CAREER: Towards Dark Energy -- A High-precision Drone-based Calibrator for Next-Generation 21cm Cosmology Experiments

过去二十年对宇宙的测量表明，近75%的宇宙是由一种神秘的成分构成的：暗能量。与“常规物质”不同，暗能量似乎与引力相反，并导致宇宙加速膨胀。这是非常令人困惑的，为了解释它，我们可能不得不重写粒子物理学，甚至是万有引力定律。回答“什么是暗能量”这个问题的最好方法是在更久远的时间里测量宇宙的膨胀。最近，天文学家意识到，射电望远镜有可能比任何其他类型的望远镜更好地进行这种测量。这是以前从未尝试过的，我们现在正在建造望远镜来实现这一目标。然而，要想成功，我们需要理解望远镜的“光束”（就像一束光一样，望远镜指向一个地方并聚焦在那里，我们必须知道这束光在天空中是什么样子的）。为了这项事业拨款，PI正在建造射电源，将其安装在四轴飞行器上，在望远镜上方飞行。利用射电望远镜的数据，无人机的位置（带有特殊位置传感器），以及一些新的分析技术，我们将绘制望远镜光束的地图。最终的结果将是一个重要的测量，帮助我们更好地理解暗能量。宇宙的加速膨胀是在1998年通过超新星测量发现的，目前还没有关于这种加速的基本理论得到实验验证。因此，宇宙学家在我们的宇宙学模型中加入了一个新的成分（“暗能量”），但解释加速膨胀的基本理论要么涉及(i)对广义相对论的修正，要么涉及（ii）一种新的粒子，它可以预测膨胀率的独特时间依赖性。两者都将彻底改变我们目前的物理基本理论模型。为了回答这些问题，我们必须测量过去110亿年宇宙的膨胀历史，并确定暗能量对膨胀的影响。对此最有希望的途径来自于对星系的测量（在广阔的天空中测量100,000个星系）。不幸的是，由于探测器技术的限制，当前和下一代的光学星系调查在他们能看到的时间上受到了限制。射电望远镜没有这些限制，它提供了一个有希望的解决方案，可以在宇宙的大部分时间内对星系进行射电调查。这种测量以前从未尝试过，我们现在正在建造射电望远镜来实现这一目标。然而，我们已经知道，要想成功，我们必须非常了解望远镜的特性，特别是望远镜的“光束”（就像一束光一样，望远镜指向一个地方并聚焦在那里，我们必须知道这束光在天空中是什么样子的）。对于这项CAREER拨款，PI正在使用四轴无人机开发一种波束测绘仪。PI的团队将在望远镜上方的无人机上飞行一个射电源，并使用无线电数据和无人机的位置来绘制波束的地图。这需要开发一种稳定的噪声源，用于无人机的飞行，改进传感器以提高无人机的定位精度，以及新的分析技术——特别是利用在天线附近进行的测量来推断它们在天空中的样子的数学转换。最终的结果将是一个重要的测量宇宙学射电望远镜和波束映射技术，被证明在暗能量科学目标所需的高精度工作。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Measurements of the Universe over the last two decades have shown that nearly 75% of the Universe is made of a mysterious component: Dark Energy. Unlike "regular matter", Dark Energy seems to act against gravity and is causing the expansion of the Universe to accelerate. This is very confusing, and to explain it, we may have to re-write particle physics or perhaps even the laws of gravity. The best way to answer the question "What is Dark Energy?" is to measure the expansion of the Universe farther back in time. Recently astronomers realized that radio telescopes have the potential to make this measurement better than any other type of telescope. This has never been attempted before, and we are now building telescopes to achieve this goal. However, to be successful we need to understand the telescope "beam" (like a beam of light, a telescope points at one place and focuses there, and we must know what that beam looks like on the sky). For this CAREER grant, the PI is building radio sources to put on quadcopter drones to fly above the telescope. Using the radio telescope data, the location of the drone (with special position sensors), and some new analysis techniques, we will make a map of our telescope beam. The end result will be an important measurement that helps us understand Dark Energy better. The accelerated expansion of the Universe was discovered from supernova measurements in 1998 and no fundamental theories for this acceleration have yet been experimentally verified. As a result, cosmologists inserted a new component ("Dark Energy") into our cosmological models, but the underlying theories to explain the accelerated expansion involve either (i) a modification to General Relativity or (ii) a new particle which would predict a unique time dependence of the expansion rate. Either would revolutionize our current models of fundamental theories of physics. To answer these questions we must measure the expansion history of the Universe across the past 11 billion years and pin down the influence of Dark Energy on the expansion. The most promising path for this comes from measurements of galaxies (measuring 100, 000 galaxies across a wide swath of the sky). Unfortunately, current and next-generation optical galaxy surveys are restricted in how far back in time they can see due to limitations in detector technology. Radio telescopes do not have these limitations, and offer a promising solution to make radio surveys of galaxies across most of cosmic time. This measurement has never been attempted before and we are now building radio telescopes designed to achieve this goal. However, we already know that to be successful we must understand our telescope characteristics extremely well, in particular the telescope "beam" (like a beam of light, a telescope points at one place and focuses there, and we must know what that beam looks like on the sky). For this CAREER grant, the PI is developing a beam mapper using a quadcopter drone. The PI's team will fly a radio source on the drone above the telescopes and use the radio data and drone position to make a map of the beam. This requires developing a stabilized noise source to be flown on the drone, improved sensors for better drone position accuracy, and new analysis techniques - particularly a mathematical transformation to use measurements made near the dish and infer what they look like on the sky. The end result will be an important measurement for cosmology radio telescopes and a beam mapping technique that is demonstrated to work at the high precision required for Dark Energy science goals.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.