We present a complete computation of the scalar power spectrum in the inflation without inflaton (IWI) framework, where the inflationary expansion is driven solely by a de Sitter (dS) background and scalar fluctuations arise as second-order effects sourced by tensor perturbations. By explicitly deriving and numerically integrating the full second-order kernel of the Einstein equations, we obtain a scale-invariant scalar spectrum without invoking a fundamental scalar field. In this framework, the amplitude of the scalar fluctuations is directly linked to the scale of inflation. More precisely, we show that matching the observed level of scalar fluctuations ${\mathrm{\Delta }}_{\mathcal{\varphi }}^2(k_{*})\approx {10}^{-9}$ at cosmic microwave background scales fixes the inflationary energy scale $H_{\inf }$ as a function of the number of observed e-folds $N_{\mathrm{obs}}$. For $N_{\mathrm{obs}}\simeq 30–60$, we find $H_{\inf }\simeq 5\times {10}^{13}\mathrm{GeV}–2\times {10}^{10}\mathrm{GeV}$, corresponding to a tensor-to-scalar ratio $r\simeq 0.01–5\times {10}^{-9}$. In particular, requiring consistency with instantaneous reheating, we predict a number of e-foldings $\mathcal{O}(50)$ and an inflationary scale $H_{\inf }\simeq {10}^{11}\mathrm{GeV}$. We also incorporate in our framework the quantum break time of the dS state, and we show that it imposes an upper bound on the number of particle species. Specifically, using laboratory constraints on the number of species limits the duration of inflation to $N_{\mathrm{obs}}\lesssim 126$ e-folds. These results establish the IWI scenario as a predictive and falsifiable alternative to standard inflaton-driven models, linking the observed amplitude of primordial fluctuations directly to the quantum nature and finite lifetime of dS space.

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• Inflation
• Quantum cosmology