Investigating dark energy by electromagnetic frequency shifts
  The observed red shift $z$ might be composed by the expansion red shift
$z_{\rm C}$ and an additional frequency shift $z_{\rm S}$, towards the red or
the blue, by considering Extended Theories of Electromagnetism (ETE). Indeed,
massive photon theories - the photon has a real mass as in the de Broglie-Proca
theory or an effective mass as in the Standard-Model Extension (SME), based on
Lorentz-Poincar\'e Symmetry Violation (LSV) - or Non-Linear Electro-Magnetism
(NLEM) theories may induce a cosmological expansion independent frequency shift
in presence of background (inter-) galactic electromagnetic fields, and where
of relevance LSV fields, even when both fields are constant. We have tested
this prediction considering the Pantheon Catalogue, composed by 1048 SNe Ia,
and 15 BAO data, for different cosmological models characterised by the absence
of a cosmological constant. From the data, we compute which values of $z_{\rm
S}$ match the observations, spanning cosmological parameters ($\Omega$
densities and Hubble-Lema\^itre constant) domains. We conclude that the
frequency shift $z_{\rm S}$ can support an alternative to accelerated
expansion, naturally accommodating each SN Ia position in the distance-modulus
versus red shift diagram, due to the light-path dependency of $z_{\rm S}$.
Finally, we briefly mention laboratory test approaches to investigate the
additional shift from ETE predictions.
