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在大约138亿年前,我们的宇宙从一个量子点,在不到十亿分之一的一兆分之一的一兆分之一秒内膨胀到其初始体积的一百亿亿倍(据估计),然后根据已知的物理规律它以稳定的速度继续扩大。

这就是宇宙膨胀的故事, 宇宙大爆炸理论的现代版本。这个短时间内的、令人吃惊的爆发式增长符合所有现有的宇宙学数据,而且对宇宙的广大、平坦和缺乏优先方向做出了解释。但是作为宇宙为何和如何诞生的解释,这种膨胀显得时间太短。它提出的问题——宇宙为什么会急剧增长,它是如何开始的,(如果有的话)在它之前发生了什么——在上世纪80年代这种理论出现后就一直让宇宙学家困惑。“对于这一时期的宇宙膨胀,我们有非常有力的证据。” 纽约大学宇宙学家Matthew Kleban表示,“但我们并不知道,或者说我们从根本上对于宇宙膨胀并没有太多的想法。”

第 1 段(可获 2 积分)

要了解宇宙的起源,今天的宇宙学家都在极力求证宇宙膨胀的动力,被称为 “inflaton”,它被设想为作为一个能量场渗透空间,并驱使它分离,专家表示宇宙膨胀的运行就像一个时钟,每一刻宇宙都在膨胀,并且直到它停止前时间是几乎完美的稳定。理论家如Kleban,就像钟表匠一样,设计了数百种不同的模式,从而复制出大爆炸的发条。

像许多宇宙的钟表匠,Kleban是弦理论的专家,他是“万物理论”的主导者,试图描述大自然中所有的距离、时间和能量。已知的物理方程在应用于微小的、转瞬即逝的、激烈的宇宙大爆炸时会被动摇,在这一过程中,他们努力地把大量的能量投入到无限的空间和时间。但弦理论却在其中蓬勃发展,它假定出额外的空间维度使得能量扩散。熟悉的点粒子在最高能量和缩放级别成为了一维的“弦”和高维的“膜”,所有这些都是穿越十维的景观,这些振动、起伏的齿轮可能就是为大爆炸这个时钟的运行提供动力。

第 2 段(可获 2 积分)

最近的一个下去,在他的办公室里,Kleban在黑板上勾勒出了最新的宇宙膨胀的设计,它的长度代表了宏观现实的三个空间维度,而它的周长标志着其他弦理论认为存在的六维空间,但它太小了。在圆柱体的一边,他画了一个圆,这是Kleban的计时器:一个自然形成和膨胀的膜。因为在它的内部形成了一个新的宇宙,它的能量逐步在每次膨胀过程中都有规律地绕着圆柱体旋转,并与之重叠。当“膜”的能量慢慢减退,就像时钟停止,膨胀就结束了。这是一些坚持弦理论的宇宙学家所赞誉的计划。“我认为这一切的发生是非常合理的。” 他说。

第 3 段(可获 2 积分)

A sketch by the string theorist and cosmologist Matthew Kleban of his Big Bang model known as unwinding inflation.

构建弦理论的物理学家和宇宙学家Matthew Kleban的大爆炸模型被作为解除膨胀的模型。Olena Shmahalo/Quanta 杂志

虽然Kleban承认现在就对他或其他人下定论还为时过早,因为计划还在进行当中。

膨胀的高速运转的记录可以通过星系的分布、跨越宇宙的星系团和超星系团中看出来。这些结构(和其中所有的一切,包括你)都是“时钟的错误”,就像新泽西州普林斯顿高等研究院的宇宙学家Matias Zaldarriaga所说。也就是说,时间本质上是不确定的,所以宇宙膨胀的速度在不同的地方和时刻稍有不同,产生密度变化。指针的跳动也可以被看作是能量的迸发,双粒子自发地出现在“膨胀区域”然后伸展开来,就像是一个正在膨胀的气球的两点。这些粒子在亿万年的过程中重力成长为银河结构,今天,跨越天空中最大距离的结构来自于最早的宇宙膨胀使其的量子涨落,而距离最近的结构则是后来产生的,这个嵌套分布在所有宇宙距离尺度 “清楚地告诉你时间在一分一秒流逝,” IAS理论物理学家Nima Arkani-Hamed表示,“但它不会告诉你它是由什么构成的。”

第 4 段(可获 2 积分)

为了发条的逆向工程,宇宙学家正在寻找一组新的数据,他们的计算表明,星系和其他结构不仅是随机分散于天空中,相反,它们有轻微的倾向是被安排在更复杂的布局中:三角形、矩形、五边形和其他各种各样的形状,这不仅可以追溯到大爆炸的时钟中的量子抖动,更重要的是齿轮的转动。

梳理宇宙三角形状态和其他形状——它已被命名为 “非Gaussianities” ,与随机分布的Gaussian钟曲线进行对比——将需要更精确的宇宙观测。因此,日益敏感的试验需要制定时间表。“我们将会比现在获得更多的信息,而且我们将会比现在拥有更高的灵敏度去探索微妙的影响。” 约翰霍普金斯大学的宇宙学家Marc Kamionkowski表示。与此同时,理论家们在确定寻找什么形状以及如何寻找它们这些方面取得了重大进展,“这是一个伟大的对复兴的理解。” Eva Silverstein表示,他是斯坦福大学的宇宙学家,设计了Kleban使用的三维绕线机制以及多种时钟模式。

第 5 段(可获 2 积分)

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对于非Gaussianities严谨的研究开始于2002年,当时令人尊敬的IAS理论家Juan Maldacena计算出了所谓的“引力场”:由于宇宙膨胀中重力不可避免的影响,在天空中存在的三角形和其他形状的最小数目。宇宙学家们一直在努力计算引力场十多年了,因为它将为实验提供一个具体的目标,如果到达了引力场,仍然没有检测到三角形,Maldacena解释说,“那么宇宙膨胀理论就是错误的。”

第 6 段(可获 2 积分)
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Eva Silverstein, a professor of physics at Stanford University, has developed many string inflationary models, including some that are currently being tested.

斯坦福大学物理学教授Eva Silverstein开发出许多弦暴涨模型,包括一些目前正在测试中的模型,这是SLAC国家加速器实验室档案和历史办公室的好意。

当Maldacena第一次计算引力场时,发现它实际上似乎是一个遥远的目标,当时,所有关于宇宙诞生的精确数据都来自对“宇宙微波背景”的观测,这是天空中最古老的光,它照亮了一个二维切面的婴儿宇宙,因为它出现在宇宙大爆炸380000年之后。基于有限数量的初生结构出现在这个二维空间,它们似乎不可能被安排在三角形和其他形状中,这在统计上是确定的。但Maldacena的工作提供了理论计算的工具,更明显的是指出由于影响比重力更大而可能存在的天空。这促使研究人员设计出更好的方式来寻找信号。

第 7 段(可获 2 积分)

Maldacena完成这个计算的一年后,Zaldarriaga和同事发现测量构成宇宙“大结构”的星系核星系团的分布将比观测宇宙微波背景产生更多的形状,“这是一次三维与二维的争论。” 美国宇航局喷气推进实验室的宇宙学家Olivier Doré表示,他提出了在大型结构中研究非Gaussianities的工作。“如果你开始计算就像是星系研究的三维空间的三角形,真的有很多可以计算的数据。”

第 8 段(可获 2 积分)

在天空中测量更多的形状概念将会揭示大爆炸的更多细节,这在量子物理学的核心原则中被称为“统一性”,统一性决定了宇宙中所有可能的量子态的概率要统一,从现在到永远,因此,存储在量子态中的信息,永远不会丢失,只会变得杂乱。这意味着所以关于宇宙诞生的信息都在其当下状态下被编码,在宇宙学家们更准确地了解前者后,他们更能了解后者。

第 9 段(可获 2 积分)

但是大爆炸的细节是如何形成三角形和其他形状从而进行编码的呢?根据Zaldarriaga所说,Maldacena的计算“解开了这一疑惑”。在量子力学支配的宇宙中,自然界所有的元素都是交叉连接的,以不同程度的概率变形并相互作用。这包括膨胀场、引力场以及其他存在于原始宇宙中的事物:在这些区域所产生的粒子会分散开,彼此形成三角形或其他几何形状,就像台球桌上被击散开的球。

第 10 段(可获 2 积分)

Triangles_In_The_Sky_Desktop.png

Lucy Reading-Ikkanda为Quanta 杂志提供

这些动态行为将会与量子抖动混合于在膨胀场出现的粒子对,在天空中产生所谓的“两点相关性”。例如,一对粒子可能已经出现在其他的一些原始区域,而这对粒子的其中一个可能会衰变为两个膨胀粒子,而另一个会衰变为单一的膨胀粒子,在天空中形成三点相关或三角形。或者,两个神秘粒子可能会相撞,从而分裂为四个膨胀粒子,形成四点相关性。罕见的还会产生五点、六点甚至更多的相关性,这和它们的数量、大小和内部角度编码的类型和产生的粒子有关。统一性原则认为通过更精确地计算形状,宇宙学家将能还原出越来越细致的原始宇宙模型,正如欧洲Large Hadron Collider的物理学家为了验证他们已知的粒子理论而寻求如何通过收集统计粒子形态和分散碰撞过程中的新的证据。

第 11 段(可获 2 积分)

根据Maldacena对引力场的计算,其他研究人员发现,很多简单的膨胀模型比最低限度产生更为明显的非Gaussianity。像Silverstein和Kleban一样的宇宙学家一直努力解决他们模型所产生的截然不同的三角形模式,他们预计在未来几年将变得越来越可预测。2014年开始进展迅速,当时在南极点的一个小实验的基础上作出了关于宇宙诞生的重大发现。宇宙学的三角形模式引起了广泛兴趣,即使它最终被证明了这是令人失望的。

第 12 段(可获 2 积分)

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随着2014年3月17日这一消息开始散布开来,宇宙膨胀的“确凿证据”被监测出来,斯坦福大学新闻办公室在YouTube上发布了一个庆祝视频。在视频中,宇宙学家Andrei Linde(膨胀宇宙学理论的先驱之一)和他的妻子,弦理论和超引力理论的宇宙学家Renata Kallosh,在家门口的台阶上见到了他们在斯坦福大学的同事Chao-Lin Kuo和随行的摄制组并接受了采访。

“这是在第二点的五西格玛。” Kuo在视频里表示。

“发现呢?” Kallosh问道,她拥抱了Kuo,而Linde惊呼道:“什么?”

第 13 段(可获 2 积分)

人们知道BICEP2是由Kuo协作主导的实验,它监测到宇宙微波背景中的漩涡图案,它将被印在称为“原始引力波”的时空涟漪中。而这些只能出现在宇宙膨胀时期,当螺旋状粒子出现在引力场然后经过拉伸,永久冻结成宇宙的形状。

在接下来的场景里,Linde和他的妻子以及来访的客人们一起喝着香槟。在上世纪80年代初,Linde、Alexei StarobinskyAlan Guth和其他年轻的宇宙学家提出了宇宙膨胀理论,作为被动摇的19世纪30年代出现的大爆炸理论的修补,大爆炸理论认为宇宙从“奇点”(一个密度无穷大的点)向外扩展,但这并不能解释为什么宇宙在成长过程中没有发生扭曲,而宇宙膨胀理论为这些问题作出了巧妙的解释,BICEP2的发现也使得该理论被证实。“如果它是正确的,” Linde对着镜头说,“这将会是颠覆我们对自然认识的时刻,让我们拭目以待吧,我们只是希望这不会是一场玩笑。”

第 14 段(可获 2 积分)

对很多研究人员而言,最令人兴奋的发现是漩涡信号的强度,它可以被衡量为 r = 0.2。研究表明,宇宙膨胀发生在一个能源规模极高和是这一现象出现的最早的时刻,在能源域的重力、弦的影响、膜或其他新事物,都会变得强大。宇宙膨胀的能源规模越高,膨胀和其它原始成分间的交错会更多,结果将会是在天空中形成明显的三角形和其他 非Gaussianities形态。

第 15 段(可获 2 积分)

“在进行BICEP之后,我们停止了手头上的工作开始思考宇宙膨胀的问题,” Arkani-Hamed表示,“宇宙膨胀就像是拥有一个巨大的粒子加速器,它比你在地球上的能量尺度要高得多。” 他说问题在于这种加速器是如何运作的,“如果真的有某些外来的东西 [在膨胀区域附近],我们应该如何去寻找它。”

随着这些研究的进行,更多关于BICEP2的研究细节逐渐显现,很明显,这一发现确实是大自然的玩笑:在南极的团队通过望远镜发现的是星系的尘埃而不是原始引力波的影响,一时间痛苦和愤怒交织。两年过去了,原始引力波仍然没有被发现。一月,BICEP2的继任者BICEP/Keck Array发现 r 值不可能超过0.07,它降低了宇宙膨胀的能量规模上限,使其进一步低于弦或其他外来的物理现象的规模。

第 16 段(可获 2 积分)

Juan Maldacena, a professor of physics at the Institute for Advanced Study, pioneered the study of cosmological non-Gaussianities.

高等研究院的物理学教授Juan Maldacena开创了宇宙学研究中的非Gaussianities。Andrea Kane表示。

尽管如此,许多研究人员已经意识到信息中潜在的三角形和其他非Gaussianities形态,很明显,这些来自宇宙膨胀时期的“化石”值得去挖掘,即使它们比BICEP2预想的要埋藏得更深。“是的,r值降低了一点。” Maldacena表示。但在他看来这并不算糟糕:相对较高的能量规模仍然是可能的。

第 17 段(可获 2 积分)

In a paper last spring that drew on previous work by other researchers, Maldacena and Arkani-Hamed used symmetry arguments to show that a key feature of string theory could manifest itself in triangles. String theory predicts an infinite tower of “higher-spin states”—essentially, strings vibrating at an infinitely rising sequence of pitches. So far, no fundamental particles with a “spin” value greater than two have been discovered. Maldacena and Arkani-Hamed showed that the existence of such a higher-spin state would result in alternating peaks and troughs in the strength of the signal produced by triangles in the sky as they grow more elongated. For string theorists, this is exciting. “You can’t build a consistent interacting theory of such a particle except if you have an infinite tower of them” like the tower in string theory, explained Daniel Baumann, a theoretical cosmologist at the University of Amsterdam. Finding the oscillatory pattern in the triangles in the sky would confirm that this tower exists. “Just seeing one particle of spin greater than two would be indicative of string theory being present.”

第 18 段(可获 2 积分)

Other researchers are pursuing similarly general predictions. In February, Kamionkowski and collaborators reported detailed information about primordial particles that is encoded in the geometry of four-point correlations, which “get interesting,” he said, because four points can lie flat or sweep into the third dimension. Observing the signals predicted by Arkani-Hamed, Maldacena and Kamionkowski would be like striking gold, but the gold is buried deep: Their strength is probably near the gravitational floor and will require at least 1,000 times the sensitivity of current equipment to detect. Other researchers prefer to tinker with bespoke string models that predict more pronounced triangles and other shapes. “So far we’ve explored only, I think, a very small fraction of the possibilities for non-Gaussianity,” Kamionkowski said.

第 19 段(可获 2 积分)

Meanwhile, Linde and Kallosh are pushing in a totally different direction. Over the past three years, they’ve become enamored with a class of models called “cosmological alpha-attractors” that do not predict any non-Gaussianities above the gravitational floor at all. According to these models, cosmic inflation was completely pure, driven by a solitary inflaton field. The field is described by a Kähler manifold, which maps onto the geometric disk seen in Escher’s drawing of angels and devils. The Escherian geometry provides a continuum of possible values for the energy scale of inflation, including values so low that the inflaton’s cross-wiring to the gravitational field and other primordial fields would be extremely weak. If such a model does describe the universe, then swirls, triangles and other shapes might never be detected.

第 20 段(可获 2 积分)

Linde isn’t bothered by this. In supporting the alpha-attractor models, he and Kallosh are staking a position in favor of simplicity and theoretical beauty, at the expense of ever knowing for sure whether their cosmological origin story is correct. An alpha-attractor universe, Linde said, is like one of the happy families in the famous opening line of Anna Karenina. As he paraphrased Tolstoy: “Any happy family, well, they look in a sense alike. But all unhappy families—they’re unhappy for different reasons.”

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Will our universe turn out to be “happy” and completely free of distinguishing features? Baumann, who co-authored a book last year on string cosmology, argues that models like Linde’s and Kallosh’s are too simple to be plausible. “They are building these models from the bottom up,” he said. “Introducing a single field, trying to be very minimal—it would have been a beautiful model of the world.” But, he said, when you try to embed inflation into a fundamental theory of nature, it’s very hard to engineer a single field acting by itself, immune to the effects of everything else. “String theory has many of these effects; you can’t ignore them.”

第 21 段(可获 2 积分)

And so the search for triangles and other non-Gaussianities is under way. Between 2009 and 2013, the Planck space telescope mapped the cosmic microwave background at the highest resolution yet, and scientists have since been scouring the map for statistical excesses of triangles and other shapes. As of their most recent analysis, they haven’t found any; given the sensitivity of their instruments and their 2-D searching ground, they only ever had an outside chance of doing so. But the scientists are continuing to parse the data in new ways, with another non-Gaussianity analysis expected this year.

第 22 段(可获 2 积分)

Hiranya Peiris, an astrophysicist at University College London who searches for non-Gaussianities in the Planck data, said that she and her collaborators are taking cues from string cosmologists in determining which signals to look for. Peiris is keen to test a string-inflationary mechanism called axion monodromy, including variants recently developed by Silverstein and collaborators Raphael Flauger, Mehrdad Mirbabayi, and Leonardo Senatore that generate an oscillatory pattern in triangles as a function of their size that can be much more pronounced than the pattern studied by Arkani-Hamed and Maldacena. To find such a signal, Peiris and her team must construct templates of the pattern and match them with the data “in a very numerically intensive and demanding analysis,” she said. “Then we have to do careful statistical tests to make sure we are not being fooled by random fluctuations in the data.”

第 23 段(可获 2 积分)

Some string models have already been ruled out by this data analysis. Regarding the public debate about whether string theory is too divorced from empirical testing to count as science, Silverstein said, “I find it surreal, because we are currently doing some traditional science with string theory.”

Now under construction, the Large Synoptic Survey Telescope in Chile will be used to map 20 billion cosmological objects starting in 2023.

Now under construction, the Large Synoptic Survey Telescope in Chile will be used to map 20 billion cosmological objects starting in 2023. LSST Project/NSF/AURA

Moving forward, cosmologists plan to scour ever larger volumes of the universe’s large-scale structure. Starting in 2020, the proposed SPHEREx mission could measure non-Gaussianity sensitively enough in the distribution of 300 million galaxies to determine whether inflation was driven by one clock or two cross-wired clocks (according to models of the theory known as single- and multi-field inflation, respectively). “Just to reach this level would dramatically reduce the number of possible inflation theories,” said Doré, who is working on the SPHEREx project. A few years further out, the Large Synoptic Survey Telescope will map 20 billion cosmological structures. If the statistical presence of triangles is not detected in the universe’s large-scale structure, there is yet another, perhaps final, approach. By mapping an ultra-faint radio signal called the 21-centimeter line, which is emitted by hydrogen atoms and traces back to the creation of the first stars, cosmologists would be able to measure even more “modes,” or arrangements of structures. “It’s going to have information about the whole volume of the universe,” Maldacena said.

第 24 段(可获 2 积分)

If or when triangles show up, they will, one by one, reveal the nature of the inflaton clock and why it ticked. But will enough clues be gathered before we run out of sky in which to gather them?

The promise of unitarity—that information can be scrambled but never lost—comes with a caveat.

“If we assume we can make perfect measurements and we have an infinite sky and so on,” Maldacena said, “then in principle all the interactions and information about particles during inflation is contained in these correlators”—that is, three-point correlations, four-point correlations and so on. But perfect measurements are impossible. And worse, the sky is finite. There is a cosmic horizon: the farthest distance from which light has had time to reach us, and thus, beyond which we cannot see. During inflation, and over the entire history of the accelerating expansion of the universe since then, swirls, triangles, quadrilaterals and other shapes have been flying past this horizon and out of sight. And with them, the subtlest of signals, associated with the rarest, highest-energy processes during inflation, are lost: Cosmologists will never be able to gather enough statistics in our finite patch of sky to tease them out, precluding a complete accounting of nature’s fundamental constituents.

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In his paper with Maldacena, Arkani-Hamed initially included a discussion of this issue, but he removed most of it. He finds the possibility of a limit to knowledge “tremendously disturbing” and sees it as evidence that quantum mechanics must be extended. One possible way to do this is suggested by his work on the amplituhedron, which casts quantum mechanical probabilities (and with them, unitarity) as emergent consequences of an underlying geometry. He plans to discuss this possibility in a forthcoming paper that will relate an analogue of the amplituhedron to non-Gaussianities in the sky.

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People vary in the extent to which they are bothered by a limit to knowledge. “I’m more practical,” Zaldarriaga said. “There are, like, tens or many tens or orders of magnitude more modes that in principle we could see, that we have not been able to measure just because of technological or theoretical inability. So, these ‘in principle’ questions are interesting, but we are way before this point.”

Kleban also feels hopeful. “Yeah, it’s a finite amount of information,” he said. “But you could say the same thing about evolution, right? There’s a limited number of fossils, and yet we have a pretty good idea of what happened, and it’s getting better and better.”

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If all goes well, enough fossils will turn up in the sky to tell a more complete story. A vast searching ground awaits.

Original story reprinted with permission from Quanta Magazine, an editorially independent publication of the Simons Foundation whose mission is to enhance public understanding of science by covering research developments and trends in mathematics and the physical and life sciences.

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