宇宙的起源

左連凱

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世紀天文學的一個重大發現表明，宇宙中的所有星系似乎都在退離地球。1920年前后，盡管人們尚未認識到螺旋星云就是星系，但觀察到了它們的多普勒紅移。1930年代初，埃德溫·哈勃和M. L.赫馬森證實，星系越遠，退離得越快。原來它們不僅離地球而去，而且離彼此也越來越遠，也就是說，宇宙在膨脹。科學家推斷，宇宙很久以前一定十分緊湊密集，由于它快速膨脹，能量和物質也迅速膨脹。這種膨脹的開始叫作宇宙大爆炸。

根據該理論，大爆炸后的最初幾分鐘里，能量、物質和物理定律的作用方式在亞原子層面發生了巨大變化。這最初的幾分鐘過后，宇宙的基礎物質如氫、氦、鋰的比例就確定了。一切都高密、高溫，所以輻射主導著早期宇宙，不存在穩定的非離子化原子。宇宙是不透明的，意味著釋放的能量很快被吸收，然后重新釋放。隨著宇宙的膨脹，密度和溫度持續下降。大爆炸發生幾十萬年后，宇宙變得透明，溫度下降到足以使電子和原子核結合形成穩定的原子。這時，被束縛的輻射自由逸出。

1940年代，喬治·伽莫夫等人預測，應該能觀測到這種逸出輻射的殘余。他們開始尋找這種背景輻射，這時物理學家阿諾·彭齊亞斯和羅伯特·威爾遜用射電望遠鏡不經意間發現了它。

2003年，美國國家航空航天局的威爾金森微波各向異性探測器對這種宇宙微波背景輻射的溫度進行了測量，精確至百萬分之一度以內。根據這些測量結果，科學家推斷宇宙有137億年的歷史，第一代恒星在大爆炸僅僅2億年后開始形成。

2014年，在南極洲用望遠鏡觀測的科學家聲稱他們發現了宇宙膨脹（大爆炸后的10-32秒內，宇宙迅速膨脹）的直接證據，這有助于解釋宇宙背景輻射變化非常小的原因。后續觀察質疑這一結果，更精密的測量便納入計劃之中。

一個與此有關的謎團是，證據顯示暗物質和暗能量無法直接觀測到。星系的旋轉曲線和星系團的力學特性揭示了暗物質的存在。暗物質可能由氣體、大量冷而致密的物體（如死星）甚或亞原子粒子組成。對遙遠的Ia型超新星的研究證明了暗能量的存在，表明宇宙膨脹在加速而不是放緩。暗能量似乎影響了宇宙的結構，使空間膨脹速度越來越快。可見物質似乎只占宇宙總質量的4%左右，宇宙中的其他物質以暗物質（27%）和暗能量（68%）的形式存在。