Dark Acoustic Oscillations as an Early-Universe Explanation of the DESI Anomaly

TL;DR

The DESI DR2 anomaly is explored as an early-Universe effect from dark acoustic oscillations (DAO) near the BAO scale rather than a sign of late-time evolving dark energy. It derives analytic expressions for the apparent BAO shifts: isotropic $\Delta\alpha_{\text{iso}} = - \left( \frac{r_D - r_B}{r_B} \right) \left( \frac{A_D}{A_B} \right) \mathcal{D}(\Delta r, \Sigma^2)$ and anisotropic $(\Delta\alpha_\parallel(z), \Delta\alpha_\perp(z)) = - \left( \frac{r_D - r_B}{r_B} \right) \left( \frac{A_D}{A_B} \right) (\mathcal{D}_\parallel(z), \mathcal{D}_\perp(z))$. Using Planck 2018, DESI DR2, and Pantheon+ data, the DAO scenario improves the fit by $\Delta \chi^2 \simeq -6$ for the positive branch and $\Delta \chi^2 \simeq -5$ for the negative branch; the preferred DAO amplitude is percent-level ($A_D \sim 0.01$) and the DAO–BAO offset is $\Delta r/r_B \simeq 0.18 \pm 0.10$ (positive) or $-0.41 \pm 0.11$ (negative), with the negative branch lying in the DRMD-predicted range $\Delta r/r_B \in [-0.48,-0.32]$. The results connect the DESI anomaly to early-Universe physics and offer a falsifiable target for future full-shape analyses, potentially resolving the CMB–BAO vs SH0ES tensions in the DRMD or Atomic Dark Matter contexts.

Abstract

DESI DR2 data have been widely interpreted as evidence for late-time evolving dark energy (DE) with an apparent phantom crossing. Here we investigate an alternative explanation, based on early-Universe physics. If dark acoustic oscillations (DAO) are close in scale to baryon acoustic oscillations (BAO), they can bias the extraction of the BAO scale from the peak in the galaxy correlation function. This leads to an apparent shift in the inferred distance if the superposition of BAO and DAO features is misinterpreted as being due to BAO only. Taking this shift into account, we find that a DAO with percent-level amplitude can reconcile DESI DR2 with Planck 2018 as well as Pantheon+ supernovae data, with fit improvement at a similar level as compared to evolving DE. Notably, a DAO feature with the required properties has been predicted in a previously proposed scenario that resolves the Hubble tension via a pre-recombination decoupling of dark matter and dark radiation (DRMD). The presence of a DAO feature close to the BAO peak can be scrutinized with future full-shape galaxy clustering data from DESI and Euclid.

Figures (5)

• Figure 1: Illustration of the monopole of the galaxy correlation function for a $\Lambda$CDM model (blue) as well as $\Lambda$CDM+DAO (orange) with parameters $r_D=1.1 \, r_B$, $\tilde{A}=A_D/A_B=0.25$, $\Sigma_{D,\text{Silk}}=4$ Mpc/$h$ and non-linear damping of the BAO and DAO peaks obtained from IR resummation Blas:2016sfaIvanov:2018gjr. Dotted lines are BAO and DAO peaks obtained from the corresponding oscillating component of the linear power spectrum, respectively. The offset between the blue and orange vertical lines shows the shift $\Delta\alpha_\text{iso}(z)\times r_B$ from (mis-)interpreting the combined BAO+DAO peak as BAO only according to \ref{['eq:alphaiso']}. $\Lambda$CDM parameters correspond to the Planck 2018 best fit Planck:2018vyg. The shaded bands provide a rough indication for the amount of variation in peak normalization and width that could be absorbed by varying the nuisance parameters used in the fit of the theoretical template to data, assuming the prior ranges employed for the DESI analysis DESI:2025qqy. The redshift and galaxy bias are chosen to match those of the DESI LRG1 sample.
• Figure 2: BAO distances $D_V(z)/r_d$ (upper left), $D_M(z)/r_d$ (lower left) and $D_H(z)/r_d$ (lower right), relative to the baryon drag horizon $r_{B}$, that are inferred when (mis-)interpreting a combined BAO+DAO peak as the BAO (orange lines), compared to DESI DR2 measurements DESI:2025zgx as well as the prediction from a $\Lambda$CDM model with parameters matching Planck 2018 Planck:2018vyg (blue lines). For the DAO we assume the same parameters as in Fig. \ref{['fig:xi0']} for illustration, $r_D=1.1 \, r_B$, $\tilde{A}=A_D/A_B=0.25$, $\Sigma_{D,\text{Silk}}=4$ Mpc$/h$. The upper right panel shows SN data by Pantheon+ compared to the predictions of the two models. Notably, including the DAO in the fit to DESI DR2 BAO data allows for cosmological parameters close to those preferred by Planck (in particular for $\Omega_m$), restoring consistency with SN data as well in this way. All quantities are normalized to a standard $\Lambda$CDM model with best-fit parameters from DESI DR2.
• Figure 3: 1D posterior distributions of the DAO parameters $\Delta r/r_B=(r_{D}-r_{B})/r_B$ and $\tilde{A}=A_D/A_B$, as well as $r_{B}$, and $\Omega_m$ from a combined analysis with Planck 2018, DESI DR2, and Pantheon+. The data fit shows a preference for a non-vanishing $\tilde{A}$ corresponding to DAO with an approximately $10- 20\%$ larger ('positive branch' in orange) or $40\%$ smaller ('negative branch' in green) dark sound horizon $r_{d,D}\equiv r_{D}$. The black dashed line depicts the DAO prediction in the DRMD model obtained in Garny:2025kqj (with an additional prior on the absolute SN magnitude taken from Riess:2021jrx). Beyond improving the fit to DESI data, the DAO feature leads to larger values of $\Omega_m$, also improving the fit to SN data.
• Figure 4: Profile likelihood curves for Planck 2018, DESI DR2, and Pantheon+ data within $\Lambda$CDM, allowing for the presence of DAO with relative amplitude $\tilde{A}$, which biases the inference of the baryon drag horizon $r_B$. The second row depicts the relative difference between dark and visible sector drag horizon scales $\Delta r/r_B=(r_{D}-r_{B})/r_B$. The positive (orange) and negative (green) branches correspond to positive and negative values of $\Delta r / r_B$, respectively. For the positive branch a small relative displacement between DAO and BAO peaks is enough to improve the fit significantly. For the negative branch, the DAO tail is responsible for shifting the apparent BAO peak and thus a larger displacement and amplitude is necessary to achieve a similar effect.
• Figure 5: Constraints on $\Omega_m$ and the baryon drag horizon $r_B$. Contours denote the $68\%$ and $95\%$ confidence regions; for the DESI analysis, we adopt a BBN prior on $\omega_b$ following DESI:2025zgx. Left: Within $\Lambda$CDM, Planck 2018 data (purple contours) exhibit a mild tension with DESI DR2 measurements (blue contours) at the $2.3\,\sigma$ level. This tension is alleviated when the DAO feature is included (orange dashed and green dotted contours), which corrects the misidentification of $r_{B}$ and shifts the DESI constraints toward larger values of $\Omega_m$. Right: Within the DRMD model, also the CMB contours extend further into the DESI-preferred region, particularly at larger values of $H_0$.