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Physics + MathPhysics & Math

A Tour of the Multiverse

ByDelia Schwartz-PerlovThe Nature of RealityThe Nature of Reality

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Once upon a time—roughly 13.7 billion years ago, to be more precise—our universe was created in the Big Bang. From a virulent exploding fireball bursting with elementary particles, the universe evolved into the pitch-black expanse we currently observe, majestically sprinkled with star-studded galaxies. But is this really the full story? Today, cosmologists are learning that the incredible tale of our universe might be just one slim chapter in a much bigger volume: a book of universes that is infinitely large—and that is still being written.

In this video, I’ll explain why scientists think that our universe may be just one of many. I invite you to watch the pencast, then read on!

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Our entire observable universe is very large, having a diameter of roughly 40 billion light years, and encompassing hundreds of billions of galaxies, each of which may contain billions of stars. Yet in the version of cosmology that I study, called eternal inflation, this is just a small fraction of an infinitely large universe which itself is only one out of an infinite number of other universes! Each of these “other” universes is the product of its own “local” big bang. Instead of being the ultimate creation event, our Big Bang merely marks the emergence of our local universe into the far grander “multiverse,” like a lone bubble materializing in an infinite flute of fizzing champagne. Each bubble represents an infinite universe, and our cosmic champagne flute is home to an infinite number of bubbles!

A Tour of the Multiverse-schwartz-perlov-bubbles-300x212.jpg

Bubbles within bubbles: The multiverse of eternal inflation.

So where do all the bubbles come from? Why couldn’t we just have one universe? Cosmologists believe that our universe experienced a period of very rapid expansion, called inflation, in the moments after the Big Bang, and they have confirmed this with a great deal of observational and theoretical evidence. It turns out that if the universe ever underwent an inflationary stage, then, even if inflation ends in one place, elsewhere inflation will continue. Once inflation starts, it never ends! The result is a mind-boggling proliferation of universes.

But there is more than one way to make a multiverse. The multiverse also emerges from string theory, which suggests that we actually live in 10 or 11 dimensions, not the mundane four (three spatial dimensions plus time) that we’re used to! That’s seems like a stretch, but since string theory is our best candidate for getting gravity and quantum mechanics to agree with each other, we need to take the idea of extra dimensions seriously. Since we only experience four dimensions, we must ask: where did all the extra dimensions go? This is where an idea called “compactification” comes in. Physicists have been able to show that if we start with a higher dimensional world, some of the extra dimensions can be “compactified” so that we don’t “experience” them directly. However, different compactifications describe lower dimensional universes with different physical properties, so we can experience them indirectly!

It turns out that there is a huge number of different ways to compactify these extra dimensions—googols of ways, in fact! All these different options offer a huge menu of different types of possible universes.

Take eternal inflation and add it to string theory, and you have not just an enormous number of possible universes, but a mechanism for creating them. Eternal inflation sees to it that the multiverse actually gets populated by each option on the menu via an ongoing process of nested bubble nucleations.