Alternative ideas in cosmology
  Some remarkable examples of alternative cosmological theories are reviewed
here, ranging from a compilation of variations on the Standard Model through
the more distant quasi-steady-state cosmology, plasma cosmology, or universe
models as a hypersphere, to the most exotic cases including static models.
  The present-day standard model of cosmology, Lambda-CDM, gives us a
representation of a cosmos whose dynamics is dominated by gravity with a finite
lifetime, large scale homogeneity, expansion and a hot initial state, together
with other dark elements necessary to avoid certain inconsistencies with
observations. There are however some models with characteristics that are close
to those of the standard model but differing in some minor aspects: different
considerations on CP violation, inflation, number of neutrino species,
quark-hadron phase transition, baryonic or non-baryonic dark-matter, dark
energy, nucleosynthesis scenarios, large-scale structure formation scenarios;
or major variations like a inhomogeneous universe, Cold Big Bang, varying
physical constants or gravity law, zero-active mass (also called `R_h=ct'),
Milne, and cyclical models.
  At the most extreme distance from the standard model, the static models, a
non-cosmological redshift includes `tired-light' hypotheses, which assume that
the photon loses energy owing to an intrinsic property or an interaction with
matter or light as it travels some distance, or other non-standard ideas.
  Our impression is that none of the alternative models has acquired the same
level of development as Lambda-CDM in offering explanations of available
cosmological observations. One should not, however, judge any theory in terms
of the number of observations that it can successfully explain (ad hoc in many
cases) given the much lower level of development of the alternative ones.
