The First Billion Years

在我的博士课程中，我想推进我们称之为“21厘米宇宙学”的领域。宇宙学是在最大尺度上研究宇宙以及它如何随时间演化的学科。21cm指的是中性氢发出的非常微弱的信号——这个原子可以发射或吸收那个波长的辐射，只要它没有被电离。这种现象在宇宙中无处不在，然而这种效应很罕见，因此只有在观测大量气体时才能看到。早期的宇宙（年龄在100万到10亿年之间）基本上充满了这种中性的氢，我们可以利用宇宙微波背景辐射，大爆炸后几十万年发出的辐射，作为手电筒穿过气体，观察它随时间的吸收或发射行为。我认为，将经典宇宙学的方法和信息与对中性氢信号的新观测相结合，可以取得很大进展。我们对宇宙中的第一批恒星所知甚少，已经建立了大量的模型。但是利用来自宇宙微波背景偏振的信息，我们可以限制这些可能性，更好地了解那个时间。另一方面，这些新的观测结果可以用来改进我们目前对宇宙中物质分布和结构随时间增长的测量。我们经常用星系作为宇宙中物质总体分布的示踪剂。然而，它们并不能描绘出一幅完整的画面，因为星系只在特定条件下形成，我们无法观察到最微弱的星系。通过21cm信号测量中性氢给了我们一个不同的观点，并允许我们用其他方法估计和纠正问题。最后，这可以给我们关于暗物质本质的提示或限制，因为我们可以探索以前未探索过的宇宙学领域。

In the course of my PhD I want to advance the field of what we call "21cm cosmology". Cosmology is the study of the universe on the largest scales and how it evolved over time. 21cm refers to the very weak signal emitted by neutral Hydrogen - this atom can emit or absorb radiation of that wavelength as long as it is not ionized. This happens everywhere in the universe, however the effect is rare and therefore can only be seen when observing large amounts of gas.The early universe (between one million and one billion years of age) is essentially filled with this neutral Hydrogen and we can use the cosmic microwave background, radiation emitted a few hundred thousand years after the big bang, as a flashlight shining through the gas and observe it's absorption or emission behaviour over time.I think much progress can be made by combining methods and information from classical cosmology with the novel observations of the neutral Hydrogen signal. We know very little about the first stars in the universe and a wealth of models has been developed. But using information from the polarization of the cosmic microwave background allows us to limit these possibilities and get a better idea of that time.On the other hand those novel observations can be used to improve our current measurements of the distribution of matter in the universe and the growth of structures with time. We often use galaxies as tracers of the overall distribution of matter in the universe. However they do not paint a complete picture since galaxies only form under certain conditions and we cannot observe the faintest galaxies. Measuring neutral Hydrogen via the 21cm signal gives us a different view and allows us to estimate and correct problems with other methods.Finally this can give us hints or constraints on the nature of Dark Matter as we can probe previously unexplored areas of cosmologies.