Constraints on cosmological parameters and CMB first acoustic peak in conformal Killing gravity

TL;DR

Conformal Killing gravity (CKG) provides a geometric extension to general relativity that introduces a dark-sector density $\Omega_D$ in the Friedmann equations. The authors perform a joint Bayesian analysis using BAO data from DESI DR2 and SDSS DR16, SNeIa from Pantheon+ and Union3, and Planck 2018 CMB likelihood to constrain $H_0$, $\Omega_M$, and $\Omega_D$ in a flat Robertson–Walker background, finding a negative $\Omega_D$ and dynamical dark energy with $q_0<0$ and a quintessence-like $w_0$. Including CMB tightens correlations between parameters and yields results consistent with Planck for $H_0$ and $\Omega_M$, and the first acoustic peak scale $\theta_s$ remains close to Planck’s best value; a negative $z_c$ is inferred for DESI+Union3. Growth of perturbations in CKG shows no strong deviations from ΛCDM in the linear regime, though the CKG $f\sigma_8(z)$ curve suggests a modest shift in the preferred $\sigma_8$ value, motivating further detailed growth analyses. Overall, CKG appears capable of describing the expansion history from late times to recombination, motivating further exploration with expanded data and more precise growth measurements.

Abstract

In the frame of conformal Killing gravity cosmology, we performed a Bayesian analysis on two different datasets of Baryon Acoustic oscillations (DESI and SDSS DR16), two datasets of SNeIa (Pantheon+ and Union3), and using the Cosmic Microwave Background (CMB) Planck likelihood. The results for $H_0$ and $Ω_M$ in a spatially flat Friedmann-Lemaître-Robertson-Walker (FLRW) background are consistent with the $Λ$CDM scenario. We obtain a non-negligible negative value for the novel density of dark sector, $Ω_D$, and its relevance in the evolution of the cosmological observables, thus finding quantitatively what its contribution is on real data to match the standard scenario. The results confirm the dynamical character of dark energy. We also calculate the deceleration parameter $q_0$ and the present time dark energy equation of state parameter $w_0$: the latter belongs to the quintessence regime. The evaluation of the first acoustic peak of CMB places it near to the best value provided by the Planck collaboration. In this scenario, we can conclude that late time and early time data can be successfully matched under the same standard.

Figures (5)

• Figure 1: Results of the cosmological analysis: DESI+Union3 (top left panel); DESI+Pantheon+ (top right panel); DR16+Union3 (bottom left panel); DR16+Pantheon+ (bottom right panel). The contours represent the $68 \%$ and $95 \%$ probability levels, respectively.
• Figure 2: Results of the cosmological analysis combining DESI+PantheonPlus+CMB: keeping $\Omega_{\Lambda}$ fixed (left panel), and letting it vary (right panel). The contours represent the $68 \%$ and $95 \%$ probability levels, respectively.
• Figure 3: The deviation of the density contrast for CKG from the values in GR, $\delta_M(z) - \delta_{M,GR}(z)$. The first one is the numerical solution of \ref{['DELTAALL']}, the GR function is $c_2/(1 + z)$. Both are evaluated for $z>10$, with initial conditions $\delta_M(400)=0.0001$ and $\delta'_m(400)=-\delta_M(400) /401$.
• Figure 4: The function $g(z)$, Eq.\ref{['eq:fsigma8 z']}, for $\Lambda$CDM (dashed) and CKT (full).
• Figure 5: The function $f(z)\sigma_8(z)$ for $\Lambda$CDM (dotted) and CKG (full), with data points. The error bars are from Table 1 by Benisty, ref.Benisty21.

Theorems & Definitions (3)

• Remark 1
• Remark 2
• Remark 3