1. Evidence of a multiverse ↑
Earlier this month, physicists working in the UK and Canada found evidence that there may be universes beyond our own. Their research, co-authored by Stephen M. Feeney, Matthew C. Johnson, Daniel J. Mortlock, and Hiranya V. Peiris, is to be published in the journal Physical Review D and can be read for free here.
Fenney et al. have found a way to search for evidence of the multiverse predicted by eternal inflation theory. Eternal inflation refers to the inflationary epoch of the big bang, a period when spacetime expanded faster than the speed of light. This idea was first proposed by American physicist Alan Guth in 1981.
Whereas standard inflation has a beginning and an end, eternal inflation does not necessarily have a beginning and continues forever. This is because inflation can end in one region of spacetime, forming a 'pocket universe' like our own, but continue in other regions, creating an infinite amount of other pocket universes.
Eternal inflation is a consequence of a number of modern theories including string theory, which predicts that some of these universes will have different physical constants to our own. This means there could be universes with more, or less, dimensions, or a higher gravitational constant to our own.
In 2007, Guth stated that:
"essentially all inflationary models are eternal...if it starts anywhere, at any time in all of eternity, it produces an infinite number of pocket universes".
Although these pocket universes may have once been close enough to collide, no one could travel between them, even if they travelled at the speed of light forever. This is because the space between universes is expanding even faster than that.
Eternal inflation theory predicts that collisions between universes leave distinct patterns in the temperature distribution of the cosmic microwave background radiation - light that was created shortly after the big bang. NASA's WMAP (Wilkinson Microwave Anisotropy Probe) has been measuring these changes since its launch, in 2001, and Fenney et al. have created a computer algorithm that can identify the predicted effects of a collision from among this data.
Although they don't yet have enough evidence to show that eternal inflation theory is correct, they have found four regions that are not consistent with the standard theory of inflation, and these are shown in this image. The orange shape is the WMAP Cold Spot, a giant void, and the red shape had been detected previously, but the green and blue shapes are new discoveries.
Fenney et al. will soon be able to study these regions further by applying their algorithm to data compiled by the European Space Agency's (ESA's) Planck satellite, WMAPs successor, which was launched in 2009. The Planck satellite is ten times more sensitive than WMAP, and it has three times the resolution, although its data is not due to be released until 2013.
2. A Multitude of Multiverses ↑
The theory of eternal inflation is not the only theory that predicts a multiverse. There are at least five types of multiverse predicted by modern physics, and they are all compatible. Swedish-American cosmologist Max Tegmark states that:
"the key question is not whether the multiverse exists but rather how many levels it has"[9a].
Multiverse Level #1
Space is extremely large, and matter can only take a limited number of forms before things start to repeat.
Tegmark asks us to imagine a two-dimensional universe with four particles of two different types. In this universe, matter can be arranged in 24=16 possible ways, and then shapes start to repeat. This means that objects in this universe are always about four objects away from their nearest duplicate. The same principle applies to our own universe.
16 types of objects can be built in a two-dimensional universe with four particles of two different types. After this, objects start to repeat. There are about 210118 types of objects in our universe. Image credit: Helen Klus/CC-NC-SA.
Tegmark states that the observable universe contains at least 10118 subatomic particles, which can be arranged in 210118 different ways. Given the size of the observable universe, this means that we are always about 1010118 metres from our nearest duplicate.
This idea is based on the assumption that space is indeed this large, and that matter is distributed in a roughly even manner. Both of these assumptions can be tested by measuring the temperature distribution of the cosmic microwave background radiation, and data from WMAP suggests that this is the case. This is perhaps the least controversial theory that predicts a multiverse.
10118 = 1 followed by 118 zeros
= 10, 000,000,000, 000,000,000, 000,000,000, 000,000,000, 000,000,000, 000,000,000, 000,000,000, 000,000,000, 000,000,000, 000,000,000, 000,000,000, 000,000,000, 000,000,000.
1010118 = 1 followed by 10118 zeros
If this was written down, the sentence would be 1,000,000,000, 000,000,000 (one billion, billion) times longer than this one:
10, 000,000,000, 000,000,000, 000,000,000, 000,000,000, 000,000,000, 000,000,000, 000,000,000, 000,000,000, 000,000,000, 000,000,000, 000,000,000.
Multiverse Level #2
The theory of eternal inflation predicts that multiverse #1 is one of many 'pocket universes'.
All theories of eternal inflation predict that there are other pocket universes, and string theory predicts that some of these will have different physical constants to our own. This is the multiverse that Fenney et al. are searching for.
Multiverse Level #3
Quantum mechanics shows that there are an infinite amount of #1 and #2 multiverses, many of which exist in the same spacetime as our own. We are simply unaware of them.
Everything in multiverses #1 and #2 obey the laws of quantum mechanics. The Everett, or many worlds, approach to quantum mechanics argues that multiverses #1 and #2 are in a superpositional state, so that we only experience one of an infinite amount of parallel worlds that exist in the same spacetime as our own. Although this multiverse theory is often considered the most controversial, some argue that we have more evidence for the Everett approach than any of the other multiverse theories.
Multiverse Level #4
Physicists predict that some of the intelligent beings in Multiverses #1, #2, and #3 will be able to create artificial realities, inside of computers, that will be nearly identical to our own.
A number of physicists and mathematicians have suggested that we will one day be able to use quantum computers in order to create virtual reality environments that are indistinguishable from real life. We may even be able to create virtual copies of people that think and feel like 'real' people. This makes it possible that we are currently living in one of these simulations.
Multiverse Level #5
Beyond Multiverses #1, #2, #3, and #4, the ultimate multiverse is composed of nothing but mathematics, and so multiverses of every conceivable shape exist.
As our understanding of the world has developed, we have been able to use mathematical models to describe nature with increasing accuracy. This has led some to argue that a complete mathematical description of the universe would be a perfect description. The idea that the universe is really a mathematical object is attributed to Ancient Greek philosopher Plato, and is echoed in the intuitive idea that mathematical laws are discovered, rather than invented.
Tegmark states that if the universe is mathematical then "complete mathematical symmetry" suggests that multiverses containing universes of every possible shape must exist, and other multiverses may obey different physical laws to our own[9b].
3. What does all this mean? ↑
The idea that there are other universes may seem far-fetched, but all of these ideas are based on widely accepted theories, and many make unique predictions that we'll one day be able to test.
The fact that people find the concept of a multiverse so bizarre does not make it any less likely to be true. The concept of the multiverse is perhaps no less strange to us than the concept of a moving Earth was to people in the 1500s.
We only have an intuition for the physics prehistoric people needed to survive, and every time we've looked at things that are larger, smaller, faster or more massive than that, we have had to confront long accepted assumptions about the world.
History has taught us that if we are going to ask profound questions, then we shouldn't expect mundane answers.