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Our current best-fit cosmological model explains structure formation primarily through adiabatic fluctuations. Complementary and orthogonal to these are isocurvature fluctuations. Primordial isocurvature is undetected on CMB scales and often treated as a constrained nuisance within LambdaCDM, but it can also act as a structured degree of freedom with real impact on cosmological inference. In this talk I will present an isocurvature-forward program: we study CDM/axion isocurvature sourced by a new "hyperbolic geometry" mechansim with scale-dependent (often blue-tilted) spectra, construct data-ready templates, and quantify their impact within standard analyses. I will highlight an application where a subdominant CDM component carrying isocurvature compensates the power suppression from dominant warm dark matter, reopening parameter space and allowing WDM masses as low as 300 eV (about an order of magnitude below typical current WDM mass bounds). I will briefly discuss extensions to ultra-light axion (fuzzy) dark matter, and conclude with comments on unexplored degeneracies between isocurvature contributions and neutrino-mass inference. |