第一章：宇宙的组成：已知与未知的边界

当我们仰望星空，宇宙的浩瀚与神秘始终激发着人类的好奇心。然而，现代宇宙学通过一系列精密的观测告诉我们，我们所见的繁星、星系和星际气体——所有由原子构成的普通重子物质（baryonic matter）——仅仅构成了宇宙总质能密度的约5%。剩下的95%由两种神秘成分主导：暗物质（dark matter）约占27%，暗能量（dark energy）约占68%。这一惊人结论并非凭空猜测，而是基于宇宙微波背景辐射（Cosmic Microwave Background, CMB）各向异性、重子声波振荡（Baryon Acoustic Oscillations, BAO）以及Ia型超新星（Type Ia supernovae）等多重独立观测的交叉验证。这些方法如同从不同角度拍摄同一张宇宙全景图，却得出了惊人一致的结论，共同支撑起当今标准宇宙学模型——ΛCDM模型（Lambda Cold Dark Matter model）。然而，这95%的“暗宇宙”其物理本质至今仍是物理学最深刻的谜团。它们如何影响宇宙的几何结构、星系形成以及加速膨胀？本书将逐一深入探讨这些未知世界。本章首先梳理这些关键数据的来源与意义，揭示“95%未知”这一结论的科学依据，为后续章节奠定坚实的观测基础。

When we look up at the starry sky, the vastness and mystery of the universe have always sparked human curiosity. However, modern cosmology tells us that the stars, galaxies, and interstellar gas we see—all the ordinary baryonic matter made of atoms—constitute only about 5% of the universe's total mass-energy density. The remaining 95% is dominated by two mysterious components: dark matter, accounting for approximately 27%, and dark energy, making up about 68%. This astonishing conclusion is not a mere speculation but is based on cross-validation from multiple independent observational methods, including the anisotropies of the Cosmic Microwave Background (CMB), Baryon Acoustic Oscillations (BAO), and Type Ia supernovae. These methods, like taking a panoramic picture of the cosmos from different angles, yield remarkably consistent results, collectively supporting the current standard cosmological model—the ΛCDM model (Lambda Cold Dark Matter model). Yet, the physical nature of this "dark universe," comprising 95% of the cosmos, remains one of the deepest mysteries in physics. How do they influence the geometry of the universe, the formation of galaxies, and the accelerating expansion? This book will delve into these invisible worlds one by one. This chapter first sorts out the sources and significance of these key data, revealing the scientific basis for the "95% unknown" conclusion, and lays a solid observational foundation for the subsequent chapters.

宇宙质能密度的精确测量：从猜想到数据驱动

20世纪初，爱因斯坦的广义相对论为描述宇宙整体动力学提供了理论基础。然而，直到20世纪末，宇宙学家们还主要依靠理论模型和有限的观测数据来推测宇宙的组成。当时普遍认为，宇宙中的物质几乎全部是可见的恒星和气体。但随着观测技术的飞跃，特别是空间望远镜和大型巡天项目的开展，我们进入了“精确宇宙学”时代。如今，宇宙的总质能密度（total mass-energy density）可以被精确测量，其参数通常用密度参数Ω表示，其中Ω_total = 1 对应平坦宇宙。通过组合多种观测手段，我们得到了以下标准值（基于普朗克2018年数据）：

• 普通重子物质密度参数：$\Omega_b \approx 0.0486 \pm 0.0006$
• 暗物质密度参数：$\Omega_{dm} \approx 0.2589 \pm 0.0057$
• 暗能量密度参数：$\Omega_\Lambda \approx 0.6925 \pm 0.0057$

这些数值加起来约为1，表明宇宙在几何上是平坦的，符合ΛCDM模型的预言。值得注意的是，暗能量通常用宇宙学常数Λ表示，其状态方程为$w = -1$，即压强为负，导致宇宙加速膨胀。

The precise measurement of the universe's mass-energy density: from speculation to data-driven cosmology. In the early 20th century, Einstein's general relativity provided the theoretical foundation for describing the universe's overall dynamics. However, until the late 20th century, cosmologists largely relied on theoretical models and limited observational data to speculate about the universe's composition. It was commonly believed that the matter in the cosmos was almost entirely visible stars and gas. But with the leap in observational technology, especially space telescopes and large-scale survey projects, we entered the era of "precision cosmology." Today, the total mass-energy density of the universe can be measured precisely, with its parameters typically expressed as density parameters Ω, where Ω_total = 1 corresponds to a flat universe. By combining multiple observational methods, we obtain the following standard values (based on Planck 2018 data):

• Ordinary baryonic matter density parameter: $\Omega_b \approx 0.0486 \pm 0.0006$
• Dark matter density parameter: $\Omega_{dm} \approx 0.2589 \pm 0.0057$
• Dark energy density parameter: $\Omega_\Lambda \approx 0.6925 \pm 0.0057$

These numbers sum to approximately 1, indicating that the universe is geometrically flat, as predicted by the ΛCDM model. It is worth noting that dark energy is often represented by the cosmological constant Λ, with an equation of state $w = -1$, meaning negative pressure, which causes the accelerating expansion of the universe.

第一支柱：宇宙微波背景辐射（CMB）与普朗克卫星

宇宙微波背景辐射是宇宙大爆炸后约38万年时释放的光子，如今已红移至微波波段，均匀充满整个天空。CMB的温度涨落（各向异性）携带了早期宇宙的丰富信息，包括物质和能量的组成。通过测量CMB角功率谱（angular power spectrum），特别是第一声学峰的位置和高度，可以精确推断宇宙的几何形状和组分。普朗克卫星（Planck satellite）是欧洲空间局的最先进CMB观测任务，其2018年发布的数据将测量精度推向前所未有的水平。CMB功率谱中的峰值位置对应于宇宙的声学尺度（sound horizon），而峰值之间的相对高度则敏感于重子物质和暗物质的比例。具体而言，重子物质会增加第一峰的高度，而暗物质则影响后续峰值的振幅。通过拟合理论模型到观测数据，普朗克团队得到了上述密度参数。下图示意了CMB功率谱与组分的关系（此处为文字描述，实际书籍中将包含图表）：横轴是多重极矩ℓ，纵轴是温度涨落的功率。峰值位置在ℓ≈220处，表明宇宙平坦；第一峰特别高，表明存在重子物质；而第三峰相对高度则揭示了暗物质的存在。

The First Pillar: Cosmic Microwave Background (CMB) and the Planck Satellite. The Cosmic Microwave Background is the photons released about 380,000 years after the Big Bang, now redshifted into the microwave band and uniformly filling the entire sky. The temperature fluctuations (anisotropies) in the CMB carry rich information about the early universe, including its composition of matter and energy. By measuring the CMB angular power spectrum, especially the position and height of the first acoustic peak, one can precisely infer the geometry and components of the universe. The Planck satellite, the European Space Agency's most advanced CMB observation mission, pushed measurement precision to unprecedented levels with its 2018 data release. The positions of the peaks in the CMB power spectrum correspond to the sound horizon of the universe, while the relative heights between peaks are sensitive to the proportions of baryonic matter and dark matter. Specifically, baryonic matter increases the height of the first peak, while dark matter affects the amplitudes of subsequent peaks. By fitting theoretical models to the observational data, the Planck team derived the density parameters mentioned above. The diagram below illustrates the relationship between the CMB power spectrum and components (described in text; actual book will include charts): the horizontal axis is the multipole moment ℓ, the vertical axis is the power of temperature fluctuations. The peak position at ℓ≈220 indicates a flat universe; the particularly high first peak indicates the presence of baryonic matter; and the relative height of the third peak reveals the existence of dark matter.

第二支柱：重子声波振荡（BAO）——宇宙的标尺

重子声波振荡是早期宇宙中重子-光子流体中的声波传播留下的特征尺度。在大爆炸后约38万年，光子与重子脱耦，声波停止，留下了一个固定的“标准尺”——声学尺度，约为4.9亿光年。这个尺度被印在了早期宇宙的物质分布中，并在后来的星系分布中体现出来。通过测量星系巡天中星系对的关联函数，可以探测到在约1.5亿光年（共动距离）处的隆起，即BAO信号。不同红移处的BAO测量给出了宇宙膨胀历史（即哈勃参数H(z)和角直径距离D_A(z)）的直接约束。例如，斯隆数字巡天（Sloan Digital Sky Survey, SDSS）及其后续项目（如eBOSS）提供了大量星系和类星体的分布数据，精确测量了多个红移点的BAO尺度。BAO测量独立于CMB，但两者结合可以极大地限制宇宙学参数。BAO结果与CMB高度一致，进一步证实了暗物质和暗能量的存在及其比例。

The Second Pillar: Baryon Acoustic Oscillations (BAO)—The Universe's Ruler. Baryon Acoustic Oscillations are the characteristic scale left by sound waves propagating in the baryon-photon fluid of the early universe. About 380,000 years after the Big Bang, photons decoupled from baryons, and sound waves ceased, leaving a fixed "standard ruler"—the acoustic scale—of about 490 million light-years. This scale is imprinted on the matter distribution of the early universe and is later reflected in the distribution of galaxies. By measuring the correlation function of galaxy pairs in galaxy surveys, one can detect a bump at about 150 million light-years (comoving distance), which is the BAO signal. BAO measurements at different redshifts provide direct constraints on the expansion history of the universe (i.e., the Hubble parameter H(z) and angular diameter distance D_A(z)). For example, the Sloan Digital Sky Survey (SDSS) and its follow-up projects (like eBOSS) have provided extensive distribution data of galaxies and quasars, precisely measuring the BAO scale at multiple redshift points. BAO measurements are independent of the CMB, but combining them greatly constrains cosmological parameters. BAO results are highly consistent with the CMB, further confirming the existence and proportions of dark matter and dark energy.

#