Scaling the smoothness of the IGM
  We use, for the first time, the Detrend Fluctuation Analysis (DFA) to study
the correlation properties of the transmitted flux fluctuations, in the
Lyman-$\alpha$ (Ly$\alpha$) Forest along the lines of sight (LOS) to QSOs, at
different space scales. We consider in our analysis the transmitted flux in the
intergalactic medium over the redshift range 2 $\leq$ z $\leq$ 4.5 from a
sample of 45 high-quality medium resolution (R $\sim$ 4300) quasar spectra
obtained with Echelle Spectrograph and Imager (ESI) mounted on the Keck II 10-m
telescope, and from a sample of 19 high-quality high resolution (R $\sim$
50000) quasar spectra obtained with Ultra-Violet and Visible Echelle
Spectrograh (UVES) mounted on the ESO KUEYEN 8.2 m telescope. The result of the
DFA method applied to both datasets, shows that there exists a difference in
the correlation properties between the short and long-range regimes: the slopes
of the transmitted flux fluctuation function are different on small and large
scales. The scaling exponents, $\alpha_{1}$ = 1.635$\pm$ 0.115 and $\alpha_{2}$
= 0.758$\pm$ 0.085 for the ESI/Keck sample and $\alpha_{1}$ = 1.763 $\pm 0.128$
and $\alpha_{2}$ = 0.798 $\pm 0.084$ for the UVES/VLT sample for the short and
long range regime respectively. The transition between the two regims is
observed at about $\sim 1.4 h^{-1} Mpc$ (comoving). The fact that $\alpha_{1}$
is always larger than $\alpha_{2}$ for each spectrum supports the common view
that the Universe is smoother on large scales than on small scales. The non
detection of considerable variations in the scaling exponents from LOS to LOS
confirms that anisotropies cannot be ubiquitous, at least on these scales.
