  Physical objects with energy $u_w(l) \sim l^{-3w}$ with $l$ a characteristic
length and $w$ a numerical constant ($-1 \leq w \leq 1$), lead to an equation
of state $p=w\rho$, with $p$ the pressure and $\rho$ the energy density.
Special objects with this property are, for instance, photons ($u = hc/l$, with
$l$ the wavelength) with $w = 1/3$, and some models of cosmic string loops ($u
= (c^4/aG)l$, with $l$ the length of the loop and $a$ a numerical constant),
with $w = -1/3$, and maybe other kinds of objects as, for instance,
hypothetical cosmic membranes with lateral size $l$ and energy proportional to
the area, i.e. to $l^2$, for which $w = -2/3$, or the yet unknown constituents
of dark energy, with $w = -1$. Here, we discuss the general features of the
spectral energy distribution of these systems and the corresponding
generalization of Wien's law, which has the form $Tl_{mp}^{3w}=constant$, being
$l_{mp}$ the most probable size of the mentioned objects.
