Is our universe the only universe Brian Greene

[Music]

[Applause]

a few months ago

the nobel prize in physics was awarded

to two teams of astronomers for a

discovery

that has been held 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

but 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 a

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

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 as 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 einsteins 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

smoky 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 promise 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 units 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 no relevance to

anybody else

well it surely is a technical detail but

some details

really matter some details provide

windows onto uncharted realms of reality

and this peculiar number may be doing

just that

as the only approach that 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 in 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 were 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 dimension 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 could

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 with 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 turn 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 which

means

we have been asking the wrong question

instead the right question to ask is why

do we humans

find ourselves in a universe with a

particular amount of dark energy we’ve

measured

instead of any of the other

possibilities that are out there

and that’s a question on which we can

make headway because

those universes that have much more dark

energy than ours

whenever matter tries to clump into

galaxies the repulsive push of the dark

energy is so strong that it blows the

clump apart

and galaxies don’t form and in those

universes that have much less dark

energy well they collapse back on

themselves so quickly

that again galaxies don’t form and

without galaxies

there are no stars no planets and no

chance for our form of life to exist

in those other universes so we find

ourselves in a universe with a

particular amount of dark energy we’ve

measured simply because

our universe has conditions hospitable

to our

form of life and that

would be that mystery solved

multiverse found now some

find this explanation unsatisfying we’re

used to physics giving us definitive

explanations for the features we observe

but the point is

if the feature you’re observing

can and does take on a wide variety of

different values across

the wider landscape of reality then

sinking one explanation for a particular

value

is simply misguided an early

example comes from the great astronomer

johannes kepler

who was obsessed with understanding a

different number

why the sun is 93 million miles away

from the earth and he worked for

decades trying to explain this number

but he never succeeded and we know why

kepler was asking the wrong question

we now know that there are many planets

at a wide variety of different distances

from their host star so hoping

that the laws of physics will explain

one particular number 93 million miles

well that is simply wrong-headed

instead the right question to ask is why

do we humans

find ourselves on a planet at this

particular distance

instead of any of the other

possibilities and again that’s a

question we can

answer those planets which are much

closer to a star like the sun would be

so hot

that our form of life wouldn’t exist and

those planets that are much farther

away from the star well they’re so cold

that again our form of life would not

take hold

so we find ourselves on a planet at this

particular distance simply because

it yields conditions vital to our form

of life and

when it comes to planets and their

distances this

clearly is the right kind of reasoning

the point is when it comes to universes

and the dark energy that they contain

it may also be the right kind of

reasoning

one key difference of course is

we know that there are other planets out

there but so far i’ve only speculated on

the possibility that there might be

other universes so to pull it all

together we need a mechanism

that can actually generate other

universes and that takes me to my final

part part three

because such a mechanism has been found

by cosmologists trying to understand

the big bang you see when we speak of

the big bang

we often have an image of a kind of

cosmic explosion that created our

universe

and set space rushing outward but

there’s a

little secret the big bang leaves out

something pretty important

the bang it tells us how the universe

evolved

after the bang but gives us no insight

into what would have powered the bang

itself and this gap was finally filled

by

an enhanced version of the big bang

theory it’s called inflationary

cosmology which

identified a particular kind of fuel

that would naturally generate an outward

rush

of space the fuel is based on something

called a quantum field but

the only detail that matters for us

is that this fuel proves to be so

efficient that it’s virtually impossible

to use it all up which means in the

inflationary theory

the big bang giving rise to our universe

is likely not

a one-time event instead the fuel not

only generates our big bang

but it would also generate countless

other big bangs

each giving rise to its own separate

universe with our universe becoming but

one bubble

in a grand cosmic bubble bath of

universes

and now if we meld this with string

theory here’s the picture we’re led to

each of these universes has extra

dimensions the extra dimensions take on

a wide variety of different shapes

the different shapes yield different

physical features and we find ourselves

in one universe instead of another

simply because it’s only in our universe

that the physical features

like the amount of dark energy are right

for our form of life to take hold

and this is the compelling but highly

controversial picture of the wider

cosmos

that cutting-edge observation in theory

has now led us to seriously

consider one big remaining question

of course is could we ever

confirm the existence of other universes

well

let me describe one way that might one

day happen

the inflationary theory already has

strong observational support because the

theory predicts

that the big bang would have been so

intense that as

space rapidly expanded tiny quantum

jitters from the micro world

would have been stretched out to the

macro world yielding a distinctive

fingerprint

a pattern of slightly hotter spots and

slightly colder spots across space which

powerful telescopes

have now observed going further if there

are other universes

the theory predicts that every so often

those universes can collide

and if our universe got hit by another

that collision

would generate an additional subtle

pattern of temperature variations across

space that we

might one day be able to detect

and so exotic as this picture is it may

one day be grounded

in observations establishing the

existence

of other universes i’ll conclude with

a striking implication of all these

ideas for the very

far future you see we learned that our

universe

is not static that space is expanding

that that expansion is speeding up and

that there might be other

universes all by carefully examining

faint pinpoints of star lake coming to

us from distant galaxies

but because the expansion is speeding up

in the very far future

those galaxies will rush away so far and

so fast that we

won’t be able to see them not because of

technological limitations

but because of the laws of physics the

light those galaxies emit

even traveling at the fastest speed the

speed of light

will not be able to overcome the ever

widening gulf

between us so astronomers in the far

future

looking out into deep space will see

nothing but

an endless stretch of static

inky black stillness

and they will conclude that the universe

is static and unchanging

and populated by a single central oasis

of matter that they inhabit

a picture of the cosmos that we

definitively know

to be wrong now maybe those future

astronomers will have records handed

down from

an earlier era like hours attesting to

an expanding cosmos teeming with

galaxies but

would those future astronomers believe

such

ancient knowledge or would they believe

in the black

static empty universe that their own

state-of-the-art

observations reveal i suspect the latter

which means that we are living through a

remarkably

privileged era when certain deep truths

about the cosmos are still within reach

of the human spirit of exploration it

appears that it may

not always be that way because today’s

astronomers by turning

powerful telescopes to the sky have

captured

a handful of starkly informative photons

a kind of

cosmic telegram billions of years in

transit

and the message echoing across the ages

is clear sometimes

nature guards her secrets with the

unbreakable grip

of physical law sometimes

the true nature of reality beckons from

just beyond

the horizon thank you very much

brian thank you um the range of ideas

you’ve just spoken about

are dizzying exhilarating incredible

how do you think of where cosmology

is now in a sort of historical side are

we in the middle of something

unusual historically in your opinion

well it’s hard to say

when we learn that astronomers of the

far future

may not have enough information to

figure things out the natural question

is

maybe we’re already in that position and

certain deep

critical features of the universe

already have escaped

our ability to understand because of how

cosmology evolves

so from that perspective maybe we will

always be

asking questions and never be able to

fully answer them on the other hand

we now can understand how old the

universe is we can understand

how to understand the data from the

microwave background radiation that was

set down 13.72 billion years ago and yet

we can do calculations today to predict

how it will look and it matches

holy cow that’s just amazing so on the

one hand

it’s just incredible where you’ve gotten

but who knows

what sort of blocks we may find in the

future you’re going to be around

for the next few days maybe some of

these conversations can continue

thank you my pleasure thank you thank

you brian

you