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

宇宙是如何从大爆炸后的一个几乎为空的体积过渡到恒星、星系和其他天体的复杂领域的？这个未知的过程发生在所谓的宇宙黎明和再电离时代。21厘米的射电宇宙学是一个研究领域，它承诺通过研究宇宙中最丰富的元素氢的无线电信号来解开宇宙的秘密。我申请资金组建一个研究小组，利用我构思和领导的宇宙氢分析无线电实验(REACH)，专注于探测和研究来自原子氢的难以捉摸的21厘米信号。无线电干涉仪已经为21厘米的功率谱设定了上限，而单天线实验则是测量天空中所有方向平均的21厘米信号，这些实验有望更早地取得突破。EDGE实验在2018年震撼了整个领域，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。由于担心数据分析和硬件系统信号的潜在影响，包括SARAS3实验的最新测量结果与EDGE的调查结果不一致，几个小组对此提出了异议。仪器系统学的污染是“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是一项引领这一新仪器浪潮的天空平均实验，其重点已转移到数据驱动的硬件和算法设计上，重点放在仪器系统信号的检测和隔离上。为此，REACH使用完全贝叶斯数据管道来联合拟合仪器模型和天空信号模型。在这个项目中，我将带领团队对天空平均21厘米的线进行首次自信的探测和研究。

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.