Kination cosmology from scalar fields and gravitational-wave signatures

Abstract

Kination denotes an era in the cosmological history corresponding to an equation of state ω = +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 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.

Quintessential Inflation: A Tale of Emergent and Broken Symmetries

“…The measured abundance of light elements indicates that our Universe was radiation dominated during BBN [34], thus setting a limit on the duration of the kinetic dominated era discussed in the previous section [89],…”

Gravitational Waves from an Inflation Triggered First-Order Phase Transition

“…For the same reason, the strength of the GW signal will get more suppressed with longer during of the MD stage. f (Hz) In the case that the universe undergoes a KD stage after the end of inflation, the equation of state is w = 1, which is the maximal value allowed by causality [96]. In this case, the dilution of the energy density goes like ρ ∼ a −6 .…”

Cosmic Perturbations from a Rotating Field

“…The kination domination preceded by matter domination imprints a peculiar signal on the spectrum of primordial gravitational waves [59,100,101] because it changes the expansion history of the universe relative to the standard radiation-dominated universe. In our setup, because of the entropy production from the rotation, gravitational waves at high frequencies are suppressed in comparison with the spectrum shown in these references, leaving a different signature.…”

“…Also, Refs. [59,100,101] assumed that the PQ charge of the axion rotation decreases only by redshifting. As a result, unless the kination domination ends at a sufficiently high temperature, the axion rotation overproduces axion dark matter by kinetic misalignment [43].…”