导读
当我们向外看时，我们的宇宙视界是一个球形边界，由过去138亿年来光线传播的距离所界定。我曾在2014年发表的题为《宇宙的宜居时期》（见此处）的论文中讨论过这个问题，并在后续的两篇论文中进行了阐述（见此处和此处）。
The Universe started with nearly the same initial conditions in all directions and distances. As a result, cosmic time was nearly synchronized everywhere we look. This means that the Big-Bang instant in our past, when the density of matter and radiation diverged, occurred at the same time everywhere we look. As a result, light traversed the same distance towards us since that singular beginning from all directions. As we look out, our cosmic horizon is a spherical boundary, defined by the distance that light traversed over the past 13.8 billion years. That the initial conditions were similar out to much greater distances is a moot point because we cannot access these regions.
The Universe was transparent to gravitational waves since the earliest Planck time of its expansion history. But the thermal background of gravitons is extremely difficult to detect, as explained in my paper with Sunny Vagnozzi here.
The hot cosmic gas cooled and rarefied enough to become transparent to light about 400,000 years after the Big-Bang. The transition to electromagnetic transparency occurred as soon as free electrons were cold enough to combine with protons and make hydrogen atoms. We observe the resulting cosmic microwave background photosphere from the inside in the form of a spherical last-scattering surface surrounding us. This inside-out orientation is geometrically opposite to the outside-in orientation of the spherical photosphere of the Sun.
The light emitted by sources farther away from our location must have propagated for a longer travel time in order to reach us today. This means that we see earlier images of the Universe, the farther we look. Observational cosmology is like an archeological dig in which we uncover layers that are progressively more ancient as we dig deeper. The benefit of the finite speed of light is that we can unravel cosmic history by surveying the observable spherical volume of the Universe out to our cosmic horizon. This is possible just because the Universe had nearly the same initial conditions everywhere.
Over time, the small perturbations in the density of matter grew by gravity and ended up as collapsed objects on small spatial scales. Gravity is responsible for making the Milky-Way, inside of which stars like the Sun condensed, next to which planets like the Earth formed. About 15 million years after the Big Bang, the cosmic background radiation had room temperature. If Earth-like planets had existed at that time, they would have not needed to be close to stars in order for their surface to be warmed to habitable conditions. I discussed this in 2014 in a paper titled “The Habitable Epoch of the Universe” here, and in two follow-up papers here and here.
The gravitational growth of cosmic structures stopped in the second half of cosmic history, owing to the accelerated expansion of the Universe — triggered by dark energy. The largest cosmic structures to ever collapse are clusters of galaxies with a quadrillion times the mass of the Sun. We will never see more massive superclusters collapse. They are being torn apart by the accelerated cosmic expansion.
The first stars formed 100 million years later and their ultraviolet radiation started to break the hydrogen atoms into constituent electrons and protons. This so-called epoch of reionization transformed cosmic hydrogen back to a soup of free electrons and protons within a billion years after the Big Bang. The first stars represent our cosmic roots and the scientific version of the biblical phrase “Let there be light!”
The first two decades of my career in astrophysics (1993–2013) were dedicated to theoretically predicting the properties of the first stars and galaxies. These predictions were summarized in two textbooks, titled: “How Did the First Stars and Galaxies Form?” here and “The First Galaxies in the Universe” here. By now, the Webb telescope detected the first generation of stars and galaxies consistently with these expectations, as reviewed most recently here.
The latest Webb data shows that the star formation rate per unit cosmic volume grew steadily by orders of magnitude in the period of 0.1–1 billion years after the Big Bang. The formation of the first stars marks the transition of the Universe into complexity from the simple initial conditions that can be summarized on a single sheet of paper (in terms of the matter budget [dark matter and ordinary matter] and the statistics of the primordial density fluctuations). The first stars also started the production of Carbon, Oxygen, Nitrogen, Phosphorus and Sulphur, elements that are essential for the chemistry of life-as-we-know-it. The early Universe spent only a few minutes at the temperature suitable for nuclear fusion and this short time only resulted in light elements, like hydrogen and helium with a sprinkle of lithium.
The first stars were short-lived. After tens of millions of years, most of them exploded. The inability of hydrogen and helium to cool the primordial gas effectively led to the formation of massive stars which resulted in gamma-ray bursts when some of them collapsed to black holes. Also, mergers of their black hole remnants could be detected by future gravitational-wave observatories. Finally, the Webb telescope and its followers could keep pushing the earliest stellar explosions and galaxies on record, as we probe deeper into our cosmic archaeological dig.
In this essay, I featured eight amazingly beautiful watercolors from a series created and sent to me by the celebrated artist, Greg Wyatt.
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https://avi-loeb.medium.com/https://www ... sorAviLoeb
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宇宙起源于几乎相同的初始条件，无论在哪个方向或距离上。因此，宇宙时间在我们所观察的任何地方都几乎是同步的。这意味着，在我们过去的宇宙大爆炸瞬间，物质和辐射密度开始发散的那一刻，在我们所观察的任何地方都是同时发生的。因此，自宇宙诞生之初以来，光向我们传播的距离始终相同。从各个方向望去。当我们向外看时，我们的宇宙视界是一个球形边界，由过去138亿年来光线传播的距离所界定。至于更远距离的初始条件是否相似，则无关紧要，因为我们无法到达这些区域。
自宇宙膨胀史最早的普朗克时期起，宇宙对引力波就是透明的。但是……正如我和 Sunny Vagnozzi 在这篇论文中所解释的那样，引力子的热背景极难探测。
宇宙大爆炸后约40万年，炽热的宇宙气体冷却并变得稀薄，最终对光透明。这种向电磁透明的转变发生在自由电子冷却到足以与质子结合形成氢原子之后。我们观测到……由此产生的宇宙微波背景辐射光球层，从内部以球形末次散射面的形式环绕着我们。这种由内而外的排列方式，在几何上与太阳球形光球层的由外而内的排列方式截然相反。
距离我们更远的光源发出的光，必须经过更长的传播时间才能到达我们这里。我们看到宇宙的早期图像，我们看得越远。观测宇宙学就像一个考古挖掘，随着我们深入挖掘，我们发现越来越古老的层次。光速有限的好处在于，我们可以通过测量宇宙的可观测球形体积来解开宇宙历史，直到我们的宇宙视界。这是可能的，因为宇宙中几乎所有地方的初始条件都相同。
随着时间的推移，物质密度的微小扰动在引力的作用下不断增大，最终在小的空间尺度上坍缩成物体。引力造就了银河系，银河系内部凝聚了像太阳这样的恒星，而像地球这样的行星则在银河系附近形成。大约在大爆炸后1500万年，宇宙背景……圆形辐射体的温度与室温相同。如果当时存在类似地球的行星，它们无需靠近恒星，表面温度也能达到适宜居住的程度。我曾在2014年发表的题为《宇宙的宜居时期》（见此处）的论文中讨论过这个问题，并在后续的两篇论文中进行了阐述（见此处和此处）。
宇宙结构的引力增长在第二个半世纪停止了。在宇宙历史的早期阶段，由于暗能量引发的宇宙加速膨胀，宇宙结构发生了巨大的变化。迄今为止坍缩的最大宇宙结构是质量是太阳一千万亿倍的星系团。我们再也不会看到质量更大的超星系团坍缩了。它们正被宇宙加速膨胀撕裂。
第一批恒星在1亿年后形成，它们的超高密度演化紫色辐射开始将氢原子分解成组成它们的电子和质子。这个所谓的再电离时期，在大爆炸后的十亿年内，将宇宙氢重新转化为自由电子和质子的混合体。第一批恒星代表了我们宇宙的起源，也是圣经中“要有光！”这句话的科学版本。
我职业生涯的前二十年天体物理学（ 1993–2013 ）致力于从理论上预测第一批恒星和星系的性质。这些预测总结在两本教科书中，标题分别为： “第一批恒星和星系是如何形成的？”和“宇宙中的第一批星系”。到目前为止，韦伯望远镜探测到的第一代恒星和星系符合这些预期，最近一次是在这里。
韦伯望远镜的最新数据显示，宇宙大爆炸后1亿至10亿年间，单位宇宙体积的恒星形成率稳步增长，数量级呈指数级上升。第一批恒星的形成标志着宇宙从最初可以用一张纸概括的简单初始状态（以物质预算[d]表示）过渡到复杂的状态。（方舟物质和普通物质）以及原始密度涨落的统计数据）。第一批恒星也开始产生碳、氧、氮、磷和硫，这些元素对我们所知的生命化学至关重要。早期宇宙仅在适宜核聚变的温度下停留了几分钟，而这短暂的时间只产生了轻元素。就像氢气和氦气，再加上一点锂。
第一批恒星寿命很短。数千万年后，它们中的大多数都发生了爆炸。氢和氦无法有效地冷却原始气体，导致了大质量恒星的形成。当其中一些恒星坍缩成黑洞时，就会产生伽马射线暴。此外，人们还可以探测到黑洞残骸的合并现象。未来的引力波天文台将最终实现这一目标。韦伯望远镜及其后续望远镜有望继续记录最早的恒星爆炸和星系，让我们能够更深入地探索宇宙考古。
在这篇文章中，我介绍了著名艺术家格雷格·怀亚特创作并寄给我的一系列精美绝伦的水彩画中的八幅。
专业网站：
https://lweb.cfa.harvard.edu/~loeb/
https://avi-loeb.medium.com/https://www ... sorAviLoeb
https://open.spotify.com/show/1zhndXkvS ... rofAviLoeb
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原文链接：https://avi-loeb.medium.com/our-cosmic- ... 1782aa8e82
作者：Avi Loeb（哈佛大学天文系教授）
