We still can't see dark matter. But what if we can hear it?

我们依旧看不到暗物质。但假如能听到呢？

The most mysterious and yet ubiquitous stuff in the cosmos, dark matter is effectively invisible. This is simply because it doesn't interact with light. But what if instead of trying to see dark matter, scientists attempted to hear it instead?

暗物质神秘莫测又无处不在，人类无法观测到它。原因很简单，它不与光发生反应。但是，如果我们不看它，而是听它呢？

New research suggests dark matter could leave a tiny but discernible imprint in the cacophony of ripples in spacetime called "gravitational waves" that ring through the cosmos when two black holes slam together and merge. However, this is only if spinning black holes can "churn" dark matter like cosmic butter. (We'll get to that shortly.)

新的研究表明，当两个黑洞猛烈相撞并融合时，会在宇宙中激起名为“引力波”的时空涟漪；而在这些引力波交织成的“宇宙喧嚣”中，暗物质或许会留下微小却可辨的印记。当然，这有个前提：自旋的黑洞必须能像搅动宇宙黄油一般，将暗物质翻搅起来。（我们稍后会详细解释。）

The team behind this new research suggests that if two black holes merge in a region of space populated by dense dark matter clouds, then the gravitational waves emerging from the event could carry the imprint of dark matter across the universe. And it's possible, they say, that our detectors could find that imprint. This would be akin to someone coughing at a Metallica concert, and that cough being only discernible over the fury of "Seek and Destroy" or "Master of Puppets" with the most sensitive instruments.

做出此项研究的团队认为，如果两个黑洞在一片遍布浓密暗物质云的区域发生并合，那么此次事件产生的引力波，便有可能携带着暗物质的印记传遍宇宙。他们认为，现有的探测器或许能捕捉到这一印记。这就好比在 Metallica（金属乐队）的演出现场，有人突然咳嗽了一声；即便周围正回荡着歌曲《寻找与毁灭》或《傀儡大师》的狂野嘶吼，借助最精密的仪器，依然能将那声咳嗽辨识出来。

Fortunately, when it comes to detecting gravitational waves from colliding black holes, humanity's instruments, such as LIGO (Laser Interferometer Gravitational-Wave Observatory), are getting more and more sensitive all the time. And in preparation for a time when such imprints could become even more easily logged in gravitational wave data, this team developed a method that predicts just what shape a gravitational wave should take when moving through dark matter, rather than empty space.

好在，在探测这类黑洞并合引力波方面，以 LIGO（激光干涉引力波天文台）为代表的人类仪器正变得愈发灵敏。为了迎接这些印记未来能更清晰地留存于引力波数据的那一天，研究团队开发了一套新方法，精准预言了引力波在穿过暗物质、而非真空时的“模样”。

"Using black holes to look for dark matter would be fantastic," team member Rodrigo Vicente, a researcher at GRAPPA (Gravitation Astroparticle Physics Amsterdam), said in a statement. "We would be able to probe dark matter at scales much smaller than ever before."

GRAPPA（阿姆斯特丹引力与天体粒子物理研究所）研究员、这项研究的团队成员之一罗德里戈·维森特在声明中表示， “借黑洞之力搜寻暗物质，堪称完美，我们现在可以在前所未有的微小尺度上探测暗物质。”

Dark matter represents such a puzzle because, despite being invisible to us, it still "outweighs" ordinary matter by a ratio of about five to one.

暗物质之所以如此令人费解，是因为尽管它对我们“隐身”，但其质量却足足是普通物质的五倍。

Its lack of interaction with light means it can't be composed of protons, neutrons and electrons — the particles that compose atoms. That's because atoms compose all the "ordinary matter" we see around us, from stars and planets to the device you're reading this article on and our own bodies. In other words, atoms do interact with light (more technically, electromagnetic radiation). In fact, the only way astronomers know dark matter exists is via its interaction with gravity and the way this interaction curves spacetime, indirectly influencing ordinary matter and light.

暗物质不与光相互作用，这就意味着它不可能由质子、中子和电子等构成原子的那些粒子组成。原因很简单：原子构成了我们周遭所有的“普通物质”，从恒星行星，到你正用来读这段话的设备，再到我们自己的身体。换句话说，原子确实会和光打交道（严格说是和电磁辐射相互作用）。事实上，天文学家之所以确信暗物质存在，唯一依据就是它与引力的相互作用——以及这种作用弯曲时空、进而间接牵动普通物质和光的方式。

With this knowledge, scientists have been hunting for particles outside the Standard Model of particle physics that could account for dark matter. These particles have a wide range of potential masses and properties, with one hypothetical particle being the "light scalar" proposed to have a mass much smaller than that of an electron. One characteristic of the light scalar would be the fact that dark matter composed of these particles would act like coordinated waves around black holes.

基于这一认知，科学家一直在搜寻粒子物理标准模型之外的候选粒子，以期破解暗物质的本质。这类粒子的质量和性质千差万别，其中一种假想粒子便是“轻标量粒子”。据推测，其质量远小于电子。它的一大特性在于：若暗物质由它们构成，那么在黑洞周围，这些暗物质会像一群同步起伏的波一样运动。

Around a spinning black hole, rotational energy would be transferred to light scalar dark matter, amplifying its density, almost like a paddle churning cream into butter. If this dark matter "butter" gets dense enough, it could affect gravitational waves from merging black holes, leaving a telltale imprint.

当一个黑洞在旋转时，它周围的自转能会转移给轻标量暗物质，使其密度陡增，这个过程就像一支无形的桨，将奶油剧烈搅打成黄油。一旦这块暗物质“黄油”稠到一定程度，它便会干扰黑洞并合时产生的引力波，并在波形上留下一道独特的“指纹”。

After determining what this signature would look like, Vicente and colleagues searched through data gathered by LIGO and its fellow gravitational wave detectors, KAGRA (Kamioka Gravitational Wave Detector) and Virgo, focusing on 28 of the clearest signals from merging black holes. Of these, 27 appeared to have come from mergers that occurred in the relative vacuum of space. One signal, however, GW190728, first heard on July 19, 2019, and the result of merging binary black holes with a combined mass of 20 times that of the sun and located an estimated 8 billion light-years away, seemed to carry the telltale trace of this merger occurring in a region of dense, "buttery" dark matter.

在明确了这一印记的形态后，维森特与同事分析了 LIGO 及它的两台“搭档”（日本神冈引力波探测器（KAGRA）和意大利 Virgo 探测器）采集的数据，并从中筛选出28个黑洞并合最清晰的信号。在这28个信号中，有27个似乎源自近乎真空的宇宙空间。但有一个例外——引力波事件 GW190728。该信号于2019年7月19日首次被捕获，来自一个总质量约为20倍太阳质量的双黑洞并合事件，发生地距地球约80亿光年。它似乎携带着一种独特的痕迹，表明这场并合发生于一片稠密的、“黄油状”暗物质云中。

The team behind this research is quick to point out that this can't be considered a positive detection of dark matter, but does say it gives us a hint at what to look for and thus where to direct follow-up investigations — something that could be increasingly useful as dark matter detectors on Earth continue into their fifth operating run with boosted sensitivity.

不过，研究团队特意强调，这尚不能算作暗物质的“实锤”，但它确实为我们指明了探索方向，提示了后续观测的靶区。随着地面引力波探测器步入第五轮运行、灵敏度进一步提升，这一线索或将愈发重要。

"We know that dark matter is around us. It just has to be dense enough for us to see its effects," said team leader Josu Aurrekoetxea, of the Massachusetts Institute of Technology (MIT) Department of Physics. "Black holes provide a mechanism to enhance this density, which we can now search for by analyzing the gravitational waves emitted when they merge."

麻省理工学院（MIT）物理系的团队负责人乔苏·奥雷科切亚（Josu Aurrekoetxea）表示，“已知暗物质就在我们身边，只要密度足够，我们就能观察到它的影响，黑洞提供了一种提升其密度的机制，而我们如今可以通过分析黑洞并合时释放的引力波，去搜寻这种致密暗物质的踪迹。”

The team's results were published on Tuesday (May 12) in the journal Physical Review Letters.

该团队的研究结果于周二（5月12日）发表在《物理评论快报》（Physical Review Letters）上。