- I was just
thinking about what I
can say about someone
I've known as a friend
than a colleague for 35 years.
And I thought back
to the day that I
met Bob, which was almost
exactly 35 years ago.
He probably doesn't remember.
But I do because
he was busy making
a sort of secret transition to
Harvard as a tenured faculty
member, and I was
trying to decide
where to go to graduate school.
And so Bob doesn't
know, but he actually
recruited me to
Harvard, even though he
was a professor at the
University of Michigan
at the time because he
was coming to Harvard.
And I was--
I'm sorry, Bob, but--
in awe of your work
and your communication
style even then.
And I guess I'm not sorry.
I still am now.
And at that time, Bob had just
recently discovered something
called the void in Bootes.
And when I say communication
style, I mean in his writing,
as well as in his speaking,
which you'll get to hear today.
I just want to read you a
sentence from the incredibly
short abstract of this
paper that discovered
the first really gigantic void
in space that led to later what
Margaret Geller
and John Huchra did
to find this sort of bubbly
structure in the universe.
So they found the first bubble.
And the concluding
sentence of that
says, "one plausible
interpretation
of finding only one
redshift of--" by the way,
there were 133 galaxies
in this survey, which
for those of you
who are astronomers
is amazingly small number.
But anyway, so 1 of
those 133 redshifts
was where the peak of the
distribution should have been.
And instead there was a giant
gap, and that was a void.
And he wrote, "one
plausible interpretation
is that a large volume
in this region of order
1 million cubic mega parsecs
is nearly devoid of galaxies."
So this is like a
great understatement.
Like, we've just discovered the
greatest hole in the universe.
And OK, maybe it's
devoid of galaxies.
So Bob went on to do a
lot with supernovae, which
is something he's very fond of.
He was very important in the
study of supernova 1987a.
Again, those of you
interested in astronomy
might remember that.
But what he did that he's
going to talk about today
is something called
supernova cosmology, where
you use supernovae as a way
to measure very accurately
the expansion of the universe.
And he's going to
explain that to you.
And he's going to explain--
I'm sorry to steal
his punch line--
that they went out to
look for the deceleration
of the universe.
And let me just say,
I already hinted what,
but something else happened.
And the reason I mentioned
this is because many of you
have come to the other events in
this series, The Undiscovered,
this year at Radcliffe.
And I just want to say that
Bob's work and Chris Bowler's
work--
Chris Bowler, who's
over here somewhere.
I can't see him.
There he is.
Chris, who was a Radcliffe
Fellow with me in 2016,
2017, and Stuart
Firestein's work, who
you heard from back in
October at The Undiscovered,
were the three pillars of
the kind of inspiration
of The Undiscovered.
So the story is that Stuart
Firestein talks about ignorance
and how the interesting
parts of science
are the parts that
are not in the book.
And Chris Bowler came along and
said, how about this Tara ship
that I work on,
which some of you
had the privilege of
seeing in Boston Harbor
back in October, that
does things like go out
and sequence all of the plankton
in the ocean and more than 10
times increase the number of
known plankton in the ocean
by just sequencing them.
And those are examples,
that kind of science
is an example of where you
have very expert people knowing
exactly what they're
doing, and then they're
surprised by their results.
So we don't want to give
the idea that undiscovered
means just pure serendipity.
It means when a team of
scientists or a scientist
is very prepared
to do something,
and then maybe accidentally
finds something very different.
And I'll let Bob explain
why his story, which
I've told some of you briefly
in previous introductions
to these events, is
such a perfect example
of an undiscovery.
And I should give a
proper introduction
and tell you that Bob was an
undergraduate here at Harvard,
that he went to graduate
school at Caltech.
I already mentioned he was a
professor at Michigan and then
here at Harvard for 30 years.
And now he's actually in
charge of the science program
at the Moore Foundation,
the Gordon and Betty Moore
Foundation in
California, which he
tells us is why he's
wearing such a nice suit.
[LAUGHTER]
It's a lovely suit, Bob.
Anyway, I should cover
up my dirty shirt here.
[LAUGHTER]
Anyway, so he also has
won many, many prizes.
And that's why I
have to look down
because I listed just a few
so that I don't forget them,
the Dannie Heineman Prize
in Astrophysics-- that
was after winning the Caltech
Distinguished Alumni Award;
the James Craig Watson Medal,
and the Wolf Prize in Physics.
But I hope he remembers that my
favorite award that we gave him
was the year after
he discovered what
he's going to tell you today.
He remembers.
He says, the shirt.
Yeah, so we gave him a
T-shirt on the occasion
of his 50th birthday that
said, "the universe isn't
accelerating.
You're just slowing down."
[LAUGHTER]
So I would stop
there, but I just
have to read you one email
that I got this morning.
So Bob and I and the other
astronomers in the audience
get many, many emails from high
school students and students
all over the world all the time.
And just this morning
I got this message,
which Bob is going to
now answer for you.
It says, "hello,
my name is Carson.
And I am a sophomore at
Jefferson Forest High
School in Lynchburg, Virginia.
I am very fascinated
by astronomy,
and I have a question.
If space is infinite, then
how is it always expanding?
I have been wondering
about this for a while now.
And the astronomy teacher
here was unsure of the answer.
So I went to a
Harvard professor.
I understand you are extremely
busy so just get back
to me when you can."
[LAUGHTER]
So Carson, another Harvard
professor, who, by the way,
points out that in addition to
his appointment at the Moore
Foundation, he is still a
research professor at Harvard,
which is free to Harvard.
So happy for funders.
Anyway so, Professor
Bob Kirshner
will now explain to us the
expansion of the universe
and its extravagance.
So thank you, Bob.
[APPLAUSE]
- Oh, me.
Well, thank you very much.
It's great that you have
taken the effort to come here.
And I'm very pleased
to be here myself.
I'm going to try to stick to
this idea of not discovering
the thing we set out to find.
We set out to find the
deceleration, the slowing
down of the expansion
of the universe.
And as I'll show you,
we found something else.
So let's start.
As Melissa has said, I'm a chief
program officer at the Gordon
and Betty Moore Foundation.
Gordon Moore was
the head of Intel.
And that was the source of his
money doubling every 18 months.
And so he set up a
philanthropic foundation.
I'll show you the plot later.
And I'm still the Clowes
Research Professor of Science.
That means that
Harvard doesn't pay me.
I don't teach.
But they collect overhead
on my research grants.
This is, from the point of view
of a university administrator,
just about perfect.
And I don't need
an office anymore.
So there you are.
Well, let's start.
You can't go wrong with
looking back to Galileo.
And here's Galileo in Venice,
showing off his telescope
in front of the Doge's Palace.
Galileo said, "all truths
are easy to understand
once they are discovered; the
point is to discover them."
So to give you one
example of what Galileo
did with that telescope was
to take a look at the moon
and to realize that the moon
is a real place, with mountains
and pits and what they
thought were seas,
that it is a part of the
world made of the same stuff,
he thought, as we are,
which was somewhat
at variance with the
view that there were
four essences and then
the astronomical thing,
the fifth essence,
the quintessence.
So here I show you air--
easy to show you
air, an empty glass.
Is that glass half
empty or full of air--
water, fire, and earth.
And then the fifth essence--
there are five shown here--
is the black frame.
And we're going to come
back to something sort
of in the background
that is really
very important to the
structure of the universe.
Now, the idea that we can
tell what the world is made of
comes from astronomy of a
couple hundred years ago,
where you take an
object, like the sun.
And if you pass
it through a prism
or use another kind of
instrument to do it,
you can spread the light
out into a spectrum.
So you know what a
scientist's job is.
It's just take something
beautiful, the rainbow,
and to turn it into a graph.
[LAUGHTER]
And that's what you see here.
Up at the top is a graph
that shows as up and down,
where the light is bright
and where it's dim.
And you'll notice-- it's
even more conspicuous
in the illustration
down at the bottom--
that there are these lines
that go across the spectrum.
Those are certain wavelengths
or colors, energies of light,
where the light is absorbed.
And it's absorbed by the atoms
in the atmosphere of the star.
Like, that one that's
in the yellow out there.
There's a nice
sharp yellow line.
That's caused by sodium.
The elements sodium
absorbs the light.
And that's how we know
what the sun is made of.
The sun is made of
mostly hydrogen gas.
That was something which was
not obvious at the beginning
but was worked out by a
very good graduate student
here at Harvard, Cecilia
Payne-Gaposchkin.
The other things
that you see here
are elements that we know
from the periodic table.
Those are the familiar elements.
And in fact, most
of the elements
that you know about
from chemistry
are also observed in the stars.
And we'll come back
to the another use
of the spectra in a minute.
So I want to start
about 100 years ago.
Everybody knows that on a clear
summer night, if you get out
from the city, you can see
the band of the Milky Way
across the northern sky.
And this is a representation of
our own galaxy, the Milky Way
galaxy, which, of course,
no one could actually
see it all at once.
This is a picture
that stitched together
of what you would see from
the northern hemisphere
and what you would see from
the southern hemisphere.
You can't be in
two places at once.
But this gives you the idea.
It's a big flattened disk, bulge
in the middle, lanes of dust
throughout.
And what I want to
emphasize in this picture--
well, first is the idea that
100 years ago, astronomers
thought that the Milky Way was
basically the whole universe.
And as we know now,
there was a big change
in our point of
view, and I'll talk
about where that came from.
So the most important thing
in each of these pictures,
like that dark background
in the four elements,
the most important thing is
invisible or in the background.
And here the most important
thing in this picture,
for this story anyway, is
that little fuzzy thing,
a little nebula, which is the
Andromeda nebula that we'll
come back to in a minute.
Well, all right,
100 years ago, there
was also very strong advances
in theoretical science.
Here is Albert Einstein,
sort of known to everybody.
And he's thinking about
the nature of the universe.
You'll notice
something interesting
about the technology here.
This is taken with a
photographic plate.
That's a thing that
has a chemical reaction
between light-- well, the
light produces a state
of a silver salt that you can
then reduce to silver metal
and make it negative, do all
that stuff with chemistry.
And one thing about it is it's
a lot less sensitive to light
than the electronic camera
that's in your cell phone.
And I'll come back to that
revolution in a minute.
And you can see it right here
because there was somebody
seated in this chair who did
not wish to be photographed
with Einstein.
And the person behind
him disagrees with him
and is shaking his head.
The shutter is open
for quite a long time.
The big advance is that we now
can look at these photographs
and see what people
were thinking.
And what Einstein
was thinking was
that the universe, this Milky
Way that we're talking about,
must be static.
He had just invented
general relativity,
the theory of
gravity, that tells
how things pull on
one another, or really
how the presence of matter,
curved space, and objects
move through space.
And Einstein was a
very creative thinker.
And he thought, well,
how could I apply this
to the universe as a whole?
Well, of course, universe
meant the Milky Way.
So he knew that the
Milky Way didn't seem
to be expanding or contracting.
So he thought the
universe must be static.
And put in by hand an extra term
into his cosmology equations,
or the gravity equations,
to make the universe static.
And he said-- can you
read this from the back?
Yes, no?
Oh, too bad because
I was going to say I
was translating from the German.
[LAUGHTER]
It says, "that term
is necessary--"
I'm going to do it later.
"That term is necessary
only for the purpose
of making possible a
quasi-static distribution
of matter, as required by the
fact of the small velocities
of the stars."
So at least when
he wrote the paper,
he was thinking, in
this very empirical way,
that he somehow had to
solve a problem and make it
so the universe was static.
Who broke this
problem wide open?
Well, it took a lot of people.
But one of the people
at the beginning
is Henrietta Swann Leavitt, who
worked at the Harvard College
Observatory.
She lived on Gardam
Street in the place
where the Curious
George house is
and worked at the observatory
for $0.25 an hour.
But the observatory director
only got $3 an hour.
Are you listening to this,
everyone, $3 an hour?
And she noticed that when you
studied a certain set of stars
that were all at
the same distance,
that she could tell that these
stars, which were vibrating,
getting brighter and dimmer,
that the ones that were
brighter had longer periods.
That is it took longer for
them to get bright and dim.
Just like a big bell
takes long and has a lower
note than a little bell, the
big stars, the luminous stars,
had a longer period
of vibration.
And that turned out
to be very important.
Because in astronomy,
we're always
getting confused about what's
nearby and what's far away.
You look at two
stars, and you don't
know whether you're
looking at a firefly that's
in the foreground or a
star 100 light years away
in the background
until you can sort out
what those objects really are.
And the observatory
director, who
was earning this
princely sum, here
is, Harlow Shapley,
used the variable stars
to try to map out the Milky Way.
And he showed that we don't live
in the center of the Milky Way.
We live out in the suburbs
and something like this.
Now, this is not a
picture of our galaxy--
the students are onto this--
because, of course, we
have no way to get outside.
The best we could do is
that sort of edge-on picture
that I showed you earlier.
And it turns out
Cambridge is not actually
the hub of the universe.
It is somewhat out
in the outskirts.
So I'm going to
emphasize instruments.
I talked already for a
second about photography.
And of course, one
of the things that
changed this subject of how
to use those variable stars
and how to map
out the structure,
the distances in the
universe was the invention
of big telescopes.
So here's a big telescope.
That's the 100-inch-- that's
the diameter of the mirror--
telescope at Mount Wilson,
which is up above Pasadena
in California.
So that telescope was
built in the same era
that I'm talking about, nearly
100 years ago, about 100
years ago.
And it was used to measure
stars, the same kinds of stars
that at Henrietta Leavitt
had been talking about
in other nebulae to see
how far away they were.
So again, the most important
thing in the picture
is kind of inconspicuous.
It's the chair.
[LAUGHTER]
The chair is where the
astronomer sits and uses
a big photographic plate to
take an image of a galaxy.
And by taking pictures
night after night,
Edwin Hubble, who
used this telescope,
was able to search for the
same kind of variable stars
in the Andromeda nebula,
the thing I pointed out
a minute ago, a few minutes
ago, for the same kinds of stars
that Henrietta Leavitt
had seen more nearby.
And so here's a
very famous picture
that he took with that
telescope in October of 1923.
And you can see there's the
fuzzy thing in the middle.
So it's sort of like the
picture I showed you before.
And he's marked, up on the
upper right, a particular star.
And he was looking for
stars that changed,
that got bright, novae.
But he realized after looking
at this plate and the ones
he had before, that it
wasn't just a new star.
It was one he had seen before,
that it was a variable.
And it was a variable of
the type that turns out
to be the same as the type that
Henrietta Leavitt had seen,
only it was a lot dimmer.
Well, that means it
was a lot farther away.
If you look out over
a desert landscape
and there are cars out there,
the ones that are nearby
look bright.
The ones that are
distant look faint.
And what he found was that the
distances in the two galaxies
were much bigger than the
distance across our own Milky
Way.
So Shapley was very impressed
with how big the Milky Way was.
And he thought those nebulae
were just some little things
at the edge.
But the distances we
measure in units of the time
it takes light to get
from one place to another.
The unit of distance we
have, a foot is the distance
that light travels
in a nanosecond,
in a billionth of a second.
A foot is used in the
United States and in Liberia
as a unit of distance.
[LAUGHTER]
The distances of
the stars that you
see when you go out tonight--
well, not tonight,
but on a clear night--
might be 100 or
1,000 light years.
That means the light took 100
or 1,000 years to get here.
The diameter of the
Milky Way is something
like 100,000 light years.
But what Hubble found
was these stars, which
were the same kinds of stars
but hundreds of times fainter,
must be 10 times as far away,
roughly speaking, millions
of light years away.
So this really was
a big surprise then,
that the universe was
a much bigger place
than people had thought.
Well, what about Einstein?
Well, Einstein-- here you
see a picture of Einstein
at the Mount Wilson Observatory.
That's Hubble hovering
over his shoulder.
And that guy looking
really worried
is the observatory director
because he is afraid
that this theorist is going to
somehow damage the instrument.
[LAUGHTER]
So the universe is much bigger
than people thought in 1915,
let's say.
But there's something
else, and that
is that you can tell
from the spectrum
that I talked about
before, the thing that
let you see what chemicals are
in the atmosphere of a star,
you can tell whether things
are coming toward you or away
from you.
There's a shift
in the wavelength,
the color of the light, that's
due to objects approaching
that look bluer.
Objects that are
receding look redder.
We know this from sound.
Everybody knows the
sound of a car going
by, [IMITATES PASSING CAR].
It goes from a high pitched to
a low pitch as it goes past you.
And if the speed of light
were a million times lower,
highways would look like this.
The cars coming toward you would
have blue lights on the front.
And the cars going away from
you would have red lights.
Oh, they do.
[LAUGHTER]
Well, OK, that's for
a different reason.
But anyway, the idea is
you can tell whether things
are coming or going.
Here is a fellow called Vesto
Melvin Slipher at the Lowell
Observatory.
And he's using an instrument,
one of these devices
that takes the spectra
of stars, of galaxies.
And he was measuring the
velocities of the galaxies.
And what he found was
something really weird.
He found that there are
a few coming and a few--
but almost all the
galaxies that he measured,
and he measured a
couple of hands full,
were moving away from us.
That's kind of strange.
So Hubble was measuring
distances to galaxies.
Slipher was
measuring velocities.
Now, many of you know
where this is going.
Hubble is famous
for Hubble's law,
which relates the velocities
and the distances.
And in 1925, Hubble
had all the information
he needed to make the
diagram that shows that we
live in an expanding universe.
Why didn't he do it?
Well, it turns out he
wrote that in his book.
In the book, The Realm of the
Nebulae, which he wrote later,
he said, "it was
natural inertia."
Students, pay attention.
If, for example, an
assignment is overdue,
what is the correct
thing to say?
Natural inertia.
[LAUGHTER]
It was "natural inertia in the
face of revolutionary ideas--"
that's the general
relativity-- "couched
in the unfamiliar language of
general relativity discouraged
immediate investigation."
[LAUGHTER]
When he's talking about
himself, he always
uses the passive voice.
He says "discouraged
immediate investigation."
So here's the diagram that
in 1929 he finally got around
to making, where you plot--
you know what a
scientist will do.
You get two lists of numbers.
You plot one against the other.
You make a graph.
Here's velocity
against distance.
And you can see that there's
some relation between them.
The things that
have small distances
have small velocities.
In fact, a few of
them are coming
toward us, which was very
puzzling to people later.
And as you go
farther out, you're
looking at objects that
are moving away from us.
So it isn't a static universe.
This cosmological constant
that Einstein put in by hand
into his general
relativity equations
was something that he
discovered 10 years later
he didn't really need.
So here are the
players in the game.
Here's Einstein.
He's next to the
observatory director
now in his good suit, still
smoking a pipe, though.
And over here's Edwin Hubble.
And here's Humason, his
assistant, who actually
measured a lot of the spectra.
And George Ellery
Hale, who's the man who
is responsible for building
the 100-inch telescope,
is up on the wall.
He's patting Hubble
on the head--
good work.
[LAUGHTER]
So if the universe
is not static,
then you don't need the
cosmological constant.
And if you look at the
literature, or the writing
about it, people
say, well, Einstein
called it his greatest blunder.
And I got really
interested and tried
to track down where he said it.
Turns out one place where
somebody says it pretty clearly
is George Gamow, who's a
notoriously unreliable witness.
But in his autobiography,
which was written by him--
only he could write that--
he says, "Einstein's
original gravity equation
was correct, and
changing it--" putting
in the cosmological
constant-- "was a mistake.
Much later, when
I was discussing
cosmological problems
with Einstein--" I
really wanted to put that in
my book but couldn't do it.
"Much later, when
I was discussing
cosmological problems
with Einstein,
he remarked that
the introduction
of the cosmological term was
the biggest blunder he ever
made in his life.
But this blunder,
rejected by Einstein,
is still sometimes used by
cosmologists even today.
And the cosmological constant
denoted by the Greek letter
lambda rears its ugly head
again and again and again."
George, calm down.
So that's my story.
It's going to be that
we thought we were going
to measure a universe that was
slowing down due to gravity,
but the cosmological constant
has reared its ugly head again
only this time with evidence.
So let me say a little
bit more about that.
We live in an
expanding universe.
You may be a bit puzzled.
And in fact, the
student who wrote in
is clearly a little
bit puzzled about this.
The view, my
experience has been,
that most faculty
at Harvard have
is that they personally are
the center of the universe,
and it's also true
for undergraduates.
Well, anyway, that
many people think,
well, it's you
that's at the center.
But no.
If you have a universe
that's stretching out
in all directions, if you
imagine this, then, in fact,
your neighbors will be
moving away from you.
And the ones who are
more distant neighbors
will move being moving
away from you faster.
If I could have the power vested
in me by the reverend board
of overseers to double
the size of this room,
the person who's
sitting next to you
would end up two seats away.
The person who
was two seats away
would end up four seats away,
moving away twice as fast.
The person who was four seats
away would end up eight.
What you would see is exactly
what we see in the universe
that you actually live in--
Hubble's law.
So when we see a bunch
of galaxies out there
and we measure
the velocities, we
can use that to understand
how the universe is expanding.
That's the point of all this.
Well, what about the
cosmological constant?
At first, people thought,
well, that's what
makes the universe expand.
And so you can
see here a cartoon
from a popular publication
in the Netherlands that says,
it shows a person--
that is Willem de Sitter--
in the shape of the
Greek letter lambda.
Get it?
And you can see that that's
making the universe swell up.
You can read the
Dutch because English
is a dialect of Frisian.
It says what does
[INAUDIBLE] the ball up?
What makes the universe
expand or swell up?
That's got to be lambda.
No other answer can be given.
Pretty good.
But they gave up on it very
quickly because Einstein said,
no, you don't need lambda.
So let's get rid of it.
And after that-- this
Sitter and Einstein
discussing this matter.
Well, discussing something.
And they basically said,
that cosmological constant,
which we put in to make the
universe static, is a bad idea.
Let's get rid of it.
So I was in Washington over the
weekend at the National Academy
meeting.
Here's Einstein.
Well, it's not Einstein.
It's a statue of Einstein.
And he's holding a tablet
which he brought down, well,
not really, he had wrote on.
And here are some
equations on here.
So a serious talk has
to have equations.
And here you see an
equation that you recognize,
E equals EMC squared.
That has something to do
with the generation of energy
in stars, how mass can
be converted to energy.
That was too far out for
the Nobel Prize committee.
But the one above they
understood pretty well.
This is about the
photoelectric effect.
I was talking about chemistry
for detecting light.
This is how you
can measure light
by its physical interaction.
The light comes in, and
it kicks out an electron.
And up at the top is
the basic equation
for general relativity.
And there's no lambda in it.
There's no lambda in
it, at least there.
So basically, by
about 1930, people
had stopped talking about
the cosmological constant.
Einstein and de Sitter had
made up a kind of universe
where gravity was
the dominant thing,
and it would slow
down the expansion
of the universe as a whole.
And people thought that
what they ought to do
is go out and measure that.
Now, there were some heretics.
I always like to show
a clerical heretic.
Here's Georges Lemaitre,
who was a Belgian priest who
came to study at Harvard.
But Harvard did not
give out the PhD.
That was thought to be too
Germanic at the time he
was here.
So he enrolled in
MIT, even though he
was working with people at the
Harvard College Observatory.
And he had some really
profound insight into this.
He said, well, everything
happens as though there's
an energy in the
vacuum that's not zero,
and that the
cosmological constant--
this is a very modern
way of talking about it.
But the cosmological
constant is a way
of thinking about
the energy associated
with gravity in the vacuum.
And he says this
essentially is the meaning
of the cosmical constant lambda.
So Einstein really
gave up on it.
Here's a famous postcard he
wrote to Weyl, a mathematician.
It's in German.
And it says here at the bottom,
"if there's no--" if there's
a quasi--" if there's--" what?
"If we live in a
quasi-static world,
then away with the
cosmological constant."
That's basically what it says.
Go quickly because you don't
know what you're talking about.
Here we are.
So we have a universe
that's expanding.
It has possibly this model
that Einstein and de Sitter had
thought about, where the
universe would expand
but slow down over time.
One other thing that's
really interesting
that you can get from the
measurements of the motions
of the galaxies is
that they're not only
expanding with the
rest of the universe,
but they're moving with
respect to each other,
and the stars in them
are moving around because
of the force of gravity.
And here's Fritz Zwicky, who
was pioneer of dark energy,
a pioneer of
supernova explosions
that I'm going to be
talking about in a minute,
and a pioneer of rude gestures.
It turns out that he
referred to his colleagues
at Caltech as spherical
bastards, he said.
They're bastards anyway
you look at them.
[LAUGHTER]
But what Zwicky
saw was that if you
looked at the motions
of galaxies with respect
to one another within a
big cluster of galaxies,
that there was more
mass there than you
could account for by the stars.
He thought there had to be extra
mass, which was dark, which
he called the dark matter.
And today that's how we think
about the clusters of galaxies,
where here you see not only
these big yellow things are
the galaxies, but gravitation
is actually warping the light
from behind the galaxies.
There's some things
in the background
making those beautiful
arcs that you see.
And those are a signature of
the presence of extra mass.
We also know from
X-ray observations
of the hot gas that's in
clusters that this is real.
That there's a lot more
mass in galaxy clusters
than you have accounted
for by the stars.
It's also true on the scale
of individual galaxies.
Vera Rubin, who worked
very hard on measuring
the motions inside galaxies,
showed that they are in orbits.
The stars in them
are in orbits that
require much more
mass than you would
have from the luminous
stuff that you see.
So this idea that the
universe has dark matter in it
was part of the story of kind
of figuring out, well, you'd
like to find out how much
dark matter there is.
One way to find out would
be to look at this expansion
and to see it slowing down.
So that became something
we all wanted to try to do.
Here's a radio
observation of a galaxy,
in fact, the same one I
showed you a minute ago.
And it's been encoded so that
the radio emission, which
is shifted to the
longer wavelengths,
is shown in red and the shorter
wavelengths in the blue.
And you can see that
this galaxy's rotating.
The one side's
coming toward you,
while the other
side is going away.
So we know that this is true
for galaxies and clusters
of galaxies, that the universe
has got a lot of stuff
that we don't see.
How do you understand this?
Well, so what we
see are the stars.
And what it's telling you
about is the dark matter
that's inside them,
inside the galaxies.
Well, it's always good to slow
down and go to the art museum.
And here you see
something which tells you
a similar story,
which is when you
look at a mountain range in
the moonlight, what do you see?
You see the reflected
light from the snow
that's on the mountain range.
But that is not the mountain.
The mountain is the
stuff under the snow.
And in the same
way, what you see
is the light from the stars.
But most of the mass
is not visible to you.
It's the dark matter.
And here is a bad haiku, "deep
snow traces rock, always winter
never spring,
mountains do not melt."
Huh?
Bad but mine.
So that's the idea.
The visible matter
traces the dark universe.
And if we want to see the
history of the universe,
we should look at
that stuff, look
at the effect of that stuff.
So you might ask yourself, well,
all right, if this is real,
why can't we measure it--
I mean, in the
laboratory here on Earth?
If it's in our
galaxy, the Earth is
going around the
center of our galaxy.
We ought to be in a kind
of fog of this dark matter,
and it ought to come
whizzing in the front,
and we ought to be able somehow
to figure out how to detect it.
People have tried to do that.
Let me just say you
don't necessarily
expect the results to
come in immediately.
You may be waiting in the
rain for quite a long time.
Astronomers knew that
the speed of light
was finite in the 1600s.
And it took 150 years before
you could make a laboratory
measurement to measure the
speed of light because it's very
fast, a foot in a nanosecond.
And they didn't have clocks
that were good to a nanosecond.
You had to figure out a very
clever way to do the timing.
It was done at the
Paris Observatory.
So the same thing for
the dark matter, people
have been very clever.
And we've known pretty
well that there is
dark matter since about 1933.
And if you read the
literature, everybody
who works in this field
expects to see it next week.
Isn't that right?
And they never do.
Or they haven't done yet.
So here's a
particular experiment,
a jug of xenon in
a mine underground
to get away from cosmic rays.
And no dark matter seen yet.
But you can at least
measure how much
there is, if you could
measure the slowing
down of the universe.
So here's something.
Dark matter now has a
day, Dark Matter Day.
Their slogan is "don't
be afraid of the dark."
[LAUGHTER]
And I think it's the
day before Halloween
or some other symbolic day.
Anyway, how to measure
the cosmic deceleration.
So here's where the
supernovae come in.
I talked about measuring
distances with the Cepheid
variable stars.
If you want to measure
the cosmic deceleration,
you've got to measure it
over a much longer distance.
You need a much brighter
thing to do that.
And it turns out the
supernovae are a million times
brighter than the stars that
Henrietta Leavitt was studying.
I'll show you one in a second.
And you can see them
1,000 times as far away.
So Hubble was
measuring distances
of millions of light years.
1,000 times a
million is a billion.
So you can measure the distances
over billions of light years.
And it turns out
that's enough to be
able to see the subtle effect
of gravitational slowing down.
It would be enough to
see the subtle effect
of gravitational
slowing down if you
had enough of these objects.
Hubble knew about supernovae.
He said, "supernovae can be
detected at immense distances.
And in principle, they are
a criterion of distance
about as reliable as that
of total luminosities
of the nebulae."
That's what he was using
to measure distances.
"Actually, however--" he
was from Missouri, but he
went to Oxford as
a Rhodes scholar.
And he came back saying things
like "actually, however."
[LAUGHTER]
"Actually, however, the
maxima are so seldom observed
and the supernovae
themselves are
so rare that they
contribute very
little to the present problem."
He was right.
They were really hard to find
using the old technology,
using the photographic plates
and things that people had
available to them at that time.
There had been supernovae
in our own galaxy.
Well, here's a picture of
a supernova in a galaxy.
You can see it's
quite a bright thing.
And as Hubble says,
they're about as reliable
as the luminosities
of the nebulae.
Here's one in our own galaxy.
If they're everywhere
in the universe,
they ought to be in our galaxy.
This is a taken
with an iPhone of--
[LAUGHTER]
--Tycho, Tycho Brahe
and his fanciful etching
of his Uraniborg
Castle in Denmark.
And you can see
somewhere up there
in the constellation
of Cassiopeia
that spot, which was the site
of a new star which we know now
was a supernova, in 1572.
So there have been these
things in our galaxy.
We can study the nearby ones.
If you go now and you look
with radio telescopes and X-ray
telescopes and optical
telescopes at that site,
here's what you see.
You see a beautiful
wrecked star.
The green fluffy stuff is
the interior of the star.
It's all moving outward.
There's a bright rim that's due
to the radio emission, where
particles are accelerated to
nearly the speed of light.
This is the remnant
of Tycho's supernova.
And we can still see
it now 400 years later.
So we know about what these
phenomena are a little bit.
They're beautiful things.
And they lead us
to beautiful ideas
because they allow us to see
what the universe is made of.
So here's another
supernova in a galaxy.
If you take the
velocity of the galaxy
by measuring its spectrum,
and you infer the distance
from the apparent
brightness of the supernova,
you can make a plot like
the one that Hubble did.
And here it is.
This is early days in
our work on supernovae.
But anyway, it's a
pretty good diagram.
One thing that's
kind of fun about it
is that you see that
little red square
down there in the corner.
That's where Hubble's
Hubble diagram
was, the one that I
showed you 20 minutes ago.
And what we're seeing
is that the supernovae
make a nice linear relation
over velocity and distance
over a much bigger distance, out
to a billion light years or so.
So what if you could
measure 2 billion
or 3 billion or 4 billion
years into the past?
Could you see the signature
of this slowing down?
And the answer is, yeah, if
you've got enough of them,
and you did a good enough job.
So here in 1995,
my colleagues and I
wrote a telescope proposal.
You have to write in and
say why they should give you
the telescope instead
of somebody else.
And what you see is this line
of the linear Hubble expansion.
It's not obvious enough.
We write out "linear
Hubble expansion."
And then as you see, if
you go to the far side
of this diagram, that
those lines diverge.
Those are different
possibilities
for the universe-- slowing down
a lot, slowing down a little,
slowing down not at all.
And the upper ones are
things that would speed up.
But of course, that would
require a cosmological
constant, and Einstein told
us that was a terrible,
terrible idea, and we shouldn't
think about it at all,
so we didn't.
What about the technology?
Well, here's my thesis
advisor in the 1990s.
And he is holding one
of the world's largest
digital devices, a CCD--
at the time-- a CCD that
had 0.24 megapixels.
You can't even buy one
that's that small anymore.
And of course, it's the
advance of technology
that I alluded to
earlier that has made it
so these chips, which
are fabricated exactly
the way computer chips
are fabricated-- they're
made of the same materials.
The same kinds of
techniques are used--
has really advanced.
In fact, Moore's law, Gordon
Moore of the Gordon and Betty
Moore Foundation,
is the relation
between the number of
transistors on a chip and time.
And you can see for this, that
it's been going up really fast.
This is on a kind of
scale where the separation
is a factor of 10.
You can see it's 1,000,
10,000, 100,000, 1 million,
10 million, going on up.
Interestingly
enough, Moore's law,
that has led to the incredible
change in our ability
to do computational work,
also had an interesting effect
on Gordon Moore.
And at about 2000,
he got into the zone
where he had
billions of dollars,
and he thought I
don't need anymore.
And so he set up the foundation,
the philanthropic foundation
that I work at.
So that was a good thing.
The change from itty-bitty
detectors to great big ones
was really important.
Here's a picture of John
Tonry, one of our colleagues.
And as I mentioned, I was
at the National Academy
over the weekend.
John was inducted into
the National Academy.
He's a Harvard graduate.
And here's how we got going
using those bigger detectors.
The problem is how do
you make that measurement
and find the supernovae
to do the job that I
was talking about?
So here's someone.
That's what I used to look like.
And I'm with Brian Schmidt,
who was a graduate student.
And here he is
explaining how easy
it's going to be to
write the computer
code to take a
digital image and take
another different digital
image, subtract them, and find
the new object.
So that's how it works.
You take a picture.
So epoch 1 might be a month ago.
And here is epoch 2.
Well, if you look at
those two pictures,
you'll have a hard time telling
if there's a new object there.
But if you take the subtraction
of the two, it's pretty good.
And there's a nice
red circle around it.
This is about 1/1,000 of the
image area of the detectors
that we actually use.
If you had to do
this, 1,000 times that
times 10 fields every
night, the graduate students
wouldn't do it.
And so Brian Schmidt
is not a lazy person.
But he said, I'm going to write
some code and make this happen.
So this is sort of the beginning
of big data in astronomy.
Well, it's not really
all that big anymore.
But we thought it was big data.
Now, to be fair, there was
another group in the field.
So that's Brian in the shorts.
And that's Saul
Perlmutter, who led a group
at the Lawrence
Berkeley Lab, that
was doing something similar.
And you can see that there
was a friendly competition
between them.
Well, they were trying to
do something very similar.
And in 1997, I thought
the game was over,
that they had beat
us, and that they had
discovered cosmic deceleration.
So there's a paper.
It's in The
Astrophysical Journal,
where they say that the
universe is omega matter.
It's nearly one.
That means the universe has
the density to slow it down
in the way Einstein
and de Sitter
were talking about for a
lambda equals 0 cosmology,
no cosmological constant.
And it says the results
are inconsistent
with lambda-dominated
cosmological constant,
low-density, flat
cosmologies that
have been proposed to reconcile
the ages of globular cluster
stars with higher
Hubble constant values.
That last line is exactly what
we think is the correct answer.
[LAUGHS] But it's not.
Well, the data they had
were inconsistent with that.
And we were in the same game.
Here's a paper from 1997,
where Brian is the lead author.
And it says, the HIgh-Z
Supernova Search--
that's our name for ourselves--
measuring cosmic deceleration.
So that's my point, that that's
what we were looking for.
We thought we'd been
beaten to the punch.
But-- and this is the
nadir of the talk.
I mean, really,
this is the worst.
But it's going to
get better very soon.
Here's Adam [? Reyes, ?]
who was a graduate student.
There's me in my vest.
You wanted to see the vest?
I got the vest.
And Adam, who was
a student at MIT,
always kept a lab notebook.
And we had a little bit of data.
And it looked like in our data,
that when you did the analysis,
this omega matter, the
matter in the universe,
had to be negative,
which sounded crazy.
But it's what you
would get if you
analyzed without
taking into account
a cosmological constant.
It's that instead of
slowing down over time,
it's speeding up over time.
And so here's the evidence
that we had back then, 1998.
The blue dots are ours.
The red dots are theirs.
They have more but
bigger scatter.
Well, there you go.
And what you see is
that we're trying
to tell which of those
lines is the right one.
Down here we've taken
out the 45 degrees.
And you can see that the
horizontal line isn't the best
fit.
The points are more up above it.
And this dashed line down here
that curves downward, that's
what you'd get in a
decelerating universe.
That is really
not the right fit.
And even without the powerful
tools of statistical analysis,
you can see that the
points light up above,
that's where they'd be if
the universe was accelerating
over time.
How do you think about this?
Well, a supernova goes off.
The light's coming toward you.
If the universe is expanding,
it takes time for the light
to get to you, and if
there's a certain distance.
If it's expanding faster,
then it has to go farther.
And it takes longer, and
it's farther away and dimmer.
So the signature was that the
supernovae were a little bit
dimmer.
This was a big surprise to
Einstein, as you can see here.
Although, Einstein had been
decomposing for some time.
[LAUGHTER]
But the way we think
about it is that there's
a kind of tug of war
between the dark matter
and the dark energy, dark matter
trying to slow things down,
dark energy trying
to speed things up.
In the early universe,
the matter is ahead,
and it slows things, down but
it gets diluted over time.
If the thing that's
speeding things up
is really the cosmological
constant, it's constant.
So even though the
universe is getting bigger,
the energy density of
the cosmological constant
is staying the same.
The vacuum energy's
staying the same.
And that leads to a universe
that will slow down for a while
and then accelerate.
And it looks like that's
the kind of world we're in.
So here's a kind of
complicated looking diagram.
But it shows how
much dark energy
and how much dark matter.
Those are the points
that we had back in 1998.
Here's more or less
where we are now.
The thing we've done over the
last 20 years is get more data.
If you have a signal that
is kind of surprising,
there'll be many people who
will be skeptical of it.
And that's a good thing.
But if you get more data,
and the single gets stronger,
that's a very good thing.
And here, it's not only
the supernova data,
but data from studies
of the cosmic microwave
background, the glow from
the Big Bang, the study
of galaxy clustering, that all
converge on a answer that looks
like we have a universe
that is mostly dark
energy, partly dark matter,
and just a pinch of the stuff
that we're made of, just
a pinch of the stuff
that we're made of,
ordinary matter.
Now, this is a big deal.
So I show you here a picture
of Brian and Adam and me.
We're in a hotel.
But the hotel is in Stockholm.
And then we put on
our fancy suits.
And Saul and Adam and Brian
all won the Nobel Prize.
And we stood around their suits.
So that's good.
We have much better data now.
And the evidence for
acceleration has not gone away.
You can't see it.
But I can.
That line is curved upward
in the way the ones that
have acceleration are.
This bottom panel
shows the difference
between the model that we're
talking about and the data.
And you can see
it's nice and flat.
There's a little bit of
noise but pretty good.
You could do this now.
So here's a set of data of
redshifts and distances.
And the line that's
ticking across
is what you would predict
for different amounts
of the dark energy.
And what you can
see is it has zero.
It starts off at 0, and
that's not a good fit.
Oh, that's better.
One more time-- so that's
no good, no good, no good,
no good.
Keep going.
Keep going.
Oh, it's getting better, better,
better, better, better, best--
too much.
So you can actually measure
how much of acceleration
there is, how much of
the dark energy there is.
Let me put this together
like this, no more blunders.
Never mind this.
I've been working on this.
So as I said, I'm now
working in philanthropy.
And so now I always
think about money.
And if you look on the
back of the $1 bill,
there's all kinds
of mystic stuff,
including this weirdo pyramid,
in which the proportions--
I've I listed out
the proportions
as the volume of the pyramid
of what the world is made of.
And it turns out that it's
about 2/3 dark energy,
this weird stuff that we
just discovered 20 years ago.
It's almost a third
dark matter, which
is not the stuff of
the periodic table.
And the atoms that make up
the intelligence, therefore
with the gleaming eye
that you see there,
of the universe, the
stars that we know about,
the planets that we know
about, all the things that we
see glowing are
just a tiny fraction
of what's really there.
And the history
of the universe is
determined by this unseen
stuff, the dark matter
and the dark energy.
And the luminous stuff is
sort of along for the ride.
So we think we were
in a universe that's
been changing over time, slowing
down first, and now speeding
up.
And if you want to find out
what the dark energy is,
of course, in the old days,
you could just Google it.
And that was before
all the institutions
that had spent a lot of money
on dark energy research put up
their own websites that
explained everything and made
them seem like the discover.
I mean, they did a good job
on explaining it to everyone.
In the old days,
you could get this
from American
Hydroponics, which was--
it's plant food for growing
marijuana in your closet.
[LAUGHTER]
And it says, "the
specialized processes here
are responsible for the very
distinct odor of dark energy."
OK, so I made it sound like
we've got a new picture,
and it's just great.
And we do have a new picture,
and it is just great.
But we still don't know
whether the dark energy
is the cosmological
constant or something else.
The thing I kind
of skipped over is
work I'm doing now to
try to measure that.
But those of you who've
been reading the newspapers
and science things in
the last week or so
might have seen this.
This is about the rate
of expansion measured
with the Hubble Space Telescope
in the nearby universe
compared to the distance to
the scattering of the microwave
background.
And it doesn't quite fit.
It's not quite what you'd get
for the cosmological model I
described so
glibly, the one that
has dark matter, dark energy,
and a cosmological constant
for the dark energy.
So the wheel is still in spin.
We do not know what
the dark energy is.
And we're going
to find out either
by a direct approach of making
those measurements better,
or perhaps by some of these
slightly indirect ways.
So the universe
that we live in is
a universe that is ruled, in
a way, by the unseen things.
So the mountain is not the snow.
The snow is the
thing on the surface.
That's what we see.
But there's a
substance beneath it
that is invisible to
us, in the same way
that dark energy is
not something we see,
but we see the effects of it.
So the artist here, of course,
is trying to depict the wind.
And you don't see the wind
when you look outside.
What you see is the tree
moving because of the wind.
And in the same way, we
talked about the dark energy
as accelerating the universe,
as seeing the effect of it
in the diagrams that
I've showed you.
Well, you may say,
well, that seems
like kind of a weird universe.
That's not the one
I would make up.
Well, you're right, of course.
And here is a T-shirt that
embodies that thought.
Here is what it says.
"The universe is under no
obligation to make sense,
but students getting a PhD are."
And apparently I
said this somewhere.
[LAUGHTER]
And it was on
these T-shirts that
were at an institution of
research that had a gift shop.
And you could go
in and buy them.
And so I went in there and I
said, I am Robert P. Kirshner.
[LAUGHTER]
And they said, oh,
that's very good.
They said, we'll give
you a good discount.
[LAUGHTER]
What kind of science is this?
When I was president of the
American Astronomical Society,
I would stand
shoulder to shoulder
with heads of other
learned groups,
and we would talk to
congressmen or philanthropists.
And we would explain to them
what science was good for.
And the arguments that
go over big with Congress
are to talk about technology.
That is the economic
benefit of science.
To talk about
defense, that if you
don't understand the
world well and have
good detectors and
powerful weapons and so on,
it's a dangerous
world out there.
A lot of the congressmen
are kind of old,
so medicine is very
popular with them.
[LAUGHTER]
And so in a way,
they're asking us
to promise that science
will make everyone
rich and safe and immortal.
Well.
I'm in favor of all
those things, of course.
But I think we do this work
for a different reason.
I think we do this work
because we're curious.
And if we were rich and
safe and immortal and bored,
that would not be
an ideal world.
And I think we have confidence
that if people go out
and do scientific research
for the motive of finding out
how the world works, that
all those other things
will come along with it.
So I think doing scientific
work for the joy of finding out
how the world works is something
that we should take care of,
not forget.
And now I'm in a
position to help
a little bit to make it happen.
Thank you very much.
[APPLAUSE]
- Well, I'm an astronomer,
so I should know the answer
to this question.
But I didn't
discover dark energy.
So I'm going to ask you.
So when the expansion
of the universe
was discovered back
in Hubble's day,
and Einstein suddenly
decides, as we hear it,
that the cosmological constant
wasn't needed anymore, well,
why is that the case?
If everybody thought that
the universe was static,
and instead they found out
that it's actually expanding,
but at that time
they didn't really
know why it was expanding.
- Right.
- We didn't really
understand the Big Bang.
So why was he so
quick to give up
on the cosmological constant?
Why didn't he just say, aha,
the cosmological constant
is pushing the universe apart?
You alluded to it.
- Yeah, I showed
you that cartoon.
I showed you that cartoon.
And it's de Sitter who is the
person quoted by the newspaper
saying it's the
cosmological constant that
is making the universe expand.
So that's just the
right thing you said.
And then very shortly after
that, Einstein and de Sitter
got together and they
wrote a paper that said--
well, the legend
is that Einstein
said about this paper,
which they wrote together
that said forget about
the cosmological constant,
let's just use this model that
doesn't just set it to zero,
it just leaves it out entirely.
The legend is that
Einstein said,
well, I never really
liked that paper,
but I did it because
de Sitter wanted it.
And de Sitter said, oh, I
didn't really like that paper.
But I did it because
Einstein insisted.
I think it's a good question.
I think the fact that this
has been rehabilitated
and is now seen, looking
back, as the origin
of the modern ideas-- that's
why I showed you the Lemaitre
quote.
That's totally different
than what somebody
would have done in 1990.
In 1990, we all
would have said, oh,
cosmological
constant, last refuge
of scoundrels, terrible idea.
Every time someone has brought
it up it's been a bad idea.
So I think we probably ought
to do a little more homework
on the question you're asking,
what is the origin of the idea?
How much did it fizzle out?
Lemaitre, who still was
talking about this, of course,
went out of astronomy and into
university administration.
I'm not casting
aspersions, but, you know.
And he was very influential
in getting computing machines
into the University of
Leuven where he was a dean.
But he didn't have
students, and he
wasn't out there talking
about this very much.
But the idea didn't
completely go away.
So I think you
raised a good point.
Why did it go Away I think it
was the authority of Einstein
and de Sitter.
And they said there's no
reason, no theoretical reason
to have this, so
let's get rid of it,
the idea that's expressed
in that postcard to Weyl.
- I wrote a paper
about a week ago
that they had discovered
a second galaxy that
lacked dark matter, like a
signature of dark matter.
And I was a little bit
confused by it because--
- Yeah, me too.
- I thought that every galaxy
was formed by dark matter.
And so all of a sudden, there's
a second galaxy now that's
looked a little different.
- Yeah.
So that's a very good question.
And of course, our
knowledge of the dark matter
for each individual
galaxy is not very good.
Most of them are not
studied very thoroughly.
But people have been looking at
galaxies, especially galaxies,
real little ones--
real little ones,
these dwarf galaxies.
And they seem to have a ratio
of regular matter to dark matter
that's pretty high.
Or the way it's usually
put in the press
is that dark matter
is really low.
So I think there's more to
know about how galaxies form.
And there's more to know
about the whole range
of possibilities.
On the average, in the bulk,
the presence of the dark matter
is there's no doubt about it.
But in particular places,
it may be that those ratios
are different.
And this seems to be
evidence that that's so.
So you're right to be puzzled.
And these things are
not very well known yet.
But it does show
that there might
be more to learn about how
the dark matter interacts
and how it's connected to the
process of galaxy formation.
That would be the
thing to say, I think.
- I have a second question.
But it might be a
little bit dumb.
But--
- That one was awfully good.
So on average--
[LAUGHTER]
- Well, so if we
can't see dark matter,
does that mean
dark matter exists
in a different dimension of--
- It wouldn't have to.
It could be just a particle.
Like, there are
neutrinos, which don't
interact very much with matter.
But it can't be the neutrino
because we know a little bit
about the mass in the neutrino.
It's not the right value.
But it could be a particle,
a subatomic particle,
that's not like neutrons
and protons and electrons
but sort of another family.
So it could exist
in our dimensions
but be a different
kind of particle that
only interacted by gravity.
- It's getting a
little bit late.
So we'll just take the three
questions from the three
who are standing up.
And then we'll--
- I don't know much
about this stuff
because I'm a microbiologist.
But I have a question.
- [INAUDIBLE] microphone.
- Oh, sorry.
I said I don't know much
about this because I'm
a microbiologist.
- Uh-oh.
- But I have--
- He's setting me up.
- I have one question.
Does lambda, does the
cosmological constant
have a dimension?
- Does it have a dimension?
You mean a size?
- No, I don't mean size.
I mean, is it energy?
Is it mass?
- Yeah, it would be mass energy.
It would be-- yes,
it would be mass.
Yes, it would be energy.
It would be an energy
associated with the vacuum.
- And I've read
that it is something
to do with the oscillation
of quantum particles
coming in and out of existence.
- Yeah.
If you imagine the
quantum picture
of the world on a
small enough scale,
you would have particles
and their anti-particles
being created and
destroyed all the time.
That's the kind of picture
of the vacuum that we have.
So the vacuum's not featureless.
The vacuum's not
without properties.
But on average, it
could have zero energy
and still have those properties.
That's the quantum picture.
Now, the question is, how
does it look for gravity?
When you go to the scales--
there is a natural scale.
Maybe this is what
you're asking.
There's a natural scale.
It's called the
Planck length, which
is the distance on which these
quantum effects for gravity
would be very important.
And the problem is,
the conceptual problem,
the intellectual problem is that
we don't have a good quantum
theory of gravity
that allows us to talk
about this in the same way.
If you make a naive estimate of
what the energy density should
be associated with the Planck
length and with gravity,
it's 10 to 120th times
bigger than the value
that we find from this
astronomical measurement.
So you've put your finger on it.
There's a quantitative
problem, which lies right
at the heart of
theoretical physics,
and for which at
the moment there's
no satisfactory explanation.
So there's really
something great to do.
That guy with the ignorance,
he was onto something.
- Hi, Bob.
So if dark matter is
everywhere, then presumably it's
also part of biology.
So how do you see it
manifesting itself
in living systems
such as ourselves?
And how come we biologists
haven't seen it yet?
- Yeah.
Well, the density's pretty low.
And the interaction
is very weak.
So the idea is that like
neutrinos could go right--
are--
neutrinos from the sun are
going right through your body
right now.
And they're not affecting
you because the probability
that a particle
will hit something
as it goes through
you is so low that you
need a light year of lead not
a meter of water to catch one.
You can build devices
to see neutrinos.
And these are weakly
interacting particles.
We don't know the interactions
that are associated
with the dark matter.
But they might be
something like that.
In which case, there would
be just negligible effects.
They don't deposit any energy.
They don't do anything to you.
So I did a calculation.
When there was a
supernova in 1987,
I did a calculation
of all the eyeballs
and of all the
people in the world.
Who would have a flash due
to the neutrino interaction?
And the answer was 10 people.
[LAUGHTER]
Yeah.
Erwin.
- I have a comment first.
- Sure.
- You're a brilliant
talker, brilliant speaker.
I loved every minute of it.
I have an historical question.
- Yes.
- Before Hubble discovered
the expansion of the universe,
two people, Alexander Friedman
in the USSR and George Lemaitre
in Belgium, predicted that
the expanding universe
was a solution of
Einstein's equations
without the
cosmological constant.
What do you know--
what is known, if anything,
about whether Hubble
was aware of either their
works before he published
his paper, which showed
the expanding universe
and has a graph that has an
embarrassing mistake in it?
- Yes, I know.
Well, I would say this.
I looked into this a little.
You could see Hubble was very
apologetic about his knowledge
of general relativity.
He never studied these things.
And so I think it was
seen by him as something
he had to rely on the
authority of others.
And in 1928, there was
an IAU meeting in Leiden.
And de Sitter was in charge
of the cosmology section,
or whatever it was
called exactly.
Hubble attended that.
And there was a
prediction de Sitter had,
another model, in
which there would
be a quadratic dependence.
That is there would
be redshift that would
go like the distance squared.
And the legend is,
and it may even
be true, that
Hubble-- well, it's
certainly true that Hubble
went to that meeting.
It's certainly true that
he talked to de Sitter
and that he subsequently
came back and told Humason,
we better look for
this de Sitter effect.
So I think what Hubble was
looking for was the parabola.
But what he found was the line.
So at least he
had the good sense
to only show that and
not a fit to a parabola.
But his theoretical
underpinnings were very slim,
I think.
Whereas, as you as you correctly
point out-- well, look,
Lemaitre was the one who really
understood both the physics
and knew the
astronomy well enough.
And there's, of course,
this famous footnote
to his French version of his
paper about the expanding
universe, where he worked out
what the ratio of distances
and velocities was, the thing we
would call the Hubble constant.
And Lemaitre worked it out
but using Hubble's data.
And there's a complicated
story of publication,
when Eddington, who is the
head of The Monthly Notices,
got a kind of interesting
letter from Lemaitre.
He said, I sent you the
solution three years ago.
He did have it published
in English but then
without that footnote.
And for a while people thought
there was some conspiracy,
that somehow Hubble
suppressed this.
But in fact, Lemaitre is the
one who took the footnote out.
He said, well, Hubble
has better data,
and he's shown it more clearly.
So I think you're asking a
really interesting question,
that how much did
Hubble understand
of what he was doing?
And I think it's fairly limited.
And the evidence
is in his book, I
think, where he tells you
how hard this was for him,
that Lemaitre was someone
who had both the physics
understanding and the
astronomical knowledge.
Einstein had the
physics intuition,
but he didn't really know
the astronomy all that well.
And so it was a kind
of a funny moment,
where sort of who you
were mattered a lot.
And I think the fact
that Lemaitre kind of
left the field
was something that
made a big difference
to how people
thought about these ideas.
- Was Lemaitre at that '28
conference, and did he speak?
- Ah, good question.
I don't know.
I don't know.
But there is an attendance list.
It's in the Harvard College
Observatory Library.
- [INAUDIBLE] haven't
memorized that.
- No, of course not.
But you can find the minutes
of the meeting, yeah.
I looked it up one
time a long time ago.
- Fantastic.
Well, we have three
things we have to do.
First, we need to
thank Bob again.
So thank you.
[APPLAUSE]
