MALE SPEAKER: Our guest today is a renowned astronomer,
and a member of the National Academy of Sciences,
with award-winning accomplishments documented
in about 700 research papers.
As the Richard and Rhoda Goldman Distinguished Professor
in Physics and Physical Sciences at UC Berkeley,
he's been voted the best professor on campus
a record nine times, and was named the Carnegie-CASE
National Professor of the Year among doctoral institutions
in 2006.
In 2011, the Nobel Prize in Physics
was awarded to the leaders of two teams,
for the discovery of the accelerating
expansion of the universe.
Those two teams had one member in common, and here he is.
Please welcome Dr. Alex Filippenko.
DR. ALEX FILIPPENKO: So, I'm very pleased to be here today.
This is my first time at the Googleplex,
and it's been fun wandering around and seeing
your neck of the woods here.
What I'd like to describe today is the discovery
of the accelerating expansion of the universe.
And the reason that I wanted my slides,
I mean I can tell the 5 or 10 or 15 minute version
without any props whatsoever to various people,
but since this is a Google audience,
I thought I'd actually use graphs,
which is something you can't do for the general public,
of course.
But I wanted to give you some of the inside scoop of how
the work was done.
So if we could have the next slide, please.
This is unfortunate because I've got 100 slides or something,
so you're going to have to listen to my cues.
I'll say Next, all right?
Go ahead, next.
This is going to happen really a lot,
so you're not going to be able to read the newspaper
or something while I'm giving my talk, OK?
Thank you so much.
Anyway, in 2011 the Nobel Prize in physics
was given to these three gentlemen.
The Nobel Prize went to the leaders of the two teams
that discovered the accelerating expansion of the universe.
Saul Perlmutter, at the Lawrence Berkeley Lab and the Department
of Physics at UC Berkeley, was the head of the Supernova
Cosmology Project, and I was associated with that team
initially.
And then there was Brian Schmidt's High Redshift
Supernova Search Team, with which I was later associated.
And then my post-doc, Adam Riess,
who did a lot of the work in the mid- to late 1990s,
he actually made the measurements of the data
when he was a post-doc in my group.
And he was the first author on the Schmidt team paper,
and I'm glad that he was awarded the prize as well.
It's unfortunate that the prize can
go to only three people, at most.
This is unlike the Peace Prize, which
is handled in a very different way.
That's the rule, or at least the tradition.
But these gentlemen understood that without the other 48 of us
working in the trenches, the discovery would never
have been made.
So they spent a good fraction of their prize money
flying the rest of us out to Stockholm in December of 2011
to participate in Nobel Week.
And that was just an amazing set of celebrations and parties--
everyone just sort of cheering for science.
It was like the Swedish Super Bowl.
Anyway, if you could go to the next slide please,
you can see the High Redshift Supernova Search Team
right after the awarding of the gold medal
to Adam Riess and Brian Schmidt.
And the rest of us like to say that besides the handshake
from the King of Sweden, the gold medal,
and a share of the $1.5 million, we experienced everything else
that Nobel Week was all about.
So we got most of what there was to offer.
But it was really a lot of fun.
If we could go to the next slide, please.
The story starts of course with Edwin Hubble,
who discovered the expansion of the universe.
Next, please.
He examined galaxies, which are these giant collections
of hundreds of billions of stars, gravitationally bound
together.
And he devised a way to determine their distances.
I'll discuss that method in a few minutes.
He also looked at their spectra.
Next, please.
And the spectra were all red shifted.
That discovery was actually made by Vesto Slipher
some one decade earlier.
So these various spectral lines, due for example
to neutral sodium and hydrogen and singly ionized calcium,
the whole pattern was shifted to the red.
And he found-- next, please-- that the relatively
nearby galaxies had small red shifts for their spectra.
And-- next slide, please-- the more distant galaxies, which
typically appear smaller and fainter in the sky
had bigger red shifts.
And this can be interpreted in terms of a motion.
And you've all heard the audible Doppler effect, of course.
But in fact, we now know-- and this
is something that Hubble himself resisted for awhile-- we
now know-- next slide, please-- I'm
just showing you a graph of a low red shift galaxy,
in fact one that's very near us.
It's moving away very little-- zero kilometers per second
is very little.
And then one that's moving away at a tenth
of the speed of light.
And you can see the same pattern of lines, but shifted over.
OK, next please.
We now know that this red shift, the cosmological red shift,
is caused by the expansion of space itself.
So it's not as though the galaxies
are moving through some pre-existing space,
like bullets flying around outside.
Rather, it's a stretching of space itself.
And we can tell by, for example, looking at the surface
brightness of galaxies, looking at the brightness per unit
area, it turns out that the brightness per unit area,
as a function of red shift, differs
if you have a truly expanding universe versus a universe
through which objects are moving.
And the data agree with the expansion
of space interpretation for the cosmological red shift.
Next slide, please.
So from our perspective, here we are in the middle
and all these other galaxies are moving away from us
with a speed that's proportional to their distance,
because the stretching of space is greater for the more
distant ones than for the nearby ones.
And there's something a bit strange about this diagram
as I've drawn it.
What's weird about it?
Yeah, we're in the middle.
Why would that be?
Do these other galaxies not like us?
Do we smell?
Is it something we said?
Or are all these other galaxies lactose-intolerant?
Get it?
Milky Way galaxy, lactose-intolerant?
Anyway.
When I tell my students at Cal about cosmology
and the expanding universe, I say, what is it?
Are we from Stanford or something?
With due apologies to the many of you who I'm sure
are Stanford alumni, or even on the faculty like Peter.
It's truly an outstanding institution, it really is.
Just not quite as outstanding as Cal.
But anyway, we don't think we're in any central location.
Next slide, please.
This is a property of any uniformly homogeneously
expanding universe, such that any galaxy,
any raisin thinks it's at the center
because the dough is uniformly filled with yeast,
and you let it bake for an hour.
And let's say it doubles in size in that time.
You see all these other raisins going away,
but your friend sees the same thing.
And by the way, the universe is either infinite
or it wraps around itself, so don't
worry about the finite size here.
So there is no center, at least not in the dimensions
that we can physically probe.
There may be a mathematically valid center.
For example, here's a two dimensional universe.
It's closed.
The laws of physics in this hypothetical universe
only act within the rubber itself.
And the creatures in this universe
could figure out the equation for their universe.
R is a constant.
Theta and phi go through 0 to 2 pi and 0 to pi, so R theta phi.
But they only live on a two-dimensional surface.
Or x squared plus y squared plus z squared
is a constant square of the radius.
Again, three variables, three spatial dimensions, but they
only live in two of them.
The center of this balloon is the center
of this expanding universe.
That's in a mathematically describable, but physically
inaccessible dimension.
So we may live in a three dimensional version of this.
We actually don't know the true shape of our universe,
so we're not sure.
Next slide, please.
In any case, we're not in any central location.
So here's what I never show to the general public--
a graph of Hubble's law, the recession speed.
Oh, you've got one.
Great.
Thank you so much.
Let's see if it goes backwards.
Ooh, it does go backwards.
Goes forward, too.
Thank you so much.
Thank you.
It's good-- there's a time symmetry in physics.
At the microscopic level, if you reverse the arrow of time,
everything looks the same, which is
why entropy and the arrow of time is such a bizarre thing,
right?
But anyway, here's Hubble's law.
The speed of a galaxy caused by the expansion of space
at any given time is proportional to its distance,
with the constant of proportionality
being given by Hubble's constant.
And it's a constant only in the sense
that it's the same everywhere in the universe at a given time.
The value actually changes with time,
so the knot here just means the constant at the present time.
And you get the recession speed just
by multiplying the speed of light by the red shift.
OK, so that's Hubble's law.
And with today's telescopes, we've
measured the current rate of expansion pretty well.
It's, for those who care, of order 70 kilometers per second,
per megaparsec.
Megaparsec is about a 3 and 1/4 million light-years.
But we expect the expansion rate to change with time.
And this is quite obvious.
The universe has galaxies in it.
All of them are going to be pulling on each other.
That should slow down the rate at which
the universe is expanding.
This is a clear prediction in general relativity.
It's true also in Newtonian gravity.
If I toss the apple, the mutual gravitational attraction
between the Earth and the apple slows it down,
brings it to a halt.
Eventually it comes back.
So in the case of the universe, if the density of the universe
is sufficiently high, then the slowing down
of the rate of expansion will be quite large.
And eventually the universe will stop and reverse its motion.
So it'll collapse in on itself.
It will implode.
So you could start with a Big Bang,
with a Big Crunch, a dense hot state.
Or you could say, Big Bang, Gnab Gib,
which is Big Bang backwards.
OK, so in that kind of a universe, in an empty universe,
you have no slowing down.
But if you have a dense universe, then the separation
between two arbitrary clusters of galaxies--
the so-called scale factor-- it increases, but at a slower rate
and then reaches some maximum height, then comes back down.
That's of course a graph of this apple's motion.
So in that kind of a universe, you'd
be looking at the galaxies, you'd be lying on your back.
They're getting fainter and smaller with time.
You would say it's a good universe.
And then you would notice something a bit peculiar.
And right around now, you'd start
getting a little bit nervous.
And then it would be, oh, goodbye cruel world.
So that's what the universe would do.
That would be the fate of the universe
if we live in a dense universe, a universe where
the average density of gravitating matter
is greater than some critical density.
This is known as omega matter.
So if omega matter is greater than 1,
you have this critical universe-- overly critical
universe that collapses on itself.
But you could have a universe that
doesn't have quite so much matter.
And it would slow down the expansion,
and it would asymptotically approach 0 as time approaches
infinity.
So this would be a universe in a sense like the apple thrown
at a speed equal to the escape speed from the earth.
It keeps on going away forever.
It doesn't quite ever stop.
And then you could have a low density universe,
which is analogous to an apple thrown
at a speed greater than the escape speed.
So it keeps on going away forever, asymptotically
approaches some non-zero positive velocity.
And most measurements of the matter density
suggested that we live in a universe that's only 30%
of this critical density.
So if that's the case, and you're lying on your back
and looking up at the galaxies, they'll
keep on getting fainter and smaller forever, all right?
And that's a very different fate compared with the Big Crunch.
This eternal expansion is a very different fate.
And cosmologists, those who study the structure
and evolution of the universe as a whole,
want to know what the fate of the universe will be,
just because.
All right.
Well, you could measure the average density of matter,
compare it with this critical density,
and figure out what universe you're in.
But that's a hard thing to do, because matter comes in clumps.
It comes in these clusters.
And the clusters have dark matter in them.
And then there's voids that don't
have much material in them.
So it's hard to get a representative volume
of the universe.
And moreover, you're not sure you're seeing everything,
even gravitationally you're not sure you're seeing everything.
So another technique is to examine
the past history of the expansion.
And I like to go back to the apple analogy.
If I measure the apple's speed at many different times,
and its trajectory, then I can see
how much it's been slowing down, figure out
whether it will ever stop and implode on itself.
All right.
In a similar way, if we measure the rate of expansion
in the past at many times, and compare it
with what it is right now, we could
predict the fate of the universe.
The way to do this is to essentially look back
into the past and see which of these curves
the universe has been following.
Clearly it's not an empty universe.
So this is just an extreme approximation.
But the universe, if what other astronomers are telling us,
should be behaving something like this.
We should see the universe following
that kind of a trajectory.
So let's focus in on the time period before now.
We're 13.8 billion years into the universe right now.
So let's look at this, OK?
Now, the definition of the redshift is the following.
A given red shift is simply the size of the universe now,
divided by the size of the universe,
or the scale factor, as it was when the light was emitted.
So redshift 0-- well, 1 plus 0 by advanced mathematics
is 1, that just means we're looking at the universe now.
A red shift 1 means that we're looking at the universe
back when essentially all distances were
half of their present value.
So arbitrary clusters of galaxies
were separated by half of their value, of their present value.
So here we go.
There is now, t equals now.
There's redshift 0.
Suppose we look at redshift 1.
Suppose we look at galaxies at redshift 1.
All right.
Then you'll notice that the lookback time-- that
is, how far back in time we're looking--
depends on which universe we're in.
Take a look at this here.
In a dense universe, we're looking back
in time some amount, some billions of years.
And the distance is the speed of light times the time.
So if we measure the distance, we
can figure out the lookback time.
In a less dense universe, the lookback time-- the distance--
is bigger.
In an even less dense universe, the lookback time
and the distance at a given redshift are even bigger.
In an empty universe, for a given redshift,
you're seeing as far back as you could possibly see.
So we expect, then, looking at different redshifts,
we expect to measure different distances,
or different lookback times, and the universe
should be-- the data should be following one of these lines.
And the observer's view of this theoretical diagram
is the following.
At small redshifts, you just have Hubble's law.
That just means that the slopes of all these curves
are the same right now.
That's just-- the universe is expanding right now,
with whatever rate it's expanding.
That's just a given.
And two clusters of galaxies have whatever separation
they have right now.
That's just a given, too.
That's why all these models have to converge at now.
Their slopes are equal, and their separations are equal.
So the slopes differ very little at small redshifts.
That's what you're seeing down here, Hubble's law.
But as you go to bigger redshifts,
you start seeing deviations from Hubble's law.
At a given redshift-- I'm sorry, I hardly ever
use a laser pointer anymore-- at a given redshift,
you can see the dense universe-- well, that galaxy,
is at a smaller distance than in a less dense universe.
And that's a smaller distance than in a less dense universe.
The empty universe, well, that's where the distance
is the biggest for a given redshift.
So the procedure is clear.
Measure the distances of galaxies
having a wide range of redshifts and see which of these curves
the universe has been following.
Various observers told us we would find this result.
But we wanted to see.
So how do we do that?
Well redshifts are easy to get.
You just take spectra of these little blobs here,
all these are little galaxies.
There's only a couple of stars in our own Milky Way
galaxy in the Hubble ultra-deep field.
So you take their spectra, and that gives the redshift.
That's easy.
The distance is a little bit harder.
We will define, what I will call it,
luminosity distance in the stock.
That's simply the distance of an object,
d sub L, such that the light that you see,
the given brightness b, follows the inverse square law.
So that's just a luminosity distance.
There are many distances in cosmology,
and you've got our remain consistent.
So you need something of known luminosity.
You need to know L, and you measure the brightness b.
And that allows you to derive the distance.
The measurement of the brightness is easy, but how do
you know the luminosity?
Well, you need what astronomers call a standard candle,
or a standardizable candle, something
that is always the same, like a 100-watt light bulb.
And it's just like the judging the distance
of an oncoming car at night.
You've calibrated how bright the headlights
of a car of known distance are.
And you look at cars whose headlights are fainter,
and you figure out their distance.
If you're not very good at doing this almost intuitively,
you shouldn't be driving at night.
So Hubble did this with a type of star
known as a cepheid variable.
And you might think, oh it's a variable star,
so that's no good.
But it turns out that there are certain categories
of cepheid variables.
And the point is, you know what their true luminosity is.
And let's just look at that one right there.
Let's call it Mike, just for kicks, OK,
because he introduced me.
What if you know that Mike is just like Betelgeuse,
the left shoulder of the great hunter Orion?
Magnificent, luminous star Betelgeuse is.
It's relatively nearby.
We know its distance.
We know its apparent brightness.
We know the luminosity.
We know its oomph.
If we then look at Michael in that other galaxy there,
we can figure out its distance from the inverse square law.
And then if we get the same answer with star Vikram
or something like that, we start gaining confidence
that we're using the right technique.
So this is what Hubble used to determine
the distances of galaxies in the mid-1920s.
And he was the one who showed what many suspected,
and that is that these spiral nebulae are far,
far outside our Milky Way galaxy.
And our view of the universe expanded by a gigantic amount
with that realization.
Well, these galaxies are so faint and fuzzy and distant
that you can't see individual normal stars within them.
But it turns out, there is one type of star
that is visible, even from distances
of billions of light years.
And that's the supernova, an exploding star.
Very few stars explode at the end of their lives,
but those that do are critical to our existence,
because we are made of the carbon, oxygen, calcium,
iron that get spat out of exploding stars like this.
So we owe our existence to this phenomenon.
Anyway, they become very bright, in some cases--
up to a few billion solar luminosities.
So if our sun were to do this, then sunblock of 50
just wouldn't cut it, folks.
You'd need sun block, or supernova block,
of a few billion to protect yourself.
But don't worry, be happy.
Our sun isn't going to do this.
So the name of the game is to find some of these supernovae
in galaxies whose distances we already know,
because we can see relatively normal stars like Mike
and Vikram in them.
So we know the distance of this galaxy.
We measure the apparent brightness of the supernova.
That allows us to determine its true luminosity.
But it's hard to find these supernovae,
because they occur only once every few decades in a given
galaxy.
So if I were a really cruel adviser,
I would have each of my students looking
through the eyepiece of a telescope
at one and only one galaxy, preferably
at night-- you see more galaxies at night
than during the day-- until that student finds a supernova.
Then we let them graduate and move on to greener pastures.
Meanwhile I will have had decades worth of slave labor
out of said student.
Well, if there are some crimes that
are so egregious that even a tenured professor can
and should get fired, then that would be one such crime.
Of course, I could have my students
look at thousands of galaxies.
These are random, independent phenomena.
So if they look at more galaxies,
you'll find more supernovae.
But that would be considered cruel and unusual punishment
as well.
Fortunately, with modern technology,
we have a better way.
We attach CCD cameras to the eyepiece end of a telescope,
take photographs of thousands of galaxies,
and then simply look for arrows.
And where you see an arrow, you see an exploding star.
You see once, twice, three times, four times, five times.
By the process of rigorous mathematical induction,
I conclude that this must work every time.
So obviously, it's not that easy,
otherwise we wouldn't give degrees for this kind of work.
What we've done is we've developed
a robotic telescope at Lick Observatory,
just a two-hour drive from Berkeley,
or a little bit over an hour from the Googleplex here.
In fact, Mike showed me the view,
which was a bit hazy today.
Couldn't quite see it today.
But I'm sure many of you have seen it.
This is not a big telescope, but it's
been programmed to look at lots and lots of galaxies.
And it's one of several telescopes
at Lick Observatory, Mount Hamilton.
Go and visit it one of these days.
Great place to go.
And my close associate, Weidong Li,
programmed this telescope to look at nearly 10,000 galaxies
a week, and to automatically compare the new pictures
with the old pictures.
And through the magic of digital subtraction,
here's the template.
Here's the new image.
This is, by the way, a negative image.
These aren't all black holes, otherwise black holes
would be easy to find.
Anyway, so here's a supernova candidate.
And here's something that the software thought
was a cosmic ray, just a charged particle that hit the detector.
And here's a poorly subtracted star.
The image quality varies with time a little bit,
as mirrors point in different locations, their shape changes.
So you get maybe a few dozen candidates per night.
And when you're observing a few dozen-- 1,000 galaxies a night,
you only expect maybe a supernova candidate
every three or four nights.
So most of these are going to be not real things.
They could be cosmic rays, or maybe an asteroid with Earth's
name written on it, hurtling toward us.
One person's garbage is another person's gold.
Someday, astronomers and engineers
will save humanity by finding the asteroid that's
30 years from hitting us.
And then they'll go and they'll deflect it.
So anyway, we have maybe a few dozen candidates per night.
And then I use slaves-- I mean undergraduate students-- who,
with their superior eye/brain combination,
examine these candidates, determine
which ones are worthy of further investigation
because they're likely to be supernovae.
And I'm very proud of my team, because I get students, even
in their freshman and sophomore year, hands-on experience
with real data analysis.
And occasionally I've even had high school students,
and they write home to Mother when they discover a supernova,
and it's just really great, because they get very jazzed.
And most of them don't go on and become
professors of astrophysics.
That's probably good.
There's enough of us in the world.
They go on to become engineers, computer scientists,
applied physicists-- people who are more immediately
useful to society.
But the hook was the cool stuff in the cosmos,
and the research experience that they get as part of my team,
and other teams at Berkeley and Stanford
and other great institutions, is really invaluable.
And something I don't need to tell you guys,
but I think we are losing our technological edge in the US.
And we have to shape up, or we will be quickly passed up.
Anyway, so-- what's that?
A supernova?
Brightens over maybe three weeks, fades over a few months.
I'll do more questions afterwards.
I realize some of you will need to leave,
so I'm kind of rushing a little bit.
But I'll be very, very happy to answer more questions later.
But yeah, I sped up the process and that animation so as to not
bore you.
So we find them, and then it's important to take
their spectra.
And so we collect the light, for example,
with the 3-meter telescope at Lick Observatory.
And I have undergraduate and graduate students
involved heavily in that stage of analysis as well.
And the spectrum tells you what kind of a supernova
you're looking at.
There are different ways in which different kinds of stars
can explode.
And only some of them are luminous enough to be useful,
and standard enough.
If you've got light bulbs of a huge range of lumens,
that's not going to be so good.
The ones that are useful for cosmology are the so-called
type 1a's.
You have this roller coaster of a spectrum.
There's the calcium in your bones,
the oxygen that you breathe.
This is real life stuff.
We are made of star stuff, right?
So type 1a supernova is useful, because it
comes from a very weird type of a star.
Our sun will become a white dwarf in about 7 billion years.
But fear not, it won't blow up.
A white dwarf is a weird sort of matter
called degenerate matter, not because it's
morally reprehensible, but this is simply
the term quantum physicists give to a very, very highly
compressed state of matter, where all the electrons are
basically in their own little cubbyhole, pigeonhole, energy
level.
And they can't occupy the same energy
levels and the same quantum states,
and so they exert this weird degeneracy pressure
on each other.
It's truly magnificent.
It's unlike any sort of thermal pressure.
Anyway, if you have a white dwarf in a binary system that's
sufficiently tight, then that white dwarf
can steal material from the other star
and approach a mass which becomes unstable
and it undergoes a thermonuclear runaway.
And it happens at about the same mass each time,
1.4 solar masses.
And the thermonuclear runaway occurs
in about the same way each time.
And it's very luminous-- four billion solar luminosities.
So these are excellent and pretty standard candles.
And they can be seen very far away.
But we want to look at them in great detail,
because it turns out they're not perfectly standard.
So here are some spectra that my team
took a little over 20 years ago.
And we started noticing-- we took these spectra at Lick-- we
started noticing that the spectra of these things
are not all identical.
So here's a pretty normal one, 1990N,
and there's this line here.
But you see it's weaker in that supernova.
And then in the supernova it's stronger.
And then this supernova also has this weird absorption feature,
which is due to titanium, it turns out.
So they're vaguely similar, but there are differences.
Here there's some clear differences as well.
So not all type 1a's are the same.
That turned out to be a very important realization
in the early 1990s.
We can't treat them all as being identical objects.
And moreover, these guys, it turns out,
are on the whole subluminous.
These are overluminous, compared to the more or less vanilla
flavored type 1a supernova.
So if you don't take that into account,
and you're looking at some distant type 1a supernova,
and you don't know the true luminosity,
you will get an incorrect distance, and hence
an incorrect lookback time.
So it's totally important to take this detail
into consideration.
Not all the headlights are the same, simply put.
And you've got to know which kind you you're looking at.
Well, the next realization was that, oh wow.
Turns out the luminous type 1a supernovae
have slower light curves.
That is, they rise and decline more slowly
than the subluminous ones.
And the normal ones are sort of right in here.
So if you find a whole bunch of nearby ones
whose distance is known, and you calibrate this relationship,
then you can look at the distant high redshift ones,
measure the light curve and say, aha, this
is a 94-watt light bulb instead of a 100-watt light bulb.
Or maybe it's a 115-watt light bulb.
So instead of saying, well, they're
all 100 watts with a dispersion of 50 watts,
you can say no, this one's 94 plus or minus 13, or something
like that.
You really improve the precision of the distance measurements,
and hence of the lookback times.
And this is what really allowed the type 1a's to become useful
in cosmology, and to ultimately lead to this amazing discovery.
Well, this relationship here was first pointed out
by Mark Phillips and then Mario Hamuy in Chile
gathered a whole bunch of type 1a supernovae
and showed, or sort of calibrated
the relationship even better.
And then my former post-doc Adam Riess, who at the time
was a grad student at Harvard, used
their sample, the Chilean sample,
to calibrate type 1a supernovae.
And then he took a new set of type 1a supernovae.
OK, this is a different setup here, in the top graph,
and he applied this machinery.
Here we have the supernovae, if you treat them
as a standard candle-- 100 watts plus or minus 50-- you
see all kinds of dispersion here.
Moreover, there are some huge outliers.
When you take into account this luminosity light curve shape
relationship, and when you also take
into account dust and other interstellar
debris, through which the light is going--
that I won't go into the details of-- you
decrease the dispersion to one third
of what it used to be-- 15% rather than 45% or 50%.
And these outliers go right down to where they should be.
So the technique works well.
So that's what gave us confidence
that we could use type 1a's to do cosmology.
And although my robotic telescope did not
contribute to the original sample that
was used to define this relationship,
it has been used since that time to refine this relationship,
and decrease our resulting scatter more and more.
So we're calibrating these things better and better
with time.
And in fact, one of my most recent graduate students,
Mohan Ganeshalingam did a thesis on this.
And here you can see the various light curves
that we have collected with Kate and you
can see very clearly that the more luminous guys have
slower light curves than the less luminous ones.
And so here is Mohan's Hubble diagram without any correction.
It just looks like crap, sort of.
I mean, it's pretty bad.
And you can't do precision cosmology with that.
You can tell the universe is expanding,
but you can't do precision cosmology.
Whereas when you apply our machinery, look at that.
I mean, that's fantastic.
This is a relatively nearby sample,
but this is the kind of confidence
we need to get before going on and doing this great redshifts.
Now, it turns out that Wayne Rosing, who
used to be Vice President of Engineering here, has gone on
and started this thing called the Las Cumbres Observatory
Global Telescope Network.
He's got a network of little telescopes
all over the world, with which we
can follow variable stars and exploding
stars throughout the 24-hour day/night cycle.
Their motto, I think, is the sun never
rises on the Las Cumbres Observatory Global Telescope
Network.
So Wayne, partly based on the success
we had with our Kate telescope-- and in fact,
he very generously helped fund that
telescope-- he has now globalized this whole thing
and is pursuing one of his passions.
So it's fantastic.
He's a good friend of mine.
Anyway, now that we've gained some confidence with the nearby
ones-- and it's 2 o'clock, but you'll permit me to go longer,
right?-- let's go and do the actual cosmology now.
Let's find these things in distant galaxies.
Now, let me remind you, there are two reasons
you want to find the distant supernovae.
The first reason is, by the observed brightness
compared to the luminosity, you get the distance and hence
the lookback time.
And from the spectrum, you get the redshift.
So the supernova is critical for two reasons-- a distance
through the inverse square law, and a red shift.
So, we find these things with a wide-angle cameras attached
to relatively big telescopes-- this is in Chile, Cerro Tololo
Inter-American Observatory-- and modern,
wide-angle CCD cameras actually are these gigantic things.
It's amazing what the silicon revolution
has done, not just for computers, of course,
but for astronomical imaging technology.
We now routinely have these 16k by 32k arrays of pixels.
And this is about the size of the full moon.
Of course, the full moon isn't rectangular,
but this is roughly the size of the full moon.
And in this picture, there are literally
thousands of galaxies.
Nearly every blob you see is a galaxy.
So if you take many such pictures
over the course of a couple of nights,
and then repeat the process three weeks later,
you will have taken the mug shot of perhaps 100,000 galaxies.
And in those three weeks, some of those 100,000 galaxies
will have produced a type 1a supernova.
Indeed, several dozen will if you do the math.
So we did that, and through digital subtraction,
here's a small subset of one of those images,
taken on the 7th of April, 1997.
Subtract that from the 28th of April,
you get a bunch of noise.
That's OK.
Any measurement process, as you know,
necessarily has some noise associated with it.
But here, cleverly placed in the middle of the square,
is something that looks like it might be real.
And here's a Hubble picture of it a few weeks later,
and it's marked with an arrow, so it's
got to be a type 1a supernova.
I wish it were that easy.
You don't know that it's a type 1a,
and you don't know it's the redshift
until you take a spectrum.
So this was my main job on both teams.
I'm a trained spectroscopist.
I've been studying supernovae since the 1980s, the mid-1980s,
and I have access to the world's biggest optical telescopes,
the twin Keck 10-meter telescopes in Hawaii.
And I urge you to visit those, if you've not done so.
They're an amazing thing to look at.
Here's Fred Chaffee, a former director of Keck,
just showing you the size scale of these gigantic eyes
that are gathering light from afar.
So with the Kecks, I could get spectra of these type
1a supernovae that at redshifts of, say, 0.455 in this case.
This is a lookback time of about five billion years.
So we're seeing this thing as it was five billion years ago.
And lo and behold, the spectrum to within the noise
looks very similar to a low redshift type 1a supernova.
So that's really great.
So here's the punchline.
Here are several of these type 1a supernovae
that have been verified through spectroscopy,
and whose redshifts have been measured.
And the punchline is that they are faint.
They're very, very faint.
And you might say, well, they're in these scrawny,
pathetic-looking galaxies that are obviously very distant.
Here you can't even see the galaxy
before the supernova went off.
Obviously, these are distant galaxies.
So the supernovae should look faint.
And that argument is correct, but the point
is, they look fainter than they had any right to be.
For a given redshift, you expect a certain range
of brightnesses, depending on what
the universe has been doing.
And the measured brightnesses were fainter than expected.
Let me show you that.
Here are the possible models that we expected.
Indeed, people told us that we would measure probably this.
a Theorists actually prefer omega matter of 1,
for reasons I can go into in the Q&A. But what we measured,
when we measured distance luminosity, distance
versus redshift, was this curve.
That wasn't-- in a multiple choice,
that was "none of the above."
And if you follow the natural progression of omega matter,
this ratio of the average of the true density of the universe,
critical density, greater than one, 1.30 negative,
negative matter density, which seems a bit odd,
because we exist.
And last time I checked, I'm gravitationally
attracted to the earth, rather than being spirited away
like on [? SpaceEx ?] or whatever.
So here's a picture from Adam Riess' lab notebook,
when he was in my group in the fall of '97.
He has this little thing here.
Omega matter is negative .36.
Oh, gosh, I hadn't taught him well yet.
It should be negative 0.36, because the decimal points
are easy to lose, right?
I hope you've all been taught that.
But, anyway.
So there it is, negative matter.
Well, the negative sign is simply
indicating that we're witnessing an accelerating universe rather
than a decelerating universe.
The wrong sign, right?
So this is kind of weird.
Well, I was actually privileged to announce
this result for the Riess/Schmidt group
for the first time at a meeting in LA.
And here's the headline that came out.
"Astronomers See a Cosmic Antigravity Force at Work."
We use this term "antigravity" hesitantly,
because people ask us, can we attach this stuff, whatever
it is, to our cars and levitate over Bay Area traffic jams?
And the answer is no.
It's either a property of space itself,
or there's so little of it that it'll never be harnessed.
Never say never.
But anyway, by December of '98, the editors of Science Magazine
proclaimed this to be the single most important discovery
in all areas of science that year.
And we were obviously very pleased by this,
but of course we're not yet sure that it's true.
But they said, well, look.
Other physicists and astronomers have
had the better part of a year to find
an obvious flaw in what you've done,
and no one has found an obvious flaw.
So either you're right or you're wrong for some subtle reason
that will end up teaching us something
interesting about the universe.
And indeed, of course that's generally how science operates.
Now, the caricature of Einstein looks
surprised here, not because he's blowing
multiple universes out of his pipe.
You might not have known that there are multiple universes,
and they come from the pipes of theoretical physicists.
Well, the first statement might be true.
And I'll be happy to come back and tell you
about the possible multiverse.
But he's surprised because this one universe
is expanding faster and faster with time,
rather than more and more slowly,
as one would expect in normal general relativity.
And he's doubly surprised because he
has a sheaf of papers here where there's an equation-- lambda
equals 8 pi g, Newton's constant,
times the density of the vacuum.
And you might wonder, who's this bozo from Berzerkely
talking about the density of the vacuum.
You were taught on your mother's knee that the vacuum is sheer
emptiness-- zero, zilch, nada.
How can it have a non-zero energy or matter density?
Well, this was Einstein's idea, not mine.
And he was a lot smarter than I am.
So the point is this.
Back in 1917, when he developed the general theory
of relativity, its basic attribute--
that is, gravity pulls-- remains the same as in Newtonian
gravity.
So the universe should be collapsing in on itself,
because all the galaxies are pulling on each other.
Or, maybe the universe was born with a big bang.
And in that case, the universe should be expanding.
But in any case, it should be a dynamic universe,
yet there was at that time no evidence
for a dynamic universe.
People thought it was static.
And Einstein himself felt that the static model
was very aesthetically pleasing.
So he conjured up something that was not aesthetically pleasing,
by his own admission.
The cosmological constant, which in this case, it repels.
It's the opposite of gravity.
So if you have something pulling down and something pulling up
and the net force is zero, you can get a static universe.
Or, if the universe started in an expanding state,
then you could have a negative cosmological constant.
And that could nullify the expansion.
So you could get a static state.
Turns out this is an unstable solution.
You can't have a static universe for very long in this case,
because if you perturb this galaxy a little bit inward,
it turns out that the gravitational force increases,
and it continues to go inward.
And conversely, if you perturb it outward a little bit,
then it turns out it continues to go outward.
So it wasn't a very satisfactory solution in many ways.
It also implied that the density of the vacuum
is non-zero, which seemed weird.
There was no experimental evidence for that.
And moreover, something that seemed so finely tuned
like this seemed ad hoc.
It seemed arbitrary.
It did not make the equations wrong.
The cosmological constant can be thought
of as a constant of integration.
So that in and of itself isn't so weird, other
than that it's a vacuum energy that's non-zero.
But even if there is this vacuum energy, why the world should
its value-- why in the world should
the value of the constant be tuned to exactly match
the attractive force of gravity?
It just didn't seem right.
So Einstein never liked it.
He reluctantly included it in his solutions for the universe.
A dozen years later, Hubble discovered
that the universe isn't static after all.
So the whole physical and philosophical motivation
for this cosmological constant, this ugly fudge factor,
disappeared.
Einstein renounced the idea, supposedly,
as having been the biggest blunder of his career.
Because had he not introduced it, he could have been famous.
He could have predicted that the universe is in a dynamic state.
So here he is, sad that he ever introduced the idea.
I don't know that that's what he's thinking,
but it might be what he's thinking.
What have we done the better part of a century later?
We've reincarnated the idea.
Not to give a static universe, but one
which, on the largest scales, beyond about 100
million light years, is accelerating in its expansion.
So here in this room, the down arrow dominates.
Everywhere in our solar system, it's down.
Everywhere in our galaxy, down.
But as you get to distances of 100 million light years
or more, the up arrow dominates and you
have an accelerating universe.
And so Einstein's biggest blunder, as he referred to it,
may have been, in some ways, his greatest intellectual triumph,
conjuring up such a thing.
And if he were around right now to see the evidence that we
and others have amassed, maybe his reaction
would be something like what was in Science Magazine.
So we now interpret this omega matter less than 0
to instead be the cosmological constant is positive,
or something like it.
It's not necessarily Einstein's cosmological constant.
It could be something having a similar effect
that on the biggest scales, it leads
to this acceleration, which is kind of weird.
Anyway, you might worry that, well, OK,
this is only out to four or five billion light years in time--
I'm sorry, four or five billion years in time.
What would be the predicted effect
if we were to look back farther in time,
if something like the cosmological constant
were really operating.
And there the idea is actually pretty simple.
The curve looks something like this.
The universe decelerates originally, and then
after a while starts accelerating.
And the reason for that is that long ago,
when galaxies were closer together,
their gravitational attraction for each other
was bigger than it is now.
And the repulsion, if it's a property of space,
or something within space, was relatively minor.
As the universe expanded, the gravitational attraction
declined with time.
It's a 1 over r squared force.
But the repulsion increased with time.
If the repulsion is caused by something in space,
or a property of space itself, than the more space there is,
the greater is the repulsive effect.
So in a sense, gravity is coming downward.
Antigravity, if you will, is going upward.
And at some point, they cross.
And that's where the universe starts accelerating,
right around here somewhere.
But the clear prediction was that if we look far enough
back in time, we should see the phase at which the universe was
decelerating, if we found something
like the cosmological constant.
So Adam Riess became principal investigator
of a Hubble project that we initiated to find and monitor
very distant supernovae-- seven, eight billion, nine billion,
or even ten billion light years away.
And we found that indeed, the data
followed the predictions of early deceleration.
So we witnessed the phase of early deceleration,
and then about five billion years ago,
that transition to acceleration.
A transition from deceleration to acceleration--
that's mathematically known as a jerk.
In fact, a jerk is any time you have a nonzero third derivative
of position.
So you have position, velocity, acceleration, jerk.
I actually didn't know that until we made this discovery.
So in a sense, we witnessed the cosmos
as having gone through a jerk.
And the headline that came out in the New York Times
was "A 'Cosmic Jerk' That Reversed the Universe."
And there's my former post-doc Adam Riess.
So I start getting these phone calls--
hey, who's this jerk you work with who
reversed the expansion of the universe?
Anyway, this isn't the greatest photo.
And Adam's mother was not pleased by this juxtaposition.
Right?
You never read the articles in newspapers,
unless you're really interested.
You go flipping through them, you look at the headlines,
look at the pictures.
And only a small subset do you have time to read.
Well, so then what is this stuff that's causing this effect?
It's clearly not the visible matter of the universe,
because all normal visible matter pulls.
It's also not antimatter.
You might think antigravity, antimatter.
Good try, but antimatter has a positive gravitational
attraction.
It's also not dark matter.
Many of you have heard of dark matter.
You can see it there, there, there.
Lots of dark matter.
It's what binds galaxies and clusters of galaxies together.
And I can't help but tell you a small aside about one
of my heroes, Fritz Zwicky, an astrophysicist at Caltech
who died just before I got there.
Actually, a few years before I got there.
He was the first to look at clusters of galaxies
and realize that the individual galaxies are moving around so
quickly that there's got be sort of additional material
there, holding the whole thing together.
Otherwise, they'd go flying apart.
And yet we see lots of clusters of galaxies.
They're not just chance, superpositions of galaxies
passing through the night.
So he suggested that there might be such a thing as dark matter.
And he was routinely ignored, but he
was decades ahead of his time on a number of issues.
One reason he was perhaps ignored
was that he was somewhat arrogant and abrasive.
Somewhat is being a little bit polite.
And he did not have a great opinion
of the intellectual capacity of his colleagues at Caltech.
And, you know, Caltech's a pretty brainy place.
So they didn't take well to this implied criticism.
And here, he may be showing you what
he thinks of their typical brain size.
I mean, I don't know that that's what he's thinking,
but he's on record as having referred to his colleagues
as "spherical bastards."
Because they're bastards anyway you look at them.
And of course, a sphere is the only object
that looks the same from all directions, right?
I would not recommend that you refer to your friends
as spherical bastards, or your Google colleagues here.
You will quickly end up friendless, or colleague-less.
But anyway, I like Fritz a lot.
Anyway, so dark matter helps bind clusters
of galaxies, and even galaxies together.
Indeed, it's critical to the formation
of clusters of galaxies and galaxies.
We wouldn't be here if it were not for dark matter,
even though we're not made of dark matter.
But the acceleration has to be something entirely different.
It has to be something that pushes in a sense, but not
enough to disrupt galaxies and clusters of galaxies.
So the name that was given to it, for better or worse,
was dark energy.
It's dark, we don't see it.
It's also mysterious-- in that sense, it's dark.
Yet it's clearly some form of energy.
But the reason a lot of us are not
entirely happy with that term is that if there's
one equation that even people on the street often know,
it's e equals mc squared, or at least they've heard of it.
So we're forever being asked, are dark energy and dark matter
just two different sides of the same coin?
And the probable answer is no, even though we don't yet
know what the dark energy is.
Anyway, in honor of the Nobel Prize and stuff,
Noelle, my wife, who's here, made this t-shirt,
"Dark Energy Is The New Black."
And she gave it to the rest of us,
who did not get a share of the one and a half million bucks.
But you gave it to the team leaders, as well.
And she even got one to the King of Sweden, we think,
if the courier gave it to him.
So what is the dark energy?
The honest truth is, we don't know.
They're really hundreds of possibilities.
The possibility I like most, and the one
that's still consistent with the data,
is that the dark energy is essentially
the zero point energy of the vacuum.
If you have quantum fluctuations, which we know
occur-- they affect the energy levels
of the electrons in a hydrogen atom, for example.
That's the Lamb shift.
Then you have this energy and it can cause an expansion of space
faster and faster if there's a positive net energy.
But theorists had always said that, well,
for every positive energy fluctuation,
there's a negative energy one.
In the parlance of string theory,
these are thermionic and bozonic fluctuations,
it turns out, if you want to look them up.
And people had always assumed that they all balance out,
and the energy of the vacuum is 0.
And the main reason for thinking that
is that the back of the envelope calculation
suggests that vacuum energy should
be 10 to the 120th power, which is 20 orders of magnitude
bigger than a real Google.
Or I shouldn't say a real Google,
the original Google, right, is ten to the hundredth.
The vacuum energy should be 10 to the 120th, 10 to the 122,
I think, if you want three significant digits.
And that wouldn't even allow us to form.
We wouldn't be here.
Stars wouldn't be here.
Galaxies wouldn't be here if that were the case.
So people had always assumed it's 0.
But if it's not 0 by some weird circumstance
that we don't yet understand, then it
turns out space itself has the desired property of expanding
faster and faster with time.
It's a very weird thing.
And the data are consistent with this interpretation.
A lot of theorists don't like this interpretation.
They think it might be some sort of a new field, a little bit
like the Higgs field that's been in the news
so much the past year or two.
And in fact, a whole category of models of this order known
as quintessence, like the Aristotelian fifth essence.
You know, earth, air, fire, and water, and the quintessence
out there.
And there are hundreds, if not thousands
of candidate theories.
And the problem is, all of them have
additional bells and whistles that
are not super well physically motivated.
And the data do not agree any better with those ideas
than they do with the simple cosmological constant.
So I'm betting on the cosmological constant,
but I may well be wrong.
What we're doing now is trying to trace,
in greater detail, the expansion history of the universe
to set observational constraints that will be used to rule out
some of the candidate theory.
So that's what we're busy doing now.
The other possibility, other than dark energy,
is that general relativity is wrong.
For completeness, I do need to mention this.
Most theorists don't like this possibility,
but it's not yet completely ruled out.
So maybe there isn't a dark energy.
Maybe instead, general relativity is wrong.
But that would be a pretty exciting thing too.
Because we really do think general relativity
is very beautiful, and has an amazing theoretical
underpinning.
Finally, you might worry that all these conclusions are just
based on supernovae.
I mean, that would be pretty weak.
I mean, what if there was something different about them
long ago.
They used to be less luminous than they are now.
We have to look for that possibility.
And there are many things I'm leaving out of this talk.
That's why they pay us the big bucks, right?
Yeah, right.
So anyway-- in academia.
So to figure out the details, right?
They pay us the big bucks.
So in science, as you all know, the more important
the discovery is, the more important
it is to verify it through completely independent
techniques.
And so because there were two teams that
made the same announcement at virtually the same time,
a lot of physicists and astrophysicists took note.
They started testing this conclusion.
And let me just briefly tell you about some
of the other tests which lead to the same conclusion, basically.
One is by looking at the early afterglow of the universe,
the cosmic microwave background radiation.
You can set lots of interesting constraints
on the properties and constituents of the universe.
And in particular, here's a map I'm sure all of you have seen.
It's a baby picture, an infant picture of the universe,
where the temperature of the universe,
to a good first approximation, is 2.7 degrees Kelvin.
But at the one part per 10,000 down
to one part per 100,000 level, there
are small variations in temperature.
That's what you're seeing here.
They correspond to small variations in density.
That's good, because it's from those variations in density
that galaxies and clusters of galaxies
were able to gravitationally form.
So if the universe were completely smooth
from the beginning, we wouldn't be here talking about it.
So from studies of the angular sizes of these little freckles,
you can tell that the universe, over large distances,
is Euclidean.
It's flat.
The sum of the interior angles of a triangle
is always 180 degrees, for example.
And in general relativity, that can only
be the case if the density of the universe
is the critical density.
And yet, we know the density of normal matter is only 30%
of the critical density.
So in a sense, you need 70% of something else.
That's consistent with the supernovae.
The something else being dark energy.
The other point is, you take those little fluctuations,
and then you let gravity do its thing over billions of years.
And gravity, that great sculptor,
sculpts the galaxies and clusters of galaxies and voids.
And when you do numerical simulations, starting
with this picture, which is a map of the variations when
the universe was 380,000 years old,
and let gravity do its thing, and you
don't include dark energy, the final distribution
of galaxies and clusters of galaxies in voids
differs, in an observationally significant way,
from the observed structure of the universe.
Whereas when you include dark energy in the mix,
then the final structure ends up looking
like the observed structure.
And this is the observed structure,
based on measurements of literally millions of galaxies.
So that's another piece of evidence
that the dark energy really does exist.
And there are other pieces of evidence as well.
So that's why we really believe it's true.
We think that the composition of the universe
looks something like this right now.
Dark energy is 70% of the total pie.
And we don't exactly know what it is.
In fact, we really don't know what it is.
Dark matter is most of the remainder,
and we're not sure what that is either.
We think it might be little particles left over
from the big bang.
WIMPS-- weakly interacting massive particles--
but they've never been detected in a laboratory.
I'm becoming mildly concerned about that.
The ordinary matter is only 5% of the total contents.
And the easily visible ordinary matter is only half a percent.
So in a sense, we're the debris of the universe.
We're the afterthought of creation.
That's not to say you're not important to yourselves,
your loved ones, your families, your friends.
But you're not made of the dominant stuff of the universe.
The dominant stuff is the dark energy,
followed by dark matter.
And we don't know what they are.
So for the kids-- I go and talk to kids sometimes
about this-- I tell them, if anyone tells you
physics is dead and there's nothing left
to be learned about the universe,
you tell them, what about the origin and detailed nature
of 95% of the contents of the universe?
So this is one reason cosmology remains so exciting.
We don't know what these things are.
And moreover, dark energy may be one of the few observable clues
that will allow us to discard some of the ideas
for quantum gravity, of which string theory is
a generic umbrella.
Quantum mechanics works great on small scales.
General relativity works great on large scales,
but the two together are in violent disagreement.
We are after a quantum theory of gravity.
Any quantum theory of gravity that does not
account for the acceleration is not a viable candidate.
So this is one of the few observational tests
we have of quantum theories of gravity.
So because of its importance, it was recognized with the Nobel
in 2011, and also because it had been confirmed in so many ways.
We don't know what the dark energy is,
but we are pretty sure at this point
that the universe really is accelerating in its expansion.
So here's a brief history of the universe,
just to close things off.
We think it began with some sort of a quantum fluctuation,
maybe out of nothing.
And then there were quantum fluctuations
within this rapidly expanding universe
that led to small variations in the distribution of matter,
which then grew over time in a universe that was decelerating.
And then in the last five billion years
or so, it had started accelerating.
And recent data have led to an update
of the age of the universe from 13.7 to 13.8 billion years.
And it did not take us 100 million years
to gather those data.
It's just that-- anyway.
So the universe aged 100 million years in the last year or two.
So that's the history of the universe.
What will be its future?
How will the universe end?
Well, if the dark energy really is the cosmological constant,
it'll never go away.
It will continue to be repulsive.
The universe will expand faster and faster with time.
A runaway universe, I call it.
So if you want to see a galaxy like this
with your very own eyes through a telescope,
go up to Lick Observatory, or Chabot,
or Morrison Planetarium, or your local astronomy club,
and look through a telescope soon-- within the next few tens
of billions of years.
Because beyond that time, all those galaxies
will be whisked away to such great distances
that we will no longer see them.
There's a possibility, of course,
that the universe will recollapse,
because we don't yet quite know what the dark energy is.
But I think it's going to expand forever.
Robert Frost didn't know of these two possibilities
when he wrote his famous poem Fire and Ice.
I think it was actually based on something in Dante's Inferno.
But in retrospect, it's very relevant to what
I've told you today.
The poem goes something like this.
Some say the world will end in fire.
Some say in ice.
From what I've tasted of desire, I
hold with those who favor fire.
But if it had to perish twice, I think I know enough of hate
to say that for destruction, ice is also great
and would suffice.
So Frost would prefer the recollapsing universe
that ends up hot and compressed.
But if the universe and he had to perish twice,
then eternal expansion would be OK with him.
And that's perhaps appropriate given his name, Robert Frost,
right, and ending in ice.
Well, anyway, this research couldn't
have been done without the help of many, many people
and federal foundations.
I'm very, very grateful to them.
I'm also grateful to all the opportunities
I've had with like the Hubble Space Telescope, the Keck
Observatory.
And in particular Lick Observatory,
because this is where a lot of the data
for the nearby supernovae was gathered.
And this is the observatory where we really
train our students and post-docs.
And they can be principal investigators
of their own projects, rather than just helping me do things
that I define from start to finish.
So they grow in their intellectual independence
by using these facilities.
And my colleague Jeff Marcy has discovered lots of exoplanets
there.
So I'm very much a supporter of Lick Observatory.
I'm actually currently president of the board of an organization
called Friends of Lick Observatory.
And we're actually trying to raise money
to support Lick Observatory, in part
because the University of California
is highly financially squeezed right now
and has told us that we're doing great science,
but they can simply no longer support the Observatory
anymore.
So if you'd like to join Friends,
let me know or you know which search engine to use
to find a website where you'll find this.
And if you want to support a place that's
cutting edge in its technology and stuff, let me know.
I'm also happy to arrange a visit
for Google employees and their friends.
It's a great place to go.
You can look through the old 36-inch James Lick
refractor at whose base James Lick is buried.
That's a wonderful experience.
And at any time, if you guys want to set up a Google Chat
and ask questions of me, or just ruminate about the universe,
I'm happy to do that kind of thing as well.
So there I am.
You can find me on Google+.
And it's been great visiting.
I hope there'll be a little bit of time for questions.
But thank you all for coming.
AUDIENCE: So in your graph, you have dark energy
as 70% of the content of the universe,
Dark Matter is 25%-- but 70% of what?
What is [INAUDIBLE]?
DR. ALEX FILIPPENKO: So let me go back to this pie.
The dark energy is 70% of the universe.
70% of what?
The pie is the whole thing.
Whatever the density of the universe is,
OK, that's the whole pie.
Now, that density we now know was
very close to the critical density between what
we call a positively curved universe,
a hypersphere-- like the three dimensional balloon--
and a negatively curved universe.
Hyperbolic space.
Closest two-dimensional example I can think of for you
is a horse's saddle.
So we're right on the dividing line.
We're a flat universe over very large distances,
ignoring black holes and stars and all that.
Light travels along Euclidean straight lines.
So it's 70% of that density.
But the point is, is this pie could apply to the universe,
regardless of what its density is.
It's just, here's the whole pie and there are its constituents.
AUDIENCE: [INAUDIBLE]?
DR. ALEX FILIPPENKO: It's energy en masse,
through e equals mc squared.
It doesn't really matter which one you're talking about.
So this has me and you in it.
That's in the form of ordinary matter.
But it also has something that we
think doesn't have a matter component.
It is a pure energy, sort of like light
is a pure form of energy.
Has zero rest mass.
But dark energy is not light.
It's just-- it's a different form of pure energy
which has what we call negative pressure.
And in general relativity, it's the negative pressure
that causes the acceleration.
AUDIENCE: So I've been somewhat confused for a while,
I guess, about the distinction between things
in space-- galaxies and whatnot moving through space
and apart from each other-- as opposed to space itself
expanding.
And so you're describing this expansion
that causes redshift and space itself expanding.
But then you're talking about the gravitational attraction
that would cause deceleration, pulling things back together.
That sounds like it would be the object in space pulling things
together.
DR. ALEX FILIPPENKO: So that's a good, fine point.
It turns out that in general relativity,
you can consider a uniform distribution of matter.
That's the simplest universe.
And the self-gravity of the universe
itself causes the fabric of the universe
itself to slow down in its stretching.
This is actually part of the framework
of general relativity.
That, in other words, if we had found that the universe isn't
behaving this way, we would have been surprised,
based on general relativity.
Now, within the universe, you can also have pulling motions.
So the Andromeda galaxy, our nearest big neighbor,
is in our local cluster, our local group, it's called.
It is moving toward us.
In fact, we're going to merge in about four billion years.
And after a couple of billion years
of looking like a train wreck, we
will turn into a different type of galaxy altogether,
an elliptical galaxy.
So those are motions through space.
They're called peculiar motions.
And they're caused by individual galaxies, or clusters
of galaxies, pulling on each other.
Nevertheless, for the universe as a whole,
you can treat that matter as being uniformly spread out
and ask yourself, what effect does it
have on the universe as a whole?
And you get these different amounts of deceleration.
Or in the case of dark energy, acceleration.
Nevertheless, there are peculiar motions within the universe,
as well.
It's like the raisins tugging on each other,
and moving through the dough.
That's happening as well.
Yes.
AUDIENCE: So the space between us and the Andromeda galaxy
is probably still expanding, although--
DR. ALEX FILIPPENKO: Yeah.
The space between us and the Andromeda galaxy is expanding,
but the problem is that the gravitational attraction
is so great that it greatly dominates.
But yes, strictly speaking, if there were no dark energy,
we would be attracting that Andromeda galaxy a little bit
more than we are.
You're absolutely right.
But it's a negligible effect within a cluster of galaxies,
because the matter density is so much greater
than this dark energy density, whatever the dark energy is.
Yeah.
AUDIENCE: If space itself is expanding,
how does that relate to the microwave background?
DR. ALEX FILIPPENKO: Yeah, if space itself is expanding,
how does that relate to the microwave background?
AUDIENCE: Did the temperature [INAUDIBLE]?
DR. ALEX FILIPPENKO: The microwave background
is the stretched electromagnetic radiation from long ago.
So when the universe was 380,000 years old,
its temperature had dropped to about 3,000 or 4,000--
about 3,000 degrees Kelvin.
At that point, electrons combined
with protons for the first time ever,
a process interestingly known as recombination.
Because plasma physicists can ionize gases and then
they watch them recombine in their labs.
But it's happening for the first time.
At that point, the universe becomes
transparent to radiation.
Because you don't have all these free electrons
that can easily scatter electromagnetic radiation, OK?
At that point, 3,000 degrees, well that's about the sun.
It's half the sun's temperature.
So the light waves back then were optical and near infrared.
They have stretched a factor of 1,000.
In fact, the stretching factor since that time is 1,079.
We now know it quite precisely.
So the waves have stretched by a factor of 1,079.
Their typical wavelength right now
is in the microwave region of the spectrum.
They preserve their black body spectral energy distribution.
It's just that now that black body corresponds
to 2.7 degrees, which, you will realize,
is 3,000 divided by 1,079.
So the microwave background is the microwave background
because of the stretching.
It's the wavelength that it has because of the stretching.
AUDIENCE: [INAUDIBLE]?
DR. ALEX FILIPPENKO: So the temperature
is going through a jerk in the sense
that the universe is expanding faster now than it would have
in the absence of dark energy.
So indeed, the temperature of the universe
is going down faster than it would
have in the absence of dark energy.
Yeah.
So the temperature, in a sense, went through a jerk as well.
Yeah
AUDIENCE: Is that detectable?
DR. ALEX FILIPPENKO: Is it detectable?
Very good question.
I believe the answer is that in principle it's detectable.
But it has not yet been detected.
The way to detect it is interesting.
And indeed, there were detections of the microwave
background long before Penzias and Wilson.
It turns out that certain clouds of gas in interstellar space
had cyanogen molecules-- CN-- as well as others.
And there are certain transitions in the cyanogen
molecule that are excited by microwave wavelength photons.
But they're excited more if the temperature of the universe
were slightly higher than the current 2.7.
So astronomers looked at these clouds at redshift 1,
when by the way the temperature would
have been 5.4-- 1 plus z times the current temperature--
and lo and behold, the transitions were clearly
stronger back then, because the absorption lines produced
by those clouds of gas were stronger than would have been
the case had the temperature back then been 2.7.
So in fact, we've measured not only the microwave background,
but changes in its temperature.
What has not been done is studies
of enough molecular clouds at a variety of redshifts
to see a transition in the rate of change of the temperature.
But in principle, I see no reason that couldn't be done.
That is very cool.
You're the first-- I can tell I'm at Google.
You're the first person ever, ever,
to have asked me that question.
So I feel like I'm on a thesis exam or something
here, where you have to answer these kinds of things
really well.
Yeah.
Other questions?
Right there, yes.
AUDIENCE: How do we know the basic nature
of the cosmic microwave background that
is coming from a blast from the past,
as opposed to, say, the void glowing?
DR. ALEX FILIPPENKO: Yeah, yeah.
So various people have tried to think
of ways of reproducing the microwave background
radiation through local sources.
And in fact, iron whiskers, turns out,
radiate in a way that has some good properties.
You can't get a perfect single temperature background
if you've got objects at different redshifts glowing,
due to whatever process.
You'd need to somehow finagle them all
to have a slightly different intrinsic spectrum, such
that when you add them all together
at these different redshifts, all their contributions,
they end up giving you a black body that
is better than the thickness of the pen with which you draw it.
That's how good the data are.
I mean, the measurements from Coby and Planck
and the W map satellites are absolutely perfect
black bodies.
And so we've thought of no way of doing that.
And yet, if the universe did begin
in a hot compressed state, for which we have other evidence--
primordial abundances of hydrogen, deuterium,
things like that as well-- then you
would expect this afterglow.
Indeed, George Gamow and his students and post-docs
predicted it.
So it's a natural prediction of an expanding universe.
And it's enormously difficult and ad hoc
to conjure up in any other way, is what I would say, I guess.
Yeah.
AUDIENCE: So probably a naive question about redshift.
So the light waves generate long ago
because of the stretch of the space got longer light wave.
DR. ALEX FILIPPENKO: That's right.
AUDIENCE: Wavelengths.
But when the space is stretched, doesn't the rule
for example, the way we measure length, also get stretched?
DR. ALEX FILIPPENKO: Yeah, the lengths of the rulers
are not stretched.
Indeed we're not expanding.
I mean, you might after a big lunch,
but that's your fault, not the universe's fault.
You're held by electromagnetic forces.
And again, it's a spring that's so tight
that you can't stretch it.
We're held to the earth by gravity
in a way that's much greater.
It easily overcomes the natural tendency of space to expand.
So even our local group of galaxies is not expanding.
Indeed, within it, Andromeda is coming toward us.
So if Hubble had only looked at the Andromeda galaxy,
he would have concluded that the universe is collapsing in
on itself.
The expansion of space is only visible starting,
oh, 4 or 5 million light years out.
That's where the density of material
and the strength of gravitational fields and things
like that become low enough that the stretching of space
can start taking over.
But we and the rulers are not stretching.
They're held together by a spring that's
so tight that you can't stretch it.
AUDIENCE: Or can I say that actually the stretch is there.
It's just too small-- dominated by the other forces.
DR. ALEX FILIPPENKO: Yeah, the stretch is there.
That's another way of saying it.
But it's so overpowered by the other forces
that it's not noticeable.
I'm very careful to say it that way,
because even in some textbooks, you
read the incorrect statement that, oh yes,
of course our galaxy is expanding.
But because our galaxy is only 100,000 light years across,
if you put 100,000 light years into Hubble's law,
you get a negligible amount of expansion.
That is a wrong explanation.
You cannot put 100,000 light years into Hubble's law.
Because that would imply that our galaxy is stretching
with the rest of the universe unimpeded.
And that's not true.
It is being impeded by its local gravity.
So just watch out.
There's that subtle difference.
And I didn't want you to walk away
thinking that it's only that our galaxy is small,
and therefore you can't notice the stretching
over a distance of only 100,000 light years.
It's more than that.
It's that our galaxy impedes the stretching because
of its self-gravity.
AUDIENCE: Light is redshifted by the expansion,
then it seems like that light is going to be losing energy.
Does that energy go somewhere?
DR. ALEX FILIPPENKO: Ooh!
Boy oh boy.
The light is indeed losing energy.
That's right.
Light has energy, and therefore in fact
has a gravitational effect.
In fact, you could have a black hole whose interior
consists entirely of light, before it
went into the singularity and got eaten.
But you can imagine creating a black hole out of light,
because light has energy and therefore it bends space-time.
So in fact, what happens is, in an expanding universe,
the light itself-- forget about anything else in the universe--
the light itself is impeding the expansion.
The light is doing work on the universe.
And that is one way of thinking of the redshifting of light.
It is losing energy, because it's
doing work on the universe.
Isn't that a cool thing?
That's an interesting insight.
Yeah.
Very cool.
And the universe is pretty cool too, at 2.7 degrees now.
MALE SPEAKER: So, we're out of time now.
We need to get Dr. Filippenko to his next meeting.
So thanks again.
DR. ALEX FILIPPENKO: Well, thank you so much.
Thank you.