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The minimal left-right symmetric model (mLRSM) provides an elegant and testable framework for addressing the origin of neutrino masses. We examine the constraints on the sub-GeV right-handed (RH) neutrino in the type-II seesaw scenario of the mLRSM without left-right mixing, taking limits from collider searches, meson decays, supernovae, neutrinoless double beta (0νββ) decay and cosmology. Specifically, we derive the 0νββ decay constraints using the advanced effective field theory approach and up-to-date nuclear matrix element calculations. Besides, we update the SN1987A cooling bound with the state-of-the-art simulations, provide new constraints from the energy deposition in the supernova ejecta, and incorporate the stringent RH neutrino lifetime upper limit τ ≲ 0.023 s from the big bang nucleosynthesis. Our results identify the parameter region compatible with all current experimental and observational constraints, where the RH neutrino mass lies between 700 MeV and 1 GeV and the RH W boson mass is slightly below 20 TeV. This region is exclusively probed by the future tonne-scale 0νββ decay experiments, providing a unique window to test the mLRSM and the possible origin of neutrino masses.

We consider the impact on cosmological first-order phase transitions (FOPTs) of low-temperature thermal corrections to the effective potential. These are corrections from degrees of freedom whose field-dependent masses in the true vacuum are much larger than the nucleation temperature, though in the false vacuum the field-dependent masses may be much smaller than the nucleation temperature. We focus on the regime in which the thin-wall approximation is valid and in which these corrections are small enough that they do not alter the vacuum structure of the theory. Although the general form of these corrections to the thermal effective potential can be quite complicated, we argue that the net effect of all such corrections can be well modeled in this limit with a single new parameter. We determine the shift in the parameters of the FOPT in terms of this new parameter and the impact on gravitational wave signals and cosmological observables.