Astronomy and Cosmology with the Planck Experiment

宇宙微波背景(CMB)为热大爆炸模型提供了有力的证据。现在宇宙学家们普遍认为，宇宙始于大约130亿年前的一种炎热、致密的状态，我们称之为大爆炸。宇宙随后膨胀和冷却，在这个过程中产生了质子、中子和电子等亚原子粒子。质子和中子在大爆炸后的几分钟内结合在一起，形成了轻元素。然而，电子需要数千年的时间才能结合在一起。在这段时间里，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(以光子形式辐射)。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)光子与物质分离。这种辐射随后进一步冷却，今天我们以2.7K左右的微波测量这些光子。然而，宇宙学家很快就了解到，CMB的温度在整个天空并不均匀。他们发现的第一件事是，CMB在天空的一个方向上更热，在相反方向上更冷。这一效应的幅度约为平均温度的0.1%，是由于我们相对于背景辐射的运动(从这一点可以看出我们的太阳系相对于CMB的运动速度约为370公里/S)。然而，直到1992年，宇宙微波背景探测卫星才首次探测到较小尺度上的温度各向异性(其幅度仅为平均温度的0.001%左右)。来自COBE的结果被用来表明，大爆炸立即伴随着一段短时间的快速膨胀，称为“通货膨胀”。正是这段时期的通货膨胀导致了温度的波动。这些波动最终是通过引力坍塌形成我们今天看到的星系等结构而增长的。中巴第二代卫星WMAP于2001年发射，提供了详细的气温波动全天地图。这些结果被用来对宇宙的几何结构、物质的数量以及哪些膨胀模型与数据兼容施加严格的限制。第三代卫星普朗克于2009年发射升空。普朗克将以比WMAP更高的灵敏度和角度分辨率绘制天空地图。它还将提供对CMB的偏振的准确测量。CMB是偏振的(大约是平均温度的0.0001%！)这是由于在去耦合期间光子与电子的散射所致。偏振信号非常微弱，很难测量。此外，CMB中有两种极化信号，宇宙学家将其分为所谓的E模和B模。后者甚至更弱，但探测到它将是非常令人兴奋的，因为这个信号直接与导致膨胀的物理联系在一起。

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.