Аннотации:
We investigate the effects of short axion kination eras on the energy spectrum of the primordial
gravitational waves corresponding to modes that reenter the Hubble horizon at the postelectroweak
symmetry breaking epoch well within the radiation domination era. Our main assumption is the existence
of an extremely weakly coupled hidden sector between the Higgs and the axion, materialized by higher
order nonrenormalizable dimension six and dimension eight operators, active at a scale M of the order
20–100 TeV. This new physics scale M, which is way higher than the electroweak scale, is motivated by the
lack of new particle observations in the large hadron collider to date beyond the electroweak scale. Once the
electroweak symmetry breaking occurs at T ∼ 100 GeV, the axion potential acquires a new minimum due
to the new terms generated by the electroweak breaking, and the axion oscillations at the origin are
destabilized. In effect after some considerable amount of time, the axion rolls swiftly to its new minimum,
experiencing a short kination epoch, where its energy density redshifts as ρa ∼ a−6. After it reaches the new
minimum, since the latter is energetically less favorable that the Higgs minimum, it decays to the Higgs
minimum and the Universe is described again by the Higgs minimum. The axion returns to the origin and
commences again oscillations initiated by quantum fluctuations, redshifts as dark matter, and the same
procedure is repeated perpetually. These short axion kination eras may disturb the background total
equation of state parameter during the radiation domination era, changing it from that of radiation w ¼ 1=3
to some value closer to the kination value w ¼ 1. We examined the effect of a value w ¼ 1=2 on the energy
density of the primordial gravitational waves. As we show, the energy spectrum of the gravitational waves
mainly depends on how many times the short axion kination epochs occur, on the inflationary theory, on the
actual value of the background equation of state parameter during the short kination eras, and finally on the
reheating temperature. Our findings indicate a characteristic shape in the energy spectrum that can be
observed in future gravitational wave experiments. We however disregarded the contribution of
the electroweak phase transition on the gravitational waves for simplicity and transparency of our
results
