The evolution of neutral hydrogen over the past 11 Gyr via HI 21 cm
  absorption
  We present the results of a blind search for intervening HI 21 cm absorption
toward 260 radio sources in the redshift range 0<z<2.74 with the Green Bank
Telescope. The survey has the sensitivity to detect sub-damped Lyman-alpha
(DLA) systems for HI spin temperatures $T_s/f$ = 100 K, and despite the
successful re-detection of ten known 21 cm absorbers in the sample, we detect
no new absorption lines in the full survey. Sources detected in 21 cm
absorption were also searched for hydroxyl (OH) 18 cm absorption and we
re-detect 1667 MHz OH absorption toward PKS 1830-211. We searched for
intervening HI 21 cm absorption along the line of sight in each source
achieving a total redshift coverage of $\Delta z$ = 88.64 (comoving absorption
path of $\Delta X$ = 159.5) after removing regions affected by radio frequency
interference. We compute a 95% confidence upper limit on the column density
frequency distribution $f(N_{\rm HI})$ and set a statistical constraint on the
spin temperature $T_s$ in the range 100-1000 K, consistent with prior
redshifted optical DLA surveys and HI 21 cm emission observations at the same
redshifts. We infer a value for the cosmological mass density of neutral gas,
$\Omega_{\rm HI}$. Through comparison with prior $\Omega_{\rm HI}$
measurements, we place a statistical constraint on the mean spin temperature of
$T_s/f$ = 175 K. Our derived $\Omega_{\textrm {HI}}$ values support a relative
mild evolution in $\Omega_{\textrm {HI}}$ over the last 11 Gyr and are
consistent with other methods that measure $\Omega_{\textrm {HI}}$.
