Studying Type II supernovae as cosmological standard candles using the
  Dark Energy Survey
  Despite vast improvements in the measurement of the cosmological parameters,
the nature of dark energy and an accurate value of the Hubble constant (H$_0$)
in the Hubble-Lema\^itre law remain unknown. To break the current impasse, it
is necessary to develop as many independent techniques as possible, such as the
use of Type II supernovae (SNe II). The goal of this paper is to demonstrate
the utility of SNe II for deriving accurate extragalactic distances, which will
be an asset for the next generation of telescopes where more-distant SNe II
will be discovered. More specifically, we present a sample from the Dark Energy
Survey Supernova Program (DES-SN) consisting of 15 SNe II with photometric and
spectroscopic information spanning a redshift range up to 0.35. Combining our
DES SNe with publicly available samples, and using the standard candle method
(SCM), we construct the largest available Hubble diagram with SNe II in the
Hubble flow (70 SNe II) and find an observed dispersion of 0.27 mag. We
demonstrate that adding a colour term to the SN II standardisation does not
reduce the scatter in the Hubble diagram. Although SNe II are viable as
distance indicators, this work points out important issues for improving their
utility as independent extragalactic beacons: find new correlations, define a
more standard subclass of SNe II, construct new SN II templates, and dedicate
more observing time to high-redshift SNe II. Finally, for the first time, we
perform simulations to estimate the redshift-dependent distance-modulus bias
due to selection effects.
