We generally believe that there is only one Universe containing all the space, matter, and energy. Numerous changes occurred in the Universe since the Big Bang 13.8 billion years ago. Still, the scientists are discovering new phenomena occurring around. Currently, we can see an enormous amount of the Universe which is about 46 billion light years in all directions. While the fabric of space is continuously expanding since the beginning of the Universe.
Are there more Universes beyond Ours?
As the scientists discover new galaxies in the space and the new phenomena occurring every day, they think that there might be more universes like our own. According to them, there might be a larger space-time structure with a countless number of universes embedded within it. If our best theories come out to be correct, then our one Universe is not enough.
If we look into the beginning of the Big Bang, the scientists suggest that it was a hot bath of radiations, particles, and antiparticles. At that time, everything was massless and moving around at the speed of light. Due to which, the temperature and the energy of the particles were extremely high: around 1028 K. Moreover, spontaneous creation due to particle collisions and annihilation of particle/antiparticle pairs used to occur rapidly and frequently.
Cosmic Inflation Theory
Now let’s have a look at how the Universe cools and expands. How it creates more matter than antimatter and eventually forms nuclei and atoms, stars and galaxies, and gravitational clumps?
The scientists describe a theory of cosmic inflation to discuss the conditions responsible for the origin of the hot Big Bang. According to this theory, the universe could be the energy inherent to the fabric of space before the Big Bang occurred. This energy got bound up in the radiations, particles, and antiparticles when the Big Bang originated.
The scientists think that if the whole energy of the Universe gets wrapped up in a form which is inherent to space itself, it will still expand. However, it might not become cool or less dense as the post-Big Bang Universe does. It might expand exponentially instead. This means that after a small but fixed time duration, the Universe might get double in size. According to their estimate, this time duration could be around 10-35 seconds. So, as 10-34 seconds pass by, the size of the Universe might increase by a factor of 210, or 1024.
The key realization of the cosmic inflation theory is that the energy density of the Universe remains constant. Therefore, the energy inherent to space remains the same in every region despite the increase in the volume of the Universe. So, if the Universe inflates, it will simply create more and more Universe that would be still inflating.
Based on the concept of constant energy density, the point arises that there may be only one Universe. Since the volume of the Universe increases tremendously and relentlessly without a bound. This happens only until the inflation comes to an end. And when the inflation ends, all the energy inherent to space converts into radiations, particles, and antiparticles. This means that the end of inflation coincides with the beginning of the Big Bang.
Quantum Nature of Universe:
As we all know that, all the matter that exists physically inherently quantum in nature. This includes all the particles and fields. The quantum particles are strange as they also possess wave-like properties. Another important characteristic of the quantum particles is that their positions are no longer fixed in the Universe. So, they are described by using a probability distribution. To describe the wave characteristics of the quantum particles, we use wave function.
Now if we consider the quantum nature of the universe, it is not that simple as for a free electron. This is because our Universe is continuously undergoing expansion and inflation according to the cosmic inflation theory.
As our space expands and inflates, various changes occur in different regions of the space. Let us consider some of the regions undergoing inflation and expansion randomly. In some of the regions, the inflation ends. While in others, the inflation continues, however, it looks like close to an ending. And in some other regions, it continues on more robustly.
Now let us work out the probabilities, for all viable models of inflation. It shows that the amount of space where inflation occurs and doesn’t end always increases over time. The regions where inflation occurs and it ends, we get a Big Bang. However, outside of all these regions, there will be a place where inflation didn’t end. Space continues inflating there. With each new instance, the inflation will end or it will continue on farther into the future.
The regions where the inflation ends, we get a Big Bang where it does. And the regions where inflation continues will be surrounded by a broader space-time. If we further elaborate, there will be regions in each of the inflating regions, where inflation would end and a Big Bang will occur. Each of those numerous Big Bangs will give rise to their own observable Universe, just like ours. However, those universes will be having a different starting point and different initial conditions for each region.
All such regions will be separated by more inflating space. Furthermore, no two Universes will ever collide or interact with each other. This is how the concept of eternal inflation arises leading to the existence of the multiverse and the existence of many disconnected Universes.
If we accept that inflation is the stage which occurred in the past prior to the Big Bang, and the Universe formed is inherently quantum in nature, then the existence of a multiverse can’t be denied. Although we are not able to observe other Universes yet, we can still observe a large number of evidences for inflation. This indirectly points to the inevitability of other universes. This means that our Universe is one of the countlessly many Universes, where all the universes are embedded in an eternally inflating and expanding background.