Model-independent parameters from DESI DR1 LRG2 with an explicitly varying power-spectrum shape

Abstract: Distinguishing between dark energy and modified gravity as the origin of the accelerated expansion requires the cosmic distances and the growth of structure to be measured in a model-independent way. Such measurements are difficult to perform while the primordial shape parameters are varied at the same time, and those parameters are therefore usually held fixed at the values determined by cosmic microwave background experiments. In this work, we present a methodology that constrains the shape parameters simultaneously with the model-independent distance and growth parameters, using an emulator built upon a hybrid model in which perturbative predictions are supplemented by templates measured from $N$-body simulations. {Because these templates are measured rather than expanded, the model carries non-linear corrections beyond the order at which its perturbative part is truncated, so that its accuracy on small scales is set by the simulations rather than by the reach of the expansion.} A Gaussian process emulator supplies these templates rapidly across the space of scale-dependent parameters, which makes the joint analysis computationally feasible, and the emulator is trained and validated over a range spanning $5σ$ of the Planck constraints. Applying the method to the LRG2 sample of DESI DR1, we obtain measurements of the Alcock--Paczynski parameters $α<em>\perp$ and $α</em>{||}$ and of the growth rate $fσ_8$. {With the tight Planck prior adopted here the constraints differ from those obtained with the shape parameters held fixed by less than a twentieth of their own uncertainties, so that the choice of template contributes at that level to the error budget of the measurement. Projected onto flat $Λ$CDM after the fit, the three measurements are mutually consistent, with $χ<sup>2=0.7$ for two degrees of freedom, and require $h=0.724\pm0.017$.