The Quintom theory of dark energy after DESI DR2

TL;DR

This paper reviews the Quintom dark energy model, its theoretical basis, observational evidence from DESI DR2, and various extensions, emphasizing its ability to cross the $w = -1$ boundary.

Contribution

It provides a comprehensive staged review of Quintom cosmology, including theoretical constraints, models, and implications for the evolution of dark energy.

Findings

DESI DR2 data suggests dark energy's equation-of-state parameter evolves across $w = -1$.

No single canonical field can smoothly cross the $w = -1$ boundary due to a no-go theorem.

Multiple viable Quintom models include two-field, higher derivatives, and modified gravity frameworks.

Abstract

Observations from DESI DR2 are challenging the $\Lambda$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.