转载自https://www.inverse.com/science/low-frequency-gravitational-waves，原文章标题为FINDING THIS COSMIC PHENOMENON COULD UNLOCK MYSTERIES OF THE ANCIENT UNIVERSE，作者为PASSANT RABIE，发布时间为2021年10月20日。

Astronomers are on the verge of unlocking an entirely new way to observe the universe.

天文学家即将解锁一种新的观测宇宙的方式。

Since the first detection of gravitational waves in October 2015, scientists have been listening in to these cosmic hums caused by massive, violent events such as the merger between two black holes.

从2015年10月首次探测到引力波以来，科学家一直在收听这种如两个黑洞合并这样的大质量的、剧烈的事件引起的宇宙之声。

But scientists still can’t detect these waves at low frequencies that are often the result of even more massive objects colliding with one another or events that took place shortly after the Big Bang.

但是科学家仍然无法探测低频的引力波，这种引力波通常是质量更大的物体相撞或者宇宙大爆炸后不久发生的事件造成的。

A team of researchers from the University of Birmingham suggests combining different methods to detect ultra low-frequency gravitational waves that hold the mystery of ancient black holes and the early universe.

伯明翰大学的一个研究团队建议结合不同的方法来探测极端的低频引力波，这种引力波含有古老的黑洞和早期宇宙的奥秘。

Their work was published Monday in the journal Nature Astronomy.

他们的研究成果被发表在了《Nature Astronomy》上。

WHAT ARE LOW-FREQUENCY GRAVITATIONAL WAVES?

什么是低频引力波？

Astronomers have mainly relied on electromagnetic radiation, or light, to study objects in space. But as light travels towards us, it interacts with different elements in outer space, including dust, obscuring our view of the cosmos.

天文学家主要依赖于电磁波来研究空间中的物体。但是当光向我们传播时，它会和太空中的包括尘埃在内的不同现象发生作用，这模糊了我们观测宇宙的视野。

Gravitational waves are a way to listen to the universe rather than see it. These hums are caused by the accelerated masses of cosmic beings, which send out ripples through spacetime at the speed of light. Scientists can listen in on these echoes of the cosmos thanks to the Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors and the Virgo detector.

引力波是一种听见宇宙而不是看见宇宙的方式。这些“声音”是由宇宙中大量物质的加速造成的，它们以光速在宇宙时空中掀起涟漪。得益于雷射干涉重力波天文台（LIGO）和室女座干涉仪，科学家可以听到这些宇宙的回响。

But most of the gravitational waves detected so far have been of higher frequencies in the millihertz. Meanwhile, low-frequency gravitational waves, which are in nanohertz frequencies, are much more challenging to detect.

但是现在探测到的大多数引力波频率都比毫赫兹更高。与此同时，纳赫兹频率的低频引力波的探测更加富有困难和挑战性。

Frank Ohme, leader of the Independent Max Planck Research Group for Gravitational Physics, explains that they oscillate faster or slower depending on what causes the gravitational waves.

马克思·普朗克独立引力物理研究小组的领导人弗兰克·欧姆解释这些引力波频率的高低取决于引起它的原因。

“The effect is the same; it's got to stretch and squeeze space and time,” Ohme tells Inverse. “The low-frequency ones want to do it slower, so it takes a lot longer for things to squeeze and stretch than the high-frequency ones.”

“效果是一样的：它在拉伸和挤压时空，”欧姆告诉Inverse，“这些低频的想要更慢，所以与高频的相比花费了更多的时间来挤压和拉伸。”

While high-frequency gravitational waves are caused by ordinary stars or smaller black holes between 20 to 30 solar masses, low-frequency waves are caused by the merger of supermassive black holes, which can be millions or billions of times the mass of the Sun.

高频引力波是由普通的恒星和更小的黑洞（质量在20到30个太阳质量之间）产生的。低频引力波是由特大质量的黑洞融合产生的，它们的质量可以是数百万或者数十亿倍的太阳质量。

Scientists also believe that low-frequency gravitational waves could come from events taking place shortly after the Big Bang, long before galaxies were formed.

科学家也相信低频引力波可能来自大爆炸后不久发生的事件，远早于星系形成。

HOW TO DETECT LOW-FREQUENCY GRAVITATIONAL WAVES

如何探测低频引力波

Christopher Moore, a researcher at the Institute for Gravitational Wave Astronomy & School of Physics and Astronomy at the University of Birmingham and lead author of the paper, has been studying gravitational waves for several years.

这篇论文的主要作者、伯明翰大学引力波天文研究所和物理与天文学院的研究者克里斯托弗·摩尔已经研究引力波好几年了。

“I’ve long been interested in gravitational waves,” Moore tells Inverse. “But for most of my time, low-frequency waves have been a niche interest with a lot less attention than the high-frequency stuff, but I think that’s really starting to change.”

“我一直对引力波很感兴趣，”摩尔告诉Inverse，“但是在我的大部分时间中，与高频引力波相比，低频引力波是一个投入了少得多的注意力的小众兴趣。但是我认为这一点确实开始改变了。”

The primary method used to detect low-frequency gravitational waves is through pulsars, compact, highly magnetized stars that rotate while emitting a regular pulse of radio waves. Scientists look for any fractional change to the timing of the pulsar’s beam that gravitational waves may cause.

探测低频引力波的主要方法是通过脉冲星，脉冲星是一种旋转的高密度、强磁场的中子星，并会发出规律性的电磁波。科学家在寻找任何可能是引力波导致的脉冲星波束的微小的变化。

“Nature has been kind enough to give us rapidly spinning millisecond pulsars, which are extremely good clocks — they rotate in a very, very stable way which makes them extremely good timekeeping instruments,” Moore says. “If a gravitational wave were to come across the Earth, you'd see the clocks speed up and slow down but in different ways.”

“自然足够友善地给了我们快速旋转的毫秒脉冲星，它们是很好的时钟——它们以一个非常固定的方式旋转，这让它们成为了非常好的计时工具。”摩尔说，“如果一束引力波穿过地球，你会看到时钟以不同的方式加速和减速。”

But while that may be the leading way to detect low-frequency gravitational waves, the authors behind the new study argue that it’s not enough since it doesn’t specify the cause behind the waves.

虽然这可能是探测低频引力波的主要方式，但是这个它的研究者争论这种方式并不足够，因为它并没有详细说明引力波的起因。

Instead, they suggest combining different methods to determine the source of low-frequency gravitational waves.

作为替代，他们建议结合不同的方法查明低频引力波的来源。

In January, the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) detected what may be hints of low-frequency gravitational waves by studying signals from distant stars, but those are yet to be confirmed.

在一月，北美纳赫兹引力波天文台（NANOGrav）通过研究来自遥远星球的信号探测到了可能是低频引力波的线索，但是它们还没被确认。

“So what we were really trying to do in this paper is to see if there's any other probe, apart from pulsar timing, any other instrument, any other experiment, any way of trying to detect gravitational waves that could help, even a little bit,” Moore says.

“所以我们在这篇论文中实际在尝试去做的是寻找是否有任何除了脉冲星计时之外的任何其它可能会有帮助的设备、实验或方法来探测引力波，即使只是一点点。”摩尔说。

One suggestion is combining the pulsar data with observations made by the European Space Agency's Gaia mission, which has the ambitious task of creating a three-dimensional map of the Milky Way.

一个建议是将脉冲星数据和欧空局（ESA）盖亚（Gaia）计划的观测结合，盖亚计划有创建银河系三维地图的宏大任务。

The authors also suggest looking into Big Bang nucleosynthesis, a model of the early universe based on how many different atoms existed shortly after the Big Bang.

作者还建议调查宇宙大爆炸的核合成，一种基于宇宙大爆炸后不久存在的原子种类数量的早期宇宙模型。

“So neither of those methods can detect gravitational waves yet, but they can place limits at different frequencies,” Moore says.

“所以这些方法都不可以探测到引力波，但是可以对不同的频率进行限制。”摩尔说。

Although the paper does not come up with conclusive answers, it is a first step in conducting future studies on low-frequency gravitational waves.

虽然这篇论文并没有得到结论性的答案，它仍是未来实施研究低频引力波的第一步。

WHY DO WE STUDY GRAVITATIONAL WAVES?

我们为什么要研究引力波？

Since researchers first detected gravitational waves, these ripples through spacetime have opened up a new field for observing the universe.

自从研究人员第一次探测到引力波，这些穿过时空的涟漪开启了观察宇宙的一个新领域。

And now, as scientists are on the verge of unlocking low-frequency gravitational waves, it’s an exciting time to be listening in to the cosmos.

现在，科学家在解锁低频引力波的边缘，这是一个令人激动的听到宇宙的时刻。

“We just tap into the really massive black holes that we know exist in the universe, but we’re not exactly sure how many there are and how heavy they are,” Ohme says. “And because they are so heavy, the gravitational waves they create not only are of lower frequencies but also are super, super loud intrinsically.”

“我们只利用了我们知道存在的大质量黑洞，但是我们并不确切地知道它们数量有多少、质量有多大。”欧姆说，“因为它们是如此的重，它们创造的引力波本质上不仅频率低，还非常的强烈。”

“So the heavier the black holes are, the larger spacetime distortion they create, and therefore we can look further out into the universe,” he adds.

“所以黑洞质量越大，就能创造越大的时空扭曲，因此我们可以向宇宙看得更远。”他补充。

But for low-frequency gravitational waves to be informative, scientists have to know their source.

因为低频引力波可以提供许多信息，所以科学家必须知道它们的起源。

“And that’s really the point, are we looking at an astrophysical signal coming from black holes in the local universe, or are we looking at a cosmological process happening, happening much closer to the Big Bang, much further back in time?” Moore says.

“并且这确实是重点，我们在看向来自黑洞的天体物理学信号，还是我们在看向发生的宇宙演变过程，在时间上比宇宙大爆炸更加接近还是更加遥远？”摩尔说。

Moore predicts that scientists are on the verge of the first confirmed detection of low-frequency gravitational waves, which may help us peer further out into the universe or learn more about how supermassive black holes came to be in the first place.

摩尔预测科学家在首次确认探测到低频引力波的边缘，这可能可以帮助我们看向更远的宇宙或者了解更多关于大质量黑洞如何形成的知识。

“It’s a completely new way of doing astronomy,” Moore says. “That’s one of the things that makes it really exciting.”

“这是一种全新的天文研究方法，”摩尔说，“这是它确实令人激动的原因之一。”

Abstract:

Gravitational waves at ultra-low frequencies (≲100 nHz) are key to understanding the assembly and evolution of astrophysical black hole binaries with masses ~106–109M⊙ at low redshifts1–3 . These gravitational waves also offer a unique window into a wide variety of cosmological processes4–11. Pulsar timing arrays12–14 are beginning to measure15 this stochastic signal at ~1–100 nHz and the combination of data from several arrays16–19 is expected to confirm a detection in the next few years20. The dominant physical processes generating gravitational radiation at nHz frequencies are still uncertain. Pulsar timing array observations alone are currently unable21 to distinguish a binary black hole astrophysical foreground22 from a cosmological background due to, say, a first-order phase transition at a temperature ~1–100 MeV in a weakly interacting dark sector8–11. This letter explores the extent to which incorporating integrated bounds on the ultra-low-frequency gravitational wave spectrum from any combination of cosmic microwave background23,24, big bang nucleosynethesis25,26 or astrometric27,28 observations can help to break this degeneracy.

（第一次做翻译，质量并不高，如发现错误请在评论区中指出，感谢帮助）