(bright music)
- Hi.
Welcome to the second Hitchcock Lecture
by Professor Peebles.
(clapping)
That's Professor Peebles down there.
I am George Smoot.
I'm the local, but I'm the person here
who probably known him the third longest
or the second longest, I don't
know if Mark has known him,
Mark Davis has known him
longer or not than I have.
But I get the pleasure of introducing him,
and yesterday you heard this
fairly formal introduction
and you see in the little
brochure you got when you came in
that he's highly honored in a lot of areas
and been a pioneer in the field.
But they actually left
some of the stuff out
that he did that was
very influential to me
and I know to Mark Davis and
to a number of other people.
There's a whole book that they left out
called Physical Cosmology
which came out in 1971.
And that book was extremely formative
to a whole generation of people.
So you look sort of up in
the history of cosmology
and everything and you
get statements like this.
According to many books
and surveys of cosmology,
including reviews of a more
or less historical nature,
'cause you know, it always
more or less, right?
The field of cosmology only
became truly scientific
about 1965, primarily as a result
of the discovery of the
cosmic background radiation,
and they give sample (mumbles).
To me, I noticed that, but it wasn't,
cosmology didn't really
become serious field
until I read Jim's first book,
because there was the stuff laid out
with a huge amount of
math, but enough words
that I could understand it to know
whether it was worth my trouble
of learning the math, right?
And each of us, Mark learned
a certain kind of math,
I learned a certain kind of math.
And it was that book and
the book by Steven Weinberg
and a couple articles by Dennis Sciama
and particularly Jim's book
and the articles by Sciama
that got me interested in the
cosmic microwave background,
and the first experiment
that we did in that area,
we named the ether-drift experiment
right after the title of
your subchapter in the book.
And Jim wasn't remembering
how long he's known me,
but I stole time out of one of his talk
almost three decades ago at
the American Physical Society
meeting in Washington, D.C.,
which was the big-time
meeting in those days.
And he had an invited talk,
and we had new results
saying we know that the earth is moving.
Finally Galileo is right.
And he graciously, as he always does,
gave up time to this young post-doc
who just asked him,
graciously gave up some time
so that I could present the results,
because it was too late for me
to get a talk in the meeting and so forth.
And then he's regretted it ever since,
so he promised to roast me tonight,
but (laughs) we'll see what happens.
But I have a couple more
things to say about him,
and that is, you will see from his talk
how good it is as a public
talk and everything,
but to physicists, he's what
we'd call elegantly casual.
And let me explain that in more detail.
He goes and gives these simple,
he'll go up to the board,
he'll do the simple calculations for you
and it'll be, well, it has to be this
by dimensional analysis, or
this is the simple calculation.
And the answer will
come up exactly correct,
no factor of two, no pi, nothing,
just exactly correct, and he
does that time after time.
And as teachers, we
occasionally do that, you know,
we don't do Chandrasekhar,
we do the Landau description
and get the limit on the
mass of a neutron star,
or we do the bending of light by the sun
or something like that
by dimensional analysis
and we kind of get the right answer
if we fudge it a little bit, and so forth.
But he does it time after time,
and I always wonder how many nights
he spent up doing those
calculations correctly
to then work out how to do that.
Because it's just beautiful
to see it come out that way,
because you see the
physics of what goes on,
and that's what's characterized his books
and that's characterized what
he's done over this time,
and again, he's doing
that in these lectures.
And so I say the public
can appreciate him,
the scientists can appreciate him,
and there's a lot of us in
here and all across the world
that came into cosmology
because we read your books
and we followed your example and so forth.
And so it's a pleasure for
me to be here and do that.
And then the only bad
thing I can say about you,
and you already put it in the biography,
is you're a Canadian,
just like our chancellor.
And so, I mean, that's like...
(laughing)
So rather than take much more time out,
'cause I already owe him some time,
let's have Professor Peebles come up here
and give the second lecture.
(clapping)
- Thank you, George.
Friends and colleagues, thank you
for coming in out of a
beautiful day to this dark room.
It's evidence, clearly, that you share
our fascination with the world around us.
I like to begin with a general comment
about the nature of this science.
Cosmology, the study of the large-scale
nature of the universe,
is a physical science,
and it operates by general rules
that are familiar from any
other branch of science.
You're free to introduce
conjectures, postulates, ideas.
They should be, and generally are,
treated for what they are,
just-so stories, until they can be tested.
A successful test, a
prediction that is made
and successfully
confirmed by measurements,
confirms our belief that
we're on the right track
with this just-so story.
A failure is a positive step
because it tells us we'd
better change the story.
Continued successes without
the need to change the story
convinces us we're on the right track
to a good approximation to reality.
You'll notice that in this description,
it clearly follows that
there is no absolute truth
in physical science, because after all
we cannot make an experiment
that is infinitely accurate.
Instead we are searching for
successive approximations.
That is why I can stand up here and claim
that cosmology at the present day
is remarkably successful,
even though it has
some pretty wild just-so
stories buried in it.
So that's gonna be my real challenge,
to convince you of those two aspects.
We have a good approximation to reality,
but we have a long way to go.
So to begin, the universe
is clumpy on small scales.
Here we are, a concentration in a room
in the suburban sprawl of the Bay Area
on a planet around a star
in a galaxy of stars in the
local supergalaxy of galaxies.
But the large-scale distribution
is somewhat different.
You'll notice that in this map
showing the distribution
out to a fixed depth
of this distribution of
galaxies across the sky
there are clumps, and I'm going
to be drawing your attention
in a little while to
this particular clump.
There are other clumps
and there are wiggles
and fascinating texture,
but you also notice,
I trust, that this part of the world
looks a lot like that part of the world.
This leads to the first law of cosmology,
the first conjecture, if you will.
The universe is uniform
in the large-scale average,
no center, no edge.
The second law is that
the universe is expanding.
Here a description, one observes
that the light from distant galaxies
is shifted toward the
red, as if Doppler shifted
by motion of the galaxies away from us.
The interpretation of this observation
is that the universe is expanding.
Here is a two-dimensional illustration
of this expanding universe model.
I remind you that here we are pretending
we live in two dimensions and not three,
that you live on the
surface of that balloon
and that you agree not to ask me
what's off the surface of the balloon.
You live on two dimensions.
Suck it up, that's where you live.
(laughing)
The balloon is being blown up.
That means that if you sit on
a galaxy and look around you,
you see the galaxies nearby moving away
and you see more distant galaxies
moving away more rapidly.
That's the expansion of the universe.
Notice that if you go to another galaxy
and repeat the measurement,
you'll see the same thing.
Here is a universe that
is uniform, no center,
and yet expanding in such a way
that everybody sees the
galaxies moving away from them
as if they were at the
center of the universe.
But they're not, there is no center.
Good.
The second law of thermodynamics.
Now we come to some conjectures.
One may ask, does it not mean
that if the universe is expanding,
that it's growing less dense
and it used to be more dense?
Fred Hoyle, on the left
here with Tommy Gold
and Hermann Bondi, not in the photograph,
pointed out that doesn't follow at all.
They said, let us introduce a postulate,
a just-so story, if you will.
Suppose that matter is
spontaneously being created
at a definite, exceedingly small rate.
The newly created matter will by gravity
pool together in clouds,
the clouds may collapse
to become galaxies of stars,
and these young galaxies
will fill the voids that are opening up
as the older galaxies move apart.
You will thus produce a
universe in a steady state,
expanding yet not evolving.
It's arguably a deeply beautiful idea.
Somehow, perhaps for
anthropomorphic reasons,
it's sort of unnerving to think
that our universe is not forever.
But I can tell you, evidence that it is
of compelling interest to many people,
eternal inflation is just
a variant of this picture.
But this classical steady state cosmology
has very clearly failed,
and here is the reason.
Space is filled with
radiation with an intensity
at each wavelength that
follows this curve.
It's a very special curve.
It is how radiation will relax
if sufficiently absorbed and re-radiated
so that the radiation can relax
to statistical equilibrium,
so-called black body radiation
or thermal radiation,
This radiation at a temperature
of 2.7 degrees above absolute zero.
How did this radiation
reach this wonderfully
characteristic state,
characteristic of equilibrium?
Surely because the radiation
was absorbed and re-emitted many times.
But we know the universe as it is now
is pretty close to transparent
at these wavelengths.
You know that because you can see
distant galaxies emitting in the radio.
The clear inference is that this radiation
is a fossil remnant from a time
when the expanding
universe was dense and hot
and able to relax to
statistical equilibrium,
a fossil, therefore, that
is in stark contradiction
to the steady state cosmology.
This is wonderful science.
A postulate is introduced and demolished.
We've made progress.
We have now the notion
of an expanding universe,
and it fits the observation
of that radiation.
That's good, but of course it's also
in a sense another just-so story,
particularly when you consider
that when we look into
the details of this theory
of the expanding and evolving universe,
we need to introduce our
own two new just-so stories,
that most of the mass of the universe
is not in the form of
which you and I are made,
that you and I are made of material,
the jargon is baryons, that occupy only
about five percent of
the mass of the universe.
The rest is in two new hypothetical forms,
dark matter and dark energy.
Isn't that embarrassing?
Absolutely.
Does it mean we're on
the totally wrong trail?
Absolutely not.
We are just trying to get
successive approximations to reality.
So we begin with the dark matter issue.
On the right you see
Fritz Zwicky, a Swiss,
many years at Caltech
and Southern California,
a somewhat irascible figure,
but also a deeply intelligent
and original astronomer.
I spent about 10 years of my life
working with the data in this book
of the catalog of the nearby galaxies.
He predicted the
existence of neutron stars
as remnants from exploding
stars, remnants of supernovae.
He predicted that the bending of light
by mass concentrations would
produce astronomical lenses.
I'll show you an example in a minute.
And he was the one who pointed out
that this cluster of galaxies
which I pointed out to you
on the beginning slide
has a curious property.
One can measure the relative
velocities of the galaxies
by looking in the differences
of their Doppler shifts.
A galaxy moving away from
us has its light shifted
toward the red, moving toward
us, shifted toward the blue.
From the observations of those shifts,
one could get the
velocities of the galaxies
in this cluster, and one can also compute
the mass seen in the stars in
the galaxies in this cluster,
and one sees a distressing inconsistency.
Much more mass is
required than is observed.
The interpretation in
those days is missing mass.
We've missed some of the mass.
We've made great progress;
these days we call it dark matter.
We still don't know what it is.
But we can be very sure of one thing,
it's not the baryons of
which you and I are made.
There is no form that could be hidden
in there in that amount.
There are other examples
of this phenomenon.
Here, oh, and here is an example
of Zwicky's gravitational
bending as astronomical lenses.
Here is an image of the center
of a cluster of galaxies.
It's a negative; where the image is dark
there are lots of stars.
The arc is in fact one
galaxy in the background
seen in a horribly distorted image.
Light from that distant object,
which is about in the
center of this image,
comes toward us at the upper part,
moving slightly upward and to the right.
But gravity bends it, pulls it over
and directs it to our eye.
Light rays coming down
from the galaxy slightly
and to our right are bent up and over
and produce the image on the bottom.
Here we see a horribly
distorted image of a galaxy.
It's a gravitational lens.
But what is really striking about this
is how smooth that arc is.
The bending angle is almost
identically the same
going around that arc.
The bending angle is the same,
but the mass distribution
in the seen material
is wildly inhomogenous.
How in the world could that be?
The mass isn't mainly where the stars are.
There's a smooth component to mass.
Another illustration.
Here on the left, an optical
image of three galaxies.
It's a negative again,
dark where there are stars.
On the right an image taken
with a radio telescope
that is sensitive to the light emitted
by hydrogen atoms at
21-centimeter wavelength.
You see that there is a
clear physical connection
between these three galaxies.
They're not just an accidental
projection on the sky,
they're close, and they're doing a dance.
But if you add up the mass you
see in those three galaxies
and compare it to the velocity
with which they're moving,
you see these guys shouldn't
be doing a dance at all,
they should be flying apart.
The standard interpretation,
the hypothesis,
the just-so story is, there's
a lot of dark matter in there
that's holding them together.
It's not in the galaxies,
it's in the general soup
connecting the galaxies.
Here's another galaxy.
Vera Rubin, at the Carnegie
Institutions in Washington, D.C,
founded by Andrew
Carnegie, the robber baron,
but you know, old dirty money
becomes new clean money.
(chuckling)
He founded the Department
of Terrestrial Magnetism;
the name hangs on, it's a charming
indication of past occupations.
Now this is a world-class center
for many branches of
science, including astronomy
and including Vera Rubin,
who measures in this example
the motion of material in
the outskirts of a galaxy.
And she finds that this material is moving
in a nice circular orbit, part
of the disc of the galaxy,
but with a disconcerting observation
that the material is
moving much too rapidly
to be held in a circular
orbit by the seen mass.
Again, we need a hypothesis: dark matter.
As I said, this dark matter
can't be baryons in any known form.
Instead we have ideas that it might be
other forms of matter, we
have other just-so stories.
A big effort to check on some
of these big just-so stories
is here collected from some
webs, from some posters.
The key point: if standard
gravity physics is right,
and it has passed demanding tests,
then matter is not dominated
by the baryons you and I
and stars and planets are
made of, so what is it?
One possibility, classes of particles
that interact only very weakly
with material of which we are made,
but occasionally will
hit an atomic nucleus
and give it a little kick.
So perhaps if that's the case,
you will be able to
detect this dark matter.
Just take a crystal of material
and wait for it to ring unexpectedly
because it's been hit by
a dark matter particle.
Now, you don't do this in
the surface of the earth.
Too much radiation floating around.
You go deep in a mine that's clean,
and you'd cool the detector down
as close to zero as you can get it,
and you build very sophisticated equipment
to look for those occasional
ringing of the apparatus.
Bernard Sadoulet is one of
the leaders of this operation.
I couldn't find him in this
picture of co-conspirators.
- He's upper left.
- Upper left?
That's not Bernard, is it?
- Think that's him.
- Oh.
Okay.
(crosstalk drowns out speaker)
- [Audience Member] He doesn't
look good in a hard hat.
- He doesn't look good in a hard hat.
Many of these people do.
Most of them are much younger than Bernard
and are very capable, very
dedicated, very brave.
They're going after a just-so story.
It'll be wonderful if they
have a positive detection.
If they get a negative result,
that'll be wonderful too.
It'll tell us, change our
just-so story and improve it.
This is the best and the brightest.
They deserve our support,
and we know that no matter
how the experiment comes out,
they're gonna go on to do
great things to the world.
We must also postulate
that about two thirds
of the mass of our universe is in
yet something even more bizarre.
So here we go, suspension
of disbelief required here.
Upper left, Albert Einstein,
upper right, Willem de Sitter,
a Dutch astronomer, director
of the Leiden Observatory.
Albert Einstein invented in 1916, '15,
a new theory of gravity,
general relativity theory.
He applied it to his notion of a universe
that was uniform, and
found to his distress
that this universe is
inconsistent with his assumption
that of course the world is forever.
That led him to adjust his theory
by putting in a new term that
he called the cosmic term,
that had come to be called for a long time
the cosmological constant term,
is now called dark energy.
You see the great advance
in science in that.
Ever more romantic names.
Now, the game is this.
Einstein visualized a
universe of this sort,
but he assumed it's obvious
the universe is forever.
But his theory said no, this universe
will collapse under the force of gravity.
Gravity pulls things together.
That is a bad thing, it
would seem to Einstein,
and so he adjusted this theory
to put in this cosmic term
that acts as a repulsion
to push apart the tendency
of gravity to draw together.
That is the notion of Einstein's universe.
The pressure is held in that balloon
just such that the pressure,
which is the operation
of the cosmological constant term,
is just such as to balance the
attraction force of gravity.
It was the Belgian Georges Lemaitre,
whose picture I will show you in a moment,
who pointed out that this
is an unstable situation.
Touch that static universe a little bit
and it'll either collapse or expand.
By the time he made that point,
it was known that the
galaxies are moving apart
as if the universe was expanding.
A brilliant connection made,
the universe is expanding.
That led to Professor
Dr Willem de Sitter's
comment in a Dutch newspaper.
You see his shape; it is
the Greek letter lambda,
which was Einstein's
symbol for this new term
in his equations, and the
quote from the newspaper is,
"Who, however, blows up the
ball, the expanding universe?
"What makes the universe
expand or swell up?
"That is done by the lambda,"
Einstein's cosmological constant.
"Another answer cannot be given."
As soon as this point was made,
people recognize that you don't
really need lambda at all.
If you're willing to accept
that the universe is expanding,
then let it expand, and let
it do it under gravity alone.
Gravity will slow the expansion,
but under the right initial conditions
the universe will expand for a long time.
Bob's your uncle.
We don't need lambda.
Here is some correspondence
between Einstein
and Georges Lemaitre, the Belgian
who discovered this connection
between the astronomers'
observations and Einstein's theory.
I was told yesterday I should have made
a few comments about that clerical collar.
He was an abbe of the
Roman Catholic Church.
He had absolutely zero problem
with the balancing of
science and religion.
He was deeply influential in
the Vatican and respected,
he was a deeply influential
and original physicist.
He in writings gave his opinion
of how you deal with these
two aspects of his work.
He said, "If you wish to swim,
"then you would be well advised to do it
"in ways that believers and nonbelievers
"have found to be effective."
I would make it more stark and to say
that if I decided I'm not gonna swim
like all of those people who
believe in another religion,
then there's a distinct possibility
I'm soon gone, isn't that right?
And by the same argument, Lemaitre said
if you would do science, then
you will do it in the manner
that people, believers and unbelievers,
have found to be effective.
Is that so wrong?
To me, perfect science.
Here is a correspondence just
after the Second World War
in which Lemaitre makes the point,
we needn't get into the technical details,
that Einstein's cosmological term
just acts like a
distributed energy density,
uniformly distributed through space,
with some peculiar properties,
let's not bother with them.
It sets the zero level of energy.
Einstein replied with
some complimentary works,
"Thank you for your very kind letter,
"I doubt that anyone has so carefully
"studied the cosmological implications
"of the theory of relativity as you have."
He was being nice, but I think
he was also being correct
and proper in saying that.
But then the line I wish
to draw your attention to,
"Since I have introduced this term,
"I had always a bad conscience."
Einstein hated the cosmological term.
To him, it was an extra
appendix that wasn't needed.
Other great physicists
echoed Einstein's view,
in particular Pauli,
great Austrian physicist.
Einstein was soon aware of
these new possibilities,
that is to say get rid of lambda,
and completely rejected
the cosmological term
as superfluous and no longer justified.
I fully accept this
standpoint of Einstein's.
One of the great physicists of all times.
A little pompous, but
there it is, he meant it.
He also had a reason for rejecting it,
namely a really ridiculous
problem with quantum mechanics.
You've heard of zero point energy.
Take two hydrogen atoms,
bring them close together,
they'll get stuck and
make molecular hydrogen.
In computing the binding
energy you have to take account
of the fact that there is a
so-called zero point energy.
Sometimes it's discussed as being a result
of the uncertainty principle,
you can't make sure
that the atoms are really at the center
of their potential well and
not moving at the same time.
The result is a little
bit of uncertainty in both
that adds up to a little bit of energy.
I don't know that I grab that.
Just live with it, there
is zero point energy
in any degree of freedom of motion.
The electromagnetic
field in quantum physics
is a field, and it's also particles,
just as particles are fields.
Was that somebody?
(laughing)
The electromagnetic
field is well described
by standard quantum mechanics.
It has lots and lots of zero
point modes of oscillation
that produces an energy density
that is ridiculously large,
so big as, Pauli is recalled as quoting
that the energy density in
the electromagnetic field's
zero point energy ought to be so large
they will curve space time so strongly
that our universe won't
even reach to the moon.
It's absurd.
His response, we can't live with this.
It's not there.
This zero point energy of
the electromagnetic field
would act as Einstein's cosmic term.
Many people recognized
this, including Lemaitre,
but the person who made it visible to all
is Yakov Zeldovich, shown here dancing
with his wife at a banquet in Hungary.
He started out, like other Russian,
Soviet physicists, working
on the nuclear bomb,
was a big factor in the
nuclear hydrogen bomb design,
soon sickened of that, turned to physics,
just as Zakharov sickened of it
and turned to political
concerns, social concerns.
The consequence was that at
the time of this photograph
he was still not allowed
to leave the Soviet Union
because he knew too much.
Not until close to the
fall of the Soviet Union
was he allowed to get out of the country.
Deeply original physicist, and the one
who firmly fixed in our mind,
you had better learn to live with
the dark energy concept, because
it exists as a possibility,
the genie has been let out of the bottle
and it is not easy to force it back in.
Well, you can see that we need
some help from the observations.
We have these hypotheses,
we've got to test them
and be guided by what is observed.
Here is an idea behind a test
about our notions of the universe.
Recall that Euclid's postulate
or self-evident statement,
parallel lines never meet.
But that's because Euclid
was thinking in flat space.
In general relativity theory,
space time can be curved,
and that means that lines
that start out straight
and move in a straight line can cross.
That's an effective curvature.
You've seen in whenever you peel an orange
and you've seen the segments
with straight lines,
each a great circle, coming
together and crossing.
That has an important effect.
Look at the light coming
from a distant galaxy.
Curvature may do two things.
It may bend the light rays
as you see in the top illustration
or as in the lower illustration.
The effect of that curvature is dramatic.
In the upper case, the galaxy
appears bigger on the sky,
so you're gathering more light from it
so it appears much brighter in the sky
than in the lower case, where you see
you're gathering light
from a much smaller cone,
and so the galaxy is smaller
on the sky and therefore fainter.
Those two effects can
be tested, of course.
Here is Edwin Hubble, left-hand person,
standing in 1931, looking
at the projected design
for a great new telescope.
He and a few others were convinced
that it might be possible
to look for this effect
and to test these ideas about
general relativity theory
and about the expanding universe,
but it would require a big new telescope.
It was the 200-inch telescope
to be placed on Mount Palomar,
its installation greatly
delayed by the Second World War.
And I can't just help
but pause to remind you,
this great telescope is still in operation
and doing wonderful science.
Show me a particle accelerator
that was designed in
1931 that's still in use.
They're all in the Smithsonian.
(chuckling)
(mumbles)
Allan Sandage.
1961, the 200-inch has been
completed, commissioned
and is in successful operation.
Let us attempt to continue
Hubble's great program,
let's look at distant galaxies,
measure how bright they are,
estimate how far away they are,
look for this brightening or dimming
due to the curvature of space time.
A bit of data, needn't concern us
except to point out that
along the vertical axis
you are looking at the
amount of shift of the light
due to the expansion of the
universe that I mentioned,
along the horizontal axis
how bright the galaxy is.
For reasons that are lost
in the mists of time,
bright is over here
and faint is over here.
And the different curves corresponding
to different hypotheses about
the nature of the universe.
So he made great progress,
but he didn't get any definitive answers.
Here we are in about the present epoch,
late 1980s, early 1990s.
Saul Perlmutter, at the
laboratories just up the hill,
initiated the Supernova Cosmology Project
with the idea that instead
of looking at galaxies,
look at stars that have
exhausted their supply
of nuclear fuel and have
exploded to make a supernova.
Classes of these supernova
have pretty standard
intrinsic luminosities, and so if you look
at how bright it is on the sky,
you can look for these effects
of curvature of space time.
What's pictured here is the debris
left over after the supernova,
the material that has
been ejected from it.
This is big science, and it required
lots of work by lots of people.
Above is a photograph of Saul's group,
another group based mainly
in Cambridge, Massachusetts
and in Hawaii, and in
Australia, Siding Springs.
Competed, have joined forces,
they've grown other groups
and have obtained a bottom line.
These measurements require
Einstein's cosmological constant,
that is to say Lemaitre's dark energy.
Let's not go into the details.
There it is.
There was a hypothesis introduced
and a test, and it worked out.
Now, of course that's only one test,
and you always have to
be a little bit careful
about what all of this might mean,
but I guess I can't resist pointing out
a wonderful historical situation.
The evidence from these
observations of these supernovae
by Saul Perlmutter and his
team, Brian Schmidt and his team
is that the universe
at the present time is,
the expansion of the universe these days
is not slowing due to gravity,
but rather speeding up
due to the repulsive effect
of Lemaitre's dark energy.
That's a remarkable thing.
It is just what de Sitter said in 1930.
But of course de Sitter
was talking was talking
about something very different;
in those days people had the idea
that originally dark energy
and gravity were in balance,
the balance broke, and the
expansion started accelerating.
The new picture, in the beginning
the universe was very dense,
gravity was very strong,
but it was expanding very rapidly
and dark energy had no particular effect.
But as the universe
expanded, gravity grew weaker
'cause things were moving further apart,
dark energy started to become important,
and in this just-so story, gravity is now
becoming sub-dominant to the
repulsive effect of gravity
and we have exactly the situation,
"What makes the universe
expand or swell up?
"That is the lambda."
Now, it didn't use to be, but it is now.
Curious how things work out.
Well, we need to check.
And we've got a positive check,
but one check often doesn't convince.
Possibly there's some systematic error.
Let's look around for other checks.
Fortunately we have them.
And they deal with the evolution
of this clustering of
matter which you see here,
uniform on large scales,
clumping on small scales,
and in particular with the
evolution of this clustering,
which is here illustrated.
Below, the distribution of galaxies
in the universe as it is now
out to a given limiting depth.
I should remind you not to look at this S.
That's the disc of our galaxy,
in this projection curved
and distorted greatly.
That disc contains dust, and the dust
obscures the light from distant galaxies,
so that's an artifact of where we live.
Here and here we have a good view out
and you see uniformity on large scales,
clustering on smaller scales.
Above, the thermal radiation.
I said earlier that we live in a universe
that contains thermal
radiation smoothly distributed.
It's not exactly smoothly distributed.
The temperature varies across the sky
by about a part in a hundred thousand.
Hot and cold spots now
measured and shown above.
This radiation was last
appreciably disturbed
by the irregular mass
distribution, the theory says,
when the universe was one
thousandth its present size.
You see therefore that we have a measure
of how clustering has evolved
as the universe has expanded,
and that measure can be
compared to our hypotheses
to decide how the universe is behaving
and whether or not our theories,
including dark matter and dark energy,
are on the right track.
Here's an illustration.
It's an old one, 1977, we were
still using IBM punch cards,
but never mind, it tells
in a simple way the story.
Start with a section of the universe
that is close to uniform, place
these mass points at random,
follow the evolution of the universe
as the whole universe
expands and of course
this section expands,
through an expansion factor
of eight, I think it was, and then 16.
You then compare the
distributions of points
at the three times by expanding
this plot to this size
and shrinking this one to this size,
so you've taken out the general
expansion of the universe
and you see remaining that the universe
used to be smooth and is growing clumpy.
The universe is gravitationally unstable.
A further point is
illustrated in the bottom row.
Here the initial conditions are the same
but the mass of each particle
is half that in the middle illustration.
You notice the dramatic difference.
The growth of clustering of matter
depends on the mass of the universe.
It's important to notice that small clumps
are still visible because
they happened fast.
In a low-density universe,
the expansion rate
soon becomes large compared
to the effective gravity
and the clustering is turned off.
This then is a very important
point to bear in mind.
The universe is growing from bad to worse,
but we all observe that.
This evolution of the clustering
depends on the properties of the universe,
and if we can measure that evolution
we will have a handle on the
properties of the universe.
So here, this is getting a little turgid.
I've got to make several points.
The first I've already described.
The departures from a uniform
mass distribution grow.
The universe is getting more messy.
Galaxies are not forever,
they didn't exist,
nor did stars, when the
universe was very dense.
They formed as the universe expanded,
cooled, and mass fluctuations grew
and coalesced into stars and galaxies.
Second, the rate of this growth depends
on the properties of the universe,
things like the mass density,
the dark energy density.
Third, in the early universe,
baryons are ionized by the hot radiation,
the free electrons scatter the radiation,
and the result is that baryons
and radiation act like a fluid.
That fluid is squishy.
I at first thought I should bring along
and example of a squishy
fluid, a bowl of Jell-O.
Would you imagine, please, a
bowl of Jell-O, and shake it.
What do you see?
You see oscillations.
These oscillations have
a definite property.
Their wavelengths are fixed
by the size of the bowl.
I was persuaded that Jell-O tends to melt
and so I shouldn't do that,
so I have instead brought
along this musical instrument.
Now, this is not gonna be pretty,
you will understand, but it's
making a very important point.
So I make noise
(whistling)
and I make a sound.
(whistling)
I change the boundary condition.
(low whistling)
I get a different sound.
I send in noise.
(blowing raspberry)
(whistling)
I select out of that noise certain tones.
Mark is looking (laughs)
a little disgusted, but
(laughing)
this is good science, you will agree.
I am in this device favoring
some wavelengths and suppressing others.
In the expanding universe,
there is a similar effect
of boundary conditions.
Musicians all understand
boundary conditions.
It's the top and bottom of
the strings of the violin,
it's the organ pipe's length of this sort.
Boundary conditions in the
expanding universe are,
and the universe started expanding
from a uniform state,
because it's unstable,
since it's fairly smooth now,
it had to have been very smooth back then.
Second, when the universe
has expanded and cooled
so the temperature is 3,000
degrees above absolute zero,
when it was about one
thousandth its present size,
the baryons, plasma recombined,
made neutral atoms and
decoupled from the radiation.
So you stop the oscillation.
The result is that favored modes grow,
disfavored modes are suppressed.
Here is a photograph of some people
who figure in all of this.
On the left Bob Dicke, my teacher,
a great physicist and
deeply admirable person.
On the right a close friend and colleague
for many years, David Wilkinson.
You'll see some more of him.
In the middle, Jer Yu,
my first graduate student
at Princeton, now high in the ranks
of the City University of Hong Kong.
Jer Yu and I worked out this rigging
and the choice of favored
and disfavored modes,
the analog of this musical instrument.
Favored modes, disfavored modes.
The effect of those favored
and disfavored modes
is to make favored and disfavored modes
of distribution of the thermal radiation.
This is a modern picture,
this is what we now know,
but we knew then, of
course, that the favored
and disfavored modes in the
wavelengths for the matter
make favored and disfavored modes
for the disturbance of the
distribution of the radiation.
Apparent early on, therefore,
that we'll learn a lot
if we can measure these
modes of oscillation,
and so people started measuring.
Here's David Wilkinson again
with another friend and colleague,
Bruce Partridge, on the left.
The clipping is from a newspaper.
I do not wish to comment on the caption,
but I can't resist drawing your attention
to the handwritten note on the side,
Denver Post, July 24th, 1967.
I can't read it.
Oh, Harold, of course.
Harold and Thelma Wilkinson
happened upon this article
at the Brass Lantern Post
restaurant in Kimball,
Nebraska while traveling.
Dave's mum and dad were made proud.
They are setting after the great goal
of discovering these irregularities
in the radiation distribution,
which they'll compare to the theory
and see whether our hypotheses
are in line with reality.
Here is David again in the '70s.
I forgot to remind you, now,
notice the width of the tie,
okay, this is '60s.
Notice the length of the hair.
David on the right,
(chuckling)
Peter Saulson, a graduate
student, in the middle,
and Ed Chang, another
graduate student, on the left,
and an instrument that would
be taken up in a balloon
to look for this variation
in the temperature
of the radiation across the sky,
and it oughta be there and oughta teach us
about how the universe is behaving.
Here is a project report for
a satellite that became COBE.
We mentioned yesterday that this guy
was project scientist, and he is pictured
in the next transparency.
This is John Mather; he
received a Nobel Prize
in physics just a few weeks ago.
Here is another PI for the experiment
I'm going to be describing in a moment,
George Smoot, who you've just seen,
a Nobel Prize in physics
just a few weeks ago.
The third PI, Mike Hauser,
another good friend
and he deserves a Nobel Prize too
for magnificent work, but you can't do it,
can't have them all.
David Wilkinson, Ed Chang you
saw, and a cast of thousands.
Not exactly big science,
but a hardworking group.
And here is an illustration
of the situation
just before the satellite
that these people built, tested and flew
and used for observations was launched.
1990, an article by Joe Silk,
who was a professor of astronomy here,
now the professor of astronomy in Oxford
in England, and I wrote.
You can't read anything on here,
you're just supposed to notice
that it's a pretty full page.
The columns are sets of hypotheses
about the nature of the universe.
Some have dark matter, some don't.
The rows are observations,
and the entries each are grades
for how well the hypothesis
fits the observation.
The details are of interest
entirely and only to
historians of science.
Some of these things are
woefully misconceived,
others more or less on the mark, I guess.
But that's not the point.
We needed the help of the observations
to sort out all of these
possibilities, and we got 'em.
Upper left, the red symbols
are the COBE results
for which George Smoot
received a Nobel Prize
as PI for this marvelous experiment.
Down below, the map showing the variations
of the temperature across the sky.
You understand the mean
has been subtracted,
these are variations of a
part in a hundred thousand.
You notice to the right,
people had jumped on this
and were trying very hard to
look at what the anisotropy is,
the fluctuations across the sky
on smaller scales, without much success.
Nothing was believable.
You might also be able to see
blue lines running through.
Those are different theories
based on different of the
hypotheses in that last chart.
You can see how good measurements
are going to straighten this out.
Already COBE pointed us
in the right direction.
The observations by Smoot et
al pointed us in the direction
of one of those sets of
hypotheses in that last slide,
the now very successful cold
dark matter dark energy theory.
So let us look at the next step.
It's obvious we've got to
do even better than COBE.
You know it can be done, now do it.
This is a proposal for a
next-generation experiment.
It was called MAP,
Microwave Anisotropy Probe.
I couldn't resist showing you this memo
dated 1991, even before
COBE had its first results,
the result of a proposal
by David Wilkinson
to build a satellite to
do even better than COBE.
Involved in this first project
were Ed Wollack, then a graduate student,
Norm Jarosik and Lyman Page,
then instructors at Princeton
along with David Wilkinson.
They went down to Washington,
D.C. to talk to NASA
and to Martin Marietta,
one of the Beltway bandits
but an expert on the technology
these people would use.
And I just wanted to point
out the opening statement.
"They seem to like the science."
(chuckling)
Oh my god, this is spectacular science.
And indeed, people liked it.
The proposal was submitted,
the group formed,
and here is a group photograph.
David Wilkinson is seated on the far left,
you can see Lyman Page seated
with the Scandinavian sweater,
above to the right Ed Wollack,
whose name you saw earlier.
Norm Jarosik far right, standing,
and others, Licia Verde, now
at University of Pennsylvania,
Peiris, now at University of
Chicago if I remember right.
And some results.
Just spectacular.
This is, again, the
texture of our universe,
but looked at in a different way,
the results of the disturbance
to this thermal radiation
by the irregularity in
the mass distribution
back when the density of the
universe was very much higher
and it was able to
interact with radiation.
I just can't resist the comparison
between what people were doing
in 1967 and what they got.
There is progress.
And an illustration of,
I should have remembered to show you.
You've seen this before.
And now I wish to boast a bit.
Notice how wonderfully well
the theory follows the measurements.
Noisy in here.
It even improved since
this transparency was made
just a year ago, less than a year ago.
But that's okay, it's
spectacular agreement.
I wanted to show you, though,
an example of yet another test.
Take the theory as it now is assumed to be
and notice that we have this ringing
that favors discrete modes
of variation of the mass distribution.
And notice that if those
modes are evenly placed,
it will have the implication
that there's a correlation
between the mass fluctuation here
and a mass fluctuation over here
at a really well defined length,
about 400 million light years.
Go measure that correlation,
and there it is.
It showed up.
I point this to you
only as an illustration
of the situation illustrated
on this exceedingly busy slide.
Now, here's the situation.
We have lots of postulates.
In those postulates, for example
that there is dark matter,
we have to say how much?
Dark energy, baryons, how much of each?
Initial conditions, do we
really have them right?
All of these parameters can be adjusted
so as to fit this curve,
but you don't have
complete freedom to adjust.
First, you can only
adjust this curve so much
to fit the observations, and then second,
we have lots of other
checks on what is going on.
I showed you one.
There's a long list of others.
And the situation now is very
clear and straightforward,
clear and straightforward.
We have redundantly and overwhelmingly
tested this set of hypotheses,
this just-so story,
and it's passed many tests.
We've got to accept,
this is a good approximation to reality.
No one should ever tell
you that this is reality,
this is the absolute truth,
but it is a very good approximation.
That, of course, is all
we could ever hope to ask.
So I'd bring now a summary pair of slides
just to comment on the
way things have gone.
Up above, this dream of testing
ideas of curved space time
and of the effect in the curved
space time of dark energy.
We have one may say here
three generations illustrated.
First generation, 1930s,
Georges Lemaitre, a wonderful theory.
Edwin Hubble, a wonderful telescope
that may be able to test the theory.
30 years later, in the
early 1960s, Allan Sandage.
The telescope has been
built, let's use it.
Let's learn how to measure these galaxies
and make these tests.
Another 30 years after that,
Saul Perlmutter and co-conspirators,
let's do it even better
by looking at supernovae,
and at last success.
Through three generations,
one at last succeeded
in making the measurements.
These people are tenacious.
They're tough, and they
succeed, sometimes.
Zwicky.
He disapproved of many things.
Actually he was always polite to me,
but you had to be careful.
I don't know whether
he would have approved
of the way we are treating his
dark matter, but there it is.
This story would not work
if the dark matter he noted
and if the dark matter
that Vera Rubin saw had not been present.
Of course you only knew
that the dark matter
had to be there because of these tests,
including David Wilkinson and the 1967s
learning how to make these measurements,
this group, and here is George again.
Longer hair there.
- It was different times.
- Different times, yes, yes.
Your tie is gone now.
In fact, you didn't have it there.
Made these measurements and showed us
the way the world really is.
And some more thoughts.
We have open problems.
Although we have brilliant successes
in checking our ideas about the curvature
of space time, we have deep problems.
We needed to postulate dark matter.
We've gotta find it, and
people are working hard on it.
We have this ridiculous dark energy
that great authorities were
willing to assure you is absurd.
Einstein, Pauli.
But yet it should be there.
And yes, it should be there,
and my goodness, it's there.
But what is it?
We don't know.
We can only leave it to groups
such as those on the
upper right to find out
by getting new hints, test new hypotheses
and finally find some success.
In short, cosmology has
had a brilliant run,
but I think it's by no means over.
There are lots of open questions
for the next generation.
Thank you.
(clapping)
- [Audience Member] A
very elementary question,
what is the difference between
dark matter and dark energy?
- It's elementary, but really important.
Dark matter has no pressure to speak of.
It's able to cluster very easily.
Dark energy has so much pressure,
it's almost totally
incapable of clustering.
Furthermore, the pressure has
a bad sign, it's negative.
It's material with a
bizarre set of properties.
It's called dark energy in part
because it just has the properties
you would expect for zero point energy
from all of the fields
of standard physics,
except for the wrong value.
It acts like an energy,
but to give it that name
is somewhat misleading; it
suggests we know what it is.
That's quite wrong.
Ask the question again
in another generation.
You might get a better answer.
(chuckling)
- [Audience Member]
Yesterday you suggested
that we should perhaps write
to our congressman or somebody
to support the idea that
basic science is useful,
and it occurred to me it might be useful
to have a couple of
examples, and maybe more.
I'd like to give two examples
of basic science which have
turned out to be useful.
If you use a GPS system to
drive around in your car,
it won't work without general
relativity corrections.
I don't remember what the error is,
perhaps someone knows here in the room,
but it's a big error.
The second example is, if
you go in for what's called
an MRI scanner, they took the nuclear
out of nuclear magnetic resonance imaging.
- Yes.
- And that works
because the nucleus, like a proton,
is spinning around, has angular momentum
like a top with a magnet attached.
And those were basic things
which were discovered
at the beginning of the last century,
and it took a bit of computers
to get the magnetic
resonance imaging to work.
What could you withdraw
from your experience,
not necessarily from cosmology,
but what would you emerge for people
to put in their letters
to the congressman?
- I'm not sure if I
would put it in a letter
to the congressman, but
the example I often use,
your examples are excellent.
I may even steal them.
But the one I use is a cell phone.
Now, I am not at all
convinced that cellphones are
an entirely unmixed blessing,
(laughing)
but I am deeply admiring of the fact
that it that little handheld device
we see evidence of the
ability of engineers
to control the behavior of molecules
to make that liquid crystal display,
to control crystals and electrons
to move around and do logic
and to remember my phone
number and dial it,
the ability of electromagnetic fields
to control molecules and electrons
and to send and receive signals.
It's spectacular.
And where would faith-based
science have taken cell phones?
(laughing)
(clapping)
(bright music)
