The intergalactic medium over the last 10 billion years I: Lyman alpha
  absorption and physical conditions
  The intergalactic medium (IGM) is the dominant reservoir of baryons at all
cosmic epochs. We investigate the evolution of the IGM from z=2-0 in 48 Mpc/h,
110-million particle cosmological hydrodynamic simulations using three
prescriptions for galactic outflows. We focus on the evolution of IGM physical
properties, and how such properties are traced by Ly-alpha absorption as
detectable using HST/COS. Our results broadly confirm the canonical picture
that most Ly-alpha absorbers arise from highly ionized gas tracing filamentary
large-scale structure. Growth of structure causes gas to move from the diffuse
photoionized IGM into other cosmic phases, namely stars, cold and hot gas
within galaxy halos, and the unbound and shock-heated warm-hot intergalactic
medium (WHIM). By today, baryons are roughly equally divided between bound
phases (35%), the diffuse IGM (41%), and the WHIM (24%). Here we (re)define the
WHIM as gas with overdensities lower than that in halos and temperatures >10^5
K, in order to more closely align it with "missing baryons". When we tune our
photoionizing background to match the observed evolution of the Ly-alpha mean
flux decrement, we obtain a line count evolution that broadly agrees with
available data. We predict a column density distribution slope of -1.70 for our
favored momentum-driven wind model, in agreement with recent observations, and
it becomes shallower with redshift. With improved statistics, the frequency of
strong lines can be a valuable diagnostic of outflows, and our favored wind
model matches existing data best among our models. The relationship between
column density and physical density is fairly tight from z=2-0, and evolves as
rho N_HI^0.74 10^(-0.37z) for diffuse absorbers. Linewidths only loosely
reflect the temperature of the absorbing gas, which will hamper attempts to
quantify the WHIM using broad Ly-alpha absorbers. [Abridged]
