Where did the Universe come from?
Assuming the Big Bang is a valid theory of the creation of Earth and the Universe, then where did the original mass come from, that formed everything that we see today?
First of all, note that mass and energy are equivalent. So, the total mass of the Universe need not be conserved even though the total energy (taking into account the energy that is equivalent of the mass in the Universe) is conserved. Mass and energy are related by the famous equation E=mc2. Hence if there is enough energy, photons can create matter-antimatter pairs. This is called pair production and is responsible for the mass in the Universe.
As to where everything came from, there is no conclusive opinion. One idea was that the Universe was created from vacuum. This is because according to quantum theory, the apparently quiescent vacuum is not really empty at all. For example, it is possible for an electron and a positron (a matter antimatter pair) to materialize from the vacuum, exist for a brief flash of time and then disappear into nothingness. Such vacuum fluctuations cannot be observed directly as they typically last for only about 10-21 seconds and the separation between the electron and positron is typically no longer than 10-10 cm. However, through indirect measurements, physicists are convinced that these fluctuations are real.
Hence, any object in principle might materialize briefly in the vacuum. The probability for an object to materialize decreases dramatically with the mass and complexity of the object. In 1973, Edward Tyron proposed that the Universe is a result of a vacuum fluctuation. The main difficulty of this proposal is that the probability that a 13.7 billion year old Universe could arise from this mechanism is extremely small. In addition, physicists would question Tyron's starting point: if the Universe was born from empty space, then where did the empty space come from? (Note that from the point of view of general relativity, empty space is unambiguously something, since space is not a passive background, but instead a flexible medium that can bend, twist and flex.)
In 1982, Alexander Vilenkin proposed an extension of Tyron's idea and suggested that the Universe was created by quantum processes starting from "literally nothing", meaning not only the absence of matter, but the absence of space and time as well. Vilenkin took the idea of quantum tunneling and proposed that the Universe started in the totally empty geometry and then made a quantum tunneling transition to a non-empty state (subatomic in size), which through inflation (the Universe expands exponentially fast for a brief period of time which causes its size to increase dramatically) came to its current size.
Another idea is from Stephen Hawking and James Hartle. Hawking proposed a description of the Universe in its entirety, viewed as a self-contained entity, with no reference to anything that might have come before it. The description is timeless, in the sense that one set of equations delineates the Universe for all time. As one looks to earlier and earlier times, one finds that the model Universe is not eternal, but there is no creation event either. Instead, at times of the order of 10-43 seconds, the approximation of a classical description of space and time breaks down completely, with the whole picture dissolving into quantum ambiguity. In Hawking's words, the Universe "would neither be created nor destroyed. It would just BE."
So, the origin of mass in the Universe and the Universe itself is quite speculative at this point. If you are interested, you can read Alan Guth's book "The Inflationary Universe", page 271-276. You can also read Hawking's "A brief history of time: From the Big Bang to black holes" page 136.
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