What if somewhere out there, there was an exact replica of yourself reading this very sentence right now, but instead of finishing the sentence, they stop right here to go do something else? What if there was a version of you that was a rockstar, a president, a cartel leader or an astronaut? Well, according to popular theory, there is. He/she is about 101029 meters from where you are sitting right now.
Common knowledge tells us that the universe is everything there is, right? So if that is true then we can’t have two universes, because that would be everything there is, and our universe would only be half of what there is.
The problem here is terminology. Physicists speaking informally often say “universe” when they really mean ‘observable universe’ (also known as the Hubble volume) which is the part of the universe that we are able to see. If there was life on the edge of our observable universe, they would experience a different observable universe then we do, simply due to their position. So, in that sense, it is perfectly acceptable to talk about multiple observable universes.
That idea is relatively well understood. When physicists talk about multiple universes, they usually do not refer to such a simple concept.
So with that out of the way, lets dive right into the three leading multiverse theories.
Level 1: Regions beyond our Cosmic Horizon Theory
There are infinitely many planets, with infinitely many people who have the same exact memories, experiences, names, and looks as you do. They come from the same family, and they live in the same city. If space is infinite, it means infinite probabilities, which means anything that is slightly possible becomes a certainty. It’s like asking yourself “what are the chances ____ happens?” Well, somewhere out there, the chance is 100%. This is the level one multiverse theory.
Although this seems absolutely crazy, this cosmological model is the simplest and the most popular theory on the subject.
In order for this theory to work, two things must be assumed.
- The universe is infinite
- The universe is uniformed. (Meaning that every single combination of particles will take place in an infinite universe.)
But what is ‘infinite’ and how big is space?
The observational universe grows by a light year every single year, so we keep uncovering new things within our universe. In school everyone is taught about simple Euclidean space, which states that every point in three-dimensional space is determined by three coordinates. So it’s really difficult to imagine how space could not be infinite. However, in 1915, Einstein came along with his theory of general relativity, which allowed space to be finite by being differently connected than Euclidean space. Einstein’s theory stated that the shape of space resembled a four-dimensional sphere, or a doughnut shape, so that traveling in one direction would ultimately bring you back to where you started from in the opposite direction.
The physics description of the world is usually split into two parts: initial conditions, and laws of physics specifying how the initial conditions evolve. Our observable universe’s gravitational clustering formed galaxies, stars, planets and other structures. Because of this, it is assumed that every possible matter configuration occurs in some Hubble volume far away, and it’s also assumed that we should expect our own Hubble volume to be relatively typical – at least typical among Hubble volumes that contain life.
Now for some numbers.
This theory states that about 101029 meters away, there is an exact replica of you. At about 101091 meters away, there is a sphere of radius 100 light-years that is completely identical to the one centered here, so everything we experience in the next century will be identical to those of our counterparts. At about 1010115 meters away, there is an entire Hubble volume that is exactly identical to ours.
Level 2: Bubble Universe theory
Level two is like level one, but instead of just one universe, it is an infinite set of distinct universes, some with different dimensionality and different physical constants. This is what is predicted by cosmic inflation.
Let’s break down cosmic inflation really quick.
In the beginning of the 20th century, astronomers started to notice that galaxies were moving away from each other, and the further apart they were, the faster they moved. They theorized that this meant that the universe was expanding and that at some point in the past, it must have been very small, dense and hot. So they called this idea the Big Bang.
In the 1960’s, two astronomers discovered something called the Cosmic Microwave Background radiation (CMB) which was a faint glow and temperature that seemed to permeate the entire universe. The CMB supported the Big Bang because the only way two extremes of the universe could have the same exact temperature was if at one point in the past, the extremes were close enough together to act on each other.
The CMB theory traced the origins to 380,000 years after the Big Bang occurred, the point in time where matter began to clump together enough for light to move around freely. Today, inflation is the best theory of how the universe formed in those early moments of expansion. Inflation is the mathematical details that explain what happened before the 380,000 year horizon of the CMB.
When the universe was small, it followed the rules of quantum physics. Energy rose out of nothing, particles came in and out of existence, and their probabilities all mixed together. Inflation theorized that the universe expanded faster than the speed of light, and that this rapid acceleration created ripples in the fabric of space-time. These fluctuations from which the ripples were born expanded to form peaks and troughs in the universe which allowed matter to clump into the matter that we see today.
As stated, level two contains infinite distinct universes, each with different dimensionality and different physical constraints. The other domains however, are infinitely far away in the sense that you would never get there even if you traveled at the speed of light forever. This is because the space between our level one multiverse and its neighbors is still undergoing inflation, which causes space to keep stretching and creating volume faster than light can travel through it.
Kind of depressing.
Besides just allowing space to expand, cosmic inflation also does other things like create “bubbles” within space that are not affected by inflation. These bubbles are where universes are made, just like ours. They form by breaking off of other bubbles in a chain reaction. This chain reaction never stops, and the number of bubbles in space is infinite.
This is actually really kind of terrifying, because it means that there is no beginning of time and there wasn’t just one Big Bang; there is and always will be an infinite number of inflating bubbles just like the one that we live in that have each experienced their very own Big Bang.
Besides just being characterized by the particles that make it up and the dimensions that it holds, the universe we live in is also characterized by a set of dimensionless numbers that are known as physical constants. Physical constants are physical quantities that are generally believed to be both universal in nature and constant in time. Some examples of physical constants are the speed of light (c), the gravitational constant (G), and Planck’s constant (h). There are models in the level two theory in which such continuous parameters vary from one post-inflationary bubble to the next.
All in all, the level two multiverse is way more diverse than the level one multiverse. The level two multiverse contains domains where not only can the conditions differ, but the dimension it is in, the particles that make it up, and the physics it experiences can also differ.
This means that there can literally be a universe where the planets are all made out of candy and inhabited by lava lamps.
Let’s take a quick little breather here before we go any further.
You know that feeling you get when you stand up from the couch after laying down for a long time? Ya know, when everything starts to go black, you get light-headed, grasp onto the nearest object and desperately fight the urge to sit back down? Well, that’s kinda what it feels like to read about the level three theory.
Level 3: Quantum Universe Theory
The third theory is one in which the parallel universes aren’t so far away.
The fundamental equations of physics appear to be what mathematics would call ‘unitary’ meaning that the equation will always give an outcome for each possible situation. If this is true, then the Universe is constantly branching into parallel universes. Whenever a quantum event appears to have a random outcome, all outcomes occur, one in each branch.
The strange part about this is that there are quantum events that correspond to situations that seem to make no sense, such as you being in two different places at once. Take for example, schrödinger’s famous thought experiment, where a contraption kills a cat if a radioactive atom decays. Because the radioactive atom eventually enters a state of decayed and not decayed, it produces a cat which is both dead and alive. Only upon opening the box does one event play out, and thus the universes split.
When talking about parallel universes, it is important to distinguish between two different ways of viewing a physical theory: the outside view of a mathematician studying its mathematical fundamental equations called the bird perspective, and the inside view of an observer living in the world described by the equations, called the frog view.
From the bird perspective, the level three multiverse is surprisingly simple: there is only one reality and it evolves smoothly over time without any sort of splitting or parallelism. The abstract quantum world described by this evolving reality has a large number of parallel story lines that are constantly merging and splitting.
From the frog perspective, however, each observer sees only a small fraction of the full reality. They only see their own Hubble volume (level 1) and this view will prevent them from perceiving level III parallel copies of themselves. When they are asked a question, they make a decision and answer the question. From a bird perspective, their single past branches into multiple futures. From the frog perspective though, each copy of them is unaware of the other copies and they perceive this quantum branching as simply a slight randomness. Afterwards, there are multiple copies of themselves that have the same memories up until the point when they answer the question.
As I bring this article to a close, I think it is important to remember just how little we know about the Universe we live in. We are a floating rock with life on it, surrounded by other floating rocks that may or may not have life on it, revolving around a floating rock that is on fire – and the rest is just a blur.
I am going to the kitchen, now, to eat a sandwich.