Most scientists now believe that the Universe, as we know it, began with a Big Bang. The idea is that, if we could somehow "run the movie of time backwards", we would see that the Universe was physically smaller in the very distant past. If this assumption is true, then at some specific moment even before that, all of the material of the Universe must have existed at a single point in space.
Therefore, in "real" forward time, everything of the Universe would have seemed to have burst out of this single point in space, and that's why it is called a Big Bang.
Physicists have speculated about the details about how this could actually happen. They suggest that there were momentary stages where only energy could have existed, then, as this incredibly hot environment cooled down a little, actual material (protons, electrons, neutrons) came into existence, then the outward momentum of all the original contents continued to carry everything outward to eventually be distributed throughout the Universe as we now see everything.
Keep in mind that this sort of subject only involves very little actual fact and a LOT of speculation on the part of the scientists! No one does or even can ever know the details of what would have actually happened in those first fractions of a second.
All of the currently supported theories about the very beginning of the Universe require completely ignoring a number of the basic principles of Physics. The field of Physics was build on ideas like the Conservation of Energy and the Conservation of Momentum. It seems really troublesome to just dismiss such concepts, and instead claim that the circumstances were so unusual that different rules applied! The premise described below does not seem to have these problems. It seems to comply with both Conservation of Energy and Conservation of Momentum. In that, I believe it has potential value.
To suggest how incomplete the logic necessarily is in these matters, I will now suggest an additional feature that does not seem to be included in such speculations, even though it must certainly have been true, if a Big Bang event occurred.
A similar logical situation exists for us on Earth. Say you decided to go on a long trip, and you decided to go EXACTLY west. No matter what you encounter, oceans, mountains or whatever, you continue to go exactly west. Well, after traveling about 25,000 miles, exactly west, (in a direction that you believe to be an exactly straight line) you will likely come upon very familiar territory, where you started from! You would arrive home from the east! If you hadn't known that the Earth was a giant ball, which you had gone around, you would certainly be really confused at how you could have arrived back home after going absolutely and perfectly straight west continuously.
More than that, if your home town had significantly changed during your long journey, you might not even recognize it and keep going. After going around the earth fifty times, you might conclude that the Earth is certainly larger than 50 x 25,000 or 1,250,000 miles! Of course, you would have been wrong!
Obviously, we can see that you hadn't been precisely going perfectly west after all! All along, without knowing it, you had been very slightly going downward as well. In a mile of walking exactly west (on a perfectly round, perfectly smooth earth), it turns out that you actually also curved downward around 16 inches without knowing it.
This is essentially the same kind of effect on that light beam that seems to be going absolutely straight through the Universe. The actual curvature of its path would not even be noticeable to us at all, but even if we could sense it, the curvature would be so small as to be virtually unnoticeable.
OK. The preparation is now in place! Now imagine that REALLY early Universe, where everything that exists could still fit in an imaginary "box" a mile on a side. Inside that Universe, gravitation would be incredibly strong, because all the mass that will ever exist is all so close together in there. All that stuff we just considered about a (straight) light beam must be true in that Universe. So, if a light beam was aimed in any direction, it would ACTUALLY be continuously bent back by that gravitational effect to permanently loop back and forth through that Universe (until it eventually ran into some object and got absorbed). Since everything is much closer together, gravitation effects are especially strong, so the curvature would be very severe. So, how long would the light beam seem to be, from the thing that created it? If that beam never ran into anything, the light beam would appear to travel forever IN A STRAIGHT LINE! Even though the entire Universe could fit inside an imaginary box a mile on a side!
Again, this is just like the trip on the Earth's surface. What is the longest trip you could ever make on Earth? Well, it could be infinitely long. Even though the Earth could fit in a really big box, you would be able to travel exactly west forever on its surface. The reality would be that you would wind up going around and around the Earth, but the path could be infinitely long.
Here is my logic about that concept. It is also speculation, but it seems at least as valid as the speculations of others in the field.
Our known Universe seems to be virtually entirely MATTER, with only the tiniest occasional amounts of ANTI-MATTER in it. But, the two are essentially identical. In principle, if enough anti-matter had accumulated somewhere in the Universe, it could have gravitationally collapsed and formed a star, just like all other stars (and the Sun) began. Such an anti-matter star would appear exactly like any other star we could ever see, and there would be no way that we could ever know that it was anti-matter rather than matter, by its light or its motion.
But, if such an anti-matter star actually existed, it could not exist very long in our matter Universe. The "empty" Universe is actually filled with all kinds of loose atoms and simple molecules, and meteoroids and asteroids and comets and probably planets. There are therefore a LOT of loose things just wandering around space, and most of these things must certainly be MATTER. When such an object would get pulled into an anti-matter star by its gravity, the matter of the object would combine with some of the anti-matter of the star to instantly annihilate each other, both instantly disappearing and creating a LOT of new energy. It seems certain that an anti-matter star or planet would soon be entirely annihilated in this way in our matter Universe.
That implies that virtually everything in our Universe is MATTER. So, what about considering an initial raw energy source that spontaneously divided itself into a LOT of MATTER, which went in one direction, and an EXACT SAME amount of ANTI-MATTER, which went in exactly the opposite direction? On a nuclear scale, physicists regularly see such energy (photons) spontaneously divide into electron-positron pairs (electrons and anti-electrons) exactly in this way. I am just proposing that it had occurred (somehow) on a grand scale.
This concept could then explain why all the material of the Universe seemed to appear at the same moment at the same place, if a Big Bang type of event actually occurred. Actually, it could also represent an ongoing process, where new material continues to enter our Universe! It just seems to me to present a more logical explanation of how it might have happened.
There is a really interesting consequence of this premise! Precisely the same number of protons and electrons and anti-protons and anti-electrons would have to have been formed. The physical law of conservation of momentum would insist that each such pair of object must necessarily move off in precisely opposite directions. From standard symmetry arguments of Physics, I don't see how it could be avoided that our MATTER Universe and the proposed ANTI-MATTER Universe must be precise "inverted mirror images" of each other! They would even have PRECISELY the same initial conditions in each of them. Therefore, all of the later gravitational interactions would have to have proceeded identically in each of them. That would imply that both of these mirror image Universes must progress absolutely identically. The materials that gravitationally came together five billion years ago to form the Sun must have been matched in the anti-Universe in the forming of the anti-Sun! And then, the anti-Earth, and anti-oceans, and anti-dinosaurs, and anti-people, and anti-computers, and anti-YOU! It seems to me that the initial symmetry of the beginning of the two Universes must necessarily have set the conditions for absolutely identical progression in both of them. This is essentially implying that the anti-ME is now typing these letters on the keyboard of his anti-computer on the anti-Earth in that anti-Universe!
More than that, I have no way of even establishing that I am the original! HE certainly thinks of himself as the original and so he considers ME as the anti-HIM!
Fortunately, the two hypothetical Universes could never meet each other, or even be in any contact with each other, because any such contact would necessarily involve mutual annihilation of the message or the person or the Universe! If they should ever meet, every particle in our Universe would exactly meet its anti-particle, and the existence of the entire Universe(s) would end. Still conserving Energy and Momentum. So, there's absolutely no way to ever confirm or deny my speculation about the anti-ME!
I find that part interesting to think about, but essentially irrelevant. More important to me is that a matter/anti-matter pair of Universes allows a logical description to be possible as to where everything originally came from!
OK! This accounts for TWO mirror-image Universes. But the beginning of this said there might be four. What gives?
Well, the premise as presented still required the pre-existence of a lot of energy, that somehow spontaneously split apart into the matter and anti-matter. Could that original energy that is necessary also be explained?
Another characteristic of nuclear physics is that it is possible for two (identical) photons to meet each other and just disappear! Essentially, this is like two identical waves in water or in a vibrating guitar string or anywhere else, which are exactly out-of-phase with each other. If two such identical waves are traveling in exactly the same direction and they are exactly out-of-phase (by 180°) then they completely "cancel" each other and disappear.
One of the central understandings of nuclear physics is that any event that occurs could occur as seen is real time or it could also occur as if time was running backwards. This seems to imply that, if two photons can completely annihilate each other, leaving nothing, then a beginning "nothing" must also be able to split into two photons. Again, things like the various conservation laws of physics would insist that the two photons thus created are (1) absolutely identical, but of opposite phase; and (2) traveling away from the point of creation in precisely opposite directions.
Therefore, my speculation on the beginning of everything adds one more step prior to the division of initial energy into matter and anti-matter Universes. The new step is an earlier division of NOTHING into mirror-image, identical energy streams traveling in precisely opposite directions from the original point of origin.
This actually provides, not only the energy stream necessary to later create our matter Universe and my hypothetical anti-matter Universe, but a SECOND identical (mirror image) energy stream going in the opposite direction. It seems to me that, due to the standard symmetry arguments, whatever cause OUR original energy stream to divide itself into matter and anti-matter, would also have occurred under the identical conditions of the OTHER original energy stream. That seems to imply to me that TWO MORE identical Universes must have come about. And, since the symmetry arguments must still be precisely true for both of these division stages, the four resultant Universes must necessarily have progressed precisely identically. Wow! That means there are probably FOUR MEs typing away! (The ME here, the energy-anti-matter me, the anti-energy-matter me, and the anti-energy-anti-matter me!) The four of us would all experience the exact same sensations, in precisely the same ways. Even though I (this me!) refer to anti-matter and anti-matter Universes, the me that is is in any of the other Universes also thinks of himself as being in the matter Universe, so they would refer to me as being in an anti- Universe!
As I keep repeating, ANYONE's opinion on such things is sheer speculation, because there is so little actual evidence that exists about these matters. But I have always liked the elegance of this approach because it begins with absolutely NOTHING, and accounts for all the mass and energy that we know exists in our Universe, where other theories, like most Big Bang theories all just see all that mass and energy just popping up without explanation. Their theories always say that the laws of physics just didn't apply at that time, to allow their speculation to be possible. I have always been uncomfortable with subjectively claiming that the laws of physics either apply or don't apply based on things some scientist might want to have happen. At least, my theory here does NOT require weird or different laws of physics to have ever existed. That single reason is why I see potential value in this theory!
C Johnson, Physicist, Physics Degree from Univ of Chicago