Testing Fundamental Physics with B-modes of Cosmic Microwave Background anisotropy

我们生活的宇宙的起源和它存在的早期时代一直令人着迷。随着对宇宙微波背景的发现和观测，现在可以拍摄宇宙大约40万岁时的“照片”。虽然这张照片一开始是可见光，但宇宙的膨胀冷却了光线，并以微波的形式到达了我们今天的地球。早期宇宙的物理过程印记在宇宙微波辐射强度和偏振的空间涨落中。我们认为原始引力势波动是强度和极化的“直”模式，而原始引力波看起来像“卷曲”模式。这些被称为B模式，但我们还没有见过它们。我的研究目标是致力于检测它们。你不能只看到宇宙微波背景，因为我们银河系中的许多天体物理源在这些频率上也是明亮的。其中两个是高度极化的--与银河系磁场对准的热尘埃，以及带电粒子在磁场中旋转时产生的同步辐射。它们比我们正在寻找的信号亮得多，但它们发出的光的颜色可以将它们与宇宙微波背景区分开来。这有点像在晴朗的夜晚试图辨认出城市中的银河系，被周围的彩色灯光蒙蔽了双眼，但现在是银河系让我们盲目。它似乎有可能表征尘埃和同步辐射的偏振，并试图将其从照片中移除。尘埃颗粒像无数的指南针一样与磁场对准，通过它们产生的光的偏振来重建我们银河系中磁场的方向是可能的。这给出了来自我们周围大尺度磁场特征的天空中平滑的偏振分数图案，一旦减去它，其余的就是相当随机的，可以作为通常的噪声来处理。同步加速器的偏振分数模式具有相似的特征，但物理更复杂，需要进一步研究。我建议研究这两个天体物理前景，并利用我们获得的知识开发新的方法来消除它们。随着西蒙斯天文台的建造和CMB-S4的提出，我们将拥有一幅非常低噪音的微波天空图像，而能否看到原始宇宙微波背景将取决于我们能否很好地减去前景。作为这两个合作组织的成员，我与我的同事在建模和数据分析方面进行合作。如果我们看到原始引力波，这将是我们在理解早期宇宙以及驱动它的基本物理方面向前迈出的一大步。这可能是对高能级通胀的“确凿证据”，也可能像弦理论模型倾向于的那样，将通胀率推低。

The origins of the universe we live in and the early epochs of its existence has always fascinated people. With discovery and observations of the cosmic microwave background, it is now possible to take a "photograph" of the universe when it was about 400,000 years old. While the picture starts as a visible light, universe expansion cools the light down and it reaches us in the form of the microwaves at the present day. Physics processes in the early universe get imprinted in spatial fluctuations of the intensity and polarization of the cosmic microwave radiation. We see primordial gravitational potential fluctuations as intensity and "straight" patterns in polarization, while primordial gravitational waves would look like "curly" patterns. These are known as B-modes, and we have not seen them yet. The goal of my research is to work on their detection. You do not get to see just cosmic microwave background, as a lot of astrophysical sources in our galaxy are also bright in these frequencies. Two of them are highly polarized - the thermal dust that aligns itself with galactic magnetic field, and the synchrotron radiation produced by charged particles as they rotate around in the magnetic field. They are much brighter than the signal we are looking for, but they can be distinguished from the cosmic microwave background by the colour of light they emit. It is a bit like trying to make out a Milky Way on a clear night in a city, being blinded by coloured lights all around you, except now it is the Milky Way that blinds us. It appears to be possible to characterize the dust and synchrotron polarization, and to try to remove it from the picture. Dust grains align themselves to magnetic field like countless compass needles, and it is possible to reconstruct the direction of the magnetic field in our galaxy from polarization of light they produce. This gives a smooth polarization fraction pattern on the sky sourced by large-scale magnetic field features around us, and once that is subtracted the remainder is quite random, and can be dealt with as the usual noise. Polarization fraction pattern of synchrotron traces similar features, but the physics is more complex, so it needs further investigation. I propose to study these two astrophysical foregrounds, and develop new methods to remove them using the knowledge we gained. With Simons Observatory being build, and proposed CMB-S4 we will have a very low noise picture of the microwave sky, and the ability to see primordial cosmic microwave background will depend on how well we could subtract the foregrounds. As a member of both collaborations, I work with my colleagues on modelling and data analysis. If we see primordial gravitational waves, it would be a big step forward in understanding the early universe, and fundamental physics driving it. It could be a "smoking gun" confirmation of inflation at high energy scales, or it could push the scale of inflation low, as string theory models tend to prefer.