[论文解读] Measuring the expansion history of the Universe with cosmic chronometers

M. Moresco|arXiv (Cornell University)|Dec 2, 2024

Cosmology and Gravitation Theories被引用 5

一句话总结

本论文将宇宙时钟作为一种与宇宙学无关的方法，通过对大质量、被动星系的差分年龄测定来测量 H(z)，并详细介绍选择标准、年龄测定方法，以及当前/未来的精度。

ABSTRACT

As revealed by Hubble in 1928, our Universe is expanding. This discovery was fundamental to widening our horizons and our conception of space, and since then determining the rate at which our Universe is expanding has become one of the crucial measurements in cosmology. At the beginning of this century, these measurements revealed the unexpected behavior that this expansion is accelerating and allowed us to have a first glimpse of the dark components that constitute $\sim$95\% of our Universe. Cosmic chronometers represent a novel technique to obtain a cosmology-independent determination of the expansion of the Universe, based on the differential age dating of a population of very massive and passively evolving galaxies. Currently, with this new cosmological probe it is possible to constrain the Hubble parameter with an accuracy of around 5\% at $z\sim0.5$ up to 10-20\% at $z\sim2$. In this Chapter, the cosmic chronometers approach is presented, describing the method and how an optimal sample can be selected; it is then discussed how the most recent measurements of the expansion history of the Universe have been obtained with this approach, as well as the cosmological constraints that can be derived. Particular attention will be given to the systematics involved in this approach and the treatment to properly take them into account. We conclude by presenting forecasts that show how future spectroscopic surveys will significantly boost the accuracy of this method and open the possibility to a percent determination of the Hubble constant, making cosmic chronometers a powerful independent tool to derive information on the expansion history of the Universe.

研究动机与目标

• 动机：需要独立探针来约束宇宙膨胀历史，超越 CMB/超新星/BAO，并解决哈勃不张力。
• 介绍宇宙时钟作为一个差分年龄测定方法，用以在尽量少的宇宙学假设下测量 H(z)。
• 描述筛选标准，以获得一组纯净的大质量、被动演化星系（CCs）的样本。
• 介绍三种主要年龄测定方法（全谱拟合、Lick 指数和 D4000）及其如何给出差分年龄。
• 讨论系统误差、模型依赖性以及对未来 surveys 改进的预测。

提出的方法

• 通过差分年龄推导 H(z)，公式为 H(z) = -(1/(1+z)) dz/dt（式3）。
• 使用极大质量、被动演化星系的最佳宇宙时钟样本来追踪差分年龄。
• 采用三种年龄测定方法：(i) 使用包含星族合成谱（SPS）模板和代码（如 pPXF、BEAGLE）的全谱拟合（FSF）；(ii) 将 Lick 指标分析与高分辨率 SPS 模型进行比较；(iii) 使用经标定的关系 D4000 = A(Z,SFH)·年龄 + B 及其差分形式（式5）对 D4000 断裂进行分析。
• 结合多准则筛选（形态、颜色、sSFR、发射线、以及高质量质量/速度分散）以最大化样本纯度。

Figure 1: Stacked spectrum of a sample of massive and passively evolving galaxies, obtained from the combination of $\sim 10^{5}$ spectra from the Sloan Digital Sky Survey. It is evident the red continuum, as well as the typical absorption features characterizing the spectra of passive galaxies (sho

实验结果

研究问题

• RQ1 宇宙时钟能否提供对扩张历史 H(z) 的独立于宇宙学的测量？
• RQ2 获取一组纯净的大质量、被动星系以用于差分 aging 的最佳多准则组合是什么？
• RQ3 FSF、Lick 指标和 D4000 方法在推导差分年龄和得到的 H(z) 约束方面有何异同？
• RQ4 CC 方法中的系统性不确定性和模型依赖性是什么，如何缓解？
• RQ5 未来光谱 surveys 提供的展望是否可能实现对哈勃常数的百分比级别测定？

主要发现

• 宇宙时钟在 z ~ 0.5 时可将 H(z) 的约束精度提升至约 5%，在 z ~ 2 时达到 10–20%（当前状态）。
• 通过结合形态、颜色（NUVrJ/UVJ/NUVrK）、sSFR 截止、光谱学（发射线限制）以及高质量质量/速度分散阈值，可以获得最佳的宇宙时钟纯样本。
• 基于 D4000 的差分年龄测量通过 dz/dD4000（式5）给出与宇宙学无关的 H(z)。
• 全谱拟合和 Lick 指标提供可替代的、兼容的差分年龄，在同一样本上应用时可得到一致的 H(z) 结果。
• 一个与宇宙学无关的年龄估计对避免循环约束至关重要，并能稳健地产生 H(z)。
• 预测显示未来的 surveys 将显著提升精度，可能达到哈勃常数的百分比级别确定。

Figure 2: Cosmic chronometers selection workflow. In the plots are shown the stacked spectra (from moresco2013 ) of differently selected samples of CC, as discussed in Sect. 3 ; an arbitrary offset has been applied to each spectrum to improve their visibility. In the left panel are shown the spectra