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,

they produce different kinds of particles --

so electrons, quarks, neutrinos, photons,

all other particles

would be united into a single framework,

as they would all arise from vibrating strings.

It's a compelling picture,

a kind of cosmic symphony,

where all the richness

that we see in the world around us

emerges from the music

that these little, tiny strings can play.

But there's a cost

to this elegant unification,

because years of research

have shown that the math of string theory doesn't quite work.

It has internal inconsistencies,

unless we allow

for something wholly unfamiliar --

extra dimensions of space.

That is, we all know about the usual three dimensions of space.

And you can think about those

as height, width and depth.

But string theory says that, on fantastically small scales,

there are additional dimensions

crumpled to a tiny size so small

that we have not detected them.

But even though the dimensions are hidden,

they would have an impact on things that we can observe

because the shape of the extra dimensions

constrains how the strings can vibrate.

And in string theory,

vibration determines everything.

So particle masses, the strengths of forces,

and most importantly, the amount of dark energy

would be determined

by the shape of the extra dimensions.

So if we knew the shape of the extra dimensions,

we should be able to calculate these features,

calculate the amount of dark energy.

The challenge

is we don't know

the shape of the extra dimensions.

All we have

is a list of candidate shapes

allowed by the math.

Now when these ideas were first developed,

there were only about five different candidate shapes,

so you can imagine

analyzing them one-by-one

to determine if any yield

the physical features we observe.

But over time the list grew

as researchers found other candidate shapes.

From five, the number grew into the hundreds and then the thousands --

A large, but still manageable, collection to analyze,

since after all,

graduate students need something to do.

But then the list continued to grow

into the millions and the billions, until today.

The list of candidate shapes

has soared to about 10 to the 500.

So, what to do?

Well some researchers lost heart,

concluding that was so many candidate shapes for the extra dimensions,

each giving rise to different physical features,

string theory would never make

definitive, testable predictions.

But others turned this issue on its head,

taking us to the possibility of a multiverse.

Here's the idea.

Maybe each of these shapes is on an equal footing with every other.

Each is as real as every other,

in the sense

that there are many universes,

each with a different shape, for the extra dimensions.

And this radical proposal

has a profound impact on this mystery:

the amount of dark energy revealed by the Nobel Prize-winning results.

Because you see,

if there are other universes,

and if those universes

each have, say, a different shape for the extra dimensions,

then the physical features of each universe will be different,

and in particular,

the amount of dark energy in each universe

will be different.

Which means that the mystery

of explaining the amount of dark energy we've now measured

would take on a wholly different character.

In this context,

the laws of physics can't explain one number for the dark energy

because there isn't just one number,

there are many numbers.