Evolution of CIV Absorbers I. The Cosmic Incidence
  We present a large high-resolution study of the distribution and evolution of
CIV absorbers, including the weakest population with equivalent widths
$W_r<0.3$~{\AA}. By searching 369 high-resolution, high signal-to-noise spectra
of quasars at $1.1\leq z_{em} \leq5.3$ from Keck/HIRES and VLT/UVES, we find
$1268$ CIV absorbers with $W_r \geq 0.05$~{\AA} (our $\sim50\%$ completeness
limit) at redshifts $1\leq z \leq4.75$. A Schechter function describes the
observed equivalent width distribution with a transition from power-law to
exponential decline at $W_r \gtrsim 0.5$~{\AA}. The power-law slope $\alpha$
rises by $\sim7\%$ and transition equivalent width $W_{\star}$ falls by
$\sim\!20\%$ from $\langle z \rangle=1.7$ to $\langle z \rangle=3.6$. We find
that the co-moving redshift path density, $dN/dX$, of $W_r \geq 0.05$~{\AA}
absorbers rises by $\sim1.8$ times from $z\simeq 4.0$ to $z\simeq 1.3$, while
the $W_r \geq 0.6$~{\AA} $dN/dX$ rises by a factor of $\sim8.5$. We quantify
the observed evolution by a model in which $dN/dX$ decreases linearly with
redshift. The model suggests that populations with larger $W_r$ thresholds
evolve faster with redshift and appear later in the universe. The cosmological
Technicolor Dawn simulations at $z=3-5$ over-produce the observed abundance of
absorbers with $W_r<0.3$~{\AA}, while yielding better agreement at higher
$W_r$. Our empirical linear model successfully describes {CIV evolution in the
simulations and the observed evolution of $W_r \geq 0.6$~{\AA} CIV for the past
$\sim12$ Gyr. Combining our measurements with the literature gives us a picture
of CIV-absorbing structures becoming more numerous and/or larger in physical
size over the last $\approx13$ Gyr of cosmic time ($z\sim6$ to $z\sim0$).
