Continuing from the last post, here are five more awesomely mind-blowing theories about parallel universes.
5. Eternal Inflation
The idea that the universe is expanding has become accepted by almost all the human population. Everywhere we look, it is obvious that space is repelling itself, creating an inflation of our universe. However, some difficulties originally arose when working with this model. Mainly, through observation it was discovered that the universe has a uniform background microwave radiation, which implies that in the early universe photons and other subatomic particles were in connection with each other and were rapidly pushed apart. In 1981 this idea was proposed as the Inflationary Model. Inflationary cosmology posits that the early universe expanded exponentially quickly within a split second of the Big Bang. After this quick time period, inflation leveled off and the normal expansion of the universe began.
Instead of the universe expanding in a smooth and uniform way since the Big Bang, theorists began working with a universe that expanded extremely rapidly right at the beginning. This theory solves a lot of problems in cosmology, including the issue of background microwave radiation as mentioned above. But, like any good theory, some problems emerged. First and foremost among these problems was the idea that inflation suddenly turned off in the early universe. This is a big problem. There seems to be no obvious mechanism for the universe to stop inflating at the rate that it inflated during the early universe.
The idea of eternal inflation seeks to deal with that issue. In eternal inflation, some parts of the universe did not stop inflating. This created pockets of space-time where inflation still occurs and spawns new universes from ours. These bubble universes represent a false vacuum (as described above). If we consider our universe to be a true vacuum (the default state), then these bubble universes will attempt to find this default state through the mechanism of inflation. Thus, inflation never stopped outright. There are still bubble universes springing off of ours and creating parallel universes.
A corollary to eternal inflation is the idea of the chaotic eternal inflation. This theory looks to answer the question of what came before the Big Bang. Theorists have proposed that our universe came into existence due to a random quantum fluctuation in a pre-existing space-time continuum. In chaotic inflation, a primal space-time continuum is filled with little “bubbles” that can become universes.  As quantum fluctuations make minute changes in the value of the scalar fields in these bubbles, eventually some gain enough energy to expand. This could be how our universe was formed, in parallel with other universes in the space-time continuum. As in normal eternal inflation, a chaotic model also includes the possibility of other universes branching off of ours.
4. Holographic Principle
Quantum information is one of the newest and most perplexing physics ideas. Simply put, the idea of information is that it is an abstract physical quantity that describes what is and what is not in a small chunk of the universe. This chunk is defined as being Planck length, or 1.616 x 10^-35 meters, which is absurdly small. Even though theorists are still trying to understand exactly what information is in a quantum framework, using this idea has been key to many ideas in modern physics. One of the key aspects of information is that it can not be destroyed by a physical process. Although the field is still developing, this tenant is not disputed.
Information theories have caused problems with older physics theories, most notably Stephan Hawking’s black hole theories. In the 1970s, Hawking was able to prove that black holes actually do radiate, contrary to popular belief. The gravitational force of the black hole can effect the quantum fields surrounding it, allowing virtual particles to become real particles and escape from the black hole. Not only does a black hole radiate, it also evaporates over time. A black hole that does not consume more matter and energy will eventually disappear. But when we consider information theories, this causes a huge problem. If black holes are sucking in stars and other forms of matter, and then eventually evaporate and disappear, where did all that information go? Any matter that passes through the event horizon seems to be lost to the universe forever, which violates physical laws. This is known as the black hole information paradox.
In order to address this problem, physicist Gerard t’Hooft looked at another mystery of black holes. Contrary to what would be expected, the maximum entropy of a black hole increases with the radius squared instead of cubed as is expected. Addressing this oddity, t’Hooft proposed that the information description of the volume of 3-dimesional space is encoded on the 2-dimensional boundary of that space. For a black hole this would be the event horizon. Later string theorists took this idea and applied it to the universe.
Leading string theorists have proposed the idea that our 3-dimensional universe is in fact a projection created from a curved 2-dimensional brane that surrounds our volume of space. By evaluating 3-dimensional general relativity problems in a 2-dimensional quantum mechanical framework, theorists have been able to solve a variety of problems that are otherwise impossible. This is an amazing step, especially considering that it combines relativity with quantum mechanics. If the holographic principle is true, our universe is surrounded by a lower dimensional universe where all the information in our 3-dimensional lives are stored.
3. Artificial Universe
I used what was written on this topic for an article that was published on KnowledgeNuts. Please click on this link to learn more about how we might be living in an artificial universe.
2. Simulated Reality
With the popularity of huge online games such as World of Warcraft or Second Life, the idea of a whole world being contained in a computer simulation is not that bizarre. As computer technology continues to increase, programs have created more and more realistic, immersive computer worlds that players can get lost in. Seeing how quickly our computer technology is expanding, theorists have had to face the idea that our world and even our universe may be a computer simulation created by a technologically advanced civilization. As computer technology marches on, it is estimated that we are more likely to be living in a simulated reality than a real one.
That would take a big computer, but according to physicist Brian Greene in his book The Hidden Reality, a computer powerful enough to run a simulation of our world would be possible. Assuming that we could only rely on modern computer technology, a computer the size of the Earth could conduct from 10^33 to 10^42 operations per second. If you take into account that a human brain does about 10^24 operations in a lifetime, and multiply that number by the 100 billion inhabitants that have walked the Earth, you get a total of 10^35 operations. At its slowest an Earth sized computer could conduct as many operations as the human race in two minutes. That is using contemporary technology and not even considering advancements like quantum computing which would only make computers smaller and faster.
How would we test whether we are in a simulated universe? It would be extremely difficult, because we have to play by the physical laws in our universe, simulated or not. A scientist looking to prove or disprove our simulated existence would face the same conundrum as a scientist trying to prove whether the laws of physics are the same everywhere. Using the tools of our system only allows us to further discover things within the system. However, some physicists have proposed that a simulated universe would have problems with radiation levels, and would also contain glitches in the system. Keep probing the simulation enough, and you will find problems or inconsistencies. Glitches can creep into the system do to an incomplete understanding of physical laws.
For humans, many laws are keeping us together that we do not understand, but if they were removed or failed, we would notice the catastrophic implications. For example, imagine living in the 1700s. Scientists back then had no understanding of general relativity or quantum mechanics. Yet, if those laws were removed, life would end. If somehow a 1700 scientist was able to create a simulated universe, he or she would not be able to codify laws that were unknown. Thus the universe would fail. A same thing might happen if our universe were simulated. An incomplete understanding of physical laws could lead to glaring glitches in the system. We would just have to find them.
1. Mathematical Universe Hypothesis
This last concept about parallel universes is more of a philosophical idea than anything. It comes mainly from the brain of Max Tegmark, the controversial Swedish physics professor at Massachusetts. Tegmark has worked and published in the field of physics from the 1990s, but recently has become very outspoken on his idea of the Mathematical Universe Hypothesis. Tegmark’s MUH argues that our universe is not just defined by mathematics, but it actually is mathematics.
In the MUH, reality is defined by what can and can not be mathematically described. But, Tegmark takes this idea even farther. According to him, our universe is made of mathematics. What we perceive as being real is the mathematical relations between objects. In these proposals, Tegmark asks that theorists strip away any notion that mathematical principles are describing an underlying structure of the cosmos. Forces do not exist to be described by mathematics. Rather, the forces are merely that name that humanity has assigned to the mathematical interaction between objects. In order to make sense of this, we need to reassess our viewpoint of what mathematics is. Instead of relying on the age-old assumption that it is the “language of the cosmos”, mathematics should actually be defined as the relations between other mathematical objects, that make up the world that we live in. Mathematics should not described as mere equations or even abstract entities, but rather concrete objects known as mathematical structures.
On the surface level, the MUH seems to represent a new revision on old school Platonism, but unlike the older Greek philosophy, the theory only talks about mathematics, not abstract Platonic ideals. The MUH may be a little hard to understand, because it is a complete re-evaluation of the universe and what mathematics actually means. Most physicists do not support the theory. It appears to be un-testable and delves into philosophical realms that physicists are generally unwilling to probe.
Even if the MUH is not well accepted, it does bring up some interesting ideas about parallel universes. In the MUH, any idea that can be mathematically defined is real. Remember, mathematics is not just some abstract entity. It is actually the structure of the universe. Thus, since theorists have been able to mathematically describe parallel universes they must exist. This includes odd ideas like a universe with nothing in it. A universe of this type is mathematically viable, thus according to Tegmark it must exist, otherwise you would not be able to calculate it.