暗能量和加速膨胀的宇宙

Progress in understanding the cosmic acceleration

1998年发现的宇宙加速膨胀是当代科学中最重大的课题之一.理论上,宇宙的加速膨胀可能意味着当前宇宙中约三分之二的能量密度是由一种新的能量组分,即暗能量所提供的也可能意味着爱因斯坦提出的广义相对论在宇宙学尺度上需要修正.暗能量和修正引力这两种完全不同的物理机制可以给出完全相同的宇宙背景膨胀历史,但却预言不同的结构形成过程.因此,我们可以通过观测宇宙的大尺度结构形成和演化来区分这两种不同的物理机制,揭示宇宙加速膨胀背后的物理规律.宇宙大尺度星系巡天是研究宇宙加速膨胀机制的重要探针之一.基于星系巡天,我们可以通过测量重子声波振荡(baryonic acoustic oscillations,BAO)和红移空间畸变(redshift space distortions,RSD)两种特殊的星系成团属性,同时测量宇宙的背景膨胀和结构形成历史,进而分别开展暗能量性质以及引力研究.SDSS(Sloan Digital Sky Survey)三期的BOSS(Baryonic Oscillation Spectroscopic Survey)巡天是近期完成的世界最大规模的星系巡天.通过对10000平方度左右天区的观测,BOSS获取了近一百万条星系光谱.基于BOSS的观测,我们对暗能量和引力性质开展了深入研究,并发现了暗能量的动力学迹象.目前正在巡天的e BOSS(extended Baryonic Oscillation Spectroscopic Survey)项目以及后续的DESI(Dark Energy Spectroscopic Instrument)和PFS(Prime Focus Spectrograph)等大型巡天将在更高的红移、以更高的精度测量BAO和RSD,这将为宇宙加速膨胀机制的研究提供关键的观测支持.

Unveiling the physics behind the accelerating expansion of the Universe, which was discovered in 1998, is one of the most challenging tasks in modern sciences. In theory, the cosmic acceleration may result from the fact that approximately two thirds energy density of the Universe is provided by Dark Energy, a yet unknown energy component, or imply that general relativity, the most successful theory of gravity within the solar system established by Einstein, may need to be extended on cosmological scales. In principle, the scenarios of dark energy and modified gravity can yield identical cosmic expansion history, but pre- dict different structure growth. Therefore, this ＂dark degeneracy＂ can be broken by studying the evolution history of the cosmic structure with large galaxy surveys. Galaxy surveys is one of the key probes for dark energy and gravity, which can be used to reconstruct the history of cosmic expansion and structure growth using baryonic acoustic oscillations （BAO） and redshift space distortions （RSD） respectively. BAO is a specific three-dimensional clustering pattern of galaxies due to interactions between photons and baryons in the early Universe. Back in the early cosmic epochs, photons scattered with electrons and electrons interacted with pro- tons, making photons and protons （baryonic matter） ＂glued＂ together. There are two opposite forces exerting on this tightly coupled plasma： the pressure of photons and gravity acting on baryons. Similar to what happens to a vibrating spring vertically fixed on the ground, the photon-baryon plasma underwent ＂Baryonic Oscillations＂, propagating waves in the same way as the sound does. Therefore, this physical process is called Baryonic Acoustic Oscillations. The effect of the BAO is to push galaxies apart until at a specific time, called the cosmic recombination, when the temperature of the Universe becomes so low that no more photon-baryon interaction can happen. Since then, the separation among gal- axies is ＂frozen＂ at a characteristic scale, which is about 150 Mpc depending on the energy content of the Universe, making it a ＂standard ruler＂ to be used to infer the expansion history of the Universe, thus BAO is one of the key probes for the nature of dark energy. RSD is another special three-dimensional clustering pattern of galaxies, but it is due to local motions of the galaxies under gravity. Suppose galaxies only co-move with the cosmic background without moving locally, the clustering of galaxies should be isotropic, in other words, there are same number of galaxy pairs along or across lines of sight of the observer. However, this is not the case in the real world. Galaxies move towards nearby galaxies due to gravity, thus they have ＂peculiar velocities＂. As we infer galaxies＇ positions from their line-of-sight velocities in astronomy, the peculiar velocities can distort galaxies＇ positions, which results in a distortion, called Redshift Space Distortions, in the 3D clus- tering of galaxies. RSD plays a key role in cosmology as it can directly be used to infer the nature of gravity （remember the peculiar motion is caused by gravity!）. The Baryonic Oscillation Spectroscopic Survey （BOSS） program, part of the Sloan Digital Sky Survey （SDSS）-III pro- ject, is the largest completed galaxy survey in the world. BOSS has enabled us to investigate dark energy and gravity to a unprecedented precision, which has implied a 3.5 sigma evidence for the dynamics of dark energy. Ongoing and forth- coming galaxy surveys including the extended Baryonic Oscillation Spectroscopic Survey （eBOSS）, Dark Energy Spec- troscopic Instrument （DESI） and Prime Focus Spectrograph （PFS）, will be able to probe the Universe at higher redshifts with much higher precision, which will be providing key observational support for the study of cosmic acceleration.