Paul M. Sutter is an astrophysicist at The Ohio State University, host of "Ask a Spaceman" and "Space Radio," and author of "Your Place in the Universe." Sutter contributed this article to Space.com's Expert Voices: Op-Ed & Insights.
It's a perennial sci-fi favorite: other worlds, other universes, other possibilities, right beyond the bounds of the known cosmos or just a flick of a magic device away. Maybe the other worlds are eerily similar to own except for some quirky little change in history. Maybe they're radically different, with new and weird and wonderful kinds of life. Maybe they're so different that the basic rules of physics don't even apply there.
I'm talking, of course, about the multiverse — the possibility that there are other distinct universes somewhere out there.
But do they exist?
The multiverse idea crops up in a few different areas of science, philosophy and even religion. Within physics itself, the possibility of other worlds appears in quantum mechanics (especially through the "many-worlds" interpretation, in which every random result in an interaction branches off new universes where each result is realized) and string theory (in which different arrangements of tiny, rolled-up extra dimensions lead to different physics in their own universes).
These ideas are fun to talk about, but perhaps the most enticing route to the multiverse is through our understanding of the earliest moments of the universe itself: the period of inflation.
As best we can tell, the exceedingly early universe underwent a period of incredibly rapid expansion: inflation. Scientists theorize that this epoch was driven by an exotic quantum field — a bubbling, frothing energy baked into the vacuum of space-time itself — that caused the universe to almost rip itself apart with the intensity of its expansion. But soon, that field lost its verve and decayed into the swarm of particles that now inhabit the present-day universe.
And it was in this epoch of inflation that a multiverse had a viable chance of appearing. Imagine the early, inflation-dominated universe as a giant loaf of rising bread. The story I just told assumes that inflation starts and stops uniformly throughout the cosmos; the whole loaf stops rising at the exact same moment.
But whatever drove inflation was a quantum field, which means we can't assume things are so simple. Some parts of the field were just randomly more energetic than others, while other spots were more sedate. That's just the random nature of life in a quantum world.
So our loaf of bread isn't exactly uniform. Some parts of it may stop rising, pinching themselves off and isolating themselves, while the rest of the loaf continues to rise. And since there will always be some part of the loaf rising faster than average, you can now imagine a loaf of bread that continues to rise for eternity, with little bits and pockets here and there randomly catching a slow portion and pinching themselves off.
Each of those pinched-off bits is its own unique universe, just one bubble of many in the multiverse. What's more, the details of the end of inflation determine a bunch of important physical properties, like the strength of the forces of nature and the masses of the particles (the details depend on the particular model of inflation). We're talking things like the mass of the electron and the number of particle species. Really important stuff.
And since each of these pinched-off, bubble universes ends inflation in its own unique and special way, each of the bubble universes gets its own unique set of physics.
Since the whole loaf of bread continues to rise (as there's always some part of it with a faster-than-average inflation rate), very quickly, you get a whole lot of universes all bubbling around each other. A multiverse. Each universe is essentially forever closed off from the others. However, there are some cases where they can intersect.
If two universes pinch off relatively soon after each other, and relatively close together, then they might briefly intersect before inflating away from each other for the rest of eternity. And if they do, that intersection might leave an imprint in the cosmic microwave background of those universes, the leftover fossil shell of light from when the universe was an infantile 380,000 years old.
Imagine two soap bubbles rubbing up against each other; this imprint will take the form of an odd-looking circle on the sky (where the "oddness" depends on what kind of universe we might be intersecting). Naturally, we haven't found any such traces of a multiverse.
Without direct evidence, it's hard to judge the viability of the multiverse-from-inflation idea. It could be a generic result of all inflation theories, so that if we are able to find stronger evidence for inflation, we might also demonstrate the existence of the multiverse. But right now, we don't fully understand the mechanics of inflation, so that's not much to go on.
And here's another question to ponder. If you changed the physics of our universe just a little bit, then life as we know it would be impossible. Why do we find ourselves in such a special universe, where our type of life can survive? Is it perhaps because of all the bubbles in the multiverse, of course we would find ourselves in one of the few capable of sparking intelligence? (Or else we wouldn't be aware of it.) Or is it something much deeper? Is this even a question for science?
Maybe another universe has all the answers.
Learn more by listening to the episode "Do We Live in the Multiverse?" on the "Ask a Spaceman" podcast, available on iTunes and on the web at http://www.askaspaceman.com. Thanks to @ProducerEv, Matthew A., Tom S., Taha, Oliver H., Wally H., Christiaan D., Keith G., Alex Z., Murtaza P., Kent L., Gabi P. and Slinkerdeer for the questions that led to this piece! Ask your own question on Twitter using #AskASpaceman or by following Paul @PaulMattSutter and facebook.com/PaulMattSutter.
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