paper

Aletheia: Emulating the non-linear matter power spectrum in the context of evolution mapping

arXiv:2511.13826 · doi:10.1093/mnras/stag927

Abstract

We present Aletheia, a new emulator of the non-linear matter power spectrum, $P (k)$ , built upon the evolution mapping framework. This framework addresses the limitations of traditional emulation by focusing on $h$ -independent cosmological parameters, which can be separated into those defining the linear power spectrum shape ( $I^_{s}$ ) and those affecting only its amplitude evolution ( $I^_{e}$ ). The combined impact of evolution parameters and redshift is compressed into a single amplitude parameter, $I ¨ _{12}$ . Aletheia uses a two-stage Gaussian Process emulation: a primary emulator predicts the non-linear boost factor as a function of ( $I^_{s}$ ) and $I ¨ _{12}$ for fixed evolution parameters, while a second one applies a small linear correction based on the integrated growth history. The emulator is trained on shape parameters spanning $\pm$ 5 $I ¨ $ of Planck constraints and a wide clustering range $0.2 < I ¨ _{12} < 1.0$ , providing predictions for $0.006 Mpc^{- 1} < k < 2 Mpc^{- 1}$ . We validate Aletheia against N-body simulations, demonstrating sub-percent accuracy. When tested on a suite of dynamic dark energy models, the full emulator's predictions show a variance of approximately 0.2%, a factor of five smaller than that of the state-of-the-art EuclidEmulator2 (around 1% variance). Furthermore, Aletheia maintains sub-percent accuracy for the best-fit dynamic dark energy cosmology from recent DESI data, a model whose parameters lie outside the training ranges of most conventional emulators. This demonstrates the power of the evolution mapping approach, providing a robust and extensible tool for precision cosmology.

13 pages, 10 figures, submitted to MNRAS. The Aletheia Python package is available on PyPI at https://pypi.org/project/AletheiaCosmo/0.1.0/. Code and documentation are available at https://gitlab.mpcdf.mpg.de/arielsan/aletheia