Recently a new claim of a multiverse has captivated many cosmologists. They claim that our currently expanding universe is not the only one, but rather that there are billions of other universes that also exist. This would mean that our universe is only one of countless universes.
When discussing this one should note that the word “multiverse” has a few meanings, as defined by cosmologists. For example, we can see our cosmic visual horizon which is at 42 billion light years away. Thus, some argue, it may be reasonable to suggest that there are many more domains like ours in our universe where the same laws of physics operate. This widely accepted view is what is called a “level 1” multiverse. However, there are cosmologists who venture further than the level 1 multiverse by suggesting that there are actually many different types of universes. These universes are speculated to have different physics, different histories, and possibly teeming with life. This is known as the “level 2” multiverse as proposed by Alexander Vilenkin. However, many find reason to doubt this level 2 multiverse, as the renowned cosmologist George Ellis, from the University of Cape Town, explains:
“Similar claims have been made since antiquity by many cultures. What is new is the assertion that the multiverse is a scientific theory, with all that implies about being mathematically rigorous and experimentally testable. I am skeptical about this claim. I do not believe the existence of those other universes has been proved—or ever could be. Proponents of the multiverse, as well as greatly enlarging our conception of physical reality, are implicitly redefining what is meant by “science” (1).
Ellis argues that if these alleged parallel universes exist beyond our cosmic horizon then how can cosmologists imply that they are, in some way, experimentally testable? Furthermore, there are several more theories of how cosmologists view this. For instance, it is theorized that some of these universes could be situated in regions of space that are far beyond our own, or that they might even exist at different points in time. Others theorize that these universes may even exist in the same space we do but in a different branch of the quantum wave function, or that they could even be completely disconnected from our spacetime. The most widely accepted is that of chaotic inflation, as Ellis goes on to explain:
“The idea is that space at large is an eternally expanding void, within which quantum effects continually spawn new universes like a child blowing bubbles. The concept of inflation goes back to the 1980s, and physicists have elaborated on it based on their most comprehensive theory of nature: string theory. String theory allows bubbles to look very different from one another. In effect, each begins life not only with a random distribution of matter but also with random types of matter. Our universe contains particles such as electrons and quarks interacting through forces such as electromagnetism; other universes may have very different types of particles and forces–which is to say, different local laws of physics. The full set of allowed local laws is known as the landscape. In some interpretations of string theory, the landscape is immense, ensuring a tremendous diversity of universes” (2).
However, these proposed theories, including the favoured chaotic inflation theory, run into some difficulties. The cosmic visual horizon is the limit to how far away we can see into space based upon signals traveling toward us at the speed of light. Since the beginning of the universe these signals have not had enough time to reach us from so far out. It is outside this cosmic horizon where parallel universes are alleged to exist, and no matter how sophisticated our technological advancements ever become, argues Ellis, it will still be beyond our capabilities: “All the parallel universes lie outside our horizon and remain beyond our capacity to see, now or ever, no matter how technology evolves. In fact, they are too far away to have had any influence on our universe whatsoever. That is why none of the claims made by multiverse enthusiasts can be directly substantiated” (3).
1. Ellis, G. 2011. Does the Multiverse Really Exist? Available.
2. Ellis, G. 2011. Ibid.
3. Ellis, G. 2011. Ibid.