Precision cosmology at high redshift with the Lyman-alpha forest

银河系，连同它所有的恒星和行星，只是宇宙中自大爆炸以来一直在膨胀的数百万个星系中的一个。我们的宇宙最初是一个热的致密等离子体，随着膨胀的继续冷却下来。物质的分布最初是非常均匀的，但引力把小的密度波动变成了我们在局部宇宙中看到的大结构：星系、星系群和超星系团。15年前，两个不同的天体物理学家团队发现，宇宙不仅在膨胀，而且还在加速膨胀。这是一个很大的惊喜！当你把一个球抛向空中时，你期望重力把它拉下来，降低它上升的速度，并最终把它带回你身边。不知何故，这似乎不适用于宇宙尺度，宇宙的膨胀并没有减慢，而是加速。宇宙的明显加速是物理学中最重要和最具挑战性的问题之一。一种可能的解释可能涉及修改目前的引力理论——广义相对论，它是由阿尔伯特·爱因斯坦在整整100年前推导出来的。或者，加速可能是由宇宙中存在的一种新的奇异成分引起的，这种成分被称为暗能量，它将起到排斥力的作用。为了找出加速的真正原因，我们需要研究宇宙的膨胀是如何随着时间的推移而变化的，尽可能追溯到过去。由于光速有限，当我们观察一个遥远的星系时，我们看到的是它几百万年前发出光时的样子。我们观察到的星系越远，我们看到的时间就越早。所以，如果我们想研究年轻时的宇宙，我们必须观察宇宙中最遥远的区域。这个项目的主要目标是尽可能精确地测量宇宙在目前年龄（估计为138亿年）的四分之一时的膨胀。为此，我将研究一种罕见的超亮星系发出的光，这种星系被称为类星体，它们非常明亮，即使它们比其他星系远得多，我们也能观察到它们。类星体发出的光并非都能到达地球。尽管星系之间的空间几乎是空的，但少量的气体（主要是氢和氦）足以掩盖一些光。这种气体在非常遥远的类星体的图像中产生“阴影”，我们可以利用这些来量化类星体和我们之间的气体密度。利用这种被称为莱曼阿尔法森林的技术，我们能够绘制出宇宙中物质分布的非常大的地图，甚至是星系之间的空间。这些地图将使我能够更准确地研究宇宙的膨胀。这项引人注目的研究将帮助我们加深对我们所生活的宇宙的理解，了解它是如何形成的，以及支配它的规律。

Tracking down the expansion of the Universe through cosmic timeThe Milky Way, with all its stars and planets, is just one of the millions of galaxies in a universe that has been expanding since the Big Bang. Our Universe started as a hot dense plasma, that cooled down as the expansion continued. The distribution of matter was initially very homogeneous, but gravity turned small density fluctuations into the large structures that we see in the local Universe: galaxies, groups of galaxies and super clusters.Fifteen years ago, two different teams of astrophysicists discovered that the Universe is not only expanding, but its expansion is also accelerating. That was a big surprise! When you throw a ball up in the air, you expect gravity to pull it down, reduce its ascending speed, and eventually bring it back to you. Somehow, this does not seem to apply on cosmological scales, and the expansion of the Universe is not slowing down, but speeding up.The apparent acceleration of the Universe is one of the most important and challenging questions in physics. One possible explanation might involve revising the current theory of gravity, general relativity, derived by Albert Einstein exactly one hundred years ago. Alternatively, the acceleration could be caused by the presence of a new exotic component of the Universe, referred to as dark energy, that would act as a repelling force.In order to find out the actual cause of the acceleration, we need to study how the expansion of the Universe has changed over time, going as far back as possible to the past. Due to the finite speed of light, when we look at a distant galaxy we are seeing it as it was millions of years ago, when the light was emitted. The further the galaxy we observe, the earlier in time we are looking at. So if we want to study the Universe when it was younger, we have to observe the most remote regions of the Universe.The main goal of this project is to measure, as precisely as possible, the expansion of the Universe when it was only one fourth of its current age (estimated to be of 13.8 billion years). To do so, I will study the light coming from a rare type of ultra-luminous galaxies, called quasars, that are so bright that we can observe them even when they are much further away than other galaxies.Not all the light that quasars emit arrives to Earth. Even though the space between galaxies is almost empty, a small amount of gas (mostly hydrogen and helium) is enough to mask some of the light. This gas creates "shadows" in the images of very distant quasars, and we can use these to quantify the density of gas between the quasars and us. With this technique, known as the Lyman alpha forest, we are able to make very large maps of the distribution of matter in the Universe, even in the space between galaxies. These maps will allow me to study the expansion of the Universe with even more accuracy.This compelling research will help us deepen our understanding of the Universe we live in, how it was formed, and the laws that govern it.