  We study the relation between the rms mass fluctuations on 8$h^{-1}$Mpc
scales and $\Omega_{\rm m}$ using the recent clustering results of XMM-{\it
Newton} soft (0.5-2 keV) X-ray sources, which have a median redshift of $z\sim
1.2$. The relation can be represented in the form $\sigma_{8}=0.34 (\pm 0.01)
\Omega_{\rm m}^{-\gamma}$ where $\gamma\equiv \gamma(\Omega_{\rm m},w)$ and it
is valid for all $w<-1/3$ models. By combining the X-ray clustering and SNIa
data we find that the model which best reproduces the observational data is
that with: $\Omega_{\rm m}\simeq 0.26$, $w\simeq -0.90$ and $\sigma_{8}\simeq
0.73$, which is in excellent agreement with the recent 3-year Wilkinson
Microwave Anisotropy Probe results.

Astrophysics