We study a minimal type-I seesaw framework in which a first-order phase transition, driven by a singlet scalar $\varphi$, produces right-handed neutrinos (RHNs) through bubble collisions, realizing a cosmic-scale collider that probes ultrahigh energy scales. The resulting inhomogeneous RHN distribution sources a novel low-frequency gravitational-wave (GW) signal in addition to the standard bubble-collision contribution. A stable lightest RHN can account for the observed dark-matter (DM) relic abundance for masses as low as $M_1\equiv m_{\mathrm{DM}}\gtrsim {10}^6\mathrm{GeV}$, with the associated novel GW signal accessible in the Laser Interferometer Space Antenna, Einstein Telescope (ET), and upcoming LIGO-Virgo-KAGRA (LVK) detectors. If the RHNs are unstable, their $CP$-violating decays generate the baryon asymmetry via leptogenesis for $M_1\gtrsim {10}^{11}\mathrm{GeV}$ and phase transition temperatures $T_{*}\gtrsim {10}^6\mathrm{GeV}$, yielding the novel GW signatures within the reach of ET, Big Bang Observer (BBO), and upcoming LVK detectors. Part of the parameter space is already constrained by the LVK $\mathcal{O}(3)$ data. If RHN decays populate a dark sector fermion $\chi$ with mass $m_{\chi }\in [{10}^{-4},{10}^4]\mathrm{GeV}$, successful cogenesis of baryons and asymmetric dark matter is achieved for $T_{*}\gtrsim {10}^7\mathrm{GeV}$ and $M_1\gtrsim {10}^9\mathrm{GeV}$, naturally explaining ${\mathrm{\Omega }}_{\mathrm{DM}}\simeq 5{\mathrm{\Omega }}_{\mathrm{B}}$. The corresponding GW signals are testable with Laser Interferometer Space Antenna, ET, and BBO. Finally, we analyze a UV-complete multi-Majoron model based on a global $U(1{)}_N\times U(1{)}_{\mathrm{B}-\mathrm{L}}$ extension of Standard Model, motivated from the hierarchy of lepton masses, in which a distinctive GW signature associated with a cosmic Majoron collider arises from scalar production during $U(1{)}_N$ symmetry breaking, detectable by BBO, ET, and upcoming LVK. Successful leptogenesis is realized for the heaviest RHN mass $M_3\sim {10}^{10}\mathrm{GeV}$ and a $U(1{)}_N$ breaking vacuum expectation value $v_2\sim \mathcal{O}(\mathrm{TeV})$ that sets the seesaw scale.

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• Baryogenesis & leptogenesis
• Gravitational waves
• Particle astrophysics
• Particle dark matter
• Particle production