Kination cosmology from scalar fields and gravitational-wave signatures
  Kination denotes an era in the cosmological history corresponding to an
equation of state $\omega=+1$ such that the total energy density of the
universe redshifts as the sixth inverse power of the scale factor. This arises
if the universe is dominated by the kinetic energy of a scalar field. It has
often been motivated in the literature as an era following inflation, taking
place before the radiation era. In this paper, we review instead the
possibility that kination is disconnected from primordial inflation and occurs
much later, inside the Standard Model radiation era. We study the implications
on all main sources of primordial gravitational waves. We show how this leads
to very distinctive peaked spectra in the stochastic background of long-lasting
cosmological sources of gravitational waves, namely the irreducible
gravitational waves from inflation, and gravitational waves from cosmic
strings, both local and global, with promising observational prospects. We
present model-independent signatures and detectability predictions at SKA,
LIGO, LISA, ET, CE, BBO, as a function of the energy scale and duration of the
kination era. We then argue that such intermediate kination era is in fact
symptomatic in a large class of axion models. We analyse in details the scalar
field dynamics, the working conditions and constraints in the underlying
models. We present the gravitational-wave predictions as a function of particle
physics parameters. We derive the general relation between the
gravitational-wave signal and the axion dark matter abundance as well as the
baryon asymmetry. We investigate the predictions for the special case of the
QCD axion. The key message is that gravitational-waves of primordial origin
represent an alternative experimental probe of axion models.
