  Within the numerically exact solution to the Dicke model proposed previously,
we study the quantum criticality in terms of the ground-state (GS) energy,
fidelity, and the order parameter. The finite size scaling analysis for the
average fidelity susceptibility (FS) and second derivative of GS energy are
performed. The correlation length exponent is obtained to be $\nu=2/3$, which
is the same as that in Lipkin-Meshkov-Glick model obtained previously,
suggesting the same universality. It is observed that average FS and second
derivative of GS energy show similar critical behavior, demonstrating the
intrinsic relation in the Dicke model. The scaling behavior for the order
parameter and the singular part of the GS energy at the critical point are also
analyzed and the obtained exponents are consistent with the previous scaling
hypothesis in 1/N expansion scheme.
