2018gv with the ESO Very Large Telescope at $-$13.6 days relative to the
$B-$band maximum light, or $\sim$5 days after the estimated explosion --- the
earliest spectropolarimetric observations to date of any Type Ia SN. These
early observations still show a low continuum polarization ($\lesssim$0.2\%)
and moderate line polarization (0.30$\pm$0.04\% for the prominent \ion{Si}{2}
$\lambda$6355 feature and 0.85$\pm$0.04\% for the high-velocity Ca component).
The high degree of spherical symmetry implied by the low line and continuum
polarization at this early epoch is consistent with explosion models of delayed
detonations and is inconsistent with the merger-induced explosion scenario. The
dense UV and optical photometry and optical spectroscopy within the first
$\sim$100 days after the maximum light indicate that SN\,2018gv is a normal
Type Ia SN with similar spectrophotometric behavior to SN\,2011fe.
