Braneworld Dark Energy in light of DESI DR2

TL;DR

This paper addresses DESI DR2 tensions with ΛCDM by modeling dark energy as a thawing scalar field propagating on a ghost-free (4+1)-D phantom braneworld. The authors solve the braneworld cosmological equations for a range of potentials (quadratic, quartic, symmetry-breaking, exponential, and axion) and show that the effective DE EoS undergoes a phantom-divide crossing, with Om diagnostics and Hubble evolution matching DESI DR2 observations. Using MCMC against DESI DR2, Union 3 supernovae, and CMB priors, they find χ^2 values for all models that are remarkably close to the CPL parametrisation, demonstrating a good fit to the data. The results provide a physically well-motivated mechanism for dynamical dark energy within higher-dimensional gravity, and point to future work on perturbations and ISW/growth signals to further test the scenario.

Abstract

Recent observational results from the DESI collaboration reveal tensions with the standard $Λ$CDM model and favour a scenario in which dark energy (DE) decays over time. The DESI DR2 data also suggest that the DE equation of state (EoS) may have been phantom-like ($w < - 1$) in the past, evolving to $w > - 1$ at present, implying a recent crossing of the phantom divide at $w = - 1$. Scalar field models of DE naturally emerge in ultraviolet-complete theories such as string theory, which is typically formulated in higher dimensions. In this work, we investigate a broad class of $thawing~scalar~field~models$, including the simple quadratic, quartic, exponential, symmetry-breaking and axion potentials, propagating on a (4+1)-dimensional ghost-free phantom braneworld, and demonstrate that their effective EoS exhibits a phantom-divide crossing. Alongside the Hubble parameter and EoS of DE, we also analyse the evolution of the $Om$ diagnostic, and demonstrate that the time dependence of these quantities is in excellent agreement with the DESI DR2 observations. Furthermore, we perform a comprehensive parameter estimation using Markov Chain Monte Carlo sampling, and find that the $χ^2$ values for all our models are remarkably close to that of the widely used CPL parametrisation, indicating that our models fit the data very well.

Figures (44)

• Figure 1: Schematic plots of different potentials considered in this work. Top-left panel shows the quartic potential (\ref{['eq:pot_quartic']}) for different values of $\lambda$. Top-right panel shows the symmetry-breaking potential (\ref{['eq:pot_SSB']}) for fixed value of $\nu$, and different values of $\lambda$. Bottom-left panel shows the exponential potential (\ref{['eq:pot_Exp']}) for fixed value of $V_0$, and different values of $\lambda$. Bottom-right panel shows the pseudo Nambu-Goldstone boson/axion potential (\ref{['eq:pot_PNGB']}) for different values of the axion mass (\ref{['eq:pot_Axion_quad_mass']}) (the quadratic approximation (\ref{['eq:pot_Axion_quad']}) is displayed in light-gray curves).
• Figure 2: The Hubble parameter relative to $\Lambda$CDM is shown for the quadratic potential (\ref{['eq:pot_Quad']}) with $m = \frac{5}{4} H_0$ ( left panel), and $m = \frac{3}{2} H_0$ ( right panel).
• Figure 3: The DE equation-of-state parameter corresponding to the quadratic potential (\ref{['eq:pot_Quad']}) is shown for $m=\frac{5}{4} H_0$ ( left panel), and for $m=\frac{3}{2} H_0$ ( right panel). Note that a larger value of the higher dimensional parameter, $\Omega_{0\ell}$, results in a lower redshift of phantom crossing.
• Figure 4: The Hubble parameter corresponding to the quartic potential (\ref{['eq:pot_Quartic']}) is shown for $\lambda = 0.05 \left( H_0/m_p \right)^2$ ( left panel), and $\lambda = 0.075 \left( H_0/m_p \right)^2$ ( right panel).
• Figure 5: The DE EoS corresponding to the quartic potential (\ref{['eq:pot_Quartic']}) is shown for $\lambda=0.05 \left( H_0/m_p \right)^2$ ( left panel), and for $\lambda = 0.075 \left( H_0/m_p \right)^2$ ( right panel). Note that a larger value of the higher dimensional parameter, $\Omega_{0\ell}$, results in a lower redshift of phantom crossing.