Dynamical Dark Energy Imprints in the Lyman-Alpha Forest

TL;DR

This work investigates whether dynamical dark energy (DDE), motivated by DESI indicating a time-varying equation of state, leaves observable imprints in the Lyman-Alpha forest. Using high-resolution, GPU-accelerated hydrodynamic simulations with CPL-based DDE, the authors compare to matched LCDM models to isolate expansion-history effects on the intergalactic medium and the Ly$\alpha$ transmitted-flux power spectrum (FPS). They find that DDE induces a scale- and redshift-dependent spectral tilt in the FPS, along with a modestly warmer low-density IGM and slightly reduced Ly$\alpha$ opacity, relative to LCDM. These signatures offer an independent avenue to test DDE, and the results highlight the need for high-precision FPS measurements and improved modeling of the UV background and radiative transfer to exploit Ly$\alpha$ forest data for constraining dynamical dark energy.

Abstract

The nature of dark energy (DE) remains elusive, even though it constitutes the dominant energy-density component of the Universe and drives the late-time acceleration of cosmic expansion. By combining measurements of the expansion history from baryon acoustic oscillations, supernova surveys, and cosmic microwave background data, the Dark Energy Spectroscopic Instrument (DESI) Collaboration has inferred that the DE equation of state may evolve over time. The profound implications of a time-variable, ``dynamical" DE (DDE) that departs from a cosmological constant motivate the need for independent observational tests. In this work, we use cosmological hydrodynamical simulations of structure formation to investigate how DDE affects the properties of the Lyman-Alpha ``forest'' of absorption features produced by neutral hydrogen in the cosmic web. We find that DDE models consistent with the DESI constraints induce a spectral tilt in the forest transmitted flux power spectrum, imprinting a scale- and redshift-dependent signature relative to standard Lambda-CDM cosmologies. These models also yield higher intergalactic medium temperatures and reduced Lyman-Alpha opacity compared to Lambda-CDM. We discuss the observational implications of these trends as potential avenues for independent confirmation of DDE.

Figures (13)

• Figure 1: A two-dimensional slice of the DESI+CMB+$\Lambda$CDM cosmology at $z=2.33$ extending across the entire simulation box. The left panel shows the gas density projected along $500\ h^{-1}\textrm{kpc}$. The right panel shows the density-weighted temperature of the gas. The filamentary nature of large-scale structure is shown with the densest regions hosting the warmest portions of the projected gas distribution.
• Figure 2: Relative expansion rate, age, and growth factor for a $w_0 w_a$CDM cosmology compared to their $\Lambda$CDM counterparts. Left: Taking the Hubble rate as a function of redshift, we calculate $[H_{\textrm{DDE}}(z)/H_{\Lambda}(z)] - 1$. The peach line shows this function for the Pantheon cosmologies, the green line for DESY5, and the yellow for Union3. Since the same present-day Hubble parameter $H_0$ is adopted in all comparisons, $H_{\textrm{DDE}}(0)/H_{\Lambda}(0)=1$ by construction. Center: Cosmic age as a function of redshift for the DDE models against their $\Lambda$CDM counterparts. Same color coding as in the left panel. All three DDE models result in a Universe that is younger at the present epoch compared to their $\Lambda$CDM counterparts ($[t_{\textrm{DDE}}(z=0) / t_{\Lambda}(0)] < 1$). Right: Relative linear growth factor $[G_{\textrm{DDE}}(z)/G_{\Lambda}(z)] - 1$ compared to their $\Lambda$CDM counterpart models. Same color coding as in the left panel. The growth factors are not normalized at a common redshift. Since the same $\Omega_{m,0}$ is used when comparing two cosmologies, differences in growth arise solely from the DDE component.
• Figure 3: Example of a synthetic Ly$\alpha$ spectrum at redshift $z=2.4$ for the fiducial DESI+CMB+$\Lambda$CDM cosmology. The top two panels display the surrounding gas and $\textrm{HI}$ density, projected along $10^{3}\ h^{-1}\textrm{kpc}$. The third panel displays a temperature projection weighted by the gas density. The fourth panel shows physical column density $N_{\textrm{HI}}$. The bottom two panels display the optical depth and transmitted flux along the skewer. The mean flux along the LOS $\bar{F}$ defines the effective optical depth $\tau_{\textrm{eff}} = -\log\left(\bar{F}\right)$ for this skewer.
• Figure 4: Distribution of gas overdensity $\rho / \bar{\rho}_b$ and temperature at $z = 3.0$ for the fiducial $\Lambda$CDM cosmology. The color scale indicates the logarithmic probability that a simulation cell falls within a given bin of the two-dimensional $\Delta$--$T$ volume-weighted histogram. The interstellar medium cool phase, where the colder/denser gas within galaxies are at $\Delta>2$, is not resolved in our simulations. White regions correspond to bins with no occupied cells, i.e., $P(\Delta, T) = 0$.
• Figure 5: Relative difference in the phase-space occupancy of gas cells between $w_0w_a$CDM cosmologies and their corresponding $\Lambda$CDM counterparts at redshift $z = 3$. The quantity plotted is the fractional difference in the joint probability distribution: $[P_{\textrm{DDE}}(\Delta, T) / P_{\Lambda}(\Delta, T)] - 1$, computed in $\log_{10} \Delta$--$\log_{10} T$ space. From left to right, the panels show the results for the DESI+CMB+Pantheon, DESI+CMB+DESY5, and DESI+CMB+Union3 models. Red (blue) regions indicate where a phase-space bin is more (less) populated in the DDE model than in its $\Lambda$CDM counterpart.