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A few months ago
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the Nobel Prize in physics
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was awarded to two teams of astronomers
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for a discovery that has been hailed
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as one of the most important
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astronomical observations ever.
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And today, after briefly describing what they found,
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I'm going to tell you about a highly controversial framework
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for explaining their discovery,
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namely the possibility
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that way beyond the Earth,
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the Milky Way and other distant galaxies,
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we may find that our universe
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is not the only universe,
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but is instead
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part of a vast complex of universes
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that we call the multiverse.
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Now the idea of a multiverse is a strange one.
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I mean, most of us were raised to believe
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that the word "universe" means everything.
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And I say most of us with forethought,
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as my four-year-old daughter has heard me speak of these ideas since she was born.
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And last year I was holding her
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and I said, "Sophia,
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I love you more than anything in the universe."
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And she turned to me and said, "Daddy,
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universe or multiverse?"
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(Laughter)
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But barring such an anomalous upbringing,
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it is strange to imagine
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other realms separate from ours,
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most with fundamentally different features,
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that would rightly be called universes of their own.
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And yet,
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speculative though the idea surely is,
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I aim to convince you
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that there's reason for taking it seriously,
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as it just might be right.
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I'm going to tell the story of the multiverse in three parts.
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In part one,
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I'm going to describe those Nobel Prize-winning results
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and to highlight a profound mystery
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which those results revealed.
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In part two,
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I'll offer a solution to that mystery.
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It's based on an approach called string theory,
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and that's where the idea of the multiverse
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will come into the story.
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Finally, in part three,
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I'm going to describe a cosmological theory
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called inflation,
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which will pull all the pieces of the story together.
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Okay, part one starts back in 1929
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when the great astronomer Edwin Hubble
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realized that the distant galaxies
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were all rushing away from us,
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establishing that space itself is stretching,
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it's expanding.
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Now this was revolutionary.
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The prevailing wisdom was that on the largest of scales
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the universe was static.
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But even so,
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there was one thing that everyone was certain of:
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The expansion must be slowing down.
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That, much as the gravitational pull of the Earth
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slows the ascent of an apple tossed upward,
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the gravitational pull
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of each galaxy on every other
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must be slowing
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the expansion of space.
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Now let's fast-forward to the 1990s
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when those two teams of astronomers
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I mentioned at the outset
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were inspired by this reasoning
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to measure the rate
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at which the expansion has been slowing.
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And they did this
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by painstaking observations
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of numerous distant galaxies,
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allowing them to chart
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how the expansion rate has changed over time.
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Here's the surprise:
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They found that the expansion is not slowing down.
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Instead they found that it's speeding up,
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going faster and faster.
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That's like tossing an apple upward
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and it goes up faster and faster.
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Now if you saw an apple do that,
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you'd want to know why.
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What's pushing on it?
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Similarly, the astronomers' results
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are surely well-deserving of the Nobel Prize,
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but they raised an analogous question.
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What force is driving all galaxies
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to rush away from every other
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at an ever-quickening speed?
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Well the most promising answer
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comes from an old idea of Einstein's.
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You see, we are all used to gravity
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being a force that does one thing,
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pulls objects together.
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But in Einstein's theory of gravity,
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his general theory of relativity,
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gravity can also push things apart.
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How? Well according to Einstein's math,
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if space is uniformly filled
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with an invisible energy,
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sort of like a uniform, invisible mist,
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then the gravity generated by that mist
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would be repulsive,
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repulsive gravity,
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which is just what we need to explain the observations.
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Because the repulsive gravity
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of an invisible energy in space --
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we now call it dark energy,
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but I've made it smokey white here so you can see it --
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its repulsive gravity
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would cause each galaxy to push against every other,
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driving expansion to speed up,
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not slow down.
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And this explanation
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represents great progress.
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But I promised you a mystery
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here in part one.
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Here it is.
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When the astronomers worked out
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how much of this dark energy
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must be infusing space
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to account for the cosmic speed up,
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look at what they found.
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This number is small.
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Expressed in the relevant unit,
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it is spectacularly small.
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And the mystery is to explain this peculiar number.
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We want this number
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to emerge from the laws of physics,
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but so far no one has found a way to do that.
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Now you might wonder,
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should you care?
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Maybe explaining this number
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is just a technical issue,
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a technical detail of interest to experts,
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but of no relevance to anybody else.
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Well it surely is a technical detail,
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but some details really matter.
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Some details provide
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windows into uncharted realms of reality,
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and this peculiar number may be doing just that,
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as the only approach that's so far made headway to explain it
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invokes the possibility of other universes --
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an idea that naturally emerges from string theory,
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which takes me to part two: string theory.
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So hold the mystery of the dark energy
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in the back of your mind
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as I now go on to tell you
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three key things about string theory.
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First off, what is it?
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Well it's an approach to realize Einstein's dream
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of a unified theory of physics,
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a single overarching framework
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that would be able to describe
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all the forces at work in the universe.
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And the central idea of string theory
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is quite straightforward.
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It says that if you examine
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any piece of matter ever more finely,
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at first you'll find molecules
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and then you'll find atoms and subatomic particles.
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But the theory says that if you could probe smaller,
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much smaller than we can with existing technology,
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you'd find something else inside these particles --
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a little tiny vibrating filament of energy,
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a little tiny vibrating string.
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And just like the strings on a violin,
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they can vibrate in different patterns
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producing different musical notes.
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These little fundamental strings,
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when they vibrate in different patterns,
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they produce different kinds of particles --
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so electrons, quarks, neutrinos, photons,
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all other particles
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would be united into a single framework,
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as they would all arise from vibrating strings.
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It's a compelling picture,
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a kind of cosmic symphony,
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where all the richness
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that we see in the world around us
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emerges from the music
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that these little, tiny strings can play.
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But there's a cost
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to this elegant unification,
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because years of research
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have shown that the math of string theory doesn't quite work.
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It has internal inconsistencies,
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unless we allow
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for something wholly unfamiliar --
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extra dimensions of space.
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That is, we all know about the usual three dimensions of space.
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And you can think about those
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as height, width and depth.
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But string theory says that, on fantastically small scales,
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there are additional dimensions
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crumpled to a tiny size so small
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that we have not detected them.
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But even though the dimensions are hidden,
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they would have an impact on things that we can observe
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because the shape of the extra dimensions
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constrains how the strings can vibrate.
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And in string theory,
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vibration determines everything.
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So particle masses, the strengths of forces,
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and most importantly, the amount of dark energy
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would be determined
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by the shape of the extra dimensions.
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So if we knew the shape of the extra dimensions,
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we should be able to calculate these features,
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calculate the amount of dark energy.
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The challenge
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is we don't know
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the shape of the extra dimensions.
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All we have
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is a list of candidate shapes
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allowed by the math.
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Now when these ideas were first developed,
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there were only about five different candidate shapes,
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so you can imagine
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analyzing them one-by-one
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to determine if any yield
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the physical features we observe.
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But over time the list grew
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as researchers found other candidate shapes.
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From five, the number grew into the hundreds and then the thousands --
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A large, but still manageable, collection to analyze,
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since after all,
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graduate students need something to do.
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But then the list continued to grow
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into the millions and the billions, until today.
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The list of candidate shapes
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has soared to about 10 to the 500.
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So, what to do?
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Well some researchers lost heart,
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concluding that was so many candidate shapes for the extra dimensions,
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each giving rise to different physical features,
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string theory would never make
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definitive, testable predictions.
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But others turned this issue on its head,
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taking us to the possibility of a multiverse.
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Here's the idea.
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Maybe each of these shapes is on an equal footing with every other.
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Each is as real as every other,
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in the sense
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that there are many universes,
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each with a different shape, for the extra dimensions.
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And this radical proposal
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has a profound impact on this mystery:
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the amount of dark energy revealed by the Nobel Prize-winning results.
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Because you see,
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if there are other universes,
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and if those universes
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each have, say, a different shape for the extra dimensions,
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then the physical features of each universe will be different,
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and in particular,
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the amount of dark energy in each universe
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will be different.
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Which means that the mystery