I will give a broad overview of the history of the Universe, starting from its birth 13.8 billion years ago to the emergence of homo sapiens just 200 thousand years ago. To make the vast cosmic distances and times more comprehensible, several useful scale models will be introduced. The Universe began with a hot "big bang" and has been expanding ever since, becoming colder and less dense on average. Gradually, the denser regions gravitationally contracted to form stars, galaxies, and clusters of galaxies that initially consisted only of hydrogen and helium. The heavier elements necessary for life were created by nuclear reactions in stars, both regular and exploding ones. As interstellar space became progressively more enriched in heavy elements, planets consisting of rocks and water could coalesce around newly formed stars, and astronomers are now searching for exoplanets that might be habitable. Life eventually developed on at least one planet, our Earth, culminating with humans after billions of years of evolution. But strong evidence suggests that the normal atomic matter of which stars, planets, and life consist now accounts for only about 5% of the Universe in any sufficiently large volume; the rest is dark matter (25%) and gravitationally repulsive dark energy (70%), two mysterious components whose physical origin and nature are not yet understood. I will end with speculations on the fate of the Universe, and of our own significance in the cosmos.
我將對宇宙演化史進行整體回顧，從138億年前的誕生談到20萬年前智慧人種的出現。我將藉用數個有用的尺度模型來讓宇宙的巨大時空更易理解。宇宙自炙熱的大霹靂開始後就一直膨脹，以致整體而言越來越冷，且密度越來越小。漸漸地，空間中密度較高區域中的氫、氦，便會透過重力塌縮而形成恆星、星系、及星系團。合成生命所需的較重元素，會透過一般性及爆炸式的恆星內核反應而產生。當星際間的重原素逐步地變多時，由岩石及水所組成的行星便可在新形成的恆星四周合併生成，而天文學家們目前即在尋找可能適居的系外行星。生物最後在至少一顆行星－地球－上發展出來，經過數十億年的演化後終於出現了人類。然而有力的證據顯示，宇宙在大尺度上，只有 5% 的物質是構成恆星、行星、及生物等的一般性原子物質，另有暗物質占 25% 、以及具有萬有斥力的暗能量占 70%，後兩者的物理本質目前尚不得而知。在演講的最末段，我會推測宇宙未來的命運、以及我們在宇宙中重要性的命運。
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It was expected that the attractive force of gravity should gradually slow down the rate at which the Universe is expanding. But observations of very distant exploding stars (supernovae) by two teams of which I was a member show that the expansion rate is actually now speeding up, a remarkable discovery that was named the "Top Science Breakthrough of 1998" and honored with the 2011 Nobel Prize in Physics to the team leaders. Over the largest distances, the Universe seems to be dominated by a repulsive "dark energy" – an idea Albert Einstein had suggested in 1917 but renounced in 1929 as his "biggest blunder." It stretches space itself faster and faster with time, eventually causing a "runaway universe" unless the dark energy later becomes gravitationally attractive. The physical origin and nature of dark energy, which makes up about 70% of the contents of the Universe, is regarded by many as the most important unsolved problem in all of physics, and it provides clues for quantum theories of gravity.
過去我們一直認為，宇宙的膨脹會因重力的拉引而減慢。但兩個獨立的研究團隊（我是其中之一的成員）透過許多遙遠超新星的觀測發現，宇宙的膨脹速率現正增加中，該發現被譽為1998年的頂尖科學突破，而兩個團隊的領導人也因此獲得2011年的諾貝爾物理獎。在大尺度上，宇宙似乎是由具有萬有斥力的「暗能量」所主宰，這個概念最早是由愛因斯坦於1917年提出，但他卻於1929年視此為大錯誤而將其摒棄。暗能量會將宇宙空間不斷地向外加速推展，除非它自己之後由斥力轉變為引力，否則將會造成所謂的「失控宇宙」。暗能量占宇宙總組成的70%，而他的物理本質已被視為整個物理界中最重要的待解難題，同時也將為重力的量子理論提供一些線索。
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