The Multiverse

FAS Astronomers Blog, Volume 31, Number 6.

This is part 5 of a 5-part series on the Universe (The Visible Universe, The Dark Universe, The Expanding Universe, The Microscopic Universe, and The Multiverse).

Define the Universe and give five examples. Many years ago, this would be a joke, but today maybe not. Recently, astronomers have been having serious discussions about the concept of a multiverse (aka parallel universes). I’m not quite sure how much of this is science and how much is speculation, but it is interesting.

In 2003, Max Tegmark introduced four possible types (or levels) of multiverses.

1. The infinite universe
2. The bubble universe
3. The quantum (many worlds) universe
4. The mathematical universe

Physicist Brian Greene goes a little deeper and, in his book The Hidden Reality, proposes nine categories of multiple universes. We won’t go into those in this article.

To set the stage for these different types of multiverses, we should first explain a little about the universe itself.

The Big Bang and Cosmic Inflation

Astronomers believe that the universe began some 13.8 billion years ago. We, thanks to astronomer Fred Hoyle, call the beginning of the universe the Big Bang. There are some features of the universe that are hard to explain with the standard theory of the Big Bang. So, in the early 1980s, Alan Guth, Andrei Linde, and others came up with an idea called Cosmic Inflation. They theorized that just after the Big Bang the universe went through a period of rapid expansion in a fraction of a second. The universe expanded exponentially and increased from the size of a subatomic particle to the size of a grapefruit (or maybe a basketball for those of us in North Carolina). In doing so any curvature was flatten out and the true extent of the universe was pushed out well beyond our horizon.

Quantum Mechanics

Quantum mechanics is the theory of the very small (subatomic particles). It says that the universe is discrete – it is made up of small buckets of energy and particles. And it is strange (very strange).

This is best exemplified by the double slit experiment. Light passes through a barrier with a slit and spreads out. The light waves then pass through a double slit. The light produces an interference pattern on a screen beyond the double slit. Because light is a wave, the waves alternate between constructive and destructive interference – bright and dark spots appear on the screen. Light is also a particle called a photon. It turns out that individual photons exhibit the same interference pattern. It even works for other quantum particles such as electrons – they also exhibit an interference pattern. It is as if the single particle goes through both slits at the same time. Here comes the interesting part. If you try and measure which slit a particle goes through, the interference pattern disappears, and the particle goes through a single slit.

Quantum theory has other strange features. One is Heisenberg’s uncertainty principle. It says that there are things we don’t know for sure. There is an inherent uncertainty in the momentum and position of a quantum particle. We can never know exactly both – there is always some uncertainty. The same holds true for a particle’s energy level and time. Therefore, quantum theory is not deterministic, it is governed by probability.

Albert Einstein didn’t believe in all this. He thought there was a deeper level to quantum mechanics that would prove to be deterministic. He is famous for saying “I don’t believe God plays dice with the universe.” However, it appears he was wrong. Steven Hawking responded to Einstein in his book The Nature of Space and Time by saying, “God does play dice, but that he sometimes confuses us by throwing them where they can’t be seen.”

The Multiverse(s)

#1 The Infinite Universe

The first type of multiverse is the infinite universe. We can see out some 46 billion light years in every direction. This sets the diameter of the visible universe at 92 billion light years. But how far beyond this does the universe extend? We don’t know. Maybe it goes on forever! If the universe is infinite, then everything repeats itself over and over again. So, the multiverse is just layers and layers of universes stretched out into infinite space.

#2a The Bubble Universe

Alan Guth proposed the first version of cosmic inflation in 1981. There were some flaws in his approach that were quickly addressed by Andrei Linde and others. Linde soon proposed concepts of inflation such as the “self-replicating universe” where bubbles of inflation continue to branch off of previous bubbles.

Therefore, the second form of the multiverse is the bubble universe. If our universe began with a bubble of cosmic inflation, there could be other universes popping into existence all over. Each universe exists in its own “bubble” of space/time with possibly its own laws of physics.

#2b The Brane Universe

There is another version of the bubble universe. String theory says that we live on a three-dimensional membrane (a brane) floating in ten- or eleven-dimension space. Each of these branes represents a separate and distinct universe.

In 2002, Paul Steinhardt and Neil Turok used this idea of branes to develop their theory of a cyclic universe. Here the Big Bang is just the collision of two three-dimensional branes. Once they collide, they drift apart, cool down, and eventually move back together for another collision. The universe is cyclic, it repeats over and over again.

#3 The Quantum Universe

The third multiverse is based on quantum mechanics.

The standard version of quantum mechanics is the Copenhagen interpretation introduced by Niels Bohr and others in the 1920s. It says that at a quantum level everything exists as a probability wave (i.e., particles go through both slits). The most famous metaphor for this is Schrodinger’s Cat. The cat sits in a box with a radioactive element. If the element decays it emits radiation and the cat dies, if it does not, the cat lives. Until we look in the box, the cat exists in a ghostly state – both dead and alive. When we perform an experiment (or look in the box), the wave collapses and a specific outcome occurs. In other words, a particle goes through a single slit – the cat is either dead or alive, but not both.

In 1956, Hugh Everett proposed a different interpretation. It is commonly called the Many Worlds interpretation. Here the outcome of every event causes the universe to branch off into different quantum worlds. In one I turned left and in another I turned right. In one the particle goes through the left slit, while in another it goes through the right. In one the cat dies and in another the cat lives.

You can find this concept in science fiction programs (e.g., Sliders, Stargate SG-1, and The Man in the High Castle). Usually there are doorways between different parallel universes that folks travel through. They typically find alternate versions of their world and themselves where everything is almost the same, but not quite.

#4 The Mathematical Universes

The last is the mathematical universes. It says that all possible universes that can be defined by mathematical analysis exist. This is because every universe has an equal possibility of being real, so if our universe is real, why not all the others?

Conclusion (Sort of)

The scientific proof of the multiverse is still out of our reach. And some, such as George Ellis, think it is far more theory than fact. But, over time, science has discovered that we do not live in a privileged place. The Earth is not at the center of the universe, the center of our galaxy, or even the center of our solar system. It is an ordinary planet orbiting an ordinary star in an ordinary galaxy in an ordinary galactic cluster. Is it also in an ordinary universe – one of many? Only time will tell.