Is the Infinite Universe Theory the Most Probable Explanation of our cosmos?
The Infinite Universe Theory states that the multiverse consists of an infinite number of universes, as well as the infinite nature of the multiverse.
The first piece of evidence that points to the infinite universe theory is Level 1 of Green and Tegmark’s levels of the universe- An extension of our universe. Level 1 states the presence of Hubble Volumes, which are spherical regions of the observable universe surrounding an observer. Furthermore, the possibility of light reflecting from any object crossing or outside the Hubble Volume is near zero, as the space between objects at the Hubble horizon and the viewer will have an average expansion rate equal to the speed of light.
The infinite universe theory can hold in terms of quantum theory and quantum states as well. For instance, The boundary condition that the number of universes of the multiverse would not depend on the value of the scale factor of a particular single universe induces the creation of multiple universes, born from universes themselves. Thus, the thermodynamic properties of these universes would also be symmetrical and uniform. Another interesting concept is the wave function. The wave function has a constant value however when measurements are made at the quantum level, the wave functions show multiple values. Max Tegmark, in his paper “Born in an Infinite Universe states that neglecting all other values obtained for the wave function is a mistake. He reaches a conclusion that these “quantum probabilities are unified with spatial observer frequencies, and the same infinite, homogeneous space that provides a real, physical collection also provides a natural measure with which to count”, thus hinting at presence of infinite universes.
Also, the cosmological constants, as well as Planck map and The Wilkinson Microwave Anisotropy Probe support acceleration and expansion for finite time.
The Penrose model helps explain the infinite size and number of universes. Roger Penrose, in his paper published in 2005, compactified space and time to generate the Penrose model. The Penrose model has 2 dimensions to it, space and time, which intersect at several points across each universe with end points representing infinite distance, infinite past and infinite future. The geometry of the model is such that more and more space and time is covered between 2 consecutive intersections, pointing to the fact that each universe is infinite in nature. Furthermore, the model is conformal in nature as it preserves the internal angles between space and time. According to the geometry of the Penrose model, only massless bodies travelling at the speed of light, or photons can reach the boundaries of each universe, while bodies with mass can never reach the edge. At the edge of a universe, however, the coordinates of time are lost, indicating that at the end of one universe you would simply go to another universe.
On the other hand, the other theories that aim to explain the cosmos are comparatively inferior as they have certain drawbacks.
The theory of bubble universes is based on the foundation of eternal inflation. What is eternal inflation? In simple terms, it means the non-uniform, indefinite expansion of the universe. Stephen Hawking, in his last paper titled ‘A Smooth exit from eternal inflation’, explains why the model of eternal inflation is wrong. Due to the non-uniform expansion, the model follows a quantum field instead of a classical one and movement of particles differ in classical and quantum fields. However, the dynamics of eternal inflation wipes out the separation between classical and quantum physics. As a consequence, Einstein’s theory of General Relativity breaks down in eternal inflation.
The concept of mathematical universes, proposed by Max Tegmark, predicts that there exists a set of universes completely independent of humans, which is represented by mathematical equations. Tegmark, in his book titled ‘our mathematical universe’, states that all mathematical structures exist and equations of M-theory that constitute the building blocks of a mathematical universe are just some mathematical structures, complicated enough for us to live in. But there is no prediction of what these structures could possibly be or its implications on the multiverse theory and physics. This, combined with the impossibility of gathering observational data, makes it an empty idea, bound by no parameters and a mere philosophical discussion.
The theory of the daughter universe functions on the principle of probability and proposes that for all outcomes of a given situation, a separate and distinct universe exists. Due to the slowing of expansion of the universe over time, Multiple universes will be located at different points in space and time. Since entanglement of universes has not been proven, this means that although different probabilities could be carried out in different universes, they will do so in a different frame of time and space and hence have no relation with each other making them independent.
Lastly, Parallel universes dictate that there exist multiple other universes in space and time that are identical to the one we are living in and the arrangement of particles are also identical. To negate this criteria, Ethan Siegel, in a paper published in 2015, compares and contrasts the number of interacting quantum particles to the rate of growth of the possible universes. Considering the age of the universe of 13.8 billion years and a finite duration of cosmic inflation, the number of possible universes results in 10^10^50 values. In comparison, there exist 10^90 quantum particles in the universe which when taken 2 at a time for interaction, results in a number much larger than the number of universes. Hence, the theory of parallel universes doesn’t correspond to our understanding of physics today.
In conclusion, the above models contradict the laws of physics understood today, making them inferior explanations of the multiverse.