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One of the coolest things about observing the universe is that the farther you look out, the older the stuff you’re looking at.
观测宇宙这件事，有许多神奇的地方，其中之一就是：观测得越远，看到的东西年龄就越大。
So in a way, telescopes become time machines.
所以，从某种角度来看，望远镜是时光机。
But that doesn’t mean there aren’t super old things right next door.
但这也并不意味着我们附近就没有年龄大的事物。
In fact, over the past decade, astronomers have found dozens of small faint galaxies orbiting our own Milky Way.
实际上，过去10年来，天文学家发现有许多规模不大、亮度不高的星系也在环绕着银河系飞行。
And they’ve turned out to be some of the oldest galaxies we’ve ever seen.
而这些星系在我们观测过的所有星系竟然算是元老级的存在了。
And by studying these neighborhood old-timers, we can get a glimpse of how the universe evolved.
通过研究这些离我们不远的元老级星系，我们可以了解到宇宙严谨的方式。
Last week in the Astrophysical Journal, a team of astronomers used these super faint galaxies to shine a light on one particular period of our history.
上周，在《天体物理学杂志》，一组天文学家通过这些亮度微弱的星系从另一个角度阐释了演进过程中一段特殊的时期。
Some of the headlines about this paper have made it sound like the researchers only just discovered these old galaxies, but we actually already knew that they existed.
文章中的一些大标题给人一种感觉：这些研究人员好像刚发现这些古老星系，但我们已经知道这些星系的存在了呀。
Until 2006, astronomers only knew of a dozen or so small satellite galaxies orbiting the Milky Way.
2006年的时候，天文学家对于环绕银河系飞行的小型卫星星系只了解那么十几个。
Our equipment just wasn't sensitive enough.
这是因为我们当时的设备还不够灵敏。
But by 2016, they’d found 54, and some of them were extremely old.
但2016年的时候，科学家发现了54个这样的星系，其中一些年纪已经很大了。
And that's just around the Milky Way.
而且发现地就在银河系附近。
Other satellites like these have also been found around the nearby galaxy Andromeda.
类似这样的卫星在仙女座附近也有找到。
And there should be plenty around other galaxies, too, we just don’t have the tech required to see them yet.
其他星系附近肯定也有很多这样的例子，只是我们的技术手段还不足以观测到所有这样的星系。
What’s new about this research is that it’s using those satellite galaxies to learn more about the epoch of reionization, a time in the universe’s history when star formation came to a near halt.
这项研究的标新立异之处在于：它是通过这些卫星星系来了解再电离时期的。所谓再电离时期，就是宇宙发展中的一个时段，这个时段里恒星会暂停形成。
About 380,000 years after the Big Bang, the universe was very, very dark.
宇宙大爆炸的38万年后，宇宙十分黑暗。
It was the beginning of the so-called “Cosmic Dark Ages,” the period when gravity hadn’t yet pulled together clumps of hydrogen and helium to form stars.
这时候正是所谓“宇宙的黑暗时期”的开始，那时候，引力还不足以将氢和氦聚集在一起形成恒星。
It lasted for about 100 million years, until the nuclear furnaces of the first stars began spewing out electromagnetic radiation of their own.
该时期长达近1亿年，直到首批恒星的核反应堆发射出自己的电磁辐射后才停止。
Some of that radiation came in the form of ultraviolet light, which had enough energy to knock electrons out of the leftover hydrogen floating around, ionizing it.
其中的一些电磁辐射是以紫外线形式出现的，而紫外线具备足够的能量，可以将残留在附近的氢原子电子弹射出来，使其实现电离。
Or more like re-ionizing it, because before the dark ages all that hydrogen started out as charged ions in the first place.
其实更像是再电离，因为在“宇宙的黑暗时期”开始之前，所有氢最开始都是以氢离子的形式存在的。
That’s why we call this period of the universe the epoch of reionization.
所以我们称这个时期为宇宙的再电离时期。
The reionized hydrogen was now so hot, and moving around so fast, that gravity couldn’t pull it together to make new stars.
经过再电离的氢离子温度极高，速度极快，所以引力作用也无法使它们聚集形成新恒星。
But not all galaxies stopped forming new stars; the most massive ones could still do that.
但并非所有的星系都会停止形成新恒星；质量大的星系就会照常形成新恒星。
Still, on an astronomical scale, most of the galaxies back then were super small, so their growth was stunted.
不过，从天文学的角度看，那时候大多数星系都还非常小，所以成长受到了限制。
It took about a billion years for everything to cool back down to the point where new galaxies could form, and most existing ones could grow.
宇宙万物过了近10亿年才能形成新星系，并产生让现有星系成长的环境。
But there’s still a lot we don’t know about the epoch of reionization.
但关于再电离时期，我们还有很多未知的地方。
For one thing, we’re not exactly sure when it happened.
比如，我们不太确定再电离时期的时间。
And that’s where last week’s paper comes in.
上周正好有一篇论文是从这一点切入的。
Even after reionization ended, even now, these small galaxies retain a record of that limited growth.
即便再电离结束后，即便是现在，这些小型星系的成长依然受到限制，这一点有证可循。
To help them read that record, the authors of this new paper combined a computer model with the real data from a collection of the dwarf galaxies surrounding the Milky Way and Andromeda.
为了助力读取这份证据，本文作者将某计算机模型与实际数据相结合，这些实际数据来自银河系和仙女座附近的一些矮星系。
Specifically, they modeled the luminosity, or light output, of each galaxy and found two distinct populations: one group of faint satellites and another of brighter ones, separated by a gap.
他们将每个星系的光度，也就是光输出进行建模，并发现2个独特的种群：两组卫星，一组亮度微弱，一组亮度更强，他们中间有一片相隔区。
That gap was caused by reionization!
这个相隔区就是由再电离引起的！
The areas with less matter in them didn’t have enough gravity to turn all that ionized hydrogen into stars, so their growth was stunted, and they stayed dimmer.
这些相隔区里物质较少，所以不具备足够大的引力，无法将经过电力的氢离子聚合形成恒星，所以它们的成长受到了限制，因而亮度就一直不是很强。
But this was more than just evidence of reionization.
这不止能够证明再电离的存在。
The team was also able to use the number of fainter galaxies to get a better idea of when reionization happened.
该研究小组还通过这些亮度较弱的星系更好地理解了再电离何时会发生。
The later it happened, the more faint galaxies you’d expect to see, because more would have been able to form before their star formation shut down.
再电离发生的越晚，星系的亮度就越弱，因为在恒星停止形成之前，本来是能形成更多星系的。
The team’s model fits best if the universe reached 100% reionization when it was about 950 million years old.
该小组模型最完美的时候当属宇宙达到100%再电离的时候，也就是宇宙9.5亿年的时候。
But that age is a lot older than other research teams have estimated.
但这个年龄比其他研究组预计的还要大很多。
A group called the Planck collaboration, for example, calculated a reionization age of only 570 million years.
比如，有一个名为普朗克的小组计算的是：再电离时期是在宇宙仅为近7.5亿年的时候。
Part of the difference might just come from the teams using different definitions, because the Planck analysis was looking to calculate the time when the universe was only 50% ionized.
存在差异可能是因为研究组用了不同的定义，因为普朗克分析组是想计算宇宙达到50%电离的时间。
But it does mean that we’ll need further studies before we can really pin down when the epoch of reionization happened.
但这确实也表明，我们还需要更进一步的研究，才能确定再电离时期。
Another reason we’ll need more research is that the data we have is incomplete.
需要进一步研究，还有一个原因：我们手头的数据并不完整。
There could be a lot more faint galaxies out there we haven’t found yet, and their properties could throw models for a loop.
应该还有很多我们尚未发现的灰暗星系存在，他们各有各的特征，加起来可以绕地球一圈了。
And, since the data only came from our local group, the time we calculate for reionization could be different from what we’d find in other places that had different sources ionizing hydrogen.
鉴于数据源头只有我们当地的小组，所以我们计算的再电离时期可能和其他有不同氢电离源头的地方不太一样。
But one way to check the validity of this research is to see if it can be used to make predictions.
但有一个办法可以检验该研究是否正确，是否可以为我所用以便做出预测。
The team used their model to propose the number of satellite galaxies that should be around the Large Magellanic Cloud, a galaxy that is itself a satellite of the Milky Way.
该研究小组通过他们的模型来判断大麦哲伦星云附近有多少个卫星星系。大麦哲伦星云本身就是银河系的卫星。
Their predicted number is 26, plus or minus 10, and with only a 68% level of confidence that it’s even within that range.
他们曾预测数量是26，上下浮动最高10个，甚至有68%的把握认为就在这个范围里。
But it is a jumping off point.
这只是个开始。
And by looking for more clues in our galactic backyard, we might someday reveal a whole lot more about what the universe was like when it first began.
通过寻找银河系里更多的线索，我们可能有一天会了解宇宙初期的更多情况。
Thanks for watching this episode of SciShow Space News!
感谢收看本期的太空科学秀。
To learn more about the epoch of reionization, you can check out our episode on how the first stars transformed the universe.
要了解再电离时期的更多信息，可以查看我们关于首批恒星是如何改变宇宙的视频。
