So. Is that it, then? Do we know everything there is to know about the origin and evolution of the Universe? Physicists have been declaring “the end of physics;” i.e., that we know all there is to know on the subject, for some hundreds of years. Not quite, I'm afraid. There are many deep mysteries in physics, and every answer raises new questions. Here are just a few of the myriad things that physicists don’t know (yet):
• The Big Bang Theory is a description of the events that occurred beginning a few billionths of a second after the origin of the Universe. What happened during those first billionths of a second?
• What, if anything, came before the Big Bang; i.e., what caused the the Universe to come into being, and where did it happen, since there was no “where” yet in which any events could happen? The Universe did not appear and begin to expand into pre-existing empty space; space itself originated at the instant of the Big Bang, and the expansion of the Universe includes the expansion of space itself.
• Did time have a beginning? Did time exist before the Universe came into existence, or did it begin with the Big Bang? Theistic religions usually claim that their gods are eternal, and at least one faction—the Church of Rome—says that while their god existed for an infinite amount of time before he created the Universe, it is forbidden to ask what he was doing before he created the Universe. I think that’s a perfectly valid question, because if the Roman god is eternal, he spent an infinite amount of time nowhere. Why did he suddenly create the Universe after an infinite time nowhere, doing nothing? How did he decide when to create the Universe? (I don't know if the Church really considers its god to be a male, but they have to use some pronoun. "It" seems too impersonal, and the Church is a male-dominated institution, so it is hardly surprising that they don’t represent their god as a goddess.) But I digress.
• What is time, anyway? The laws of physics show no preference for the direction of the flow of time; the laws work equally well whether time is seen to flow forward or backward, yet the psychological arrow of time—the way we perceive time—moves only forward. Why do we remember yesterday and not tomorrow? (Yes, that is a valid scientific question. See also thermodynamics and the arrow of time.)
• Why should there be a thing we call mathematics that can describe with such elegance the workings of the Universe?
• Is M Theory—the extension of String Theory that asserts that all of the fundamental particles and forces in the Universe are made up of infinitesimally small, vibrating strings of energy—correct? We experience three dimensions and Einstein showed that, while it is not obvious, time is our fourth dimension, with the three spatial dimensions (x, y, z) and time comprising a continuum called “spacetime”. M Theory could be the foundation of a Grand Unified Theory, yet it predicts that there are 10 or 11 dimensions. What are we to make of that? (The notion that space and time are aspects of the same thing can be a difficult concept to grasp. As far as is known, everything in the Universe except mathematical points and imaginary lines exists in three spatial dimensions. Consider a tree, perhaps. It has width x, height y, and depth z, but it also exists during a certain period in time, and a complete description of the tree would include all four of those dimensions.)
• It is a widely held principle of science that the most elegant answer, meaning the simplest answer, to a question is very probably the correct answer. See Occam’s Razor. While current theories such as General Relativity, M Theory, and quantum mechanics may be said to be mathematically elegant, the mathematics is complex, and only a handful of people understand these theories. Could it be that we will never understand the Universe at its most fundamental level, either because in the 13.75 billion years since the beginning of the Universe critical evidence that would enable us to explain the fundamentals has become blurred and diluted, or because the answer is so complex that the human mind will be unable to formulate a coherent description (theory) of the Universe’s basic laws? It is my belief that the Universe ought to be accessible to the average person (practically anyone who knows arithmetic can achieve an understanding of Newtonian mechanics) and I will be very disappointed if a Theory of Everything is derived and that theory is too complex for ordinary people like me to comprehend. There are not very many mathematical physicists in the world. Why should the workings of the Universe be accessible to only an extremely tiny fraction of the human population? My belief, that an understanding of the Universe should be accessible to everyone, has no basis other than the fact that it is the way I would like for the Universe to be. Thus, my belief is based on wishful thinking; it is not a theory.
• Organic molecules (molecules that contain carbon and that are necessary for the existence of all life that we know about) are very common throughout the Universe. Will these organic molecules always interact in such a way as to produce life if conditions in a certain place (most likely, but not necessarily, a planet) are right, i.e., if conditions are warm and wet, as they have been for most of the history of the Earth? In other words, is life common in the Universe?
• Extending the previous question—Is anyone out there? Are there other self-aware beings in the Universe? It seems that everyone says there must be, and serious scientists have invented equations showing that it is mathematically certain that there are. The reality, however, is that we have not seen any evidence whatsoever that sentient beings exist elsewhere. In the foregoing link, the physicist Enrico Fermi counters Drake’s equation by famously asking “Where are they?” Where, indeed! The size of our galaxy, not to mention the size of the Universe, makes it very possible, if not probable, that we will never know if there is anyone out there. Equally unappealing is the probability that even if we found proof (radio signals, most likely) that another civilization exists, the distance between them and us would make it impossible for us to converse with them. If they were 50,000 light-years from us and we sent a friendly greeting via a sufficiently powerful radio signal, they would receive the signal 50,000 years later. If they replied immediately we would receive the answer 100,000 years after we sent the greeting; if they were contemplative beings they might think for another 50,000 years before replying. Would the knowledge that we had sent a greeting persist on Earth for 100,000 years or more so that someone here would know to turn on a radio at the right time to listen for an answer? Consider, too, the possibility that there is an advanced civilization much closer to us—say, no more than 100 light-years away. If they're listening, they already know about us, because they have received radio signals from Earth. They could, in theory, visit us within perhaps a couple of thousand years. Would we want that? What if they are in a situation that is a popular, and quite plausible, science-fiction theme—their home planet is threatened by natural phenomena (the aging of their sun, planetological activity, climate change) or by their own actions (pollution, war). Suppose they are looking for a new home and a good supply of cheap labor. Or food. Suppose they carry pathogens that could kill us all. It has been suggested that we need to consider whether we should take steps to prevent other civilizations from detecting our presence, at least until we have detected them and had an opportunity to assess their civilization first through the study of their unintentional radio emissions. Alien visitors, too, might face risks. Perhaps our message would be a declaration of jihad or a promise of eternity in hell if the aliens aren’t the right color or of the right religion. In a world where we kill people by the hundreds of thousands over the mistaken belief that they are different from us in some way, imagine how we might react to beings who were really different from humans in their appearance and their philosophy. I think it would be a shame if a group of non-human diplomat-scientists traveled thousands of years to deliver wondrous offerings of peace and friendship to the people of Earth, only to land and promptly get lynched or imprisoned. This, too, is a plausible science-fiction theme.
Develop a theory—not a guess, but a theory—that answers any one of the questions raised above and you will have a good shot at winning the Nobel Prize in something or other.