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研究证实：我们迷失在没有方向的宇宙 | PRL 论文推荐
原创 2016-09-14
科研圈
科研圈
几位宇宙学家组成的研究团队通过分析最古老辐射——宇宙微波背景辐射（CMB，大爆炸留下的余晖）——告诉我们，宇宙是各向同性的。而宇宙演化的标准模型正是基于这样的一致性假设。
来源 Science
撰文 Adrian Cho 翻译 金庄维
根据一项最新研究，宇宙没有特殊的方向，无法定义普适的“上方”。
仰望星空时，你是否想过：宇宙在所有方向上是不是完全相同？它有没有可能像个大陀螺一样旋转？几位宇宙学家组成的研究团队通过分析最古老辐射——宇宙微波背景辐射（CMB，大爆炸留下的余晖）——告诉我们，宇宙是各向同性的。无论你从哪个方向看，宇宙都一样：没有旋转轴，也没有任何特殊方向。根据他们的估算，宇宙存在特殊方向的可能性只有1/121000，这是迄今为止宇宙各向同性的最佳证据。这一发现给宇宙学家带来安慰，因为宇宙演化的标准模型正是基于这样的一致性假设。
“这次的分析比以往全面得多，”剑桥大学宇宙学家 Anthony Challinor 说道，“宇宙在多大程度上具有各向同性？这个问题至关重要。”
哥白尼在1543 年发现地球围着太阳转，将地球和人类赶下宇宙中心的位置。这个发现进一步发展成哥白尼原理：广袤的宇宙没有中心，也没有任何特殊位置。在 20 世纪早期，随着爱因斯坦提出广义相对论、观测发现宇宙在各个方向膨胀，哥白尼原理又“进化”为宇宙学原理，它假设宇宙在任何位置、各个方向上完全相同。用更高大上的说法，宇宙是均匀并且各向同性的。
宇宙学原理也有其局限性。恒星与星系的存在显示宇宙中物质的分布并非处处相同。科学家猜想大爆炸中诞生的宇宙原本是一锅“汤”，里面均匀分布着各种亚原子粒子。在接下来的所谓“暴胀 ”阶段，宇宙剧烈膨胀（尺度随时间呈指数增长），“汤”里微小的量子涨落扩展到宇宙的尺度，由此产生的密度差异成为星系的“种子”。宇宙学的标准模型基于的假设是在大尺度情况下，这些差异微乎其微，空间仍然保持均匀并且各向同性。
事实未必非得如此。理论上讲，空间可能处处相同但仍然存在特殊的方向，就像钻石，虽然密度均匀，但碳原子沿着特殊方向排列。21 世纪初甚至出现了这种“各向异性”的征兆：NASA 的 WMAP（威尔金森微波各向异性探测器）测量结果暗示斑斑点点的 CMB 图中似乎存在某种微妙的波动，指向所谓的“邪恶轴心”（axis of evil）——大多数研究人员仅把它视为统计误差。
现在，伦敦大学学院的宇宙学家 Daniela Saadeh 、Andrew Pontzen 和同事们用迄今为止最严格的测试排除了特殊方向。他们采用欧洲航天局的Planck 卫星对 CMB 的测量结果。（Planck 从 2009 年到 2013 年采集数据，得到的 CMB 图比 WMAP 精确得多。）研究团队没有寻找 CMB 图中古怪的不对称，而是反过来进行系统的研究：他们考虑了能在宇宙中产生特殊方向的所有方式，以及这些方式如何在 CMB 上留下痕迹，接着他们在数据中寻找这些特殊信号。
各向异性的宇宙会在 CMB 上留下图样（底部）。但实际的 CMB （顶部）只有随机噪声，没有这些图样的迹象。Credits: (Top to bottom) ESA and the Planck Collaboration;D. Saadeh et. al., zenodo
例如，宇宙可能在不同方向上以不同速度膨胀。不均匀的膨胀会导致来自某些方向的辐射波长被拉伸得格外长，最终在CMB 上留下一个大“靶心”。宇宙也可能绕着某个特殊轴旋转，在CMB 上产生螺旋图样。另外，初生的宇宙还可能被引力波扭曲：引力波会在一个方向上拉伸宇宙，而在垂直方向上压缩，这类运动将在 CMB 上留下更加复杂的螺旋。研究人员试图在 CMB 上识别五种显示宇宙存在特殊方向的可能图样或“模式”。
Saadeh、Pontzen 和同事们用一台超级计算机，在 CMB 温度的随机波动中寻找潜藏的图样证据——就像静电干扰非常严重的情况下，试图在老式电视机的“雪花屏”中找出微弱的图像。他们在研究中也寻找了 CMB 偏振产生的图样（Planck 测过CMB 偏振）。五种图样中的三种“被偏振数据排除，”Saadeh 说道。
其他人也对宇宙旋转的信号做过类似的测试，但 Saadeh 他们对信号的限制较以往提高了一个数量级。在在线发表在 PRL 上的论文中，他们也对其他各种各向异性给出限制。“我们首次真正排除各向异性，”Saadeh 说道，“从前只是没有探测到各向异性。”
但是这一进展究竟有多大意义？这很难说，Challinor 说道，因为除了宇宙学标准模型，没有令人信服的替代模型精确预测各向异性的宇宙应该什么样。“问题在于，你把它和谁做比较？”他问道。但他仍表示“这个猜想（各向同性）是宇宙学的根基”，因此“检验它非常重要”。
原文链接： http://www.sciencemag.org/news/2016/09/it-s-official-you-re-lost-directionless-universe
论文基本信息
题目 How isotropic is the Universe? 作者 Seungkyu Lee,Eugene A. Kapustin, Omar M. Yaghi 期刊 PHYSICAL REVIEW LETTERS 日期 29 August 2016 摘要 A fundamental assumption in the standard model of cosmology is that the Universe is isotropic on large scales. Breaking this assumption leads to a set of solutions ot Einstein's field equations, known as Bianchi cosmologies, only a subset of which have ever been tested against data. For the first time, we consider all degrees of freedom in these solutions to conduct a general test of isotropy using cosmic microwaves background temperature and polarization data from Planck. For the vector mode (assoctiated with vorticity), we obtain a limit on the anisotropic expansion of (sigma mu/H)0 < 4.7x10-11 (95% CI), which is order of magnitude tighter than previous Planck results that used CMB temperature only. We also place upper liits on other modes of anisotropic expamnsion, with the weaker limit arising from the regular tensor mod (sigma T, reg/H)0 < 1.0x10-6(95% CI). Including all degrees of freedom simultameously for the first time, anisotropic expansion of the Univers is strongly disfavoured, with odds of 121,000:1 against.
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It’s official: You’re lost in a directionless universe
Ever peer into the night sky and wonder whether space is really the same in all directions or whether the cosmos might be whirling about like a vast top? Now, one team of cosmologists has used the oldest radiation there is, the afterglow of the big bang, or the cosmic microwave background (CMB), to show that the universe is “isotropic,” or the same no matter which way you look: There is no spin axis or any other special direction in space. In fact, they estimate that there is only a one-in-121,000 chance of a preferred direction—the best evidence yet for an isotropic universe. That finding should provide some comfort for cosmologists, whose standard model of the evolution of the universe rests on an assumption of such uniformity.
"It's a much more comprehensive analysis than in previous cases," says Anthony Challinor, a cosmologist at the University of Cambridge in the United Kingdom who was not involved in the work. "The question of how isotropic is the universe is of fundamental importance." In 1543, Nicolaus Copernicus knocked Earth and humanity from the supposed center of the universe by noting that Earth goes around the sun, not the other way around. That observation gave birth to the Copernican principle, which holds that we have no special place in the infinite, centerless universe. In the early 20th century, with the advent of Albert Einstein's general theory of relativity and the observation that the universe is expanding in all directions, that idea evolved into the cosmological principle, which assumes that the universe is the same everywhere and in every direction. In fancier terms, the universe is both homogeneous and isotropic.
The principle has its limitations. As the existence of stars and galaxies shows, matter is not distributed exactly the same way everywhere. This, they assume, arises because the universe was born as a homogeneous soup of subatomic particles in the big bang. As the universe underwent an exponential growth spurt called inflation, tiny quantum fluctuations in that soup expanded to gargantuan sizes, providing density variations that would seed the galaxies. Yet, the standard model of cosmology rests on the assumption that, on the largest scales, these variations are insignificant, and space is homogeneous and isotropic.
But it doesn't necessarily have to be that way. Theoretically, it's possible that space could be the same from point to point, but still have special directions—much as a diamond crystal has uniform density, but specific directions in which its atoms line up in rows. There were even some hints of such "anisotropy" in the early 2000s, when measurements from NASA's Wilkinson Microwave Anisotropy Probe (WMAP) spacecraft suggested that some subtle undulations in the motley CMB appeared to line up along a so-called "axis of evil"—which most researchers discount as a statistical fluke.
Now, Daniela Saadeh and Andrew Pontzen, cosmologists at University College London, and colleagues have ruled out special directions with the most stringent test yet. They also use measurements of the CMB, this time taken with the European Space Agency's Planck spacecraft, which collected data from 2009 to 2013 and provided far more precise CMB maps than WMAP. Instead of looking for curious imbalances in the CMB, they systematically worked the other way around. They considered all the ways that space could have a preferred direction and how such scenarios might imprint themselves on the CMB. Then they searched for those specific signs in the data.
An anisotropic universe would leave telltale patterns in the cosmic microwave background (bottom). But the actual CMB (top) shows only random noise and no signs of such patterns. [size=0.625]Credits: (Top to bottom) ESA and the Planck Collaboration; D. Saadeh et. al., zenodo
For example, space could be expanding at different speeds along different axes. Such differential expansion would cause the radiation from some directions to stretch to longer wavelengths than in others, and the upshot would be a big bull's-eye pattern in the CMB. Or, space could be rotating about a particular axis, which would create a spiral pattern in the CMB. Finally, the newborn universe could have been agitated by distortions in space itself known as gravitational waves, which would stretch the whole cosmos in one direction and compress it in a perpendicular direction. That sort of motion would leave more complex spirals in the CMB. In all, the researchers identify five potential patterns or "modes" in the CMB that would signal some sort of special direction in space. Using a supercomputer, Saadeh, Pontzen, and colleagues look for evidence of any such patterns lurking faintly behind random variations in the CMB's temperature—a process not unlike trying to pick out a weak picture through extreme static on an old-fashioned TV screen. To give their study even more bite, they also look for accompanying patterns in the polarization of the CMB's microwaves, which Planck also mapped. For three of the five patterns, "polarization data is the killer thing," Saadeh says.
Others had performed similar tests for signs that the universe is spinning, but Saadeh, Pontzen, and colleagues improve the limit on such a signal by an order of magnitude. They also put limits on all other kinds of anisotropy, as they report in a paper in press at Physical Review Letters. "For the first time, we really exclude anisotropy," Saadeh says. "Before, it was only that it hadn't been probed." But just how significant is that advance? That's hard to judge, Challinor says, because there aren't compelling alternatives to the standard model of cosmology that predict exactly how an anisotropic universe should be. "The problem is, what do you compare it to?" he asks. Still, he notes, "this assumption is fundamental cosmology" so "it's very important to check."
Posted in: PhysicsDOI: 10.1126/science.aah7276
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发表于 2016-9-17 13:41
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