Extending CSST Emulator to post-DESI era

TL;DR

This work addresses the need for fast, percent-level predictions of the nonlinear matter power spectrum in dynamical dark energy scenarios ($w_0w_a$CDM) to capitalize on DESI DR2+CMB data. It extends the spectral equivalence method to use auxiliary $w_0w_a$CDM models (beyond $w$CDM), enabling sub-percent accuracy ($\lesssim1\%$) over the $z\le3$ range and expanding applicability to the full $2\sigma$ DESI posterior. Validation against the Kun $N$-body suite and extended dynamic dark energy simulations confirms robustness across an 8D cosmological space including massive neutrinos, with only minor extrapolation effects. This approach enhances the CSST Emulator’s utility for post-DESI cosmology, enabling precise, rapid likelihood analyses and motivating extensions to other statistics and DE models.

Abstract

The recent DESI BAO measurements have revealed a potential deviation from a cosmological constant, suggesting a dynamic nature of dark energy. To rigorously test this result, complementary probes such as weak gravitational lensing are crucial, demanding highly accurate and efficient predictions of the nonlinear matter power spectrum within the $w_0w_a$CDM framework. However, most existing emulators fail to cover the full parameter posterior from DESI DR2+CMB constraints in the $w_0\mbox{-}w_a$ plane. In this work, we extend the spectral equivalence method outlined in Casarini et al. 2016 to use auxiliary $w_0w_a$CDM models for approximating the power spectrum of a target $w_0w_a$CDM cosmology, moving beyond the previous use of $w$CDM auxiliaries. Incorporating this enhanced module, the extended CSST Emulator achieves a prediction accuracy of $\leq1\%$ over the $1σ$ confidence region from DESI DR2+CMB constraints for $z\leq3$, validated by additional dynamic dark energy simulations. The emulator's applicable parameter space has been generalized to fully encompass the $2σ$ region, greatly enhancing its utility for cosmological analysis in the post-DESI era.

Figures (5)

• Figure 1: Dark and light blue ellipses represent the $68\%$ and $95\%$ confidence level of DESI DR2+CMB constraint under the $w_0w_a$CDM model. The gray shaded region represents the original parameter range of the CSST Emulator. The black star and four colored points denote the cosmologies of the five extended dynamical dark energy simulations used to validate spectral equivalence. The colored stars and diamonds indicate the identified auxiliary $w$CDM and $w_0w_a$CDM cosmologies discussed in Section \ref{['sec:desi-era']}, respectively. Different colors correspond to redshifts $z\in[0,3]$.
• Figure 2: The accuracy of spectral equivalence between the target $w_0w_a$CDM simulations and the auxiliary $w$CDM models predicted by CSST Emulator at different redshifts.
• Figure 3: Comparison of 'cb' power spectra between the extended dynamic dark energy simulations and the auxiliary $w$CDM models predicted by CSST Emulator at different redshifts. Different colors represent different cosmologies. The dark and light gray shaded regions indicate the $1\%$ and $2\%$ difference, respectively.
• Figure 4: Similar to Figure \ref{['fig:weq-emu-vs-sim']}, but for comparison with the auxiliary $w_0w_a$CDM models.
• Figure 5: The dark and light blue shaded regions represent the extended parameter range of $w_0$ and $w_a$ utilizing $w$CDM and $w_0w_a$CDM as the auxiliary model, respectively. Orange ellipses indicate the $68\%$ and $95\%$ confidence level of DESI DR2+CMB constraint under the $w_0w_a$CDM model. The original parameter range of CSST Emulator is shown by the gray shaded region. Different panels represent different redshifts for $z\in[0,3]$.