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Invisible to the naked eye, 虽然肉眼看不见
Our night sky is scattered with the hundreds of billions of galaxies that fill the known universe. 但我们的夜空中其实散布着数千亿个星系
Like the stars, these galaxies form constellations,Hidden patterns that echo the reverberations of matter and light from an epoch long before galaxies ever formed. 这些星座中星系和恒星形成的图案隐含着 远古时代物质和光的回响
These are the Baryon Acoustic Oscillations (BAO), 它们被称为重子声学振荡（BAO）
and they may hold the key to understanding the nature of dark energy. 它们可能是理解暗能量本质的关键
翻译：Toucc 字幕：Paul_Dirac
The field of cosmology and the study of the universe on its larger scales was once the least precise in all of astrophysics. 在所有天体物理学中 宇宙学领域及其研究在大尺度上曾经是最不精确的
The impossibly vast distances made accurate measurements near impossible. 无比遥远的距离使得精确测量几乎不可能
But as our telescopes and our techniques improved over the last few decades, 但过去几十年中随着望远镜和技术的改善
Things are now very different. 今时不同往日
We live in the era of precision cosmology, in which we know the stunning detail, 我们生活于精确宇宙时代，掌握了其中惊人的细节
the properties that govern the very birth, evolution, and the end of our universe. 比如控制宇宙开始，演变，直至结束的一些属性
We talked about one of those properties in a recent episode- The Hubble constant, 上期节目我们讨论了其中的一种属性——哈勃常数
and about a growing conflict in its measured value, which hints at strange new physics. 还讨论了它引发的日渐显著的冲突 这一冲突也暗示着新的未知物理学
Today, we'll talk about another measurement that may help resolve this crisis: 今天我们要讨论的另一个测量可能有助于解决这个危机
The baryon acoustic oscillations. 那就是重子声学振荡
They are the fossils of the first sound waves in the universe, 它们是宇宙中第一个声波的化石
imprinted on the distribution of galaxies on the sky. 印刻于星空中星系的分布中
And in these patterns, we could read the expansion history of the universe. 通过这些图案我们可以读取宇宙的扩张史
Let's start at the beginning. 让我们从头开始吧
For the first few hundred thousand years in the life of our universe, 在宇宙生命最初的几十万年里
All of the space was filled with hydrogen and helium in plasma form. 所有空间中都充满着等离子体形式的氢和氦
Protons and the lightest of nuclei forged in the first minutes after the Big Bang, 在大爆炸后的几分钟，质子和最轻的原子核融合
and still so hot that no atoms could form, and electrons buzzed free of their nuclei. 但仍然太热而无法形成原子，电子也还不受原子核束缚
These particles of matter are our baryons. 这些物质粒子就是所谓的重子
There's also light, in fact, around a billion photons for every electron. 除此之外还有光，实际上每个电子对应着大约10亿个光子
but no photon is safe from a free electron. 但光子无法摆脱自由电子的束缚
Unbound electrons present a huge target to scatter any wavelength of light. 这些自由电子成为了所有波长光的一个巨大散射源
And scatter they did, constantly. 而且是持续不断的散射
A photon could barely travel any distance before colliding with an electron. 光子还没有运动多远就会撞上电子
The electrons in turn exerted their electromagnetic pool on the nuclei. 电子反过来将其电磁池施加于原子核上
We say that in this state, light was coupled with matter. 我们称这种状态下，光与物质是耦合的
And baryons and photons formed a single strange fluid: 此时重子与光子形成了一种奇怪的流体
a Baryon-Photon plasma. 重子-光子等离子体
There are three profound differences between the behaviour of matter in this state compared to the gentle gas nebula of the modern universe. 在这一状态下 物质的行为与现代宇宙中温和的气体星云相比存在三个显著差异
First: The Baryon-Photon plasma was opaque. 差异一：重子-光子等离子体是不透明的
There exists no lines of sight to anything during that time, 那时没有任何物质的视线
and we'll come back to that later. 这个之后会说到
Second: Light was able to exert an enormous pressure on this plasma, 差异二：光能够对这种等离子体施加巨大的压力
as we'll see that it'd lead to the production of colossal sound waves. 之后我们会看到它将引发强劲的声波
And third: Those sound waves travelled fast. 差异三：这些声波的传播速度极快
Rapid interaction between the charged particles of the plasma via the trapped photons meant that ripples in the plasma travelled at over half the speed of light. 等离子体中的带电粒子可以通过被困其中的光子迅速发生作用 这意味着等离子体中波纹的运动速度超过了一半光速
Mixed in this soup of baryons and photons was dark matter. 重子和光子中还混杂着暗物质
In fact, dark matter outweighs baryons by a factor of five, 实际上，暗物质要比重子多五倍
Which means it was, by far, the dominant gravitational influence in the early universe as it still is. 这意味着截至目前的研究 它是早期宇宙乃至现在还占主导地位的引力作用
But unlike baryons, dark matter does not interact with light at all. 但与重子不同，暗物质根本不会与光相互作用
Light exerts no pressure on dark matter. 光不会对暗物质施加压力
Okay, so, the universe is filled with this hot ocean of baryons, photons, and dark matter. 所以那时的宇宙是一片充满着重子，光子和暗物质的汪洋
Now, in order for it to do anything interesting, 为了让其中发生一些有趣的东西
We need one final ingredient: 我们还需要最后一个条件
Density fluctuations. 密度的涨落
A teensy bit more matter here, or a teensy bit less there, 这里多一点，那里少一点
These fluctuations probably were the remnants of random quantum fluctuations from when the universe was subatomic in size. 这种波动可能是宇宙在亚原子大小时随机量子波动的残余
And now that they've expanded enormously by cosmic inflation in the beginning instant of the Big Bang. 它们在宇宙大爆炸开始的瞬间被宇宙的膨胀放大了
Immediately after that, two competing forces began to work. 在那一瞬间，两个相竞争的力开始作用
Each over-dense region pulled gravitationally on its surroundings, drawing matter towards it. 那些较高密度的区域用引力吸引其周围物质，将物质拉向它
In particular, the dark matter flowed inwards towards this density peak. 特别是暗物质也会流向这些密度峰值区
But also at that density peak, the imprisoned photons exerted an enormous outward pressure. 同时这些高密度区中受困的光子也将向外施加巨大的压力
To equalise this pressure, radiation pushed outwards and carried the baryons with it. 为了平衡这种压力，辐射带着重子被推向外界
This resulted in an acoustic wave, 这导致了一种声波的产生
a true sound wave in the form of an expanding shell of increased density. 一种密度较高的膨胀球壳形式的真实声波
And remember, sound travelled at over half the speed of light back then, so the shell expanded fast. 别忘了，那时的声速高达光速一半以上 所以球壳膨胀得很快
But as it expanded, so did the universe. 整个宇宙也随之膨胀
As matter became more diffused, and the photons themselves were stretched, redshifted to lower energies, the universe cooled. 随着物质变得分散，光子自身也被拉长并红移至较低能量 宇宙逐渐冷却
At 380,000 years, the plasma hit a critical temperature of 3000 Kelvin, 在380000年，等离子体达到3000开尔文的临界温度
around the surface temperature of the coolest red dwarf stars. 约为最冷红矮星的表面温度
At this temperature, electrons could finally be captured by nuclei and the first true atoms formed. 在这个温度下 电子最终会被原子核捕获并形成第一个真正的原子
The baryons transitioned in phase from a plasma to a gas. 重子从等离子体转变为气体
We call this phase transition event: Recombination 我们把这个相变过程称作复合
While free electrons were able to interact with any frequency of light, 自由电子能够与任何频率的光相互作用
electrons bound into atoms are restricted to only those specific frequencies corresponding to the energy level transitions of that atom. 而原子中的电子只能与该原子能级跃迁所对应的特定频率作用
As a result, light and matter were no longer coupled. 这样，光与物质就不再耦合
The universe went from opaque to transparent over a period of several thousand years. 宇宙在几千年的时间里从不透明变为透明
As the wave of plasma and photons decoupled, 随着等离子体波和光子的解耦
light began to stream freely through the universe as the cosmic background radiation. 光开始作为宇宙背景辐射自由地穿过宇宙
But the plasma, now hydrogen and helium gas, stalled. 而那些等离子体，也就是现在的氢气和氦气则不会运动
The speed of sound dropped from half the speed of light to only hundreds of meters per second. 声速从光速的一半下降到每秒几百米
The wave of plasma-turned-gas essentially froze in its current state. 波在等离子体转为气体时冻结在了当时的状态
The radius of that shell became fixed to the rate of expansion of the universe. 球壳的半径取决于宇宙的膨胀率
And its size? 那尺寸是多大？
Well, the exact distance that sound could travel over the age of the universe at that time. 就是声波在那个宇宙年代可以传播的距离
We call this the sound horizon. 我们称其为音界
And at recombination, it was around five hundred thousand light years. 在复合时它约为50万光年
While all of this was happening, dark matter was doing its own thing. 在这些发生时，暗物质也在做另一些事
Immune to the radiation pressure, the central dark matter overdensity had continued to grow. 由于不受辐射压影响，中心的暗物质密集区不断增长
It pulled on the expanding shell, and was pulled by it. 它在拉扯膨胀球壳的同时也在被拉扯
When the expanding wave froze, 在膨胀的波冻结时
both dark matter and baryons flowed together and consolidated the new structure. 暗物质和重子流向彼此并形成了新的结构
Once more in the gravitational grip of dark matter, 在暗物质引力的再次作用下
hydrogen and helium could begin the long work of collapsing into stars and galaxies as the universe continued to expand. 氢和氦才可以在宇宙继续扩张的过程中慢慢坍缩成恒星和星系
And now over thirteen and a half billion years later, the universe is expanded by a factor of 1,100. 在135亿年后的现在，宇宙膨胀又了1100倍
So those rings should be 150 mega parsecs across, 500 million light years. 所以这些环应该长达150Mpc，也就是5亿光年
So how does this primordial history lesson help us understand the subsequent expansion of the universe? 那么这个原初历史课对我们理解之后的宇宙扩张有什么帮助呢？
Well, crazily, we can still see those rings, not made of plasma or gas, but made of galaxies. 令人震惊的是，我们现在仍能看到这些环 只不过是由星系而不再是等离子体或气体组成
Before I show you what they look like, a note of caution: 在我展示它们的样貌之前，请注意以下事项：
The story I just told you about the early development of these density fluctuations is far from the whole picture. 我刚刚所说的这些密度涨落的早期发展其实跟真实情况相差甚远
In reality, the density waves sloshed inwards and outwards. 现实情况是这些这些密度波会相互重叠
Gravity pitted against radiation pressure. 引力与辐射压相互抵抗
Density everywhere oscillated with a rate of oscillation depending on the size of the initial fluctuation. 每个地方的密度都会以一个取决于初始涨落量的速率振荡
Those are the oscillations in the baryon acoustic oscillations, 这就是重子声学振荡
and the complex signature of that sloshing is imprinted in detail on the temperature map of the cosmic microwave background radiation. 而它们重叠的复杂特性详尽地印刻在宇宙微波背景辐射的温度图中
That's something we'll explore in a follow-up episode, 这也是之后节目会继续探讨的
because it is perhaps the most powerful tool we have in cosmology. 因为它可能是宇宙学中最有力的工具
But today, we're gonna focus on this first outbound density pulse. 但今天我们要继续讨论这第一道外扩的密度脉冲
Then, what it looks like now in the arrangement of galaxies. 以及它对当今星系分布的影响
So, the acoustic shells at recombination overlapped in a complex web. 复合时这些声壳以复杂的形式相互重叠
Those rings were further smeared out over the thousands of years it took for the universe to fully transition from plasma to gas. 这些环在宇宙完全从等离子体转换为气体的数千年内进一步分散
Collapse that web into galaxies over the age of the universe, 再经过整个宇宙年龄的时间最终坍缩成星系
and at first glance, it looks like a random smattering of galaxies on the sky. 乍一看，这就像是星空中随机分布着星系
But the pattern is there. 但其中环状图案是在的
Detecting it required the most detailed surveys of the heavens ever conducted: 探测它需要对星空进行最详细的勘测
It required galaxy redshift surveys. 需要进行星系红移巡天
Redshift is just the amount by which a galaxy's light has been stretched as it travelled through the expanding universe. 红移就是星系的光穿过膨胀宇宙时被拉长的量
The more stretching, the longer that light has travelled, and so the more distant that galaxy must be. 拉长越多，则光运动的距离越长，所以星系肯定更遥远
Redshift gives distance. 红移能得到距离
And with careful measurements of galaxies' positions on the sky, 通过精细测量星系在星空中的位置
a redshift survey can produce a three-dimensional atlas of the universe. 红移巡天可以得到一个三维的宇宙地图集
With a galaxy redshift survey in hand, 有了星系的红移巡天
how do you find patterns in what looks like a random splattering of tens to hundreds of thousands of galaxies? 要如何从这数十万看似随机分布的星系中找到原本的环状图形呢？
Well, you expect that galaxies should mostly form in the centers of those primordial density fluctuations, 首先，这些星系应该主要形成于那些原始密度波动区的中心
where the dark matter was the most concentrated? 那里暗物质最为集中
But you should also have a slight overabundance of galaxies at exactly 150 megaparsecs from those clusters. 其次，在距离中心区刚好150Mpc的位置应该也能看到较多的星系
Corresponding to the acoustic rings, everything is so hopelessly overlapping. 那里对应的是音环，一切都混乱地重叠在一起
But here's what you do. 但可以这样做
In your atlas of the universe, take slices of the universe, each slice a certain distance from us. 在得到的宇宙地图集中，对宇宙进行切片 每片都距离我们一个特定的距离
Within that slice, tally up the distances on the sky between every pair of galaxies. 在某一片中计算每对星系之间的距离
You should see a lot of galaxies that are close together. 你会看到很多星系非常靠近
That's the clustering from the giant dark matter density peaks. 那是暗物质所在的密度峰值区中的星系团
They should also see a slight excess of galaxy pairs with separations of 150 megaparsecs. 此外还应该在相距距离150Mpc的位置看到星系对数目的一个小峰值
These are the galaxy pairs where one is at the center of the dark matter peak, and one is on the surrounding ring. 这是星系对中一个位于中心暗物质区，一个位于外环的情况
That bump seems small, but it's statistically very significant. 这个小凸起并不明显，但在统计学上是非常显著的
It's also exactly where we expected it to be. 它也正好出现在了我们预想的位置
This baryon acoustic oscillations signal was first spotted in 2005 这一重子声学振荡信号在2005年被首次发现
by the Sloan Digital Sky Survey in the northern hemisphere galaxies, 由斯隆数字巡天对北半球星系进行观测
and the 2dF survey in the south. 而2dF巡天对准南半球
Since then, the WiggleZ BAOs and 6dF surveys have improved the measurements. 之后的WiggleZ BAOs和6dF巡天改进了测量
Now, there was a powerful driving motivation to make this measurement and to improve it. 现在还有另一个有力的动机来驱动并改进这类测量
It was to confirm dark energy. 那就是证实暗能量的存在
Dark energy was first discovered by using distant supernovae as distance measurements to track the rate of expansion of the universe. 暗能量是在将遥远超新星作为距离测量工具 以计算宇宙膨胀率时被首次发现
Those observations revealed that the expansion rate is accelerating due to an unknown influence what we call dark energy. 这些测量结果表明 扩张速度由于某种被称为暗能量的未知影响而在不断增加
But such an incredible claim requires independent checking. 但这一惊人的论断还需要独立检验
It requires an independent test of the expansion history. Enter the baryon acoustic oscillations. 需要对扩张历史进行独立测试 重子声振荡的研究便开始了
See, we know exactly how big those acoustic rings should be. 现在我们知道了这些音环的确切大小
Our understanding of the behaviour of the original baryon photon plasma is excellent. 我们对原始的重子-光子等离子体性质的理解也很完备
So, we know how far the acoustic wave should have travelled before being frozen by recombination. 所以我们知道这些声波在被复合冻结时运动了多远的距离
And we can confirm that prediction by looking at the patterns in the cosmic background radiation. 也就可以在宇宙背景辐射上观察这些环状图案来确认这一预测
That means we know how big those rings were when they formed, 这意味着我们知道这些环在产生时的大小
and we can see how big they are at different points in the modern universe from our redshift surveys. 也能通过红移巡天得到它们在现代宇宙中各个位置的大小
These rings give us a standard ruler on the sky, spanning all of cosmic time. 这些环是星空中的又一个标尺，任何时候都是
They allow us to track the expansion rate of the universe. 它们能帮助我们追踪宇宙膨胀速度的变化
The baryon acoustic oscillations agree with and confirm what we measure using supernovae distances: 重子声学振荡与超新星得到的结果一致
The expansion of the universe is accelerating! 确认了宇宙的膨胀在加速
They confirm the existence of dark energy. 也证实了暗能量的存在
They also confirm the dark energy behaves just as is predicted by Einstein's cosmological constant. 他们还证实暗能量的性质与爱因斯坦的宇宙学常数所预测的一致
It appears unchanging over time, just as we'd expect if dark energy is the energy of the vacuum itself. 它似乎不随时间改变，暗能量就是我们所预想的真空本身的能量
Frankly, dark energy aside, just being able to see these patterns is cool enough for me. 坦率地说，不提暗能量，单单看到这些环状图案就已经很牛逼了
I mean think about it. 仔细想想
There are rings in the sky inscribed in galaxies, 星空中的星系构成了这么多环
There are rings in the sky inscribed in galaxies, frozen echoes of the very first sound waves to reverberate across space-time. 那是萦绕在时空中首个声波滞止而成的回响
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Okay, so once again, we're covering comments from two episodes.
Today, it's the 'Crisis in cosmology' and 'Negative mass perpetual motion' episodes.
That Chess Guy has a fun one. To paraphrase: As the universe expands towards the infinite future,
does that expansion outpace the probability of collapsing into a big crunch
due to everything quantum tunneling towards a single point?
Well, the answer is- it depends on what size you consider. For the entire observable universe?
Almost certainly, there could be no quantum tunneling big crunch,
but there is some smaller size that can eventually collapse that way.
Now, you need to wait 10^(10^25) years for the first quantum tunneling to turn iron stars into black holes,
and way, way longer than that for anything larger.
Netist comments that, "It can't be real because it would break our current understanding of physics"
...is hardly a good argument. Now, while I agree that this isn't a hard rule, it's a good guiding principle
to be dubious of new theories that contradict extremely well tested old theories.
Remember that things like general relativity and much of quantum field theory are verified to stunning precision.
If a new theory says, "Nuh-uh, that all wrong",
it better be able to also explain how those theories appear to be so right while being wrong.
Luis Aldamiz recalls the appropriate quote, "Extraordinary claims require extraordinary evidence."
Again, this is a guiding principle,
and it helps scientists sort through the overwhelming flood of poorly thought-out
"theories" that they get bombarded with all the time.
Seriously, I should show you guys my inbox one of these days.
Mr. Miller advises us to remember that the number zero didn't exist for thousands of years.
Ergo, time will prove negative mass' existence.
Well, first I'd say that the number zero existed, at least in a sense before we discovered it.
It's a common fallacy to draw analogies between revolutionary ideas
that took a long time to prove or discover and fringe ideas that may or may not ever be proved.
Why? Because for every revolutionary idea that was proved, right?
There are countless of fringe ideas that were wrong.
You've never heard of those ones.
Not everything that we can imagine exists.
In fact, we can imagine infinitely more things than a real.
Still, it's worth looking into the wacky ideas for the tiny fraction that do spark revolutions.
But don't get too attached to any one of them.
flux_capacitor notes an alternate model for how negative mass might behave.
In so-called bimetric gravity, you can have positive and negative masses,
but each is described by its own set of Einstein field equations.
That's kind of like having parallel spacetimes, one with positive and one with negative masses,
which can still interact gravitationally.
In these models, like masses attract and opposite masses repel,
but you don't get the crazy runaway motion
that occurs if you put both positive and negative masses in the same spacetime.
So, no perpetual motion machines.
It can also be used to explain dark energy and dark matter.
An example is the Janus model of Jean-Pierre Petit.
This is a much more sophisticated model than the one by Jamie Farnes.
It is, however, just as speculative.
Okay, this is my favorite comment ever.
Adam Houlett asks whether these explanations might be any more understandable
if the "actor" was actually knowledgeable, something-something,
translation lost because the actor is more a pretty face than an expert.
Adam, you made me blush!
Normally, it's "He looks like a regular-sized Tyrion Lannister".
Seriously, every astrophysicists, like me, craves being mistaken for an actor and I feel like I've finally made it.
[PBS Space Time outro]