Cosmological tests of quintessence in quantum gravity

TL;DR

The paper investigates quantum-gravity–motivated hilltop quintessence as a dynamical dark energy candidate by studying axion, saxion, and Higgs-like hilltop models. It introduces the Dutta–Scherrer–Chiba (DSCh) parameterisation to describe the equation of state near hilltops and validates it against explicit models, comparing performance to the CPL form. Using a CAMB-based MCMC analysis with Planck, DESI BAO, Pantheon+/Union3, and DES-Y5 data, the study derives posterior constraints on model parameters, assesses model fit via AIC, and highlights how inference is sensitive to theoretical priors. The work demonstrates a productive dialogue between quantum-gravity conjectures and cosmological observations, while outlining theoretical refinements needed to maximize the informative power of future datasets for testing dark energy scenarios in quantum gravity.

Abstract

We use a suite of the most recent cosmological observations to test models of dynamical dark energy motivated by quantum gravity. Specifically, we focus on hilltop quintessence scenarios, able to satisfy theoretical constraints from quantum gravity. We discuss their realisation based on axions, their supersymmetric partners, and Higgs-like string constructions, including dynamical mechanisms to set up initial conditions at the hilltops. We also examine a specific parameterisation for dynamical dark energy suitable for hilltop quintessence. We then perform an analysis based on Markov Chain Monte-Carlo to assess their predictions against CMB, galaxy surveys, and supernova data. We show to what extent current data can distinguish amongst different hilltop set-ups, providing model parameter constraints that are complementary to and synergetic with theoretical bounds from quantum gravity conjectures, as well as model comparisons across the main dark energy candidates in the literature. However, all these constraints are sensitive to priors based on theoretical assumptions about viable regions of parameter space. Consequently, we discuss theoretical challenges in refining these priors, with the aim of maximizing the informative power of current and forthcoming cosmological datasets for testing dark energy scenarios in quantum gravity.

Figures (18)

• Figure 7: Evolution of the equation of state parameter for the exponential potential \ref{['eq:Vexp']} and its comparison to the DSCh and CPL parameterisations. For the latter we used $\Omega_{\phi 0}$, $w_0$ obtained from the evolution with CAMB, while for CPL we fitted the linear behaviour between $a=0.5$ and $a=1$ to obtain the parameters, indicated to the right.
• Figure 8: Analytic results for $\Delta \phi_i$ obtained from eq. \ref{['eq:phii']} for the hilltop quintessence models, and posterior contours furnished by a MCMC analysis in the $K$-$\vert \Delta\phi_{i}\vert$ plane. The analytic results are represented in dark grey line using best-fit values for $\Omega _{\phi,0}$ and $w_0$ from the data combination with Union3. Dark blue shapes indicate the points corresponding to the best-fit values (see Tables \ref{['tab:Axion_table_full']}-\ref{['tab:Higgs_table_full']}) for model parameters ($\phi _0$ and $f$): circle for axion model, star for sugra model and triangle for the field theory model. In the same figure, we show with blue contours the $1\sigma$ and $2\sigma$ bounds in the $K-\vert \Delta\phi_{i}\vert$ plane from the constraints on the DS parameterisation for the data combination with Union3. See Figure \ref{['fig:phiK_DS_all']} in the appendix for the analogous figure including constraints from all the data combinations.
• Figure 9: Parameter constraints on the Axion model, eq. \ref{['axionV']} ($68\%$ and $95\%$ contours).
• Figure 10: Parameter constraints ($68\%$ and $95\%$ contours) for the Saxion model \ref{['sugraV']}.
• Figure 11: Constraints on the Higgs-like hilltop model \ref{['eq:FTV']} ($68\%$ and $95\%$ contours).