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

宇宙起源和初始演化的大爆炸模型现在是一个熟悉和充分研究的研究领域。在随后的几十亿年里，恒星和其他天体的后期演化也许更容易理解。然而，人们对这两个时期之间的时间（大爆炸后大约3.5亿年到10亿年）知之甚少。在此期间，宇宙从一个充满冷却的中性气体的巨大体积转变为我们现在可以从地球上观察到的宇宙物体的领域。起初，宇宙充满了热而稠密的电离气体雾，直到持续的膨胀和冷却使电子和质子结合并形成第一批中性原子。氢。最终，中性物质在重力的作用下聚集在一起，为核聚变的发生提供了条件，导致了第一批恒星和星系的诞生（这一时期被称为宇宙黎明）。随后，在再电离时代，这些物体加热并再电离宇宙中周围的氢。研究人员推测，从地球上可以探测到那些时期的电磁信号，频率在FM波段附近，可以用来研究早期的宇宙。我们最好的估计表明，通过使用单天线射电望远镜，我们应该能够探测到来自这一时期的难以捉摸的无线电波。然而，这至少需要将无线电接收机的响应校准到前所未有的水平。利用我为超级望远镜平方公里阵列和氢时代再电离阵列设计无线电传感器的经验，我将把我的工作重点放在开发和操作一个简单、非常稳定、易于校准的电磁波传感器上，如果在远离人为无线电信号和干扰的偏远无线电安静区（如沙漠地区）操作，它能够探测到这些极其微弱的理论信号。这个实验被称为REACH（宇宙氢分析无线电实验），它的目标是通过观察南非卡鲁半沙漠地区氢自然发射的无线电信号，为宇宙的早期时代打开一扇窗。氢是形成第一批恒星的原料但当时充满宇宙的氢云也阻止了我们直接观察第一批恒星发出的光。因此，我们的目标是通过它们与氢云的相互作用来观察这些恒星，就像人们通过观察笼罩在雾中的阴影来推断风景一样。

The Big Bang model for the origin and initial evolution of the Universe is by now a familiar and well-studied research field. The subsequent, late time, evolution of stars and other celestial objects over billions of years is perhaps even better understood. Less is known, however, about the time between these periods (from about 0.35 to 1 billion years after the Big Bang). During this time the Universe transitioned from being a vast volume filled with a cooling neutral gas to become the realm of cosmic objects that we can now observe from Earth.At the beginning the Universe was filled with a hot, dense fog of ionized gas until the continued expansion and cooling allowed electrons and protons to combine and form the first neutral atoms, eg. hydrogen. Eventually, the neutral matter clumped together under the effects of gravity, providing the conditions for nuclear fusion to occur, leading to the birth of the first stars and galaxies (period known as the Cosmic Dawn). Subsequently, these objects heated and re-ionised the surrounding hydrogen in the Universe during the Epoch of Re-ionization.Researchers speculate that electromagnetic signals from those times should be detectable from Earth at frequencies around the FM band and could be used to study the early Universe. Our best estimations indicate that by using a single antenna radio telescope we should be able to detect the elusive radio waves from this period. However, this will require, as a minimum, calibrating the response of the radio receiver to unprecedented levels. Using my experience designing radio sensors for the super telescope Square Kilometre Array and the Hydrogen Epoch of Re-ionization Array, I will focus my work on developing and operating a simple, very stable, easily calibratable sensor of electromagnetic waves capable of detecting these extremely faint theorized signals if operated from a remote radio quiet zone such as desert areas far away from human-made radio signals and interference. This experiment is called REACH (Radio Experiment for the Analysis of Cosmic Hydrogen), and it will aim at opening a window to these early epochs of the Universe by observing radio signals naturally emitted by hydrogen from the semi-desertic region of the Karoo in South Africa. Hydrogen was the raw material forming the very first stars but also these same hydrogen clouds filling the Universe at the time stop us from directly observing the light from these first stars. Thus, we aim to look at these stars through their interaction with the hydrogen clouds in the same way one would infer a landscape by looking at the shadows in the fog covering it.