An international research team led by the National Astronomical Observatories of the Chinese Academy of Sciences (CAS) has recently released new findings on dark energy detection (DESI collaboration, arXiv:2504.06118). They discovered that the equation of state for dark energy evolves over time (see Figure 1), excluding Einstein's cosmological constant at a 4.3 σ confidence level, and crossing the cosmological constant limit w=-1 during its evolution. This discovery provides significant experimental support for the Quintom dark energy theory proposed by Prof. ZHANG Xinmin’s team from the Institute of High Energy Physics of CAS.

Figure 1: Evolution of the dark energy equation of state over time, sourced from arXiv:2504.06118

What is the physical essence of dark energy? This is a major enigma of the new century. Over the past few decades, scientists worldwide have proposed various theories to interpret dark energy. A crucial quantity describing the physical properties of dark energy is its equation of state w (the ratio of pressure to energy density). Different dark energy models can be phenomenologically classified based on the evolutionary behavior of w (see Figure 2). For instance, for the cosmological constant, w remains constant over time and equals -1. For dynamical dark energy models, w varies with time, but different models predict different ranges of variation: the Quintessence theory predicts w>−1, the Phantom theory predicts w<−1, and the Quintom theory predicts that w crosses -1 during the evolution of the universe.

Figure 2: Dark energy models classified by equation of state (Credit: NAOC)

The Quintom dark energy theory was first proposed internationally by Prof. ZHANG Xinmin’s team in 2004. To date, this paper has been cited over 1,200 times. The paper was awarded the “Thomson Scientific Research Fronts Award 2008”. Over the past two decades, ZHANG has led his team in conducting in-depth and systematic research on the Quintom theory, spearheading international studies on dark energy theories where the equation of state crosses -1. In 2010, the international review journal Physics Reports published a dedicated article introducing Quintom cosmology.

Dark energy and dark matter are collectively referred to as the two new "clouds" in modern material science of the 21st century. Currently, the confidence level for the time evolution of the dark energy equation of state is 4.3 σ, which has not yet reached the 5 σ gold standard in particle physics for determining whether a discovery is "definitive". With the accumulation of DESI data and further observations from supernovae (SNe), large-scale structure surveys (LSS), and cosmic microwave background radiation (CMB, such as the AliCPT experiment for primordial gravitational wave detection), the Quintom dark energy theory will undergo further testing.