  We discuss certain issues related to the limitations of density parameters
for a ``radiation''-like contribution to the Friedmann equation using
kinematical or geometrical measurements. We analyse the observational
constraint of a negative $(1+z)^4$-type contribution in cosmological models. We
argue that it is not possible to determine the energy densities of individual
components of matter scaling like radiation from astronomical observations. We
find three different interpretations of the presence of the radiation term: 1)
the FRW universe filled with a massless scalar field in a quantum regime (the
Casimir effect), 2) the Friedmann-Robertson-Walker (FRW) model in the Randall
Sundrum scenario with dark radiation, 3) the cosmological model with global
rotation. From supernovae type Ia (SNIa) data, Fanaroff-Riley type IIb (FRIIb)
radio galaxy (RG) data, baryon oscillation peak and cosmic microwave background
radiation (CMBR) observations we obtain bounds for the negative radiation-like
term. A small negative contribution of dark radiation can reconcile the tension
in nucleosynthesis and remove also the disagreement between $H_0$ values
obtained from both SNIa and the Wilkinson Microwave Anisotropy Probe (WMAP)
satellite data.
