从星系形成到宇宙起源，该空间望远镜所获取的图像正为宇宙学家提供探索新数据及构建新理论的基础。

From how galaxies form to the beginnings of the universe, the images from the space telescope are giving cosmologists new data to explore—and theories to build out.

当夏洛特·梅森（Charlotte Mason）思考宇宙奥秘时，她喜欢涂鸦。“我是个非常视觉化的人，”她说，“我通常会画很多图，试图理解正在发生什么。”近期理论提出，那些微小的红色光点可能是被浓密气体包裹的黑洞，或许代表了一种全新的天体——“黑洞星”（black hole star），其致密气体包层会像恒星大气一样发光。但梅森认为，这些宇宙谜题背后或许还有更多玄机。她与同事近期分析了其中一个红色光点所发出的光谱。若浓密云团模型成立，则部分光线在穿过气体时应发生改变——但他们并未观测到这种现象。“那现在该怎么办？从头再来。但如果我把气体设想为块状结构，”梅森边说边在围绕黑洞的云团示意图上画出若干空洞，“我应该就能得到更接近实际观测结果的信号。” 全球各地，像梅森这样的研究人员正热切地拼合詹姆斯·韦布空间望远镜（JWST）对远古宇宙的零星观测，以构建一幅更清晰的宇宙起源图景。正如穿越数十亿光年抵达我们的光子一样，新的碎片正不断落位。 得益于JWST，黑洞的故事变得愈发复杂：它持续发现一批古老黑洞，其质量之大已超出既有理论的解释能力——远远超出。科学家主要关注影响黑洞质量的两个关键因素：黑洞“种子”在诞生之初的质量，以及此后这些种子的成长速率。然而，无论是假设黑洞初始质量就足够大，还是假设其成长速度极快，都难以解释它们如何在宇宙早期便达到太阳质量的十亿倍。 在当今宇宙中，黑洞形成于大质量恒星核心燃料耗尽后发生的坍缩。格林（Greene）指出，由于第一代恒星质量极大，它们可能留下质量高达约100倍太阳质量的黑洞种子。“我们知道这种情况确实会发生，但要让它们如此迅速地增长至十亿倍太阳质量，实在极其困难，”她说，“你必须近乎强行喂养它们。” 不过，近期计算机模拟暗示，黑洞或许存在某种“后门”机制。若吸积盘以恰好的方式膨胀，入射气体便可能压倒辐射压。此类“超爱丁顿”（super-Eddington）吸积将导致气体以异常高速率涌入黑洞。即便如此，天文学家尚不清楚早期宇宙中是否存在足够多的气体来催生最大规模的黑洞。 一些研究者认为，远古时期高密度的星团可能产生了大量黑洞种子，并迅速合并。另一些人则推测，超大质量黑洞根本并非起源于恒星。在此情形下，巨大的气体云团会直接坍缩为黑洞。这种“直接坍缩”（direct collapse）机制可形成质量达太阳10,000倍的种子黑洞。“人们在计算机中尝试模拟这一过程时，确实能生成这类直接坍缩黑洞，却无法产生足够数量来解释我们所观测到的所有黑洞，”格林表示。 一幅模拟图展示了大爆炸后最初5.5亿年内一个星系的形成过程。从左至右各面板分别代表暗物质、气体与恒星。“我们显然尚未完全理解黑洞增长方式的差异，”格林说，“因此，目前最令人兴奋的工作，便是从物理本质上弄清其中究竟有何不同？” 与看似过大的早期黑洞类似，JWST发现的许多早期星系也显得过于明亮。为探明原因，研究人员正在重新评估星系形成理论。天文学家通常用红移（redshift）来描述这些事件发生的时间，即早期天体发出的光因宇宙膨胀而被拉伸的程度。“大约在红移11（对应大爆炸后4.2亿年）时，恒星形成率开始显著上升，”她继续道，“到红移9（5.5亿年）时，我们便形成了一个典型的星系。”屏幕上显示的星系代表早期星系群体，而迄今JWST发现的最古老星系仅存在于大爆炸后2.8亿年。 该望远镜令人困惑地发现了一批明亮且极为古老的星系，起初促使部分科学家怀疑我们对基础宇宙学的理解——即支配早期宇宙中能量与物质行为的基本定律——可能存在缺陷。但在对这些原始天体进行了数年研究后，理论学家目前已提出若干模型，用以解释其亮度与丰度。“我们几乎已从‘早期星系太多’，转向‘解释它们的理论太多’，”索默维尔（Somerville）向在场听众坦言。 或许第一批星系将气体转化为恒星的效率，远高于此前预期；又或许它们经历了由湍流环境驱动的周期性恒星爆发；抑或早期恒星形成区倾向于优先孕育大质量、极高亮度的恒星。许多天体物理学家认为，上述因素乃至其他未知机制的某种组合，共同促成了这些星系的演化。 随着模型不断完善，JWST正记录下越来越多的星系。通过将其在早期宇宙中的观测结果与旨在解释这些现象的模拟进行比对，研究人员正逐步逼近宇宙黎明真实本质的答案。 一个引人入胜的新线索最近来自JWST的中红外仪器（MIRI）——一台超低温设备，可将遥远天体的光分解为光谱。MIRI揭示，早期星系并不具备科学家此前假定的统一特征。“最大的意外在于，我们在早期宇宙中观测到的星系性质竟如此多样，”阿泰克（Atek）指出，“你原本预期它们看起来会很相似。” 这种多样性或许表明恒星形成是以爆发形式发生的：星系经历恒星聚变、爆发并抛射气体云的循环，从而暂时中止恒星诞生；随后气体再度聚集，触发新一轮恒星诞生浪潮。 另一条线索来自一组氮元素丰度异常偏高的星系。

When Charlotte Mason ponders cosmic mysteries, she likes to doodle. “I am quite a visual person,” she said. “I usually draw a lot of pictures trying to understand what’s going on.” Recent ideas suggest that little red dots could be black holes cocooned in thick gas, possibly representing a completely new type of object called a black hole star, in which the tight shroud of gas emits light like a stellar atmosphere. But Mason thinks there may be more to these cosmic enigmas. She and colleagues recently analyzed the spectrum of light emitted by one little red dot. If the dense-cloud picture is correct, then some of the light should have been altered from passing through the gas—but that’s not what they saw. “Now what do I do? Start again. But now if I make my gas clumpy,” Mason said, drawing a new diagram with holes in the clouds surrounding the black hole, “I should be able to get [a signal] that looks closer.” All around the world, researchers like Mason are eagerly piecing together JWST’s glimpses of the ancient cosmos to create a clearer picture of our universe’s beginnings. And like the photons that travel billions of light-years to reach us, new fragments are constantly falling into place. The story of black holes has become more complicated thanks to JWST, which keeps spotting ancient black holes that are too big to explain with established theories—much too big. Scientists look at two key factors that influence a black hole’s size: how massive a black hole “seed” was when it originated, and how quickly these seeds grew after that. But it’s hard to explain how black holes either formed already big enough or grew fast enough to reach a billion times the mass of the sun in early cosmic times. In the modern universe, black holes form when the core of a massive star runs out of fuel and collapses. Considering the first stars were quite massive, they could have left behind black hole seeds of up to about 100 solar masses, Greene said. “We know that happens, but it’s really, really hard to get them to a billion so quickly,” she said. “You really have to force-feed them.” But recent computer simulations suggest that black holes might have something of a back door. If the accretion disk puffs up in just the right way, the incoming gas can overwhelm the radiation pressure. Such “super-Eddington” accretion would lead to gas funneling in at extraordinary rates. Even so, astronomers don’t know if there would have been enough gas around to produce the biggest black holes. Some researchers think that ancient, dense star clusters may have created lots of black hole seeds that rapidly merged. Or perhaps supermassive black holes never started as stars at all. In this case, colossal clouds of gas would have plunged directly into a black hole. This “direct collapse” mechanism can form a seed some 10,000 times the mass of the sun. “When people try to do this in a computer, they can make these direct-collapse black holes, but they can’t make enough of them to explain all the black holes that we see,” Greene said. A simulation of a galaxy forming in the first 550 million years after the Big Bang. The panels from left to right represent dark matter, gas, and stars. “There’s clearly differences in how the black holes are growing that we don’t fully understand yet,” Greene said. “So for me, the most exciting thing to do right now is try to understand, physically, what’s different?” Like early black holes that seem too big, many early galaxies spotted by JWST seem too bright. To figure out why, researchers are reassessing their ideas about how galaxies form. Astronomers generally describe the timing of these events in terms of redshift, or how much the light from early objects has been stretched by cosmic expansion. “By about a redshift of 11 [420 million years], the star formation rate starts to really pick up,” she continued. “At redshift 9 [550 million years], we make a nice galaxy.” The galaxy on the screen represented an early population, but the most ancient galaxy discovered by JWST so far existed only 280 million years after the Big Bang. The telescope’s bewildering discovery of bright, early galaxies initially led some scientists to suggest that our understanding of fundamental cosmology, the laws that govern the behavior of energy and matter in the early universe, may be flawed. But after a few years of studying these primitive objects, theorists now have several models to explain their brightness and abundance. “We almost have gone from having too many early galaxies to having too many theories to explain them,” Somerville told the room. Perhaps the first galaxies converted gas to stars more efficiently than previously thought. Or they experienced periodic bursts of star formation driven by turbulent conditions. Or maybe early star-forming regions preferentially created massive, extremely bright stars. Many astrophysicists think some combination of these factors, and perhaps others, contributed to the galaxies’ development. As these models improve, JWST is documenting more and more galaxies. By comparing what it sees in the early universe to simulations that attempt to explain why, researchers are inching closer to uncovering the true nature of cosmic dawn. An intriguing clue has recently emerged from JWST’s Mid-Infrared Instrument (MIRI), a supercooled device that can split apart the light of distant objects. MIRI has revealed that early galaxies do not have the same traits, as scientists assumed. “The main surprise is the diversity of the properties of galaxies we are seeing at early epochs,” Atek said. “You’re expecting that they would look the same.” This diversity may be an indication of star formation that occurred in bursts, as galaxies cycled through periods of fusing stars that exploded and expelled gas clouds, halting the creation of stars, only for the gas to gather again and trigger a new wave of stellar birth. A further clue comes from a group of galaxies with an overabundance of nitrogen. The presence of the element suggests that there may have been a lot of particularly massive stars in the early universe. In simulations, these massive stars generate an excess of nitrogen before exploding in supernovas and scattering the element across their host galaxies. Someday, researchers may uncover the full picture of galactic formation. Until then, they’ll continue sifting through the traces in new observations and simulations. Once the astral lights switched on, the universe transformed. Radiation from early galaxies and black holes ionized a sea of neutral hydrogen gas, carving out immense bubbles amid the cosmic haze. Researchers call this period reionization, as it was the second time the universe was ionized. It marks the end of the cosmic dark age, when the foggy abyss was devoid of stars. In many ways, those first stars are the mothers of the universe. “We’re looking back at what created us,” said Lise Christensen, an astrophysicist with the Cosmic Dawn Center. Though it’s a mournful rumination on existence—the universe as “a foul and pestilent congregation of vapors,” humanity as the “quintessence of dust”—we now understand that Hamlet’s description is more scientifically accurate than Shakespeare could have known. We are in fact made of elements forged in stars and ejected into the void as gas and dust. Unlike Hamlet wallowing in Elsinore, however, scientists who study the origins of the universe are exhilarated by these cosmic beginnings. Original story reprinted with permission from Quanta Magazine, an editorially independent publication of the Simons Foundation whose mission is to enhance public understanding of science by covering research developments and trends in mathematics and the physical and life sciences.