Gravitational Wave Echoes of the First Order Phase Transition in a Kination-Induced Big Bang
Gravitational waves (GWs) produced during first-order phase transitions (FOPTs) in the early universe provide a powerful probe of nonstandard cosmological histories. We study GW production from a FOPT ending a kination-dominated epoch in the Kination-Induced Big Bang scenario, in which a period of kination domination terminates through a phase transition that reheats the universe into radiation domination. A rolling scalar field drives the kination epoch. In the specific model we consider, its derivative coupling to a second scalar (tunneling field) dynamically traps the latter in a false vacuum, with the phase transition triggered as the kination field slows due to Hubble friction. We compute the resulting stochastic GW background from bubble nucleation and collisions, presenting analytic estimates and numerical results for the peak amplitude and frequency. In all cases we find an upper bound $\Omega_{\rm GW} h^2\lesssim 2\times10^{-7}$ from the bubble percolation condition. In the case where the false vacuum energy dominates at the transition (yet the kination field drives the FOPT), we find $\Omega_{\rm GW}h^2\gtrsim 10^{-12}$. We further find that the Hubble scale during the phase transition across a broad set of model parameters is bounded by $\mathscr{O}(10^{-13})M^2/M_{\rm Pl}\lesssim H_* \lesssim \mathscr{O}(0.1)M^2/M_{\rm Pl}$, where $M$ is the mass-scale controlling the strength of the interaction between the kination and tunneling fields. The predicted signal spans frequencies from nHz to MHz, allowing the model to explain the signal reported by Pulsar Timing Array experiments and to be constrained or probed by interferometers such as LISA, Advanced LIGO, Cosmic Explorer, and BBO. Interestingly, a FOPT can occur even if the bare tunneling potential has a single minimum, as metastability is generated dynamically by the coupling between the tunneling and the kination field.