Inflation is known to produce large infrared scalar fluctuations. Further, if a scalar field ($\chi$) is nonminimally coupled with gravity through $\xi {\chi }^{2}R$, those infrared modes experience tachyonic instability during and after inflation. Those large nonperturbative infrared modes can collectively produce hot big bang universe upon their horizon entry during the postinflationary period. We indeed find that for reheating equation of state (EOS), $w_{\varphi }>1/3$, and coupling strength, $\xi >1/6$, large infrared fluctuations lead to successful reheating. We further analyze perturbative reheating by solving the standard Boltzmann equation in both Jordan and Einstein frames, and compare the results with the nonperturbative ones. Finally, embedding this infrared reheating scenario into the well-known $\alpha$-attractor inflationary model, we examine possible constraints on the model parameters in light of the latest Atacama Cosmology Telescope (ACT), Dark Energy Spectroscopic Instrument (DESI) results. To arrive at the constraints, we take into account the latest bounds on tensor-to-scalar ratio, $r_{0.05}\le 0.038$, isocurvature power spectrum, ${\mathcal{P}}_{\mathcal{S}}\lesssim 8.3\times {10}^{-11}$, and effective number of relativistic degrees of freedom, $\mathrm{\Delta }{N}_{\mathrm{eff}}\lesssim 0.17$. Subject to these constraints, we find successful reheating to occur only for EOS $w_{\varphi }\gtrsim 0.6$, which translates to a subclass of $\alpha$-attractor models being favored and placing them within the $2\sigma$ region in the $n_s-r$ plane of the latest ACT, DESI data. In this range of EOS, we find that the coupling strength should lie within $2.11\lesssim \xi \lesssim 2.95$ for $w_{\varphi }=0.6$, and for $w_{\varphi }>0.6$, the allowed range becomes even tighter. Finally, we compute secondary gravitational wave signals induced by the scalar infrared modes, which are found to be strong enough to be detected by future GW observatories, namely BBO, DECIGO, LISA, and ET.

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• Cosmology
• Evolution of the Universe
• Gravitational waves
• Inflation