左连凯

One of the dominating astronomical discoveries of the 20th century was that the galaxies of the universe all seem to be moving away from Earth. Doppler redshifts were observed for spiral nebulae around 1920 even though they were not yet known to be galaxies. By the early 1930s， Edwin Hubble and M. L. Humason had established that the more distant a galaxy， the faster it was receding. It turned out that they were moving away not just from the Earth but from one another—that is， the universe is expanding. Scientists conclude that the universe must once， very long ago， have been extremely compact and dense， and a rapid expansion caused the energy and matter to rapidly expand. The beginning of this expansion is referred to as the Big Bang.

On the subatomic level， according to this theory， there were vast changes of energy and matter and the way physical laws operated during the first few minutes after the Big Bang. After those early minutes the percentages of the basic matter of the universe—hydrogen， helium， and lithium—were set. Everything was so compact and hot that radiation dominated the early universe and there were no stable， un-ionized atoms. The universe was opaque， in the sense that any energy emitted was quickly absorbed and then re-emitted. As the universe expanded， density and temperature continued to drop. A few hundred thousand years after the Big Bang， the temperature dropped far enough that electrons and nuclei could combine to form stable atoms as the universe became transparent. Once that occurred， the radiation that had been trapped was free to escape.

In the 1940s， George Gamow and others predicted that remnants of this escaped radiation should be observable. They had started to search for this background radiation when physicists Arno Penzias and Robert Wilson， using a radio telescope， inadvertently found it.

In 2003， NASAs Wilkinson Microwave Anisotropy Probe made measurements of the temperature of this cosmic microwave background radiation to within millionths of a degree. From these measurements， scientists were able to deduce that our universe is 13.7 billion years old and that first-generation stars began to form a mere 200 million years after the Big Bang.

In 2014， scientists operating a telescope in Antarctica claimed to have found direct evidence for cosmic inflation， the rapid expansion of the universe during the first 10-32 seconds after the Big Bang that helps explain why variations of the cosmic background radiation are so small. Follow-up observations have cast doubt on this result， and higher precision measurements are planned.

A related mystery is evidence suggesting hidden matter and hidden energy that cannot be directly observed. The presence of dark matter is indicated by the rotation curves of galaxies and the dynamics of clusters of galaxies. Dark matter may be composed of gas; large numbers of cool， compact objects like dead stars; or even subatomic particles. Evidence for dark energy is derived from studies of distant Type Ia supernovae indicating that the expansion of the universe is accelerating rather than slowing. Dark energy seems to work on the very fabric of the universe， acting as a force that increases the rate at which space expands. Visible matter seems to constitute only about 4%1 of the total mass of the universe while the rest of the universes mass is in the form of dark matter （27%） and dark energy （68%）.

20世纪天文学的一个重大发现表明，宇宙中的所有星系似乎都在退离地球。1920年前后，尽管人们尚未认识到螺旋星云就是星系，但观察到了它们的多普勒红移。1930年代初，埃德温·哈勃和M. L.赫马森证实，星系越远，退离得越快。原来它们不仅离地球而去，而且离彼此也越来越远，也就是说，宇宙在膨胀。科学家推断，宇宙很久以前一定十分紧凑密集，由于它快速膨胀，能量和物质也迅速膨胀。这种膨胀的开始叫作宇宙大爆炸。

根据该理论，大爆炸后的最初几分钟里，能量、物质和物理定律的作用方式在亚原子层面发生了巨大变化。这最初的几分钟过后，宇宙的基础物质如氢、氦、锂的比例就确定了。一切都高密、高温，所以辐射主导着早期宇宙，不存在稳定的非离子化原子。宇宙是不透明的，意味着释放的能量很快被吸收，然后重新释放。随着宇宙的膨胀，密度和温度持续下降。大爆炸发生几十万年后，宇宙变得透明，温度下降到足以使电子和原子核结合形成稳定的原子。这时，被束缚的辐射自由逸出。

1940年代，乔治·伽莫夫等人预测，应该能观测到这种逸出辐射的残余。他们开始寻找这种背景辐射，这时物理学家阿诺·彭齐亚斯和罗伯特·威尔逊用射电望远镜不经意间发现了它。

2003年，美国国家航空航天局的威尔金森微波各向异性探测器对这种宇宙微波背景辐射的温度进行了测量，精确至百万分之一度以内。根据这些测量结果，科学家推断宇宙有137亿年的历史，第一代恒星在大爆炸仅仅2亿年后开始形成。

2014年，在南极洲用望远镜观测的科学家声称他们发现了宇宙膨胀（大爆炸后的10-32秒内，宇宙迅速膨胀）的直接证据，这有助于解释宇宙背景辐射变化非常小的原因。后续观察质疑这一结果，更精密的测量便纳入计划之中。

一个与此有关的谜团是，证据显示暗物质和暗能量无法直接观测到。星系的旋转曲线和星系团的力学特性揭示了暗物质的存在。暗物质可能由气体、大量冷而致密的物体（如死星）甚或亚原子粒子组成。对遥远的Ia型超新星的研究證明了暗能量的存在，表明宇宙膨胀在加速而不是放缓。暗能量似乎影响了宇宙的结构，使空间膨胀速度越来越快。可见物质似乎只占宇宙总质量的4%左右，宇宙中的其他物质以暗物质（27%）和暗能量（68%）的形式存在。