Astronomy and Cosmology with the Planck Experiment

宇宙微波背景辐射（CMB）为热大爆炸模型提供了令人信服的证据。现在宇宙学家普遍认为，宇宙在大约130亿年前开始于一个热的，致密的状态，我们称之为大爆炸。然后宇宙膨胀和冷却，在这个过程中产生了亚原子粒子，如质子，中子和电子。大爆炸后几分钟，质子和中子结合形成了轻元素。然而，电子要经过几千年才能联合收割机。在这段时间内，温度足够高，以至于原子核和电子形成了物质辐射等离子体（辐射以光子的形式存在）。随着宇宙继续膨胀和冷却，有一段时间（大爆炸后大约30万年）光子与物质脱钩。这种辐射然后进一步冷却，今天我们测量这些光子作为微波，温度约为2.7 K。然而，宇宙学家很快就发现，宇宙微波背景辐射的温度在整个天空中并不均匀。他们发现的第一件事是，CMB在天空的一个方向上更热，而在相反的方向上更冷。这种效应的幅度约为平均温度的0.1%，这是由于我们相对于背景辐射的运动（从这个可以看出我们的太阳系相对于CMB的运动速度约为370 km/s）。然而，直到1992年，宇宙微波背景探测器（COBE）卫星才首次在较小尺度上探测到温度各向异性（幅度仅为平均温度的0.001%左右）。COBE的结果被用来表明，大爆炸是由一个被称为“暴胀”的短期快速膨胀立即进行的。正是这一时期的通货膨胀造成了温度的波动。这些波动最终通过引力坍缩而成长为我们今天看到的星系等结构。第二代CMB卫星WMAP于2001年发射，提供了详细的温度波动全天空图。这些结果被用来严格限制宇宙的几何形状，物质的数量以及哪些膨胀模型与数据兼容。2009年，第三代普朗克卫星发射升空。普朗克将以比WMAP高得多的灵敏度和角分辨率绘制天空。它还将提供CMB偏振的精确测量。CMB是极化的（在平均温度的0.0001%左右的水平上!）这是由于在去耦期间光子从电子散射。极化信号非常微弱，难以测量。此外，CMB中有两种类型的偏振信号，宇宙学家将其分为所谓的E模式和B模式。后者甚至更弱，但检测将是非常令人兴奋的，因为这个信号直接与导致通货膨胀的物理学有关。

The Cosmic Microwave Background (CMB) provides compelling evidence for the hot big-bang model. It is now generally agreed amongst cosmologists that the Universe began around 13 billion years ago in a hot, dense state which we call the big-bang. The Universe then expanded and cooled, in the process producing subatomic particles such as protons, neutrons and electrons. The protons and neutrons combined a few minutes after the big-bang to form light elements. The electrons, however, would take many thousands of years before they would combine. During this time the temperature was high enough so that the nuclei and electrons formed a matter-radiation plasma (with radiation in the form of photons). As the Universe continued to expand and cool there came a time when (around 300,000 years after the big-bang) the photons decoupled from matter. This radiation then cooled further, and today we measure these photons as microwaves with a temperature of around 2.7 K. Cosmologists soon learnt, however, that the temperature of the CMB is not uniform over the entire sky. The first thing they found was that the CMB is hotter in one direction of the sky and cooler in the opposite direction. The amplitude of this effect is around 0.1% of the average temperature, and is due to our motion with respect to the background radiation (from this one can show our solar system is moving at around 370 km/s with respect to the CMB). However, it wasn't until 1992 that the Cosmic Microwave Background Explorer (COBE) satellite made the first detection of temperature anisotropy on smaller scales (the amplitude for which is only around 0.001% of the average temperature). The results from COBE were used to show that the big-bang was immediately proceeded by a short period of rapid expansion called 'inflation'. It is this period of inflation which generated fluctuations in temperature. These fluctuations eventually grew by gravitational collapse into structures such as galaxies which we see today. The second generation CMB satellite WMAP was launched in 2001 and provided detailed full sky maps of the temperature fluctuations. These results were used to place tight limits on the geometry of the Universe, the amount of matter and which models of inflation were compatible with data. The third generation satellite Planck was launched in 2009. Planck will map the sky with much higher sensitivity and angular resolution than WMAP. It will also provide accurate measurements of the polarization of the CMB. The CMB is polarized (at the level of around 0.0001% of the average temperature!) due to the scattering of photons off electrons during the period of decoupling. The polarized signal is extremely weak and difficult to measure. Furthermore, there are two types of polarized signal in the CMB, which cosmologists separate into so-called E and B-modes. The latter is even weaker still, but a detection would be very exciting as this signal directly links to the physics which caused inflation.