点亮宇宙的黑暗时代

点亮宇宙的黑暗时代

哈佛-史密松天体物理中心新闻稿
2004年5月发布



宇宙的黑暗时代结束于氢原子云聚集在一起形成第一代恒星的时刻.氢原子云即为上面艺术家的概念图中的亮红色部分.
编辑: David A. Aguilar, 哈佛-史密松天体物理中心

当欧洲的黑暗时代随着14世纪文艺复兴的到来而终结的时候,社会被科学,艺术和文学的”新星”而照亮,如米开朗基罗,达芬奇,乔托以及但丁.无独有偶,宇宙也经历过相同的启蒙过程.大爆炸的时候,宇宙沐浴在光芒之中,随后这些光芒迅速黯淡.但是,黑暗时代以第一代恒星开始发光为标志而终结,宇宙--正如西方文明一样--步出黑暗而进入了光明的世纪.

那些想研究宇宙黑暗时代的天文学家们遇到了一个基本的难题.在被第一代恒星照亮之前,即使一个物体早已存在,你又如何观测呢?理论学家Abraham Loeb和Matias Zaldarriaga (哈佛-史密松天体物理中心)找到了一个解决办法.他们的计算表明,天文学家可以通过早期宇宙的第一代原子投射出的阴影来探测他们.

为了看到这些阴影,观测者必须研究宇宙微波背景辐射(CMB)--也即宇宙诞生时所留下来的辐射.大爆炸使宇宙充满了光和物质.随着空间的膨胀,宇宙变冷了,而光也随着波长被拉的越来越长而变的微弱,只留下宇宙沉寂在黑暗之中.

当宇宙的年龄大概是370,000年的时候,它已经冷到足以使电子跟质子结合,从而复合成中性氢原子并使得残留的CMB辐射在过去的130亿年内几乎可以毫无阻碍地在宇宙中传播.

随着时间的流逝,一些光子跟氢气体的团块相遇并被吸收.通过寻找那些光子少的地方--也就是被氢的团块挡住的地方-天文学家可以确定宇宙非常早的时候的物质分布.

“微波的天空携有巨量的信息,这些信息可以告诉我们宇宙的初始状况,且精度极高” Loeb说.


暴胀和暗物质

为了吸收CMB光子,氢的温度(特指其激发温度)必须要低于CMB辐射的温度--只有在宇宙年龄为2000万至1亿年这段时间内这种情况才会发生.巧合的是,这段时间也在恒星跟星系形成之前,因此这是一扇通向所谓”黑暗时期”的唯一窗口.

跟之前的仪器,如威尔金森微波各向异性探测器(WMAP)相比,研究CMB的阴影可使天文学家观测小得多的结构.阴影方法可以探测到小至相当于今天宇宙中30,000光年跨度的氢团块,或者等于原初宇宙仲300光年的跨度(由于宇宙膨胀尺度变大了).这样的分辨率比WMAP高1000倍.

“这个方法提供了一个指向极早期宇宙物理的窗口,称为暴胀时期,人们认为物质分布的涨落即产生于这个时期.而且,我们可以决定是否中微子或其它未知粒子构成了宇宙中暗物质的重要部分.这些问题--暴胀时期到底发生了什么,以及何为暗物质--是现代宇宙学中的关键问题,其答案将会使人们对宇宙的本性有基本的洞察”Loeb说.

观测挑战

氢原子在一个特定的波长--21厘米(8英寸)--上吸收CMB光子.宇宙的膨胀通过一个叫做红移的现象来拉伸光子的波长(因为波长越长光就越红).因此,为了观测早期宇宙的21厘米吸收,天文学家就必须搜寻更长的在6到21米之间的波长(20到70英尺),这个范围在电磁波谱的射电部分.

由于天空上其它源的前景的干扰,在射电波段观测CMB阴影并不容易.为了得到精确的数据,天文学家将不得不使用下一代的射电望远镜,如低频阵列(LOFAR)和平方千米阵(SKA).尽管观测是一个挑战,潜在的回报却是巨大的.


“有一个信息的金矿在那里等着我们去挖掘.完全的探测在实验上来说具有挑战性,但是我们知道这些信息在那里,我们可以努力在不久的将来去探测它,这也是值得的”Loeb说.

哈佛-史密松天体物理中心(CfA)总部位于Cambridge, Mass,是一个史密松天体物理天文台跟哈佛学院天文台的联合机构.CfA的科学家分为6个研究部门,研究宇宙的起源,演化及最终命运.


原文:

[size=+2]Illuminating the 'dark ages' of the universe
      [size=-2]HARVARD-SMITHSONIAN CENTER FOR ASTROPHYSICS NEWS RELEASE
Posted: May 4, 2004

登录/注册后可看大图     [size=-3]Thecosmic "dark ages" ended when clouds of hydrogen, glowing red in thisartist's conception, came together to form the first stars. Creditavid A. Aguilar, Harvard-Smithsonian Center for Astrophysics

When the European dark ages ended with the coming of the Renaissance inthe 14th century, society was illuminated by new "stars" of science,art and literature like Michelangelo, Leonardo da Vinci, Giotto, andDante. Oddly enough, the universe may have experienced the sameenlightenment. At the moment of the Big Bang, the universe was bathedwith light that quickly faded. But with the ending of the cosmic darkages as the first stars began to shine, the universe - like westerncivilization - moved out of the dark ages and into the age ofillumination.
Astronomers who want to study the cosmic dark ages face a fundamentalproblem. How do you observe what existed before the first stars formed tolight it up? Theorists Abraham Loeb and Matias Zaldarriaga(Harvard-Smithsonian Center for Astrophysics) have found a solution. Theycalculated that astronomers can detect the first atoms in the early universeby looking for the shadows they cast.
To see the shadows, an observer must study the cosmic microwave background(CMB) - radiation left over from the birth of the universe. The Big Bangfilled the universe with light and matter. As space expanded, it cooled, andthe light from the Big Bang dimmed as it was stretched to longer and longerwavelengths, leaving the universe in darkness.
When the universe was about 370,000 years old, it cooled enough forelectrons and protons to unite, recombining into neutral hydrogen atoms andallowing the relic CMB radiation from the Big Bang to travel almostunimpeded across the cosmos for the past 13 billion years.
Over time, some of the CMB photons encountered clumps of hydrogen gas andwere absorbed. By looking for regions with fewer photons - regions that areshadowed by hydrogen - astronomers can determine the distribution of matterin the very early universe.
"There is an enormous amount of information imprinted on the microwave skythat could teach us about the initial conditions of the universe withexquisite precision," said Loeb.
Inflation and dark matter
To absorb CMB photons, the hydrogen temperature (specifically its excitationtemperature) must be lower than the temperature of the CMB radiation -conditions that existed only when the universe was between 20 and 100million years old. Coincidentally, this is also well before the formation ofany stars or galaxies, opening a unique window into the so-called "darkages."
Studying CMB shadows also allows astronomers to observe much smallerstructures than was possible previously using instruments like the WilkinsonMicrowave Anisotropy Probe (WMAP) satellite. The shadow technique can detecthydrogen clumps as small as 30,000 light-years across in the present-dayuniverse, or the equivalent of only 300 light-years across in the primordialuniverse. (The scale has grown larger as the universe expanded.) Suchresolution is a factor of 1000 times better than the resolution of WMAP.
"This method offers a window into the physics of the very early universe,namely the epoch of inflation during which fluctuations in the distributionof matter are believed to have been produced. Moreover, we could determinewhether neutrinos or some unknown type of particle contribute substantiallyto the amount of 'dark matter' in the universe. These questions - whathappened during the epoch of inflation and what is dark matter - are keyproblems in modern cosmology whose answers will yield fundamental insightsinto the nature of the universe," said Loeb.
An observational challenge
Hydrogen atoms absorb CMB photons at a specific wavelength of 21 centimeters(8 inches). The expansion of the universe stretches the wavelength in aphenomenon called redshifting (because a longer wavelength is redder).Therefore, to observe 21-cm absorption from the early universe, astronomersmust look at longer wavelengths of 6 to 21 meters (20 to 70 feet), in theradio portion of the electromagnetic spectrum.
Observing CMB shadows at radio wavelengths will be difficult due tointerference by foreground sky sources. To gather accurate data, astronomerswill have to use the next generation of radio telescopes, such as the LowFrequency Array (LOFAR) and the Square Kilometer Array (SKA). Although theobservations will be a challenge, the potential payoff is great.
"There's a gold mine of information out there waiting to be extracted. Whileits full detection may be experimentally challenging, it's rewarding to knowthat it exists and that we can attempt to measure it in the near future,"said Loeb.
Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center forAstrophysics (CfA) is a joint collaboration between the SmithsonianAstrophysical Observatory and the Harvard College Observatory. CfAscientists, organized into six research divisions, study the origin,evolution and ultimate fate of the universe.

原文出自:

http://spaceflightnow.com/news/n0405/04darkages/

支持支持，只是楼主能否把原文与译文一段段的间隔着来，这样大家看着方便

2# gohomeman1
OK,下次再贴的时候就这么干.

看过了。学习了！