0:00:00.000,0:00:02.000
A few months ago

0:00:02.000,0:00:04.000
the Nobel Prize in physics

0:00:04.000,0:00:06.000
was awarded to two teams of astronomers

0:00:06.000,0:00:09.000
for a discovery that has been hailed

0:00:09.000,0:00:11.000
as one of the most important

0:00:11.000,0:00:13.000
astronomical observations ever.

0:00:13.000,0:00:15.000
And today, after briefly describing what they found,

0:00:15.000,0:00:18.000
I'm going to tell you about a highly controversial framework

0:00:18.000,0:00:21.000
for explaining their discovery,

0:00:21.000,0:00:23.000
namely the possibility

0:00:23.000,0:00:25.000
that way beyond the Earth,

0:00:25.000,0:00:28.000
the Milky Way and other distant galaxies,

0:00:28.000,0:00:30.000
we may find that our universe

0:00:30.000,0:00:32.000
is not the only universe,

0:00:32.000,0:00:34.000
but is instead

0:00:34.000,0:00:36.000
part of a vast complex of universes

0:00:36.000,0:00:38.000
that we call the multiverse.

0:00:38.000,0:00:41.000
Now the idea of a multiverse is a strange one.

0:00:41.000,0:00:43.000
I mean, most of us were raised to believe

0:00:43.000,0:00:46.000
that the word "universe" means everything.

0:00:46.000,0:00:49.000
And I say most of us with forethought,

0:00:49.000,0:00:52.000
as my four-year-old daughter has heard me speak of these ideas since she was born.

0:00:52.000,0:00:54.000
And last year I was holding her

0:00:54.000,0:00:56.000
and I said, "Sophia,

0:00:56.000,0:00:59.000
I love you more than anything in the universe."

0:00:59.000,0:01:01.000
And she turned to me and said, "Daddy,

0:01:01.000,0:01:03.000
universe or multiverse?"

0:01:03.000,0:01:06.000
(Laughter)

0:01:06.000,0:01:09.000
But barring such an anomalous upbringing,

0:01:09.000,0:01:11.000
it is strange to imagine

0:01:11.000,0:01:13.000
other realms separate from ours,

0:01:13.000,0:01:15.000
most with fundamentally different features,

0:01:15.000,0:01:18.000
that would rightly be called universes of their own.

0:01:18.000,0:01:20.000
And yet,

0:01:20.000,0:01:22.000
speculative though the idea surely is,

0:01:22.000,0:01:24.000
I aim to convince you

0:01:24.000,0:01:26.000
that there's reason for taking it seriously,

0:01:26.000,0:01:28.000
as it just might be right.

0:01:28.000,0:01:31.000
I'm going to tell the story of the multiverse in three parts.

0:01:31.000,0:01:33.000
In part one,

0:01:33.000,0:01:35.000
I'm going to describe those Nobel Prize-winning results

0:01:35.000,0:01:37.000
and to highlight a profound mystery

0:01:37.000,0:01:39.000
which those results revealed.

0:01:39.000,0:01:41.000
In part two,

0:01:41.000,0:01:43.000
I'll offer a solution to that mystery.

0:01:43.000,0:01:45.000
It's based on an approach called string theory,

0:01:45.000,0:01:47.000
and that's where the idea of the multiverse

0:01:47.000,0:01:49.000
will come into the story.

0:01:49.000,0:01:51.000
Finally, in part three,

0:01:51.000,0:01:53.000
I'm going to describe a cosmological theory

0:01:53.000,0:01:55.000
called inflation,

0:01:55.000,0:01:58.000
which will pull all the pieces of the story together.

0:01:58.000,0:02:02.000
Okay, part one starts back in 1929

0:02:02.000,0:02:04.000
when the great astronomer Edwin Hubble

0:02:04.000,0:02:07.000
realized that the distant galaxies

0:02:07.000,0:02:09.000
were all rushing away from us,

0:02:09.000,0:02:11.000
establishing that space itself is stretching,

0:02:11.000,0:02:13.000
it's expanding.

0:02:13.000,0:02:16.000
Now this was revolutionary.

0:02:16.000,0:02:19.000
The prevailing wisdom was that on the largest of scales

0:02:19.000,0:02:21.000
the universe was static.

0:02:21.000,0:02:23.000
But even so,

0:02:23.000,0:02:26.000
there was one thing that everyone was certain of:

0:02:26.000,0:02:29.000
The expansion must be slowing down.

0:02:29.000,0:02:32.000
That, much as the gravitational pull of the Earth

0:02:32.000,0:02:35.000
slows the ascent of an apple tossed upward,

0:02:35.000,0:02:37.000
the gravitational pull

0:02:37.000,0:02:39.000
of each galaxy on every other

0:02:39.000,0:02:41.000
must be slowing

0:02:41.000,0:02:43.000
the expansion of space.

0:02:43.000,0:02:46.000
Now let's fast-forward to the 1990s

0:02:46.000,0:02:48.000
when those two teams of astronomers

0:02:48.000,0:02:50.000
I mentioned at the outset

0:02:50.000,0:02:52.000
were inspired by this reasoning

0:02:52.000,0:02:54.000
to measure the rate

0:02:54.000,0:02:56.000
at which the expansion has been slowing.

0:02:56.000,0:02:58.000
And they did this

0:02:58.000,0:03:00.000
by painstaking observations

0:03:00.000,0:03:02.000
of numerous distant galaxies,

0:03:02.000,0:03:04.000
allowing them to chart

0:03:04.000,0:03:07.000
how the expansion rate has changed over time.

0:03:07.000,0:03:10.000
Here's the surprise:

0:03:10.000,0:03:13.000
They found that the expansion is not slowing down.

0:03:13.000,0:03:15.000
Instead they found that it's speeding up,

0:03:15.000,0:03:17.000
going faster and faster.

0:03:17.000,0:03:19.000
That's like tossing an apple upward

0:03:19.000,0:03:21.000
and it goes up faster and faster.

0:03:21.000,0:03:23.000
Now if you saw an apple do that,

0:03:23.000,0:03:25.000
you'd want to know why.

0:03:25.000,0:03:27.000
What's pushing on it?

0:03:27.000,0:03:29.000
Similarly, the astronomers' results

0:03:29.000,0:03:32.000
are surely well-deserving of the Nobel Prize,

0:03:32.000,0:03:36.000
but they raised an analogous question.

0:03:36.000,0:03:38.000
What force is driving all galaxies

0:03:38.000,0:03:41.000
to rush away from every other

0:03:41.000,0:03:44.000
at an ever-quickening speed?

0:03:44.000,0:03:46.000
Well the most promising answer

0:03:46.000,0:03:49.000
comes from an old idea of Einstein's.

0:03:49.000,0:03:51.000
You see, we are all used to gravity

0:03:51.000,0:03:54.000
being a force that does one thing,

0:03:54.000,0:03:56.000
pulls objects together.

0:03:56.000,0:03:58.000
But in Einstein's theory of gravity,

0:03:58.000,0:04:00.000
his general theory of relativity,

0:04:00.000,0:04:03.000
gravity can also push things apart.

0:04:03.000,0:04:06.000
How? Well according to Einstein's math,

0:04:06.000,0:04:08.000
if space is uniformly filled

0:04:08.000,0:04:10.000
with an invisible energy,

0:04:10.000,0:04:13.000
sort of like a uniform, invisible mist,

0:04:13.000,0:04:16.000
then the gravity generated by that mist

0:04:16.000,0:04:18.000
would be repulsive,

0:04:18.000,0:04:20.000
repulsive gravity,

0:04:20.000,0:04:23.000
which is just what we need to explain the observations.

0:04:23.000,0:04:25.000
Because the repulsive gravity

0:04:25.000,0:04:27.000
of an invisible energy in space --

0:04:27.000,0:04:29.000
we now call it dark energy,

0:04:29.000,0:04:32.000
but I've made it smokey white here so you can see it --

0:04:32.000,0:04:34.000
its repulsive gravity

0:04:34.000,0:04:36.000
would cause each galaxy to push against every other,

0:04:36.000,0:04:38.000
driving expansion to speed up,

0:04:38.000,0:04:40.000
not slow down.

0:04:40.000,0:04:42.000
And this explanation

0:04:42.000,0:04:44.000
represents great progress.

0:04:44.000,0:04:47.000
But I promised you a mystery

0:04:47.000,0:04:49.000
here in part one.

0:04:49.000,0:04:51.000
Here it is.

0:04:51.000,0:04:53.000
When the astronomers worked out

0:04:53.000,0:04:56.000
how much of this dark energy

0:04:56.000,0:04:58.000
must be infusing space

0:04:58.000,0:05:00.000
to account for the cosmic speed up,

0:05:00.000,0:05:02.000
look at what they found.

0:05:09.000,0:05:11.000
This number is small.

0:05:11.000,0:05:13.000
Expressed in the relevant unit,

0:05:13.000,0:05:15.000
it is spectacularly small.

0:05:15.000,0:05:18.000
And the mystery is to explain this peculiar number.

0:05:18.000,0:05:20.000
We want this number

0:05:20.000,0:05:22.000
to emerge from the laws of physics,

0:05:22.000,0:05:25.000
but so far no one has found a way to do that.

0:05:25.000,0:05:28.000
Now you might wonder,

0:05:28.000,0:05:30.000
should you care?

0:05:30.000,0:05:32.000
Maybe explaining this number

0:05:32.000,0:05:34.000
is just a technical issue,

0:05:34.000,0:05:37.000
a technical detail of interest to experts,

0:05:37.000,0:05:39.000
but of no relevance to anybody else.

0:05:39.000,0:05:42.000
Well it surely is a technical detail,

0:05:42.000,0:05:44.000
but some details really matter.

0:05:44.000,0:05:46.000
Some details provide

0:05:46.000,0:05:48.000
windows into uncharted realms of reality,

0:05:48.000,0:05:51.000
and this peculiar number may be doing just that,

0:05:51.000,0:05:54.000
as the only approach that's so far made headway to explain it

0:05:54.000,0:05:57.000
invokes the possibility of other universes --

0:05:57.000,0:06:00.000
an idea that naturally emerges from string theory,

0:06:00.000,0:06:03.000
which takes me to part two: string theory.

0:06:03.000,0:06:07.000
So hold the mystery of the dark energy

0:06:07.000,0:06:09.000
in the back of your mind

0:06:09.000,0:06:11.000
as I now go on to tell you

0:06:11.000,0:06:14.000
three key things about string theory.

0:06:14.000,0:06:16.000
First off, what is it?

0:06:16.000,0:06:19.000
Well it's an approach to realize Einstein's dream

0:06:19.000,0:06:22.000
of a unified theory of physics,

0:06:22.000,0:06:24.000
a single overarching framework

0:06:24.000,0:06:26.000
that would be able to describe

0:06:26.000,0:06:28.000
all the forces at work in the universe.

0:06:28.000,0:06:30.000
And the central idea of string theory

0:06:30.000,0:06:32.000
is quite straightforward.

0:06:32.000,0:06:34.000
It says that if you examine

0:06:34.000,0:06:36.000
any piece of matter ever more finely,

0:06:36.000,0:06:38.000
at first you'll find molecules

0:06:38.000,0:06:41.000
and then you'll find atoms and subatomic particles.

0:06:41.000,0:06:43.000
But the theory says that if you could probe smaller,

0:06:43.000,0:06:46.000
much smaller than we can with existing technology,

0:06:46.000,0:06:49.000
you'd find something else inside these particles --

0:06:49.000,0:06:52.000
a little tiny vibrating filament of energy,

0:06:52.000,0:06:55.000
a little tiny vibrating string.

0:06:55.000,0:06:57.000
And just like the strings on a violin,

0:06:57.000,0:06:59.000
they can vibrate in different patterns

0:06:59.000,0:07:01.000
producing different musical notes.

0:07:01.000,0:07:03.000
These little fundamental strings,

0:07:03.000,0:07:05.000
when they vibrate in different patterns,

0:07:05.000,0:07:07.000
they produce different kinds of particles --

0:07:07.000,0:07:09.000
so electrons, quarks, neutrinos, photons,

0:07:09.000,0:07:11.000
all other particles

0:07:11.000,0:07:13.000
would be united into a single framework,

0:07:13.000,0:07:16.000
as they would all arise from vibrating strings.

0:07:16.000,0:07:19.000
It's a compelling picture,

0:07:19.000,0:07:21.000
a kind of cosmic symphony,

0:07:21.000,0:07:23.000
where all the richness

0:07:23.000,0:07:25.000
that we see in the world around us

0:07:25.000,0:07:27.000
emerges from the music

0:07:27.000,0:07:30.000
that these little, tiny strings can play.

0:07:30.000,0:07:32.000
But there's a cost

0:07:32.000,0:07:34.000
to this elegant unification,

0:07:34.000,0:07:36.000
because years of research

0:07:36.000,0:07:39.000
have shown that the math of string theory doesn't quite work.

0:07:39.000,0:07:41.000
It has internal inconsistencies,

0:07:41.000,0:07:43.000
unless we allow

0:07:43.000,0:07:46.000
for something wholly unfamiliar --

0:07:46.000,0:07:49.000
extra dimensions of space.

0:07:49.000,0:07:52.000
That is, we all know about the usual three dimensions of space.

0:07:52.000,0:07:54.000
And you can think about those

0:07:54.000,0:07:57.000
as height, width and depth.

0:07:57.000,0:08:00.000
But string theory says that, on fantastically small scales,

0:08:00.000,0:08:02.000
there are additional dimensions

0:08:02.000,0:08:04.000
crumpled to a tiny size so small

0:08:04.000,0:08:06.000
that we have not detected them.

0:08:06.000,0:08:08.000
But even though the dimensions are hidden,

0:08:08.000,0:08:11.000
they would have an impact on things that we can observe

0:08:11.000,0:08:14.000
because the shape of the extra dimensions

0:08:14.000,0:08:17.000
constrains how the strings can vibrate.

0:08:17.000,0:08:19.000
And in string theory,

0:08:19.000,0:08:22.000
vibration determines everything.

0:08:22.000,0:08:24.000
So particle masses, the strengths of forces,

0:08:24.000,0:08:27.000
and most importantly, the amount of dark energy

0:08:27.000,0:08:29.000
would be determined

0:08:29.000,0:08:31.000
by the shape of the extra dimensions.

0:08:31.000,0:08:34.000
So if we knew the shape of the extra dimensions,

0:08:34.000,0:08:37.000
we should be able to calculate these features,

0:08:37.000,0:08:40.000
calculate the amount of dark energy.

0:08:40.000,0:08:42.000
The challenge

0:08:42.000,0:08:44.000
is we don't know

0:08:44.000,0:08:47.000
the shape of the extra dimensions.

0:08:47.000,0:08:49.000
All we have

0:08:49.000,0:08:51.000
is a list of candidate shapes

0:08:51.000,0:08:54.000
allowed by the math.

0:08:54.000,0:08:56.000
Now when these ideas were first developed,

0:08:56.000,0:08:58.000
there were only about five different candidate shapes,

0:08:58.000,0:09:00.000
so you can imagine

0:09:00.000,0:09:02.000
analyzing them one-by-one

0:09:02.000,0:09:04.000
to determine if any yield

0:09:04.000,0:09:06.000
the physical features we observe.

0:09:06.000,0:09:08.000
But over time the list grew

0:09:08.000,0:09:10.000
as researchers found other candidate shapes.

0:09:10.000,0:09:13.000
From five, the number grew into the hundreds and then the thousands --

0:09:13.000,0:09:16.000
A large, but still manageable, collection to analyze,

0:09:16.000,0:09:18.000
since after all,

0:09:18.000,0:09:21.000
graduate students need something to do.

0:09:21.000,0:09:23.000
But then the list continued to grow

0:09:23.000,0:09:26.000
into the millions and the billions, until today.

0:09:26.000,0:09:28.000
The list of candidate shapes

0:09:28.000,0:09:33.000
has soared to about 10 to the 500.

0:09:33.000,0:09:36.000
So, what to do?

0:09:36.000,0:09:39.000
Well some researchers lost heart,

0:09:39.000,0:09:42.000
concluding that was so many candidate shapes for the extra dimensions,

0:09:42.000,0:09:45.000
each giving rise to different physical features,

0:09:45.000,0:09:47.000
string theory would never make

0:09:47.000,0:09:49.000
definitive, testable predictions.

0:09:49.000,0:09:53.000
But others turned this issue on its head,

0:09:53.000,0:09:55.000
taking us to the possibility of a multiverse.

0:09:55.000,0:09:57.000
Here's the idea.

0:09:57.000,0:10:00.000
Maybe each of these shapes is on an equal footing with every other.

0:10:00.000,0:10:02.000
Each is as real as every other,

0:10:02.000,0:10:04.000
in the sense

0:10:04.000,0:10:06.000
that there are many universes,

0:10:06.000,0:10:09.000
each with a different shape, for the extra dimensions.

0:10:09.000,0:10:11.000
And this radical proposal

0:10:11.000,0:10:14.000
has a profound impact on this mystery:

0:10:14.000,0:10:17.000
the amount of dark energy revealed by the Nobel Prize-winning results.

0:10:17.000,0:10:19.000
Because you see,

0:10:19.000,0:10:22.000
if there are other universes,

0:10:22.000,0:10:24.000
and if those universes

0:10:24.000,0:10:28.000
each have, say, a different shape for the extra dimensions,

0:10:28.000,0:10:30.000
then the physical features of each universe will be different,

0:10:30.000,0:10:32.000
and in particular,

0:10:32.000,0:10:34.000
the amount of dark energy in each universe

0:10:34.000,0:10:36.000
will be different.

0:10:36.000,0:10:38.000
Which means that the mystery

0:10:38.000,0:10:40.000
of explaining the amount of dark energy we've now measured

0:10:40.000,0:10:43.000
would take on a wholly different character.

0:10:43.000,0:10:45.000
In this context,

0:10:45.000,0:10:48.000
the laws of physics can't explain one number for the dark energy

0:10:48.000,0:10:51.000
because there isn't just one number,

0:10:51.000,0:10:53.000
there are many numbers.

0:10:53.000,0:10:55.000
Which means

0:10:55.000,0:10:58.000
we have been asking the wrong question.

0:10:58.000,0:11:00.000
It's that the right question to ask is,

0:11:00.000,0:11:03.000
why do we humans find ourselves in a universe

0:11:03.000,0:11:06.000
with a particular amount of dark energy we've measured

0:11:06.000,0:11:09.000
instead of any of the other possibilities

0:11:09.000,0:11:11.000
that are out there?

0:11:11.000,0:11:14.000
And that's a question on which we can make headway.

0:11:14.000,0:11:16.000
Because those universes

0:11:16.000,0:11:18.000
that have much more dark energy than ours,

0:11:18.000,0:11:21.000
whenever matter tries to clump into galaxies,

0:11:21.000,0:11:24.000
the repulsive push of the dark energy is so strong

0:11:24.000,0:11:26.000
that it blows the clump apart

0:11:26.000,0:11:28.000
and galaxies don't form.

0:11:28.000,0:11:31.000
And in those universes that have much less dark energy,

0:11:31.000,0:11:33.000
well they collapse back on themselves so quickly

0:11:33.000,0:11:36.000
that, again, galaxies don't form.

0:11:36.000,0:11:39.000
And without galaxies, there are no stars, no planets

0:11:39.000,0:11:41.000
and no chance

0:11:41.000,0:11:43.000
for our form of life

0:11:43.000,0:11:45.000
to exist in those other universes.

0:11:45.000,0:11:47.000
So we find ourselves in a universe

0:11:47.000,0:11:50.000
with the particular amount of dark energy we've measured

0:11:50.000,0:11:53.000
simply because our universe has conditions

0:11:53.000,0:11:57.000
hospitable to our form of life.

0:11:57.000,0:11:59.000
And that would be that.

0:11:59.000,0:12:01.000
Mystery solved,

0:12:01.000,0:12:03.000
multiverse found.

0:12:03.000,0:12:08.000
Now some find this explanation unsatisfying.

0:12:08.000,0:12:10.000
We're used to physics

0:12:10.000,0:12:13.000
giving us definitive explanations for the features we observe.

0:12:13.000,0:12:15.000
But the point is,

0:12:15.000,0:12:18.000
if the feature you're observing

0:12:18.000,0:12:20.000
can and does take on

0:12:20.000,0:12:22.000
a wide variety of different values

0:12:22.000,0:12:25.000
across the wider landscape of reality,

0:12:25.000,0:12:27.000
then thinking one explanation

0:12:27.000,0:12:29.000
for a particular value

0:12:29.000,0:12:32.000
is simply misguided.

0:12:32.000,0:12:34.000
An early example

0:12:34.000,0:12:37.000
comes from the great astronomer Johannes Kepler

0:12:37.000,0:12:39.000
who was obsessed with understanding

0:12:39.000,0:12:41.000
a different number --

0:12:41.000,0:12:45.000
why the Sun is 93 million miles away from the Earth.

0:12:45.000,0:12:48.000
And he worked for decades trying to explain this number,

0:12:48.000,0:12:51.000
but he never succeeded, and we know why.

0:12:51.000,0:12:53.000
Kepler was asking

0:12:53.000,0:12:55.000
the wrong question.

0:12:55.000,0:12:58.000
We now know that there are many planets

0:12:58.000,0:13:01.000
at a wide variety of different distances from their host stars.

0:13:01.000,0:13:04.000
So hoping that the laws of physics

0:13:04.000,0:13:07.000
will explain one particular number, 93 million miles,

0:13:07.000,0:13:10.000
well that is simply wrongheaded.

0:13:10.000,0:13:12.000
Instead the right question to ask is,

0:13:12.000,0:13:15.000
why do we humans find ourselves on a planet

0:13:15.000,0:13:17.000
at this particular distance,

0:13:17.000,0:13:20.000
instead of any of the other possibilities?

0:13:20.000,0:13:23.000
And again, that's a question we can answer.

0:13:23.000,0:13:26.000
Those planets which are much closer to a star like the Sun

0:13:26.000,0:13:28.000
would be so hot

0:13:28.000,0:13:30.000
that our form of life wouldn't exist.

0:13:30.000,0:13:33.000
And those planets that are much farther away from the star,

0:13:33.000,0:13:35.000
well they're so cold

0:13:35.000,0:13:37.000
that, again, our form of life would not take hold.

0:13:37.000,0:13:39.000
So we find ourselves

0:13:39.000,0:13:41.000
on a planet at this particular distance

0:13:41.000,0:13:43.000
simply because it yields conditions

0:13:43.000,0:13:46.000
vital to our form of life.

0:13:46.000,0:13:49.000
And when it comes to planets and their distances,

0:13:49.000,0:13:53.000
this clearly is the right kind of reasoning.

0:13:53.000,0:13:55.000
The point is,

0:13:55.000,0:13:58.000
when it comes to universes and the dark energy that they contain,

0:13:58.000,0:14:02.000
it may also be the right kind of reasoning.

0:14:02.000,0:14:05.000
One key difference, of course,

0:14:05.000,0:14:07.000
is we know that there are other planets out there,

0:14:07.000,0:14:10.000
but so far I've only speculated on the possibility

0:14:10.000,0:14:12.000
that there might be other universes.

0:14:12.000,0:14:14.000
So to pull it all together,

0:14:14.000,0:14:16.000
we need a mechanism

0:14:16.000,0:14:19.000
that can actually generate other universes.

0:14:19.000,0:14:22.000
And that takes me to my final part, part three.

0:14:22.000,0:14:25.000
Because such a mechanism has been found

0:14:25.000,0:14:28.000
by cosmologists trying to understand the Big Bang.

0:14:28.000,0:14:30.000
You see, when we speak of the Big Bang,

0:14:30.000,0:14:32.000
we often have an image

0:14:32.000,0:14:34.000
of a kind of cosmic explosion

0:14:34.000,0:14:36.000
that created our universe

0:14:36.000,0:14:39.000
and set space rushing outward.

0:14:39.000,0:14:41.000
But there's a little secret.

0:14:41.000,0:14:44.000
The Big Bang leaves out something pretty important,

0:14:44.000,0:14:46.000
the Bang.

0:14:46.000,0:14:49.000
It tells us how the universe evolved after the Bang,

0:14:49.000,0:14:51.000
but gives us no insight

0:14:51.000,0:14:55.000
into what would have powered the Bang itself.

0:14:55.000,0:14:57.000
And this gap was finally filled

0:14:57.000,0:14:59.000
by an enhanced version of the Big Bang theory.

0:14:59.000,0:15:02.000
It's called inflationary cosmology,

0:15:02.000,0:15:06.000
which identified a particular kind of fuel

0:15:06.000,0:15:08.000
that would naturally generate

0:15:08.000,0:15:10.000
an outward rush of space.

0:15:10.000,0:15:13.000
The fuel is based on something called a quantum field,

0:15:13.000,0:15:16.000
but the only detail that matters for us

0:15:16.000,0:15:19.000
is that this fuel proves to be so efficient

0:15:19.000,0:15:21.000
that it's virtually impossible

0:15:21.000,0:15:23.000
to use it all up,

0:15:23.000,0:15:25.000
which means in the inflationary theory,

0:15:25.000,0:15:28.000
the Big Bang giving rise to our universe

0:15:28.000,0:15:31.000
is likely not a one-time event.

0:15:31.000,0:15:34.000
Instead the fuel not only generated our Big Bang,

0:15:34.000,0:15:40.000
but it would also generate countless other Big Bangs,

0:15:40.000,0:15:43.000
each giving rise to its own separate universe

0:15:43.000,0:15:45.000
with our universe becoming but one bubble

0:15:45.000,0:15:48.000
in a grand cosmic bubble bath of universes.

0:15:48.000,0:15:50.000
And now, when we meld this with string theory,

0:15:50.000,0:15:52.000
here's the picture we're led to.

0:15:52.000,0:15:54.000
Each of these universes has extra dimensions.

0:15:54.000,0:15:57.000
The extra dimensions take on a wide variety of different shapes.

0:15:57.000,0:16:00.000
The different shapes yield different physical features.

0:16:00.000,0:16:03.000
And we find ourselves in one universe instead of another

0:16:03.000,0:16:06.000
simply because it's only in our universe

0:16:06.000,0:16:09.000
that the physical features, like the amount of dark energy,

0:16:09.000,0:16:13.000
are right for our form of life to take hold.

0:16:13.000,0:16:16.000
And this is the compelling but highly controversial picture

0:16:16.000,0:16:18.000
of the wider cosmos

0:16:18.000,0:16:20.000
that cutting-edge observation and theory

0:16:20.000,0:16:24.000
have now led us to seriously consider.

0:16:24.000,0:16:28.000
One big remaining question, of course, is,

0:16:28.000,0:16:31.000
could we ever confirm

0:16:31.000,0:16:34.000
the existence of other universes?

0:16:34.000,0:16:36.000
Well let me describe

0:16:36.000,0:16:39.000
one way that might one day happen.

0:16:39.000,0:16:41.000
The inflationary theory

0:16:41.000,0:16:43.000
already has strong observational support.

0:16:43.000,0:16:45.000
Because the theory predicts

0:16:45.000,0:16:47.000
that the Big Bang would have been so intense

0:16:47.000,0:16:50.000
that as space rapidly expanded,

0:16:50.000,0:16:52.000
tiny quantum jitters from the micro world

0:16:52.000,0:16:55.000
would have been stretched out to the macro world,

0:16:55.000,0:16:58.000
yielding a distinctive fingerprint,

0:16:58.000,0:17:00.000
a pattern of slightly hotter spots and slightly colder spots,

0:17:00.000,0:17:02.000
across space,

0:17:02.000,0:17:05.000
which powerful telescopes have now observed.

0:17:05.000,0:17:08.000
Going further, if there are other universes,

0:17:08.000,0:17:10.000
the theory predicts that every so often

0:17:10.000,0:17:12.000
those universes can collide.

0:17:12.000,0:17:14.000
And if our universe got hit by another,

0:17:14.000,0:17:16.000
that collision

0:17:16.000,0:17:18.000
would generate an additional subtle pattern

0:17:18.000,0:17:20.000
of temperature variations across space

0:17:20.000,0:17:22.000
that we might one day

0:17:22.000,0:17:24.000
be able to detect.

0:17:24.000,0:17:27.000
And so exotic as this picture is,

0:17:27.000,0:17:29.000
it may one day be grounded

0:17:29.000,0:17:31.000
in observations,

0:17:31.000,0:17:34.000
establishing the existence of other universes.

0:17:34.000,0:17:36.000
I'll conclude

0:17:36.000,0:17:39.000
with a striking implication

0:17:39.000,0:17:41.000
of all these ideas

0:17:41.000,0:17:43.000
for the very far future.

0:17:43.000,0:17:45.000
You see, we learned

0:17:45.000,0:17:47.000
that our universe is not static,

0:17:47.000,0:17:49.000
that space is expanding,

0:17:49.000,0:17:51.000
that that expansion is speeding up

0:17:51.000,0:17:53.000
and that there might be other universes

0:17:53.000,0:17:55.000
all by carefully examining

0:17:55.000,0:17:57.000
faint pinpoints of starlight

0:17:57.000,0:18:00.000
coming to us from distant galaxies.

0:18:00.000,0:18:03.000
But because the expansion is speeding up,

0:18:03.000,0:18:05.000
in the very far future,

0:18:05.000,0:18:08.000
those galaxies will rush away so far and so fast

0:18:08.000,0:18:11.000
that we won't be able to see them --

0:18:11.000,0:18:13.000
not because of technological limitations,

0:18:13.000,0:18:15.000
but because of the laws of physics.

0:18:15.000,0:18:17.000
The light those galaxies emit,

0:18:17.000,0:18:20.000
even traveling at the fastest speed, the speed of light,

0:18:20.000,0:18:22.000
will not be able to overcome

0:18:22.000,0:18:25.000
the ever-widening gulf between us.

0:18:25.000,0:18:27.000
So astronomers in the far future

0:18:27.000,0:18:29.000
looking out into deep space

0:18:29.000,0:18:32.000
will see nothing but an endless stretch

0:18:32.000,0:18:36.000
of static, inky, black stillness.

0:18:36.000,0:18:38.000
And they will conclude

0:18:38.000,0:18:40.000
that the universe is static and unchanging

0:18:40.000,0:18:43.000
and populated by a single central oasis of matter

0:18:43.000,0:18:45.000
that they inhabit --

0:18:45.000,0:18:47.000
a picture of the cosmos

0:18:47.000,0:18:50.000
that we definitively know to be wrong.

0:18:50.000,0:18:53.000
Now maybe those future astronomers will have records

0:18:53.000,0:18:55.000
handed down from an earlier era,

0:18:55.000,0:18:57.000
like ours,

0:18:57.000,0:18:59.000
attesting to an expanding cosmos

0:18:59.000,0:19:01.000
teeming with galaxies.

0:19:01.000,0:19:03.000
But would those future astronomers

0:19:03.000,0:19:06.000
believe such ancient knowledge?

0:19:06.000,0:19:08.000
Or would they believe

0:19:08.000,0:19:11.000
in the black, static empty universe

0:19:11.000,0:19:15.000
that their own state-of-the-art observations reveal?

0:19:15.000,0:19:17.000
I suspect the latter.

0:19:17.000,0:19:19.000
Which means that we are living

0:19:19.000,0:19:22.000
through a remarkably privileged era

0:19:22.000,0:19:24.000
when certain deep truths about the cosmos

0:19:24.000,0:19:26.000
are still within reach

0:19:26.000,0:19:28.000
of the human spirit of exploration.

0:19:28.000,0:19:33.000
It appears that it may not always be that way.

0:19:33.000,0:19:35.000
Because today's astronomers,

0:19:35.000,0:19:38.000
by turning powerful telescopes to the sky,

0:19:38.000,0:19:41.000
have captured a handful of starkly informative photons --

0:19:41.000,0:19:44.000
a kind of cosmic telegram

0:19:44.000,0:19:46.000
billions of years in transit.

0:19:46.000,0:19:50.000
and the message echoing across the ages is clear.

0:19:50.000,0:19:53.000
Sometimes nature guards her secrets

0:19:53.000,0:19:55.000
with the unbreakable grip

0:19:55.000,0:19:57.000
of physical law.

0:19:57.000,0:20:01.000
Sometimes the true nature of reality beckons

0:20:01.000,0:20:04.000
from just beyond the horizon.

0:20:04.000,0:20:06.000
Thank you very much.

0:20:06.000,0:20:10.000
(Applause)

0:20:10.000,0:20:12.000
Chris Anderson: Brian, thank you.

0:20:12.000,0:20:14.000
The range of ideas you've just spoken about

0:20:14.000,0:20:17.000
are dizzying, exhilarating, incredible.

0:20:17.000,0:20:19.000
How do you think

0:20:19.000,0:20:21.000
of where cosmology is now,

0:20:21.000,0:20:23.000
in a sort of historical side?

0:20:23.000,0:20:26.000
Are we in the middle of something unusual historically in your opinion?

0:20:26.000,0:20:28.000
BG: Well it's hard to say.

0:20:28.000,0:20:31.000
When we learn that astronomers of the far future

0:20:31.000,0:20:34.000
may not have enough information to figure things out,

0:20:34.000,0:20:37.000
the natural question is, maybe we're already in that position

0:20:37.000,0:20:40.000
and certain deep, critical features of the universe

0:20:40.000,0:20:43.000
already have escaped our ability to understand

0:20:43.000,0:20:45.000
because of how cosmology evolves.

0:20:45.000,0:20:47.000
So from that perspective,

0:20:47.000,0:20:49.000
maybe we will always be asking questions

0:20:49.000,0:20:51.000
and never be able to fully answer them.

0:20:51.000,0:20:53.000
On the other hand, we now can understand

0:20:53.000,0:20:55.000
how old the universe is.

0:20:55.000,0:20:57.000
We can understand

0:20:57.000,0:21:00.000
how to understand the data from the microwave background radiation

0:21:00.000,0:21:03.000
that was set down 13.72 billion years ago --

0:21:03.000,0:21:05.000
and yet, we can do calculations today to predict how it will look

0:21:05.000,0:21:07.000
and it matches.

0:21:07.000,0:21:09.000
Holy cow! That's just amazing.

0:21:09.000,0:21:12.000
So on the one hand, it's just incredible where we've gotten,

0:21:12.000,0:21:16.000
but who knows what sort of blocks we may find in the future.

0:21:16.000,0:21:19.000
CA: You're going to be around for the next few days.

0:21:19.000,0:21:21.000
Maybe some of these conversations can continue.

0:21:21.000,0:21:23.000
Thank you. Thank you, Brian. (BG: My pleasure.)

0:21:23.000,0:21:26.000
(Applause)