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

我们所生活的宇宙的起源及其存在的早期时代一直让人们着迷。随着宇宙微波背景辐射的发现和观测，现在有可能拍下40万年前宇宙的“照片”。当这张照片以可见光开始时，宇宙膨胀使光冷却，并以今天的微波形式到达我们这里。早期宇宙的物理过程在宇宙微波辐射强度和极化的空间波动中得到了印记。我们看到的原始引力势波动是极化的强度和“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，我们还没有看到它们。我的研究目标是检测它们。你不能只看到宇宙微波背景，因为我们星系中的许多天体物理源在这些频率上也很亮。其中两个是高度极化的——与星系磁场对齐的热尘埃，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要亮得多，但可以通过它们发出的光的颜色将它们与宇宙微波背景辐射区分开来。这有点像在一个晴朗的夜晚，在一个城市里，你被周围的彩色灯光弄瞎了眼睛，试图辨认出银河系，只不过现在是银河系蒙蔽了我们。似乎有可能确定尘埃和同步加速器极化的特征，并试图将其从图像中移除。尘埃颗粒像无数的指南针一样排列在磁场上，并且有可能通过它们产生的光的偏振来重建我们银河系磁场的方向。这就给出了我们周围大尺度磁场特征在天空中产生的一个平滑的极化分数模式，一旦减去它，剩下的部分是相当随机的，可以像通常的噪声一样处理。同步加速器的偏振分数模式也有类似的特征，但其物理性质更为复杂，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立，以及CMB-S4的提出，我们将有一个非常低噪声的微波天空图像，而看到原始宇宙微波背景的能力将取决于我们能多好地减去前景。作为这两项合作的成员，我与同事们一起进行建模和数据分析。如果我们能看到原始引力波，那将是理解早期宇宙和驱动它的基本物理的一大进步。这可能是高能量尺度下暴胀的“确凿证据”，也可能像弦理论模型所倾向的那样，将暴胀的尺度推低。

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.