Cosmology with AGN dust time lags -- Simulating the new VEILS survey
  The time lag between optical and near-infrared continuum emission in active
galactic nuclei (AGN) shows a tight correlation with luminosity and has been
proposed as a standardisable candle for cosmology. In this paper, we explore
the use of these AGN hot-dust time lags for cosmological model fitting under
the constraints of the new VISTA Extragalactic Infrared Legacy Survey VEILS.
This new survey will target a 9 deg^2 field observed in J- and Ks-band with a
14-day cadence and will run for three years. The same area will be covered
simultaneously in the optical griz bands by the Dark Energy Survey, providing
complementary time-domain optical data. We perform realistic simulations of the
survey setup, showing that we expect to recover dust time lags for about 450
objects out of a total of 1350 optical type 1 AGN, spanning a redshift range of
0.1 < z < 1.2. We use the lags recovered from our simulations to calculate
precise distance moduli, establish a Hubble diagram, and fit cosmological
models. Assuming realistic scatter in the distribution of the dust around the
AGN as well as in the normalisation of the lag-luminosity relation, we are able
to constrain {\Omega}_{\Lambda} in {\Lambda}CDM with similar accuracy as
current supernova samples. We discuss the benefits of combining AGN and
supernovae for cosmology and connect the present work to future attempts to
reach out to redshifts of z > 4.
