【摘要 宇宙学家们非常关心暗能量是否为宇宙学常数, 一组来自中国的科学家利用最新重子声学震荡, 宇宙微波背景辐射, 超新星数据拟合发现, 暗能量状态方程可能在三十亿年前经历过转折. 受限于当前宇宙学实验的精度, 该发现置信度并不高(约2simga). 但未来宇宙学实验可对这一结果正确性给出决定性论断.】

Dark energy has become one of the hottest topic among modern cosmologists since the discovery of cosmic acceleration. The Cosmological Constant, firstly introduced by Einstein in 1917, is now commonly considered to be the standard candidate of dark energy due to its simple, natural form. However, cosmologists still believe that dark energy may evolve dynamically during the expansion history. If this be true, the whole evolution of our Universe would be different from the case of a Cosmological Constant. For example, if dark energy density keeps increasing, in the far future our Universe may end up with a big rip (see http://www.eurekalert.org/pub_releases/2012-07/sicp-dea071112.php.)  

Recently, a group of Chinese researchers investigated this topic via data from most recent cosmological observations. They found that, the dark energy equation of state (EoS), defined as w=ρ/p, may have experienced a turning point in the past (if dark energy is cosmological constant then there will be w=-1 throughout the expansion history of the Universe). Their work, entitled "Investigating the possibility of a turning point in the dark energy equation of state", was published in Sci China-Phys Mech Astron 2014, Vol. 57, No. 8.

The most recent baryon acoustic oscillation (BAO) data, released by the Sloan Digital Sky Survey and WiggleZ Dark Energy Survey, are combined with the cosmic microwave background data (from Planck and WMAP satellites) and type Ia supernovae data (Supernova Legacy Survey 3-year sample) to perform a joint analysis. Data are fitted via an elaborately designed parametrization to search for possible evolutions of dark energy EoS in the past, a more meticulous analysis compared with what people did in previous studies. As the authors mentioned, their parametrization is “a direct characterization of a possible turning in w” and “should be comprehensive enough given the current observational data”.

The result revealed that dark energy EoS may have undergone a turning point at about 3 billion years ago (roughly corresponding to cosmological redshift z=0.3). There may be a long period that dark energy density kept increasing. However, around 3 billion years ago, the speed of increasing slowed down due to some unknown reasons. Even more, probably dark energy density has became decreasing in the recent epoch.  

Due to the limited ability of current observational data, the significance of dynamical dark energy is not very high (only a few more than two sigma). However, the result can be proved or falsified by future cosmological data. And this topic, of course, will always be hot among scientists who are attracted by the charm of dark energy. “To reveal the mysterious veil of dark energy, we need to make an exhaustive study based on more powerful data from future experiments”, said the authors at the end of the article.

See the article: Hu Y Z, Li M, Li X D, Zhang Z H. Investigating the possibility of a turning point in the dark energy equation of state. Sci China-Phys Mech Astron, 2014, 57: 1607–1612