Thermal and nonthermal dark matters with gravitational neutrino reheating

We have discussed in detail how neutrinos produced from inflaton solely through gravitational interaction can successfully reheat the Universe. For this, we have introduced the well-known type-I seesaw neutrino model. Depending on seesaw model parameters, two distinct reheating histories have been realized and dubbed as (i) Neutrino dominating: Following the inflaton domination, the universe becomes neutrino dominated, and their subsequent decay concludes the reheating process, and (ii) Neutrino heating: Despite being subdominant compared to inflaton energy, neutrinos efficiently heat the thermal bath and produce the radiation dominated universe. Imposing baryon asymmetric yield, the <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mi mathvariant="normal">Δ</a:mi><a:msub><a:mi>N</a:mi><a:mi>eff</a:mi></a:msub></a:math> constraint at big bang nucleosynthesis (BBN) considering primordial gravitational waves (PGW), we have arrived at the following constraints on reheating equation of state to lie within <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline"><d:mn>0.5</d:mn><d:mo>≲</d:mo><d:msub><d:mi>w</d:mi><d:mi>ϕ</d:mi></d:msub><d:mo>≲</d:mo><d:mn>1.0</d:mn></d:math>. In these neutrino-driven reheating backgrounds, we further performed a detailed analysis of both thermal and nonthermal production of dark matter (DM), invoking two minimal models, namely the Higgs portal DM and classical quantum chromodynamics pseudoscalar axion. An interesting correlation between seemingly uncorrelated DM and type-I seesaw parameters has emerged when confronting various direct and indirect observations. When DMs are set to freeze-in, freeze-out, or oscillate during reheating, new parameter spaces open, which could be potentially detectable in future experiments, paving an indirect way to look into the early Universe in the laboratory.