Probing the Cosmic Dawn and Epoch of Re-ionization with the REACH experiment

宇宙是如何在大爆炸后从一个几乎空无一物的体积转变成一个由恒星、星系和其他天体组成的复杂王国的？这个未知的过程发生在所谓的宇宙黎明和再电离时代。21厘米射电宇宙学是一个研究领域，有望通过研究宇宙中最丰富的元素——氢发出的射电信号来解开婴儿宇宙的秘密。我请求资助，组建一个研究小组，利用我构思和领导的宇宙氢分析无线电实验（REACH），专注于探测和研究原子氢发出的难以捉摸的21厘米信号。无线电干涉仪已经对21厘米的功率谱设定了上限，而这是单天线实验，测量21厘米的信号在天空中所有方向的平均值，这些有希望取得更早的突破。EDGES实验在2018年震撼了这个领域，报告了一个比预期深两倍的宇宙信号，需要解释奇异的物理学。由于担心数据分析和硬件系统信号的潜在影响，包括最近SARAS3实验的测量结果与EDGES发现不一致，这一观点受到了几个团体的质疑。仪器系统的污染是“第一代”21厘米射电望远镜的主要瓶颈。为了解决这些问题，在过去的5年里出现了一种新的实验方法，催生了第二代实验。REACH是引领新一波仪器的天秤实验，其重点已转移到数据驱动的硬件和算法设计，重点是仪器系统信号的检测和隔离。为此，REACH使用全贝叶斯数据管道将仪器模型与天空信号模型联合拟合。在这个项目中，我将带领团队对天空平均21厘米线进行第一次自信的检测和研究。

How did the Universe transition from a mostly empty volume after the Big Bang to a complex realm of stars, galaxies and other celestial objects? This unknown process occurred during the so-called Cosmic Dawn and the Epoch-of-Reionziation. 21-cm radio-cosmology is the field of research promising to unlock the secrets of the infant Universe by studying radio signals from the most abundant element in the Universe: hydrogen. I request funding to assemble a research team to focus on the detection and study of the elusive 21-cm signal from atomic hydrogen using the Radio Experiment for the Analysis of Cosmic Hydrogen (REACH) that I conceived and lead. Radio-interferometers have already placed upper limits on the 21-cm power spectra, while it is the single-antenna experiments, measuring the 21-cm signal averaged across all directions in the sky, the ones that are promising even earlier breakthroughs. The EDGES experiment shook the field in 2018 reporting a cosmological signal twice deeper than expected requiring exotic physics to be explained. This has been contested by several groups amid concerns on the data analysis and potential impact of hardware systematic signals, including recent measurements from the SARAS3 experiment incompatible with the EDGES findings. The contamination from instru- ment systematics is the main bottleneck for "1st generation" 21-cm radio telescopes. Aiming to resolve these concerns, a new experimental approach has emerged over the last 5 years giving birth to a 2nd generation of experiments. REACH is a sky-averaged experiment leading this new wave of instruments, where the focus has shifted to a data-driven hardware and algorithm design focusing on the detection and isolation of instrumental systematic signals. To this end REACH uses a fully Bayesian data pipeline to jointly fit instrument models with models of the sky sig- nals. During this project I will lead the team aiming at a first confident detection and study of the sky-averaged 21-cm line.