  We study the evolution of baryonic gas before the reionization in the
lognormal (LN) model of cosmic clustering. We show that the thermal history of
the universe around the reionization can roughly be divided into three epochs:
1) cold dark age $z>z_r$, in which baryon gas is neutral, and opaque to
Ly$\alpha$ photons; 2) hot dark age $z_r > z> z_{gp}$, in which a predominant
part of baryon gas is ionized and hot, but it is still opaque to Ly$\alpha$
photons; 3) bright age $z<z_{gp}$, in which the universe is ionized highly
enough to be transparent to Ly$\alpha$ photons. In the flat cold dark matter
cosmological models given by WMAP and COBE, the difference of the two redshifts
$z_r - z_{gp}$ is found to be as large as $\sim 10$ with $z_r\sim 17$ and
$z_{gp}\sim 7$. This reionization history naturally yields a high optical depth
to the CMB $\tau_e \simeq 0.12 - 0.19$ observed by the TE polarization of the
WMAP, and a low redshift $z_{gp}$ of the appearance of the Ly$\alpha$
Gunn-Peterson trough $z_{gp} \simeq 6 - 8$ in QSO's absorption spectra. The
reason why the universe stays long in an ionized, yet Ly$\alpha$ opaque, stage
is because the first photo-ionization heats the intergalactic gas effectively
and has balanced the gravitational clustering a long period of time. Therefore,
the result of a high $\tau_e$ and low $z_{gp}$ is a common feature of all the
models considered. Besides the cosmological parameters, the only free parameter
we used in the calculation is $N_{ion}$, the mean ionization photons produced
by each baryon in collapsed objects. We take it to be 40 - 80 in the
calculation.
