黑暗世界：通向未知宇宙的神秘之旅

伤我心太深

一个影子宇宙，与我们的宇宙交织在一起，其自身却有可能拥有丰富的内容

                  黑暗世界

                                             作者：Jonathan Feng ； Mark Trodden

   

有很大一部分物质，银河系和其它星系本该由它们参与构成的，但我们却看不见。虽然银河系看上去光辉璀璨，但其实它本身是一个黑暗的世界，我们仰首太空，寻找着暗物质的踪影，它们肯定悄无声息的躲在某个角落，等待我们去发现。慢慢地，我们了解到，自己生活的世界其实非常，非常奇怪。加入我们，一起到黑暗的中心去体验一番吧！

the dark side
The train of thought to dark matter began with the discovery of radioactive beta decay in the early 1900s. Italian theorist Enrico Fermi sought to explain the phenomenon by postulating a new force of nature and new force-carrying particles that caused atomic nuclei to decay. This new force was similar to electromagnetism and the new particles to photons—but with a key twist. Unlike photons, which are massless and therefore highly mobile, Fermi argued that the new particles had to be heavy. Their mass would limit their range and account for why the force causes nuclei to fall apart but otherwise goes unnoticed. The new force is now known as the weak nuclear force and the hypothesized force-carrying particles are the W and Z particles, which were discovered in the 1980's. They are not dark matter themselves, but their properties hint at dark matter.

                            黑暗一面

    关于暗物质的很多想法，可追溯至二十世纪早期放射性 β衰变的发现。当时意大利理论物理学家恩里科·费米为了解释该现象，设想了自然界一种新的作用力和它的携带粒子，正是这些粒子导致原子核发生衰变。这种作用力类似于电磁力，而新的粒子类似于光子——只不过它们拥有一个关键的扭矩。与没有质量且流动性强的光子不同，费米认为这些新的粒子质量很大，它们的质量限制了力的作用范围，所以尽管力把原子核挤破了，但人们却一无所知。这种新的力就是现在被人们所熟知的弱核力，而假想的力携带粒子就是W粒子和Z粒子，它们于上世纪80年代被发现。这两种粒子本身不是暗物质，但它们的性质却预示了暗物质的存在。

One goal of the Large Hadron Collider is to look for those particles, which should have masses comparable to those of the W and Z. Indeed, physicists think dozens of types of particles may be waiting to be discovered—one for each of the known particles, paired off in an arrangement known as supersymmetry.

    强子对撞机的一个目标就是要寻找这样一些粒子，它们的质量与W粒子和Z粒子相当。事实上，物理学家想到有待发现的粒子多达数十种——每种都对应着一种已知粒子，它们排列起来，形成人们所熟知的超对称。

These hypothetical particles include some collectively known as weakly interacting massive particles, or WIMPs. The name arises because the particles interact only by means of the weak nuclear force. Being immune to the electric and magnetic forces that dominate the everyday world, they are totally invisible and have scarcely any direct effect on normal particles. Therefore, they make the perfect candidate for cosmic dark matter.

    这些假想的粒子构成一个集体，被人们称作“弱相互作用重粒子”，或者说叫WIMPs。其名字的得来是因为它们之间的作用力仅为弱核力。日常的电磁力对它们完全不起作用，并且它们看不见，摸不着，很少会与一般的粒子发生作用。所以它们是宇宙暗物质的理想候选者。

The WIMP Coincidence

                 “弱相互作用重粒子”巧合

Given the expected mass of WIMPs and the strength of their interactions, which govern how often they annihilate one another, physicists can easily calculate how many WIMPs should be left over. Rather amazingly, the number matches the number required to account for cosmic dark matter today, within the precision of the mass and interaction-strength estimates. This remarkable agreement is known as the WIMP coincidence. Thus, particles motivated by a century-old puzzle in particle physics beautifully explain cosmological observations.

    知道了弱相互作用重粒子质量的期望值以及它们之间相互作用的大小（相互作用的大小决定彼此湮灭的频率），物理学家们就能轻易算出还有多少弱相互作用重粒子剩余。令人惊讶的是，在质量与作用力大小精确范围内，该数量与宇宙暗物质所要求的量相当符合，这一显著一致性被称作弱相互作用重粒子巧合。于是，由粒子物理里的世纪谜团所引出的粒子观点很好地解释了宇宙观测结果。

This line of evidence, too, indicates that WIMPs are inert. A quick calculation predicts that nearly one billion of these particles have passed through your body since you started reading this article, and unless you are extraordinarily lucky, none has had any discernible effect. Over the course of a year you might expect just one of the WIMPs to scatter off the atomic nuclei in your cells and deposit some meager amount of energy. To have any hope of detecting such events, physicists set their particle detectors to monitor large volumes of liquid or other material for long periods. Astronomers also look for bursts of radiation in the galaxy that mark the rare collision and annihilation of orbiting WIMPs. A third way to find WIMPs is to try to synthesize them in terrestrial experiments.

    以下的证据同样表明，弱相互作用重粒子是惰性的。通过一个简单的计算可以推算，当你开始阅读文章到现在，穿过你身体的粒子已经有近十亿，除非你非常幸运，否则没有一个会对你身体造成显著影响。一年到头，你也许可能被一颗弱相互作用重粒子从你细胞中把原子核打出来，然后把自己的一点能量埋进去。物理学家用粒子探测器长期监控大量的液体或其它材料，以检测这大海捞针般的偶发事件。天文学家同时也在寻找突发性星系辐射，因为这标志着发生了罕见的运动弱相互作用重粒子碰撞湮灭事件。寻找弱相互作用重粒子的第三种途径是在地面通过实验人工合成它们。

Out-Wimping the WIMPs

                     比弱相互作用还弱

Recent developments in particle physics have uncovered other theoretically plausible dark matter candidates. This work hints that the WIMP is just the tip of the iceberg. Lurking under the surface could be hidden worlds, complete with their own matter particles and forces.

    最近粒子物理界里的研究成果揭示了理论上暗物质的其他候选者。这些成就说明，弱相互作用重粒子只是冰山一角。尚未浮现的也许是一个隐秘的世界，它拥有自己的物质粒子和作用力。

One such development is the concept of particles even more wimpy than WIMPs. Theory suggests that WIMPs formed in the first nanosecond of cosmic history might have been unstable. Seconds to days later they could have decayed to particles that have a comparable mass but do not interact by the weak nuclear force; gravity is their only connection to the rest of the natural world. Physicists, tongue in cheek, call them super-WIMPs.

    一种发展是认为有些作用力粒子比弱相互作用重粒子还弱。理论认为弱相互作用重粒子在宇宙诞生的第一个纳秒内就形成了，并且极不稳定。数秒到数天之后它们就衰变成质量与之相当，但彼此作用力却不是弱核力的一些粒子。万有引力是它们与自然界联系的唯一桥梁。物理学家索性将它们称为超级弱相互作用重粒子。

The idea is that super-WIMPs constitute the dark matter of today's universe. Super-WIMPs would elude direct observational searches but might be inferred from the telltale imprint they would leave on the shapes of galaxies. When created, super-WIMPs would have been moving at a significant fraction of the speed of light. They would have taken time to come to rest, and galaxies could not have begun forming until then. This delay would have left less time for matter to accrete onto the centers of galaxies before cosmic expansion diluted it. The density at the center of dark matter halos should therefore reveal whether they are made of WIMPs or super-WIMPs; astronomers are now checking. In addition, the decay from WIMP to super-WIMP should have produced photons or electrons as a by-product, and these particles can smash into light nuclei and break them apart. There is some evidence that the universe has less lithium than expected, and the super-WIMP hypothesis is one way to explain the discrepancy.

    所以，有人认为超级弱相互作用重粒子是构成当今宇宙的暗物质。它们也许会躲过审慎的观察，但是我们却能从其留在宇宙中的蛛丝马迹推导得出。超级弱相互作用重粒子一旦产生，就将以接近光速的速度飞行，所以要想使它们停下来需要时间，在此之前星系不可能形成。这一延迟使得在宇宙发生膨胀把物质驱散之前，没有足够时间让它们来得及在星系中心积聚。暗物质光环中心的密度应能反映出它们究竟是由弱相互作用重粒子构成还是超级弱相互作用重粒子构成的，关于这一点，宇宙学家们也在寻找办法进行验证。而且，在弱相互作用重粒子衰变成超级弱相互作用重粒子过程中，应该产生光子和电子等副产品，并且这些粒子也会与质量较轻的原子核发生碰撞，将它们撞碎。有证据表明宇宙中锂元素的含量比预计要少，所以假设的超级弱相互作用重粒子可以作为一种方法解释这种差异。

Dark Forces, Hidden Worlds

                    暗作用力，隐性世界

Once one admits the possibility of hidden particles with properties that go beyond the standard WIMP scenario, it is natural to consider the full range of possibilities. Could there be a whole sector of hidden particles? Could there be a hidden world that is an exact copy of ours, containing hidden versions of electrons and protons, which combine to form hidden atoms and molecules, which combine to form hidden planets, hidden stars and even hidden people?

    一旦承认有可能存在性质超越标准弱相互作用重粒子的粒子，人们自然而然会联想还有没有其它可能。是否可能存在一大批隐藏粒子？是否可能存在一个隐性的世界，它完全是我们世界的翻版，隐含着各种版本的电子与质子，然后由它们构成隐性的原子和分子，再由这些原子和分子构成隐性的行星、恒星甚至人类？

The idea is truly tantalizing. Could it be that what we see as dark matter is really evidence for a hidden world that mirrors ours? And are hidden physicists and astronomers even now peering through their telescopes and wondering what their dark matter is, when in fact their dark matter is us?

    这一概念实在引人入胜。是否有可能我们所谓的暗物质其实就是我们的世界的隐性镜像？而隐性的物理学家和天文学家此时甚至正在用望远镜仰望太空，苦苦思索着他们看到的暗物质是什么，而事实上就是我们自己？

Unfortunately, basic observations indicate that hidden worlds cannot be an exact copy of our visible world. For one, dark matter is six times more abundant than normal matter. For another, if dark matter behaved like ordinary matter, halos would have flattened out to form disks like that of the Milky Way—with dramatic gravitational consequences that have not been seen. Last, the existence of hidden particles identical to ours would have affected cosmic expansion; compositional measurements rule that out. These considerations argue strongly against hidden people.

    不幸的是，基本的观测结果显示隐性的宇宙不可能完全是我们看得见的宇宙的翻版。原因之一，暗物质的总量是一般物质的六倍，原因之二，如果暗物质的行为与一般物质类似，光环将变平并最终演变成银河系一样的大碟——这是我们尚未观测到的巨大引力作用的结果。原因之三，如果有类似于我们世界的隐性粒子的存在，它们将影响宇宙膨胀，多种观测都证明这一点。基于以上考虑，我们能证明隐性人类不可能存在。

That said, the dark world might indeed be a complicated web of particles and forces. In one line of research, several investigators, including one of us (Feng) and Jason Kumar of the University of Hawaii at Manoa, have found that the same supersymmetric framework that leads to WIMPs allows for alternative scenarios that lack WIMPs but have multiple other types of particles. What is more, in many of these WIMP—less theories, these particles interact with one another through newly postulated dark forces. We found that such forces would alter the rate of particle creation and annihilation in the early universe, but again the numbers work out so that the right number of particles are left over to account for dark matter. These models predict that dark matter may be accompanied by a hidden weak force or, even more remarkably, a hidden version of electromagnetism, implying that dark matter may emit and reflect hidden light.

    综上所述，黑暗世界也许的确是由各种隐性粒子与作用力构成的一张复杂关系网。在一个研究领域里，有些研究者，包括我们的Feng和夏威夷大学Manoa校区的Jason Kumar博士，发现基于能推导弱相互作用重粒子存在的同一超对称理论框架，也能推导出不是由弱相互作用重粒子，而是由其它多种粒子构成的理论方案。此外，在这些与弱相互作用重粒子关系不大的理论中，这些粒子通过设想的新的暗作用力，彼此之间还可能互相影响。他们发现这些力还会改变早期宇宙粒子产生和湮灭的速率，不过计算结果同样表明剩余的粒子值与当今暗物质的量相当。这些模型推测暗物质伴随着一种隐性的弱作用力，甚至更奇怪的，一种隐性的电磁机制，表明暗物质会发出和反射隐性的光。

This "light" is, of course, invisible to us, and so the dark matter remains dark to our eyes. Still, new forces could have very significant effects. For example, they could cause clouds of dark particles to become distorted as they pass through one another. Astronomers have searched for this effect in the famous Bullet Cluster, which consists of two clusters of galaxies that have passed through each other. Observations, however, show that the brief co-mingling of clusters left the dark matter largely unperturbed, indicating that any dark forces could not be very strong (view Bullet Cluster Movie on next page).

    这种“光”，对于我们当然是不可见的，所以我们的眼睛对暗物质依然是视而不见。但是新的力却依然发挥非常大的影响。例如，当它们彼此穿越时，会使得暗粒子团发生扭曲变形。天文学家正在著名的“子弹星系团”上寻找这种效应，子弹星系团由两大星系组成，它们彼此穿越。然而观测结果显示星团的瞬间交合却没对暗物质造成丝毫影响，这意味着暗作用力不会是一种很强的作用力。

Silver Bullet

                          银子弹

Is there any direct evidence for dark matter—WIMP or Super—WIMP based? The Bullet Cluster may have provided an answer. This is a pair of galaxy clusters that have collided. The collision had little effect on the stars; they passed quietly by each other. However, huge clouds of interstellar gas rammed into one another at 6 million mph, heat up to 100 million degrees celsius and fired out X-rays—(the red areas in the photo at left). It is the blue areas that are the mystery; these were detected in 2006 because of the gravitational lensing of the background stars. Despite the enormous gravitational mass, there is no interaction with anything else happening in the collision—this is very probably dark matter: unreactive particles oblivious to the firestorm around them.

    有没有直接证据显示暗物质是由弱相互作用重粒子或者超级弱相互作用重粒子组成的呢？子弹星系团也许能为我们提供证据。它们是两大星团，彼此互相碰撞，并且碰撞对恒星几乎没有什么影响，它们依然静静的彼此穿越。然而，体积庞大的星云却以600万英里/小时的速度迎头相撞，从而使它们自身温度上升到1亿度，进而向外剧烈喷发X光（图中红色部分）。真正神奇的是蓝色部分，它们因背景恒星的引力透镜效应而于2006年被探测到。除了庞大的引力质量外，碰撞的过程没有其它相互作用——这也许就是暗物质：不活泼的粒子对身边的熊熊烈焰视而不见。

One Dark Thing to Another

                  从一种暗物质到另一种

    同样引人入胜的是暗物质有可能与暗能量相互影响。现存的许多理论都觉得它们并没多大联系，但也没有确凿的证据表明它们之间必然没有联系。物理学家们现在正在考虑暗物质与暗能量之间如何相互影响。一种愿望是将它们互相配对从而方便解释宇宙中的一些问题，如它们之间的巧合——它们的密度何以互相之间成比例？暗能量的密度大约是暗物质的三倍，但这个比例系数本应是1000或1百万。如果暗物质以某种方式激发了暗能量的存在，这种巧合才说得过去。

    将暗物质和暗能量互相配对也使得暗物质粒子之间能互相影响，而这些影响在一般粒子之间一般是不存在的。近来发展的模型允许并且有时候必须要让暗能量对暗物质施加一种作用力而对一般物质没有影响。在这种作用力影响下，暗物质会试图将自己与胶着在一起的一般物质拉开。2006年，加州理工大学的Marc Kamionkowski博士和多伦多加拿大理论天体研究所的Michael Kesden博士建议观察被自己强大的邻居肢解的白矮星来获取这种效应。例如，人马座白矮星正被银河系肢解，天文学家们认为它的暗物质与一般物质正涌入银河系。Kamionkowski 和 Kesden两位博士经过计算认为，如果暗物质之间的作用力比正常物质强或弱超过4%，这两种成分的分离过程就可以观测到。然而截至目前，数据还不能反映出这种情况。

    另一种设想是暗物质和暗能量之间的联系会改变宇宙的结构，因为宇宙结构严重依赖于它的各种构成成分，包括黑暗部分。一些研究者，包括我们的Trodden博士和来自康奈尔大学的Rachel Bean、Éanna Flanagan和Istvan Laszlo等学者在内，最近用这种有效的限制条件证明了很多模型不成立。

    尽管没有结论，但理论上的复杂黑暗世界是如此的不容置疑，以致于如果暗物质最终被发现仅仅是一堆毫无差别的弱相互作用重粒子，反而会令一些研究者感到吃惊。毕竟，基于对称原理，看得见的物质由多种多样的粒子组成，它们之间的作用力也有数种之多，没有证据表明，暗物质和暗能量会有所区别。