今天随便聊聊关于宇宙的起源、人类命运这样的大话题。现在，大爆炸学说已经是普遍接受的观点。我们的宇宙，是从一个能量高度密集的状态，也就是大爆炸中诞生的。宇宙在逐渐冷却的过程中，形成了恒星、行星以及生命。普通人会有这样的疑问：如果宇宙有一个诞生的时刻，也就是大爆炸，那么在大爆炸之前是什么呢？关于这个问题，标准的答案是霍金的话：这个问题是一个伪命题，因为时间和空间从大爆炸中诞生，所以不存在“大爆炸以前”这个时间点。这就像问“南极的南边是什么”一样，因为你站在南极点上，无论向哪个方向走，都在你的北边，所以没有“南极的南边”这个概念。当然，并不是所有人都同意这个观点。同样是诺贝尔奖得主的罗杰·彭罗斯，他主张宇宙是一个循环体，在大爆炸之前，还有另一个宇宙，它也有自己的大爆炸。而近期发射成功的韦伯太空望远镜，对大爆炸后宇宙婴儿期进行的观测，发现了一些有趣的现象。其中之一是在宇宙早期就存在着结构类似银河系的成熟星系。而按照现有的理论，也就是大爆炸理论，在这一时期宇宙还不应该有这么成熟的星系。虽然这并不是证明在我们的宇宙之前还有其他宇宙的确凿证据，但至少也说明了现有的宇宙形成理论是有漏洞和缺陷的。
那么，循环宇宙理论是什么样的呢？按以前的理解，宇宙在目前处于膨胀阶段，也许在将来的某个阶段，宇宙会重新开始收缩，最终再次聚集成一个奇点，也就是下一次大爆炸的发生。这个理论听起来很科学，但其实与佛教中的轮回观念很类似。不过现在似乎遇到一个问题，那就是人们的天文观测发现，宇宙的膨胀正在加速，而这种加速是受到一种未知的暗能量驱动的。暗能量会推动空间的膨胀，如果它不会消失，那么宇宙的膨胀就将无休止地进行下去。那宇宙又如何能压缩、回缩、重新聚集成一个奇点呢？不过，彭罗斯提出了“共形循环宇宙”理论。今天听了一个节目的讲解，似乎明白了一些。因为在这个理论中，宇宙不需要重新收缩，所以虽然有点骇人听闻，但这个理论还是很有意思的。人们思考宇宙的起源总是将时间往前回溯，直到大爆炸那一刻，甚至还想探寻大爆炸发生之前的时刻。而彭罗斯的理论则是将时间往后推到无穷遥远的未来。世上的万事万物都有其寿命，太阳会在几十亿年后变成一颗红巨星，可能会吞没地球，即使没有，也会将地球烤干，那时任何生命都不可能存在了。太阳最终也会熄灭，变成一颗白矮星。而比太阳更大的恒星，则会坍缩成黑洞。也许再过上百亿年，整个银河系的恒星都会死亡，变成大大小小的黑洞、中子星和白矮星，黑洞则会吞噬其他星体。很久很久以后，宇宙中只剩下黑洞这种天体了。但是，根据霍金理论，黑洞会产生霍金辐射，慢慢蒸发，尽管这个过程非常缓慢，但即使是最巨大的黑洞，也终会有蒸发殆尽的那一天。如果连黑洞都消失了，宇宙还剩下什么呢？对了，就是那些黑洞蒸发产生的光子。也就是说，宇宙中最后的幸存者将是一片光子的海洋。了解过爱因斯坦相对论的人都知道，相对论的结论是，时间和空间会随着速度的变化而扩张和收缩。简单来说，速度越快，经历的时间就越慢；速度越快，空间对运动的物体也会发生压缩。而一旦达到光速，时间对运动者来说就会完全停滞，也就是时间不再存在了。同样地，如果速度达到光速，空间距离也会对运动者收缩为0。所以，如果宇宙最终只剩下光子，那么时间和空间也就都不存在，那不就是回到了大爆炸发生之前的状态吗？
Today, let's talk about the origin of the universe and the fate of mankind. The Big Bang theory is now generally accepted. Our universe was born out of a highly energy-dense state, the Big Bang. As the universe cooled, stars, planets and life formed. Ordinary people have this question: If the universe had a birth moment, the Big Bang, then what was before the Big Bang? The standard answer to this question is Hawking's words: this question is a false proposition, because time and space were born out of the Big Bang, so there is no "before the Big Bang" point in time. This is like asking "What is south of the South Pole", because you are standing at the South Pole and whichever direction you go is north of you, so there is no such thing as "south of the South Pole". Not everyone agrees, of course. Roger Penrose, also a Nobel laureate, argued that the universe is a cyclic body and that before the Big Bang, there was another universe that had its own big Bang. And the recently launched Webb Space Telescope, which looked at the infancy of the universe after the Big Bang, has found some interesting things. One of them is the existence of mature galaxies with structures similar to the Milky Way in the early universe. According to the current theory, known as the Big Bang theory, the universe should not have such mature galaxies at this time. While this is not conclusive proof that there were other universes before ours, it does at least show that existing theories of how the universe formed have holes and flaws.
So, what does the cyclic universe theory look like? According to the previous understanding, the universe is currently expanding, and perhaps at some stage in the future, the universe will begin to contract again, and eventually come together again into a singularity, which is the next big Bang. This theory sounds scientific, but it's actually very similar to the Buddhist concept of reincarnation. But now there seems to be a problem: astronomical observations have found that the expansion of the universe is accelerating, and this acceleration is driven by an unknown type of dark energy. Dark energy drives the expansion of space, and if it doesn't disappear, the expansion of the universe will continue indefinitely. So how can the universe compress, retract, and reassemble into a singularity? However, Penrose proposed a "conformal cyclic universe" theory. Today, I listened to the explanation of a program and seemed to understand some. Because in this theory, the universe doesn't have to contract again, so it's kind of shocking, but it's interesting. People who think about the beginning of the universe always look back in time to the Big Bang, and even to the moments before the Big Bang. Penrose's theory, on the other hand, pushed the time back infinitely far into the future. Everything in this world has a lifespan, and the sun will become a red giant in a few billion years, which may engulf the Earth, or if it doesn't, it will bake the Earth dry, and then any life will be impossible. The Sun will eventually die out and become a White Dwarf. Stars larger than the Sun, on the other hand, collapse into black holes. Perhaps in a few billion years, the stars of the entire galaxy will die, turning into black holes, neutron stars and white dwarfs, and the black holes will devour other stars. After a long, long time, black holes were the only objects left in the universe. However, according to Hawking's theory, black holes produce Hawking radiation and slowly evaporate, although this process is very slow, but even the most massive black holes will eventually evaporate. If even black holes disappeared, what would be left of the universe? That's right, the photons from those black holes that evaporate. In other words, the last survivors in the universe will be a sea of photons. Anyone who has read Einstein's theory of relativity knows that it concludes that time and space expand and contract with changes in speed. Simply put, the faster the speed, the slower the experience of time; The faster the speed, the space will also compress the moving object. Once the speed of light is reached, time ceases to exist for the person moving it. Similarly, if the speed reaches the speed of light, the distance in space will also contract to zero for the person moving. So, if only photons were left in the universe, then neither time nor space would exist, and wouldn't that be a return to the state before the Big Bang?
