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宇宙中究竟有多少个星系?这个问题网上一搜,有说1000亿的,有说2000亿的,也有说两万亿的。到底哪个数字更接近真实情况呢?
如何知道有多少星系?一个个去数显然不现实,你能想到最简单也最直接的方法,可能就是通过随机抽样来估算。这样我们就可以先对天空中某片区域拍一张照片,然后数一数里面有多少个星系,完了再根据这块天区占整个天球的比例,最后计算出个总数。
这确实是个办法,而且天文学家也是这么做的。
1995年,哈勃望远镜曾对北半球天空的一小块区域进行了长时间曝光的深度观测。那是一块面积不到满月百分之一的狭小区域,起初看起来空无一物,人们自然也没有报太大希望。然而当照片出来后,大家全都傻了眼:之前还空空如也的地方,竟然布满了密密麻麻各种天体!这张照片便是著名的“哈勃深空场(Hubble Deep Field, HDF)”,也叫“哈勃深空”。
经过统计,照片中的星系多达三千个,于是初步估算,宇宙中的星系大约在800亿个左右。三年后,哈勃又在南天区拍了张类似的照片。由于这次的曝光时间更长,这张“哈勃南天深空”拍出了更多的星系,此后通过它估算出的星系数量增加到了1250亿。
看来只要增加曝光时间,拍出更多星系应该不是问题。于是2003到2004年间,哈勃先后花了70多天,累计曝光280个小时,最终生成了一张包含近万个星系的“哈勃超深空(Hubble Ultra Deep Field, HUDF)”。这张照片预示着宇宙中的星系总数可能超过了2000亿个。
此后,虽然不同模型得出的星系数量不尽相同,但结果都大差不差。这也是“宇宙星系总数在1000亿~2000亿”这个说法的来源。
你可能也意识到了,这样得出的星系总数显然不准嘛。不管我把曝光时间加到多长,总有一些星系因为太远太小,我看不到。再加上宇宙本身也在膨胀,遥远星系发出的光会产生严重的红移,以至于很多星系别说哈勃,就是韦伯上了也无能为力。
然而这还不是最关键的。不知道你发现没有,通过照片数星系的方式存在一个大BUG:我们拍摄的照片并不能代表此刻的宇宙!
由于光速限制,星系发出的光到达地球是需要时间的,距离越远花费时间越长。这就相当于星系拍了张自拍,然后寄给我们,等我们收到照片已经是很久很久以后的事了。这就会导致一个现象:近处的星系通常看起来会比较“老”,因为照片拍的就是它们现在的样子;但远处的星系看起来就相对年轻,因为那些照片是在很久很久以前它们还很年轻的时候拍的,有些甚至是出生不久的“满月照”。这个现象意味着,在那之后的几十亿、上百亿年里,一些星系它们已经相互融合成了更大的星系,星系数量会减少;但与此同时,新的星系也在不断地形成,星系数量又会增加。也就是说,我们没办法获知此时此刻的宇宙有多少星系。
对于我们来说这似乎是个无解的问题,但科学家可不会轻易放弃,毕竟这可是一帮连宇宙多大岁数都能推算出来的人。
2016年,一篇发表于《天体物理学期刊》上的文章中,研究人员通过研究星系密度如何随宇宙变化,从而推测出宇宙中可能的星系总数。
研究人员基于大量观测数据建立的星系质量函数,在排除了宇宙膨胀影响后,计算出位于不同红移区间(也就是不同距离上)的星系密度,这样我们就对宇宙中星系数量的变化规律有了大致了解。其中影响星系数量的因素主要有两个:一个是红移,一个是星系质量。
红移很好理解,不同红移代表宇宙的不同时期,每个时期的星系数量肯定不一样。经过计算发现,星系数量其实与红移的关系并不是特别大。因为当研究人员把红移极限值设定为12时,结果只比红移极限为8的时候星系总数多了65%。
然后,星系质量呢,它代表了什么样的天体可以算作一个星系,太小的话它可能就和星团没什么区别了。所以呢,研究人员设定了100万倍太阳质量的星系质量下限,这也是一些小的矮椭球星系通常的质量下限。
计算结果显示,在宇宙诞生之初,星系密度经历了一个迅速的下降阶段,在30亿年后趋于恒定。这个结果出乎意料。你想啊,早期的宇宙充满了原始气体,此时正是大批恒星集中诞生的时候。新恒星不断出现,带来的结果肯定是星系数量不断上涨才对。但为何计算结果恰恰相反呢?
一个合理的猜测是:在这期间星系间的合并意外的频繁。因为这时候虽然有很多新诞生的星系,但是它们普遍还很小,加上此时的宇宙还没膨胀太大,仍然十分拥挤,因此星系之间很容易发生碰撞,相互融合。如果把星系质量提高一万倍,限定在100亿倍太阳质量(差不多就是大麦哲伦这种比较成熟的星系),那结果就和预想的一样了:随着时间推移,星系的数量稳步上升。因为随着小星系合并,大星系肯定越来越多嘛。
最终,研究人员以红移值8为标准,计算了红移小于8且质量在100万倍太阳质量以上的星系数量。为什么红移设定在8?前面说了,红移值对星系数量的影响不是很大,从8的时期开始(大概是大爆炸后8亿年左右),宇宙中星系的总数就已经基本固定了。再往前算意义不是很大,而且对高红移天体的观测结果本身不确定性就很大。所以,我们只需要计算红移<8的星系数量,基本上就能代表目前宇宙中的星系总数了。
这个数量是多少呢?大约2万亿!没错,比哈勃深场估算出的足足多了10倍。这意味着,通过哈勃望远镜我们只能看到宇宙中1/10的星系,另外的九成虽然理论上有看到的可能,但是因为它们个头太小或离得太远,如今的技术很难将其识别。
这其实也从一方面解释了奥伯斯佯谬的问题:如果宇宙是无限大的,那恒星也是无限多的,那么无论我们朝哪个方向望去,视线总会落到一颗恒星上,所以天空应该是亮的才对,可现实中的夜晚,天空为什么总是黑的呢?
其实都不用无限多,几万亿个星系就足以覆盖视线中的每一个点。只是因为它们太过暗淡,即使通过哈勃、韦伯这样的设备我们也无法尽收眼底。
对了,有必要说一下:本期提到的“宇宙”特指“可观测宇宙”,“2万亿”的这个结果也仅限定在可观测宇宙范围内。但是可观测宇宙的范围会随时间变化,当可观测宇宙以外的光经历了足够长的时间后,它依然有机会进入我们的视野。假如宇宙是无限的话,那是否意味着可观测宇宙的星系数量终将也是无限的呢?
答案是:不会!因为宇宙一直在加速膨胀中,如今我们可观测宇宙中的绝大部分区域,它们已经在以超光速远离我们。
目前可观测宇宙半径大约在465亿光年,假设暗能量保持不变,再过若干亿年,这个半径最终会定格在620亿光年这个极限值。宇宙中的星系数量,虽然在接下来的很长时间内仍会继续增加,但是数量的上限最终只会达到当前数量的2.36倍,也就是不到5万亿。所以这也意味着,哪怕人类文明能够永远存续下去,我们也终将无法等到一览宇宙全貌的那一天。
虽然理论上未来会有更多的星系被我们看到,但实际上,越来越多的星系它们会因为宇宙膨胀而红移越来越大。
也就是说,未来我们看到的星系很可能并不会增加,反而是已经看到的星系会逐渐淡出视野,直至永不可见。
现如今,所有红移大于1.69的星系,它们的退行速度都已超过光速。它们此刻发出的光,我们将永远无法看到。若干亿年后,当宇宙中所有星系均以超光速远离彼此时,对于之后宇宙中诞生的新文明来说,它们甚至无法得知有其他星系存在这一事实。由于发现不了空间膨胀,它们可能根本不知道有大爆炸的存在。对它们来说,宇宙将是一片永恒的黑暗,而宇宙的诞生,也将是一个永远无法解开的谜团。
既然宇宙一直在加速膨胀,那终将有一天,这个膨胀速率会大到使星系中的恒星彼此远离。
到那时它们会惊讶的发现,原本一直稳定存在的星系竟然开始了膨胀!
当漫天繁星变得不再闪耀,意味着星系已经开始解体。不过,哪怕整个可观测宇宙只剩下一颗恒星,那也不过是夜晚的天空没有了星星而已。但接下来才真正进入地狱之门:行星系统崩溃、天体瓦解,灾难接踵而至,最终整个世界乃至小小的原子都会分崩离析。难道说,宇宙的最终命运真的是“大撕裂”吗?
论文&综述:
[1] Christopher J. Conselice, Aaron Wilkinson,Kenneth Duncan, Alice Mortlock. THE EVOLUTION OF GALAXY NUMBER DENSITY AT z < 8 AND ITS IMPLICATIONS[J]. The Astrophysical Journal, 2016, 830(2)
[2] N. Metcalfe, T. Shanks, A. Campos, H. J. McCracken, R. Fong. Galaxy number counts — V. Ultradeep counts: the Herschel and Hubble Deep Fields[J]. Monthly Notices of the Royal Astronomical Society, 2001, 323(4):795-830
[3] Krauss, Lawrence M.; Robert J. Scherrer. The Return of a Static Universe and the End of Cosmology. General Relativity and Gravitation[J]. 2007, 39(10): 1545-1550
[4] J. Richard Gott III, Mario Jurić, David Schlegel, Fiona Hoyle, Michael Vogeley, Max Tegmark, Neta Bahcall, Jon Brinkmann. A Map of the Universe[J]. The Astrophysical Journal, 2005, 624(2)
新闻&文章:
[1] SPACE.COM: How many galaxies are there?
[2] WIKIPEDIA: Observable universe
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#3eb575;--weui-BRAND: #07c160;--weui-BLUE: #10aeff;--weui-INDIGO: #1196ff;--weui-PURPLE: #8183ff;--weui-WHITE: rgba(255, 255, 255, .8);--weui-LINK: #7d90a9;--weui-TEXTGREEN: #259c5c;--weui-FG: #fff;--weui-BG: #000;--weui-TAG-TEXT-RED: rgba(250, 81, 81, .6);--weui-TAG-BACKGROUND-RED: rgba(250, 81, 81, .1);--weui-TAG-TEXT-ORANGE: rgba(250, 157, 59, .6);--weui-TAG-BACKGROUND-ORANGE: rgba(250, 157, 59, .1);--weui-TAG-TEXT-GREEN: rgba(6, 174, 86, .6);--weui-TAG-BACKGROUND-GREEN: rgba(6, 174, 86, .1);--weui-TAG-TEXT-BLUE: rgba(16, 174, 255, .6);--weui-TAG-BACKGROUND-BLUE: rgba(16, 174, 255, .1);--weui-TAG-TEXT-BLACK: rgba(255, 255, 255, .5);--weui-TAG-BACKGROUND-BLACK: rgba(255, 255, 255, .05)}}@media screen and (min-width:1024px){body:not(.pages_skin_pc) .wx-root,body:not(.pages_skin_pc) body{--appmsgExtra-BG: #F7F7F7}}@media screen and (min-width:1024px){body:not(.pages_skin_pc) .wx-root[data-weui-theme=dark],body:not(.pages_skin_pc) body[data-weui-theme=dark]{--weui-BG-0: 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.6);--weui-TAG-BACKGROUND-ORANGE: rgba(250, 157, 59, .1);--weui-TAG-TEXT-GREEN: rgba(6, 174, 86, .6);--weui-TAG-BACKGROUND-GREEN: rgba(6, 174, 86, .1);--weui-TAG-TEXT-BLUE: rgba(16, 174, 255, .6);--weui-TAG-BACKGROUND-BLUE: rgba(16, 174, 255, .1);--weui-TAG-TEXT-BLACK: rgba(255, 255, 255, .5);--weui-TAG-BACKGROUND-BLACK: rgba(255, 255, 255, .05)}}@media(prefers-color-scheme:dark){.wx-root:not([data-weui-theme=light]),body:not([data-weui-theme=light]){--appmsgExtra-BG: #121212}}.rich_media_content p{clear:both;min-height:1em}td p{margin:0;padding:0}table{margin-bottom:10px;border-collapse:collapse;display:table;width:100%!important}@media screen and (min-width:0�) and (min-resolution:72dpi){.rich_media_content table{table-layout:fixed!important}}
div.autoTypeSetting24psection > p,div.autoTypeSetting24psection > section{margin-bottom: 24px;}