前不久，上海天文台张鹏杰研究员和我合作完成的一篇关于用Ia 型超新星测大尺度本动速度的论文在Physical Review D 上发表。英国Physics World (物理世界)的编辑 Hamish Johnston 见到后写信来咨询了一些问题，于7月24日写成一篇报道发表在其Headline News （头条新闻）上，不过他并未通知我们，因此直到前天我上Physics World 网站浏览时才发现。顺便说一句，这一工作鹏杰是主要贡献者也是论文的第一作者，但不知何故Hamish 来信是向我询问，报道文中也多次写成 Chen & Zhang，真令我有点不好意思。

 

我们的论文主要想法是，星系随着宇宙膨胀互相远去，但除了这一均匀的运动外，星系相对于随宇宙膨胀的共动坐标系还有运动，这是由于其邻近非均匀分布的物质产生的引力造成的，称为本动速度(peculiar velocity)。 如果能够测量本动速度在大尺度上的分布，就能测定引力分布和不均匀性的增长速度，这有助于暗能量的测定。如何测定本动速度呢？问题是对一个星系测到的光谱红移是其宇宙膨胀红移和本动速度多普勒效应的叠加，难以区分。但是，如果星系中正好爆发一颗Ia 型超新星那就不一样了。Ia型超新星的光度经修正后基本一样，因此可以作为标准烛光，因此通过测定其视亮度就可以定出我们到该星的距离。这样，我们也就知道了该点的膨胀速度，于是可以求出本动速度来。不过，为了要达到足够的精度，我们可能需要分布在全天不同红移总共大约一百万颗超新星的数据,这一要求是惊人的，举例来说，美国正在计划中的SNAP 空间望远镜，一年才约可观测两千颗超新星。LSST 望远镜可能可以观测到这么多的超新星，但为此也许就不一定有足够的持续时间去测它们的光变曲线。总之，这是一个实际的困难，但是也许不是无法克服的，特别是如果建造多个大型望远镜并持续进行几十年的观测的话。也许正是这一观测的难度，激发了记者Johnston 先生的兴趣吧。

以下是Physics World英文报道原文。

 

A multi-telescope survey of millions of supernovae could give cosmologists a better understanding of dark energy and could help explain the origins of large-scale structure in the universe, according to cosmologists Pengjie Zhang and Xuelei Chen of the Chinese Academy of Science. The proposed survey would involve measuring the “peculiar motion” of each exploding star, which would reveal new information about how galaxies are distributed throughout the cosmos.

 

Galaxies are moving away from one another thanks to the ongoing expansion of the universe. However, this expansion is not the same everywhere and each galaxy has an additional “peculiar motion”, which is caused by the gravitational pull of neighbouring galaxies.

 

Rather than isolate the peculiar motion of individual galaxies, astronomers try to determine the large-scale peculiar velocity (LSPV), which is the average peculiar velocity of galaxies relative to Earth in a certain region of the universe. They rely on knowing the distances between Earth and each galaxy —and that can be very difficult to measure precisely.

 

'Standard candles'

 

Something they can measure accurately, however, is the distance to a type-Ia supernovae — exploding white-dwarf stars that are known as “standard candles” because they always give off the same amount of light. All they have to do is measure the supernova brightness, and the dimmer it is, the farther away it is. But astronomers can also estimate the speed with which the star was moving away from Earth when it exploded by measuring the Doppler shift (or “red shift”) in the wavelength of its light.

 

If the universe was expanding in a uniform manner, all type Ia supernovae of a specific brightness would have the same redshift. However, Chen and Zhang expect that the average brightness of supernovae with the same redshift in a small patch of the sky would be different to the average brightness (at that redshift) over the entire sky. “This difference is proportional to the LSPV,” explains Chen (Phys Rev D 78 023006) .

 

Chen and Zhang believe that such a survey would be performed best using supernovae with an “intermediate” redshift of about 0.5, which dates their explosions to about five billion years ago. The survey would involve about 106 supernovae, and would require several decades of continuous observation using a number of different telescopes.

 

Christopher Gordon, a cosmologist at the University Oxford, UK, told physicsworld.com that the proposal “is an interesting idea”. However, he believes that Chen and Zhang may be overly optimistic in saying that such a survey could be completed in several decades – even when new telescopes designed specifically for finding supernovae such as the Large Synoptic Survey Telescope (LSST) come online in the next decade.

 

Smoothing dark energy

 

Still, if Chen and Zhang can persuade astronomers to do the survey, the results could shed light on the nature of dark energy — a mysterious substance that counteracts gravity and encourages galaxies to move apart. Dark energy tends to prevent galaxies from clumping and therefore should smooth out variations in the LSPV. However, some theories of dark energy predict less clumping than others.

 

A survey could also further our understanding of dark matter — a substance that interacts via gravity but not via electromagnetic radiation. Galaxies are made up mostly of dark matter, so Chen and Zhang believe that a LSPV survey could reveal much about the distribution of dark matter in the universe.

 

The survey would allow physicists to study the effects of gravity over very large length scales and could also help cosmologists understand how primordial quantum fluctuations that existed just after the Big Bang became the large-scale structure of galaxies that we observe today.