The habitability of the Universe through 13 billion years of cosmic time
  The field of astrobiology has made tremendous progress in modelling
galactic-scale habitable zones which offer a stable environment for life to
form and evolve in complexity. Recently, this idea has been extended to
cosmological scales by studies modelling the habitability of the local Universe
in its entirety (e.g. Dayal et al. 2015; Li & Zhang 2015). However, all of
these studies have solely focused on estimating the potentially detrimental
effects of either Type II supernovae (SNII) or Gamma Ray Bursts (GRBs),
ignoring the contributions from Type Ia supernovae (SNIa) and active galactic
nuclei (AGN). In this study we follow two different approaches, based on (i)
the amplitude of deleterious radiation and (ii) the total planet-hosting volume
irradiated by deleterious radiation. We simultaneously track the contributions
from the key astrophysical sources (SNII, SNIa, AGN and GRBs) for the entire
Universe, for both scenarios, to determine its habitability through 13.8
billion years of cosmic time. We find that SNII dominate the total radiation
budget and the volume irradiated by deleterious radiation at any cosmic epoch
closely followed by SNIa (that contribute half as much as SNII), with GRBs and
AGN making up a negligible portion (<1%). Secondly, as a result of the total
mass in stars (or the total number of planets) slowly building-up with time and
the total deleterious radiation density, and volume affected, falling-off after
the first 3 billion years, we find that the Universe has steadily increased in
habitability through cosmic time. We find that, depending on the exact model
assumptions, the Universe is 2.5 to 20 times more habitable today compared to
when life first appeared on the Earth 4 billion years ago. We find that this
increase in habitability will persist until the final stars die out over the
next hundreds of billions of years.
