Universe or Multiverse?

Before 100 years ago, the concept of multiverse was meaningless. According to most of the scientists, the universe was considered as infinite and boundless, it was always there as its current form and stable, yet it does not have a beginning and an end. Then it was discovered that the universe was not stable at all, but it was expanding, all galaxies and known matter are traveling apart from each other. Scientists started thinking that if universe was expanding its size, then it means all the matter was coming from a smaller point, and so the Big Bang theory was born.

According to the theory, the universe was not eternal at all, but had a beginning. And it was not endless, the total matter and energy it contains was finite. After the construction of Hubble telescope and locating it at the outer space, the orbit of earth, we had the opportunity of the most possible clear views of the universe ever. Since the light has a speed and takes longer to travel from further distances, the further out we looked, the further back in time we see. The Hubble telescope was able to see the things until 14 billion light years away, which is considered the age of the universe, the time the big bang happened.

Nobody knows what was before the big bang, or what will the ultimate fate of the universe be. Since the Big Bang the universe continues to expend, surprisingly with an accelerating speed, caused by dark energy. Because we cannot see and measure the dark energy, it is hard to say whether it is constant, increasing or decreasing. The three different scenario about the fate of the universe depends on dark energy. The most likely scenario is if dark energy is constant, then the universe will continue its expansion forever, that will be a cold and lonely place, the big freeze. Another scenario will be in action in case the dark energy increases its power by time, then the acceleration of expansion of the universe will increase so fast that at one point, the gravity will not be able to hold the atoms of matters together, this will bring an end to galaxies, stars, planets etc. even the atom itself will be split into protons neutrons and electrons and those into smaller particles, quarks. Those quarks will move so fast and apart from each other that will never be able to combine and create atoms, therefore the matter, which is called the big rip. The last possible scenario is if the dark energy is losing power, then by time, gravitational force of galaxies will eventually overcome over the impulsing force of dark energy, therefore the expansion of universe will stop at some point, and then this time the process will be reversed and the universe will start shrinking with an accelerating speed, until all matter and energy squeeze in one single point, the big crunch, which eventually will cause a new big bang.

Whatever will be the fate of the universe, we all know now that universe is not eternal, and it contains finite amount of matter and energy. This statement brings us the critical question to ask. Is the universe alone, or are there other independent universes somewhere? It is quite fair to ask this question, as the universe is finite, and if the all cosmos is infinite, therefore there can be infinite number of other universes, the multiverse. Our universe is expanding, which means all the matter we know are moving apart from each other. But if there are some other matter somewhere out that moving towards us, not the apart, that means that matter did not originated from our own big bang, but from another independent big bang, another universe. 

Unlike big bang and evolution, the multiverse is not a theory, but a hypothesis, as we have no evidence and no possible way to perform an observation to test that hypothesis at that very moment. We cannot see beyond the observable universe, but the complete darkness, we simply do not know what else is out there, or there is something else out there or not. The problem is that we might never be able to find it out, as the light has a limited speed, the empty space between those possible universes could be so extensive, that the first light originated from another big bang could reach to our observable universe after trillions and trillions of light years, a time length that could be longer that the entire possible life of our own universe.

The amount of dark energy in our universe is just the right amount to make our universe the way it is today. Our universe makes up approximately %68 dark energy, %26 dark matter and %6 ordinary matter. Dark energy is in fact, according to quantum physics, the pure energy of the perfect vacuum of space. The value of dark energy is constant everywhere of space in our universe. In the atomic level of space, it is incredibly small amount, approximately 1.10⁻¹²⁰ .This combination has the critical importance, for example, 1.10⁻¹¹⁷ a bit more powerful dark energy would still be a very small amount, but it would cause a tremendous accelerate of the expansion of the universe right after the big bang that the galaxies and stars, therefore life would never exist, and 1.10⁻¹²³ less dark energy would cause the shrinking of the universe a short time after the big bang because of the power of gravitational force, once again the universe of today would never exist either. The reason of this sensitive balance is still a mystery, however, if we live in a multiverse, then it is not a mystery at all, as there are infinite number of other universes, it is just one among the infinite number of combinations. There might be billions of other universes that the dark energy has the wrong amount, and thus those are uninhabitable, and the others, those are similar to our universe.