Cosmic Voids May Contain the Universe’s Best Secrets

宇宙空洞可能隐藏着宇宙最深奥的秘密

Nature abhors a vacuum, so the saying goes, but nobody told the universe. Space is filled with cosmic voids—vast regions mostly free of matter that have opened between dense threads of material that make up a cosmic web. 俗话说“自然界厌恶真空”，但宇宙似乎并不理会这一点。太空充满了宇宙空洞——这些广袤的区域几乎没有物质，它们存在于构成宇宙网的致密物质丝状结构之间。

Far from being vacant backwaters with little to study, these voids may hold solutions to some of the most persistent cosmic mysteries, such as the behavior of gravity, the nature of dark energy, and the so-called Hubble tension, an observational mismatch in the expansion rate of the universe that has caused astronomers’ headaches for years. 这些空洞绝非毫无研究价值的荒芜之地，它们可能掌握着解决一些长期困扰宇宙学难题的钥匙，例如引力的行为、暗能量的本质，以及所谓的“哈勃张力”——即宇宙膨胀率观测值之间的不匹配，这一问题多年来一直让天文学家头疼不已。

“With voids, we have the power to tackle most of the interesting cosmological riddles,” says Alice Pisani, a research professor in cosmology working at the Centre for Particle Physics in Marseille (CPPM) of the French National Centre for Scientific Research. She adds that because there’s less interference from matter, there’s a “high signal-to-noise” ratio in terms of what researchers can observe. “通过研究空洞，我们有能力破解大多数有趣的宇宙学谜题，”法国国家科学研究中心马赛粒子物理中心（CPPM）的宇宙学研究教授爱丽丝·皮萨尼（Alice Pisani）表示。她补充说，由于物质干扰较少，研究人员在观测时能获得更高的“信噪比”。

The advent of new telescopes and advanced simulations has supercharged this field, inspiring a growing community of scientists worldwide to specialize in voids as unique cosmological laboratories. Some experts argue we may even live inside a colossal void, a position that may alter our view of the universe in consequential ways. 新型望远镜和先进模拟技术的出现极大地推动了这一领域的发展，激励了全球越来越多的科学家将空洞视为独特的宇宙学实验室进行专门研究。一些专家甚至认为，我们可能就生活在一个巨大的空洞之中，这一观点可能会以深远的方式改变我们对宇宙的看法。

For places defined by sparseness, voids are becoming cosmological heavyweights, where the laws of physics can be observed with unusual clarity. “From a cosmology perspective, it is a very exciting time,” Pisani says. 对于这些以稀疏为特征的区域而言，空洞正逐渐成为宇宙学中的“重量级选手”，在这里，物理定律可以被异常清晰地观测到。“从宇宙学的角度来看，这是一个非常令人兴奋的时代，”皮萨尼说。

What Are Cosmic Voids?

什么是宇宙空洞？

Following the Big Bang, the universe was a uniform soup of subatomic particles. But over millions of years, as matter cooled and stabilized into atoms, the faint outlines of the cosmic web began to emerge. 大爆炸之后，宇宙是一锅均匀的亚原子粒子汤。但经过数百万年的演化，随着物质冷却并稳定为原子，宇宙网的模糊轮廓开始显现。

Over billions of years, the web gravitationally pulled gas clouds, galaxy clusters, and other cosmic objects toward its scaffolding. As more matter is drawn into the web, gaps have widened between its filaments, forming voids. 在数十亿年的时间里，宇宙网通过引力将气体云、星系团和其他宇宙天体拉向其骨架。随着越来越多的物质被吸入网中，丝状结构之间的空隙不断扩大，从而形成了空洞。

Small “subvoids” can open between galaxy clusters, where they might be only 10 or 20 million light years across. But voids can get bigger. Much bigger. The Boötes Void, also known as the “Great Nothing,” stretches across more than 300 million light years. 星系团之间可能会形成小的“子空洞”，其直径可能仅为1000万到2000万光年。但空洞可以变得更大，大得多。牧夫座空洞（Boötes Void），也被称为“大虚无”，横跨超过3亿光年的范围。

Calling them cosmic voids can be “misleading,” Pisani says, “because we end up thinking that a void means empty. But as a matter of fact, the voids that we look at are never empty. There are very tiny low-mass galaxies inside those under-dense regions.” The Boötes Void, for example, contains a few dozen galaxies—though that’s still far less than the thousands that would be expected in a similarly sized area. 皮萨尼说，称它们为“宇宙空洞”可能会产生“误导”，“因为我们最终会认为空洞意味着空无一物。但事实上，我们所观测的空洞从来都不是空的。在这些低密度区域内，存在着非常微小的低质量星系。”例如，牧夫座空洞中就包含了几十个星系——尽管这与同样大小区域内预期的数千个星系相比，依然少得多。

Because they are comparatively bereft of material, cosmic voids remained out of observational view until the late 1970s. Until that point, the positions of galaxies had been mapped as 2D points on the sky, but the development of 3D maps of galaxy distribution revealed the contours of the cosmic web for the first time, exposing the presence of voids. 由于它们相对缺乏物质，宇宙空洞直到20世纪70年代末才进入观测视野。在此之前，星系的位置仅被绘制为天空中的二维点，但随着星系分布三维地图的发展，宇宙网的轮廓首次被揭示出来，从而暴露了空洞的存在。

In recent years, a host of new telescope surveys have kicked off an explosion of new void discoveries, such as the Dark Energy Survey Instrument (DESI) in Arizona, and the European Euclid space telescope. These instruments are expected to map more than 100,000 voids in space, offering an unprecedented glimpse of these structures. Yet these surveys will still only capture a fraction of the many millions of voids that are estimated to exist in the observable universe. 近年来，一系列新的望远镜巡天项目引发了空洞发现的爆发，例如亚利桑那州的暗能量光谱仪（DESI）和欧洲的欧几里得（Euclid）太空望远镜。这些仪器预计将绘制出太空中超过10万个空洞的地图，为我们提供前所未有的观测视角。然而，这些巡天项目所捕捉到的，仅仅是估计存在于可观测宇宙中数百万个空洞中的一小部分。

“Just in the last 10 years, the field really evolved significantly with new technologies,” says Nico Schuster, a cosmologist and cosmic void expert at CPPM. “All of that really enables us to observe plenty more galaxies than we could before, and that really allows us to probe, finally, the cosmic web at a much deeper depth, and find more voids and resolve them better.” “就在过去10年里，随着新技术的应用，该领域确实取得了显著进展，”CPPM的宇宙学家兼宇宙空洞专家尼科·舒斯特（Nico Schuster）说。“所有这些使我们能够观测到比以前多得多的星系，这最终让我们能够以更深的深度探测宇宙网，发现更多的空洞并更好地解析它们。”

At the same time, better computational simulations of the cosmic web have filled in the gaps in knowledge about the evolution of voids over time, allowing scientists to model hundreds of thousands of voids, which is an order of magnitude more than simulations could compute just a few years ago, Schuster says. 舒斯特表示，与此同时，对宇宙网更完善的计算模拟填补了关于空洞随时间演化的知识空白，使科学家能够模拟数十万个空洞，这比几年前模拟所能计算的数量高出一个数量级。

This revolution has distinguished voids as “powerful cosmological laboratories,” according to a comprehensive overview of void science published in April in The Astronomy & Astrophysics Review, which was led by Pisani. 根据皮萨尼领导并于4月发表在《天文学与天体物理学评论》（The Astronomy & Astrophysics Review）上的一篇关于空洞科学的综合综述，这场革命使空洞被公认为“强大的宇宙学实验室”。

What Can We Learn From Voids?

我们能从空洞中学到什么？

Because of their relative sparseness, voids offer a rare glimpse of the “simpler” effects of gravity without all the complications inherent to chaotic massive objects, like galaxy clusters. For this reason, cosmologists look to voids to test modified theories of gravity and the limits of general relativity. In practice, researchers probe these constraints by mapping how “tracers,” such as galaxies, dark matter halos, and other objects, move through voids and comparing those observations to cosmological models’ predictions. 由于其相对稀疏的特性，空洞提供了一个难得的机会，让我们得以一窥引力的“简单”效应，而无需面对像星系团那样混乱的大质量天体所固有的复杂性。因此，宇宙学家寄希望于通过空洞来检验修正引力理论和广义相对论的极限。在实践中，研究人员通过绘制“示踪物”（如星系、暗物质晕和其他天体）在空洞中的运动轨迹，并将这些观测结果与宇宙学模型的预测进行比较，来探测这些限制。

For example, Schuster has published studies that explore pristine and simple motions of objects in voids, and their implications for studying neutrinos, which are among the lightest particles in the universe. 例如，舒斯特发表的研究探讨了空洞中天体原始而简单的运动，以及这些运动对于研究中微子（宇宙中最轻的粒子之一）的意义。

Though neutrinos are incredibly abundant—100 trillion of them pass through your body every second—they barely interact with matter. This spectral quality is enhanced further in voids, where there is almost no matter to interact with in the first place, revealing new insights into neutrino physics. 尽管中微子数量极其庞大——每秒钟有100万亿个穿过你的身体——但它们几乎不与物质发生相互作用。这种特性在空洞中得到了进一步放大，因为那里几乎没有物质可供相互作用，从而为中微子物理学揭示了新的见解。

Voids are also emerging as unique probes into dark matter and dark energy, which are two big question marks in the so-called “standard model” of cosmology, a well-corroborated framework of fundamental forces and phenomena in the universe. 空洞也正成为探测暗物质和暗能量的独特工具，这两个问题是所谓宇宙学“标准模型”中的两大问号，而该模型是目前对宇宙基本力和现象解释得最完善的框架。

They’re also the perfect spaces to explore the nature and implications of dark energy, the vague term for whatever is causing the rate the universe is expanding to speed up. Because voids don’t contain much matter, the properties of dark energy can be clocked more clearly. 它们也是探索暗能量本质及其影响的完美空间。暗能量是一个模糊的术语，指代任何导致宇宙膨胀速度加快的因素。由于空洞中不含太多物质，暗能量的特性可以被更清晰地测量出来。