The Quintom theory of dark energy after DESI DR2

Abstract

Observations from DESI DR2 are challenging the $Λ$CDM paradigm by suggesting that the equation-of-state parameter of dark energy evolves across $w = -1$, a phenomenon known as the Quintom scenario. Inspired by this development, we present a staged review of Quintom cosmology including its theoretical foundations, observational supports, and implications as well as possible extensions. We first trace the historical progression from Einstein's static cosmological constant to modern dynamical dark energy, summarizing recent cosmological constraints that favor an evolving $w(z)$ along time. A key focus is the theoretical no-go theorem for dark energy showing that no single canonical field or perfect fluid model can smoothly cross the $w = -1$ boundary. We then survey viable Quintom constructions, including two-field models, single-scalar fields with higher derivatives, modified gravity frameworks, interacting dark energy, and an effective field theory approach that unifies these mechanisms. Possible interactions of Quintom fields with ordinary matter and the potential roles in yielding non-singular universe solutions are discussed.

Figures (6)

• Figure 1: 68% and 95% marginalised posterior constraints on $w_0$-$w_a$ plane for the flat $w_0w_a$ model from the combination of DESI BAO data, CMB and SNe, for PantheonPlus Brout:2022vxf, Union3 Rubin:2023jdq and DESY5 Abbott:2024agi SNe datasets in blue, orange and green, respectively. The left panel is taken from ref. DESI:2024mwx, while the right panel is from ref. DESI:2025zgx. We have added the boundary lines to illustrate the regions of quintessence, phantom and quintom.
• Figure 2: The reconstructed EoS parameter $w$ from Gaussian process regression. The upper left panel is from Fig.2 of Yang:2024kdo, the black curve denotes the mean value, while the light blue shaded zones indicate the allowed regions at 68% confidence interval. The upper right panel is from Fig.10 of DESI:2025fii, the Gaussian process reconstruction is shown in blue, accompanied by shaded 68% and 95% confidence intervals. The bottom panel is from Fig.3 of Yang:2025mws, shows the mean values of the reconstructed dark energy EoS parameter $w$ and the normalized energy density $f_{de}$, along with $1\sigma$ and $2\sigma$ uncertainties, where $f_{de}$ is defined as $f_{de}(z)=\rho_{de}(z)/\rho_{de,0}$.
• Figure 3: Dark energy equation of state $w(z)$ reconstructed from several datasets. The results are for two different approaches: the correlation-prior method (bottom-layered, dark-blue band) and the ($w_0$, $w_a$) parameterization (top-layered, green band).The panel is from Fig.3 of DESI:2025wyn.
• Figure 4: The constraint on coefficients $\tilde{c}_1$ and $\tilde{c}_2$ from DESI+CMB data (blue), DESI+CMB+Pantheon+ data (orange), DESI+CMB+Union3 data (purple), DESI+CMB+DESY5 data (brown). We choose $a_i=0.5$ corresponding the redshift $z_i\simeq 1$ which is in the matter-dominant era. The total equation of state $w_{T0}\simeq w_0\Omega_{DE0}$, where $\Omega_0$ is the fraction of today's dark energy density, $\Omega_{DE0}\simeq0.647$.
• Figure 5: The value of factor $D$ (vertical axis) in terms of $w_0$ and $w_a$ in the allowed parameter space of DESI+CMB (left-top), DESI+CMB+Pantheon+ data (right-top), DESI+CMB+Union3 data (left-bottom), DESI+CMB+DESY5 data (right-bottom). The parameter choices are the same with Fig. \ref{['constrainG']}.