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  • A few months ago

  • the Nobel Prize in physics

  • was awarded to two teams of astronomers

  • for a discovery that has been hailed

  • as one of the most important

  • astronomical observations ever.

  • And today, after briefly describing what they found,

  • I'm going to tell you about a highly controversial framework

  • for explaining their discovery,

  • namely the possibility

  • that way beyond the Earth,

  • the Milky Way and other distant galaxies,

  • we may find that our universe

  • is not the only universe,

  • but is instead

  • part of a vast complex of universes

  • that we call the multiverse.

  • Now the idea of a multiverse is a strange one.

  • I mean, most of us were raised to believe

  • that the word "universe" means everything.

  • And I say most of us with forethought,

  • as my four-year-old daughter has heard me speak of these ideas since she was born.

  • And last year I was holding her

  • and I said, "Sophia,

  • I love you more than anything in the universe."

  • And she turned to me and said, "Daddy,

  • universe or multiverse?"

  • (Laughter)

  • But barring such an anomalous upbringing,

  • it is strange to imagine

  • other realms separate from ours,

  • most with fundamentally different features,

  • that would rightly be called universes of their own.

  • And yet,

  • speculative though the idea surely is,

  • I aim to convince you

  • that there's reason for taking it seriously,

  • as it just might be right.

  • I'm going to tell the story of the multiverse in three parts.

  • In part one,

  • I'm going to describe those Nobel Prize-winning results

  • and to highlight a profound mystery

  • which those results revealed.

  • In part two,

  • I'll offer a solution to that mystery.

  • It's based on an approach called string theory,

  • and that's where the idea of the multiverse

  • will come into the story.

  • Finally, in part three,

  • I'm going to describe a cosmological theory

  • called inflation,

  • which will pull all the pieces of the story together.

  • Okay, part one starts back in 1929

  • when the great astronomer Edwin Hubble

  • realized that the distant galaxies

  • were all rushing away from us,

  • establishing that space itself is stretching,

  • it's expanding.

  • Now this was revolutionary.

  • The prevailing wisdom was that on the largest of scales

  • the universe was static.

  • But even so,

  • there was one thing that everyone was certain of:

  • The expansion must be slowing down.

  • That, much as the gravitational pull of the Earth

  • slows the ascent of an apple tossed upward,

  • the gravitational pull

  • of each galaxy on every other

  • must be slowing

  • the expansion of space.

  • Now let's fast-forward to the 1990s

  • when those two teams of astronomers

  • I mentioned at the outset

  • were inspired by this reasoning

  • to measure the rate

  • at which the expansion has been slowing.

  • And they did this

  • by painstaking observations

  • of numerous distant galaxies,

  • allowing them to chart

  • how the expansion rate has changed over time.

  • Here's the surprise:

  • They found that the expansion is not slowing down.

  • Instead they found that it's speeding up,

  • going faster and faster.

  • That's like tossing an apple upward

  • and it goes up faster and faster.

  • Now if you saw an apple do that,

  • you'd want to know why.

  • What's pushing on it?

  • Similarly, the astronomers' results

  • are surely well-deserving of the Nobel Prize,

  • but they raised an analogous question.

  • What force is driving all galaxies

  • to rush away from every other

  • at an ever-quickening speed?

  • Well the most promising answer

  • comes from an old idea of Einstein's.

  • You see, we are all used to gravity

  • being a force that does one thing,

  • pulls objects together.

  • But in Einstein's theory of gravity,

  • his general theory of relativity,

  • gravity can also push things apart.

  • How? Well according to Einstein's math,

  • if space is uniformly filled

  • with an invisible energy,

  • sort of like a uniform, invisible mist,

  • then the gravity generated by that mist

  • would be repulsive,

  • repulsive gravity,

  • which is just what we need to explain the observations.

  • Because the repulsive gravity

  • of an invisible energy in space --

  • we now call it dark energy,

  • but I've made it smokey white here so you can see it --

  • its repulsive gravity

  • would cause each galaxy to push against every other,

  • driving expansion to speed up,

  • not slow down.

  • And this explanation

  • represents great progress.

  • But I promised you a mystery

  • here in part one.

  • Here it is.

  • When the astronomers worked out

  • how much of this dark energy

  • must be infusing space

  • to account for the cosmic speed up,

  • look at what they found.

  • This number is small.

  • Expressed in the relevant unit,

  • it is spectacularly small.

  • And the mystery is to explain this peculiar number.

  • We want this number

  • to emerge from the laws of physics,

  • but so far no one has found a way to do that.

  • Now you might wonder,

  • should you care?

  • Maybe explaining this number

  • is just a technical issue,

  • a technical detail of interest to experts,

  • but of no relevance to anybody else.

  • Well it surely is a technical detail,

  • but some details really matter.

  • Some details provide

  • windows into uncharted realms of reality,

  • and this peculiar number may be doing just that,

  • as the only approach that's so far made headway to explain it

  • invokes the possibility of other universes --

  • an idea that naturally emerges from string theory,

  • which takes me to part two: string theory.

  • So hold the mystery of the dark energy

  • in the back of your mind

  • as I now go on to tell you

  • three key things about string theory.

  • First off, what is it?

  • Well it's an approach to realize Einstein's dream

  • of a unified theory of physics,

  • a single overarching framework

  • that would be able to describe

  • all the forces at work in the universe.

  • And the central idea of string theory

  • is quite straightforward.

  • It says that if you examine

  • any piece of matter ever more finely,

  • at first you'll find molecules

  • and then you'll find atoms and subatomic particles.

  • But the theory says that if you could probe smaller,

  • much smaller than we can with existing technology,

  • you'd find something else inside these particles --

  • a little tiny vibrating filament of energy,

  • a little tiny vibrating string.

  • And just like the strings on a violin,

  • they can vibrate in different patterns

  • producing different musical notes.

  • These little fundamental strings,

  • when they vibrate in different patterns,