Good evening, virtual audience,
and welcome. Thank you so much for
joining us tonight.
My name is Kate Bruns and on behalf of
Harvard Book Store,
the Harvard University Division of
Science and the Cabot Science Library, I
am
so honored to introduce this event with
James Peebles
presenting his latest book, "Cosmology
Century",
an inside history of our modern
understanding of the universe. Tonight's
event is the next installment in our
Harvard Science Book Talk series.
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So thank you for your patience and your
understanding  - so now
i am so delighted to introduce tonight's
speaker.
If you are here watching tonight, you are
likely familiar with Professor Peebles'
world-renowned discoveries and insights
into theoretical cosmology,
including but not limited to his work
predicting the existence of the cosmic
background of radiation,
and, in dark matter, cosmic structure
formation,
the origin of galaxies and more. John 11
calls him
quote "universally admired among
cosmologists as one of the true greats
of our time".
Just last year, he was awarded
the highest honor in his field,
the Nobel Prize for Physics for his
groundbreaking work.
He's currently the Albert Einstein
Professor of Science Emeritus at
Princeton University,
and has been at Princeton now for
roughly 60 years, I
believe. Additionally, he is the author,
co-author or editor of many prior books,
including "The Large-Scale Structure of
the Universe",
"Principles of Physical Cosmology", "Finding
the Big Bang",
and a textbook on quantum mechanics.
Tonight
he will be presenting his landmark new
book, "Cosmology Century".
Of the book, Robert Kishner writes, quote,
"Peebles is the best possible guide to
the long and winding road that is the
20th century's development of
understanding the universe.
His contributions are right at the
center of this tale that now
leads reasonable people to think the
universe is governed by a tug-of-war
between the un-  unseen forces of dark
matter
and dark energy. "Cosmology Century" is a
mirror for practitioners,
and a window for the curious. We are so
happy to have him digitally here tonight,
so without further ado, Professor Peebles,
the digital podium
is yours!    --Thank you.
I am so delighted to be here and to be
able to talk to you
about what is known about the universe
on its largest observable scales.
I think it's good to start by - uh -
inviting you to consider
what you see when you look around you,
and then
consider stepping back and looking at
the bigger scale,
then step back again, and keep going.
Here is my guide to what you might see
......... - sure - ah --
I'm only surprised when these things
work.  -- so
I --I particularly like this photograph
of Planet Earth.
It was taken in the 1960s - one of the
very first
taken from far enough away that you can
see that the earth is a sphere,
surrounded by dreadfully open space.
Before this we of course knew that the
world is round and  -uh - we knew that the
next planet is a long way away,
but knowing and seeing can be different.
This
image had a great great impact on people,
It was so impressive that the
bureaucrats didn't know what to do with
it, so they classified it.
uh- Stuart Brand was one of those leading
the charge to get the images released;
he used it as insignia for his
book, "The Whole Earth Catalog" - what we
would these days term
tools for living sustainably. If you're
into that sort of thing,
turning these pages is just fantastic.
You see the
guides to all kinds of tools, all
manual.
Let us look at the next slide:
here is the largest planet in our solar
system,
very different from Planet Earth, quite
similar to the Sun,
similar composition, mostly hydrogen and
helium, trace elements of what you and I
are made of.
It is not a star because it's too light;
it doesn't have enough mass to make
enough pressure to cause
thermonuclear reactions, but it is still
cooling off from the heat of formation.
It's cooling by rising currents of warm
gas,
falling currents of cold gas: the
currents rising and falling are twisted
by the rotation
of the planet into these wonderful
patterns.
Here is our nearest star, the Sun:
the image on the right is a close-up of
a bit of the surface.
Heat is convected up through the surface
through
rising bubbles of hot plasma,
falling bubbles of cooler plasma, making
this wonderful pattern
that you see; here strong magnetic fields
have gathered
and prevent the convection causing a
cool spot -
a sunspot. Here is our neighborhood of
galactal stars:
here's the Sun --
the relative sizes of these stars are in
proportion
where the sizes are - of course, the sizes
are
greatly exaggerated relative to the
distance between stars.
The most massive star in our
neighborhood is Sirius A -
here - it is twice the mass of the sun -
that greater mass drives far great -
far greater - greater range of rates
of thermonuclear reactions that makes
it about 25 times
the luminosity of the Sun. That high
luminosity
is accompanied by a very hot surface. Hot
is blue.  -ah --
It is going to run out of nuclear fuel
well before the Sun.
When it does, it will shed some mass and
the rest will
collapse to avoid dwarf.
Sirius B over here has a mass of about
that of the Sun;
it's far more compact. It began life as
even more massive than Sirius A,
exhausted its supply of nuclear fuel
even
earlier, and collapsed, already, to a
white dwarf.
There are lots of stars less massive
than the Sun:
they burn nuclear fuel far less rapidly,
they're much less luminous,
and their surfaces are much cooler -
redder -
kind of fun to consider down here -
Proxima Centauri -
uh - that little dwarf star is known to
have a
planet circling at just such a distance,
but water on the surface of that planet
assuming there is some,
would neither freeze nor boil off.
Wouldn't you just love to go over there
and see what's happening on the surface
of that planet...
you can imagine people are giving great
thought to that, sending
masses of drones to fly by. It - it's only
four light years away so -
well, it wouldn't take that long for the
image to get back to us.
Four light years -um- it's going to take a
lot
longer to send drones all that distance -
but people can dream -
who knows - we'll see what happens.
Astronomers have established that
there are about as many planets around
stars as there are stars, so lots is
happening in this little neighborhood.
On a still larger scale, here's a picture
of the sky showing
the Milky Way: we're in a galaxy of stars -
it's flattened like a disc -
we are in the disc - we are looking
through the disc and seeing this band of
light.
You're seeing dark bands running through
the bands of light -
that's not absence of stars;  the presence
of streams of interstellar dust
that absorbs the light. We get a picture
of what a galaxy
of our sort looks like if we look at
another one nearby: the scene face on
this beautiful galaxy shows you
dark streams of dust, wound up by the
differential rotation of this
galaxy; you see blue spots -
that's where stars have formed fairly
recently; the most massive
stars are still shining;  they're
producing most of the starlight -
they're hot, they're blue, hence the blue
patches.
You see red patches - that's where these
hot stars have ionized interstellar
gas made of plasma - the recombination
line of hydrogen
produces a very strong red line - that's
recombining hydrogen. You notice a sheen
of yellow
near the center that actually continues
through the whole
galaxy: that's the light of some
thousands of millions of planets like
the Sun - of stars like the Sun.
You pause to consider that the odds are
good
that there are about as many planets
around these stars as there are stars:
that's some thousands of millions of
planets
on which all sorts of marvelous things
are happening,
that we, the human race, will never
see.  After all, this thing is some
10 million light years away - it
gives you pause.   Here is the distribution
of the galaxies around us -
another step back - the red dots you see
 - uh
are the massive galaxies, similar to the
one we just looked at
or to our Milky Way. You see two views -
projection in perpendicular directions -
so here is a particular galaxy,
NGC 6946, in one direction,
- whoop --and the other.
The black dots are smaller galaxies -
there are lots of them.
A remarkable empty space here - about a
third of the space;
one measly little galaxy in there -
Wouldn't you like to know why it got
there -- why there aren't more?
Well, this is one of the studies of the
evolution of the universe --
impressive too that another projection
you see,
a sheet of galaxies along here --
Let's look still further back:  here is a
picture of the distribution of galaxies
across the sky
out to - uh - as I'll describe later --
Galaxies are moving away from us;
these galaxies typically are moving away
from us at about seven percent of the
speed of light.
-- pretty far away --  you see
this tight knot of galaxies:
it is, in effect, a galaxy of galaxies.
But you see something new: this spot -
this spot looks much the same.
That new effect is seen better if we
look at
the radio - the centers of galaxies
contain massive compact objects - 
they're very likely
massive black holes; sometimes they
explode
and when they do they can send out jets
of exceedingly hot relativistic plasma;
it piles up into these radio lobes that
can be
very bright in the radio.
Here is a distribution across the sky of
the 10,000 or so
brightest of those radio sources. I got
to explain a lot of peculiar features:
first, there's a hole in the center -
that's not a hole in the world - that the
telescope can't - looks there -
look there. We see some incomplete
observations here -
you see one dot right there - that's the
radio source I just showed you; it's so
bright that it confuses the telescope
when looking in that general direction,
you get a hole; there's got to be another
one here but I've not quite figured out
why there's this hole.
Anyway, there are lots of fainter radio
galaxies
in stars in our galaxy and they make
this little band - that's the band of our
Milky Way -
So after all of that, you get to look
around and ask yourself what do you see.
You see here if you do a careful
statistical analysis, that this is the
same
clumpy distribution as we saw in those
earlier
maps, but we're looking through so many
clumps - clump upon clump -
that it's averaged out to a large extent
and what you see therefore is ---
nothing. This is a very deep
result. Scientists
focus on layers of perhaps -
on the natures of atoms or the way atoms
combine together to make molecules,
some so large as to make up the cells of
living beings or how those cells make up
beings such as us,
or how we organize our our cells and
societies,
or how we're organized into - into solar
systems and the solar systems
organized in galaxies, on up, and of
course you can go down
layer upon layer, things to study - but
this
is something new - nothing new -
This is why we can have a cosmology: it
is because
on these large scales the universe is
close to uniform.
It is not exactly uniform, of course, it
is lumpy on small scales,
but that large scale mean can be studied:
we can in particular observe that the
universe
appears to be expanding. This is an image
from 1930
Ithink still by far the best way to
explain what is meant by the expansion
of the universe.
Imagine --  imagine that we live not in
three dimensions but in two:
imagine we live on the surface of this
balloon.
You must ask me, so what's off the
surface of the balloon? You don't live
there, you live on the surface of the
balloon,
that's what you experience; the balloon
is being blown up
by this curious figure --a long story
which we can't get into --
another caution: as the balloon is being
blown up, the galaxies aren't expanding,
you and I are not expanding, the galaxy's
not expanding but the galaxies are
moving apart,
and you notice if you sit on any one
galaxy and look at around you
as the balloon is blowing up, you'll see
that the galaxies are moving away from
you
and you may say, well -- the universe is
expanding and it's expanding away from
me.
It's a comforting thought - but of course
you go over here and you see the same
thing:
this guy's moving away from me, these
guys are moving away from me,
and so on, the universe is expanding away
from me.
That's the way it has to be in a
universe that's uniform - 
everybody has the same experience on
average.
You might also notice that if something
is further away from me here,
well, it's moving away faster because
this is moving away from me -
this is moving away from this guy - so
this one is moving faster
than it is away from here. That's a key
law, a key property of cosmology, and it
is illustrated here
um -Astronomers have discovered that that
recession that I tried to illustrate in
the  - with the balloon -
says that the further away the galaxy
the more rapidly it's moving.
Here was a discovery plot, Edwin Hubble
in the 30s -
late 20s - Hubble with great assist from
Humason in 1936 -
the furthest of the galaxies that they
could observe is moving away
at 10 percent of the speed of light.
Wow. In 1936 people could look that far.
I'm impressed to notice that I was one
year old when
astronomers could look that far, and to
look much further than that took decades
for establishing more efficient
detectors in photographic plates.
Well, I want now
to give space to just one event in
the great progress that's been made in the
study of the
expanding universe since then.
The end of the Second World War saw a great
explosion of energy
in science and technology, from particle
accelerators to automobiles with tail
fins,
and maybe not surprising these four
people independently
decided it's time to look more closely
into this notion of an expanding
universe.
Bob Dicke, on the left, worked in war
research
in the Radiation Lab at M.I.T., radar and
the like;
Yakov Zel'dovich, on the right, did war
research in the Soviet Union
uh - ending up in the Soviet
equivalent of Los Alamos making great
contributions to nuclear weapons.
Fred Hoyle was a great
author of -great exponent on how stars
form,
how they make elements, how the elements
are distributed;
in 1948, he introduced an alternative to
the notion of the evolving universe, the
steady state theory - I'll mention it
briefly later;
but I want to talk about a paper
published in the same year, in 1948, by
George Gamow.
Gamow was a refusenik, we might say,
an emigre from the Ukraine;
before the war he made some interesting
contributions to
cosmology, but the great contribution in
1948
showed to me spectacular scientific
intuition and ingenuity. He argued
- we can't get into the tales, that's just
the basic point - he argued
that a sensible-looking universe could
evolve from a hot
dense state, along the way producing by
thermonuclear reactions a lot of helium,
nothing much beyond that, so that the universe
would begin making stars with hydrogen
and helium
and that there would be the radiation in
a hot universe; it would be [??] as the
universe expands, the radiation wouldn't
go away -
it would just cool off as the universe
expanded but it would still be there.
Alas, I think it must go with this deep
intuition,
a certain disinterest in details -
Gamow -uh  -was of that sort,
and so his ideas were not forgotten but
they were never very heavily promoted.
So now I come to the -the year 1964.
1964 -1964.
Fred Hoyle in the U.K. in Cambridge
England
uh -is becoming aware of the astronomical
evidence
that stars contain a lot of helium,
far more helium than he was expertise
on element formation and stars could
imagine forming in stars.
Where in the world did that helium come
from?
He intensely disliked Gamow's hot Big
Bang Theory,
but he's a good scientist and so he and
Roger Taylor, a colleague,
published a paper. It had a nice title -
"The Mystery of the Cosmic Helium
Abundance" -
in which they uh admitted, not very
enthusiastically, but it was there in
black and white,
maybe this is the rate the helium left
over from Gamow's hot Big Bang.
Hoyle knew very well that that
interstellar space seemed to be
warm- a few degrees above absolute zero-
because you could see absorption lines
from the molecule
cyanogen - the carbon and the nitrogen
stuck together,
not only from the ground level but from
the first excited level.
What excited that level? Well, one
possibility, it's a sea of radiation;
Hoyle knew about that earlier. I can
cite two publications,
but by 1964 he'd forgotten it.
Meanwhile in the Soviet Union you see
Yakov Zel'dovich with two of his
colleagues.
Zeldovich -uh- being an influential figure
in nuclear weapons,
had great influence. He had four
Socialist Worker Hero medals.
When he wanted something from the
bureaucracy, he put in those medals and
he would get prompt attention,
but of course the authorities would
never let him out of the Soviet Union, he
knew too much.
He found it hard to communicate outside
the Soviet Union.
Pblications back and forth were
censored. It took a long time to get
through censoring.
He had great difficulty publishing in
the journals that people
outside the Soviet Union read and yet he
made spectacular contributions to
cosmology,
but in 1964 he was under the impression
that stars contained little helium
and therefore Gamow's hot Big Bang
Theory
must be wrong. He was working on the how
you get a self-consistent theory
of a universe that expands from a dense
cold state.
It's difficult because the matter tends
to coalesce into -
into helium, under those conditions; we
don't want all helium we've got a lot of
hydrogen.
As it happens, I was -uh -in the city in-
in Princeton, New Jersey- uh- working with
these guys
on a suggestion, but by Bob Dicke. He had
decided in 1964
quite independent of Gamow, if he didn't
know about that, or if he didn't, forgot
about it
that a sensible universe would begin hot,
and he -uh- persuaded two of the young
people in his group:
David Wilkinson, here you can see he's
holding a screwdriver,
and Peter [Roll?] his plaid shirt you can
just make out -
to build a Dicke radiometer. Bob had
invented this technology during the war,
and used it to see if we might be in a
sea of thermal radiation
left over from a hot Big Bang. I -
he had, I remember the meeting when he
suggested this and I remember his
casually turning to me and saying,
"Why don't you look into these
theoretical implications?"
So Peter went off to education, David
and I for the rest of our careers
just followed Bob's suggestion.
Funny how these things can happen.
Meanwhile
in -uh - New Jersey, 30 miles from us,
the Bell Telephone laboratories were
experimenting with communication by
microwave radiation -
radiation in wavelengths of millimeters
to centimeters.
The first experiments already in 1959
showed something was wrong. The
instrument was detecting more radiation
than they could account for. The
engineers were upfront about this,
but -uh - didn't know what to do about it -
it's not their job to worry about it -
so that anomaly remained a dirty little
secret,
so as to speak, in the Bell Laboratories
until 1964,
when these two young guys, Arno Wilson -
Arnold
Penzias and Bob Wilson- resolved to track
down the source of this radiation.
They exhaustively discovered all
possibilities of
origin within the instrument or the
surroundings.
They were at their wit's end. To their
great credit,
they didn't give up and even more
important, they complained about it
until someone heard them and told them,
"You ought to get in touch with Bob
Dicke
and these guys who are doing an
experiment off in Princeton
that might be relevant to your
problem."  Those two got the Nobel Prize
for this identification; it was right and
proper, as I said - they did the right
things.
um - I must admit I've always been a
little unhappy with the Nobel committee
for not
admitting that the third person on that
nomination on that recognition ought to
have been Bob Dicke -
but, well Bob has done many things well
recognized for them -
it's all right... so I guess I have to
admit
that -uh-last fall when I was awakened at
five in the morning
uh -and- uh -was asked when I picked up the
phone
"Are you Professor Philip James Edwin
Peebles?"
and I admitted yes, I am uh -
the voice said, "Then -uh- we have voted to
present you with the Nobel Prize. Do you
accept?"
At this point I could have stopped and
said "Well, I'd like to raise the issue of
Bob Dicke"
but I didn't - I meekly accepted,
and the conversation got more friendly.
Well, to carry on -
this radiation is present.
Our thinking in 1965, when it was
recognized and all of these
elements came together was one of great
excitement and relief. In 1964 we didn't
know there was anything there to detect
it was, it was a gamble in the dark - uh -
I had spent at least as much time
preparing for a null result of the
experiment as for a positive one.
It was so exciting to know that
something was there,
something one can measure and something
that we can analyze.
Of course you don't know whether- whether
what's there is really radiation
left over from the early universe. There
is a characteristic signature however.
This radiation began as thermal
radiation,
which has a very distinct spectrum -
sometimes talk
about a Planck spectrum - you tell me the
temperature-
and remember the astronomers had a
temperature of a few degrees above
kelvin, above absolute zero a few kelvin-
I'll tell you the intensity of each
wavelength -so now go and measure the
intensity of each wavelength to see if it
follows that curve; if it does
that's pretty convincing evidence that
this is left over from the hot early
universe.
So two groups - many groups started out
trying to make this measurement - two
succeeded;
it's very difficult because you have to
observe above the atmosphere -the
atmosphere radiates,
you have to get away from that - here's
one group:
here's David Wilkinson; here's John
Mather
got the Nobel Prize for -uh - I didn't want
to show that yet -
Nobel Prize well-deserved for this
experiment -um -
second group:  Herb Gush, University of
British Columbia
western end of Canada - I'm a little
annoyed I have to keep telling people
where
British Columbia is but there it is -
This guy was a brilliant - is a brilliant
experimentalist
uh - and he had just the right skills to
do this experiment.
Here are his two able assistants: uh - Mark
Halperin on our left and -uh -
Herb - Ed [Wishnow?] above
and this is my favorite uh- photograph of scientists at work of all time.
If you knew nothing about these two
except what you could infer from this
image,
would it inspire confidence? Okay, well
you shouldn't - i shouldn't ask you such
questions. Anyway
here were the results, moving to me to
consider that both projects took
15 years from start to finish. They were
completed within a few
months of each other. Either made the
point
the radiation has the thermal spectrum
expected from radiation from the early
universe -
tangible evidence that the universe
really did expand from a very different
state.
As is mentioned here John Mather got the
Nobel Prize for this -
well well-deserved uh -
Herb Gush got our deep deep respect.
uh - Canadians are not always as assertive
as they should be but never mind -
So I think I've gone far enough in
telling you the sort of flavor
of what goes on in this subject; in fact
what goes often in any other branch of
physical science -
so i would like to conclude these
prepared remarks
by quoting from a much
deeply-admired and influential expert on
philosophy and baseball, Yogi Berra,
who made the wonderfully perceptive
comment "you can see
a lot by just looking". We made great
progress in the study of the evolving
universe
establishing that it really is expanding,
but it really is evolving,
through the work of so many people
here illustrated.
I just like to kind of look at these
people and -uh -and
admire where all went on, in the many
people who made so many great
contributions to make this all happen --
stop sharing --ah here we are
how -- are we here?  ---Yes! hello
Oh good! all right -   -That was wonderful! um-
We're now going to turn to question and
answer. If anyone has
a question they have not submitted yet
please feel free to
put it in the question box - but it looks
like
you're all right here. I'm going to start with
this one
at the top: um-  Daniel asks are you
optimistic
about understanding the nature of dark
energy in the next
20 years?
Dark matter is a little bit easier...
experiments in progress
may any day now detect it, though I will
caution you
I -  I think any day now we're going to get
a breathless announcement, "dark matter
has been detected."
The first question you've got to ask
yourself is did you detect
the dominant form of dark matter or only
a trace element?
There's going to be a lot of debate
about that, but that will be fun.
It'll be so exciting to have a new
detection if it teaches a lot it'll
stimulate our thinking
and the experiments will go on. Dark
energy,
which is just another name for
Einstein's cosmological constant,
is a deep mystery. It's been a mystery --
in fact, when I was a postdoc in Bob
Dicke's group,
i remember discussions of the deep
problem
of the quantum vacuum energy density...
easy to see with the natural value
of the quantum -the energy density -of
quantum vacuum
which you know is a very complicated
business, is some hundred orders of
magnitude bigger
than what would be allowed for
Einstein's cosmological constant.
It behaves like Einstein's cosmological
constant but it's a ridiculous value.
So why is it that we have a cosmological
constant?
We have, in effect, quantum physics
dark zero point energy, yet
it's the wrong value by a ridiculous
amount. There's a deep,
deep discovery there to be made - maybe by
the next generation
--  we leave it for them.
- Juliana is asking what are the most
pressing or interesting needs from
engineers
over the next few decades for advancing
the field of cosmology?
-Oh wow -um- you know great experiments are
going on
with space telescopes. The Hubble Space
Telescope has been such a wonderful
device,
but on the ground, people are learning
how to do almost as well
at a fraction of the cost. Meanwhile JWST
 -the next generation space telescope- will
go up;
it'll see marvelous new things. It's
pretty much a standard rule:
you look at the universe in a new way
and you will see something new and
surprising.
I tried to give you a flavor of that, but
the images I showed you -
the things around us on various scales -
when these observations are made, we're
going to be surprised, is my bet, and
we're certainly going to be edified.
There'll be a generation of observations
after that,
and certainly as -
as detectors get more and more efficient
and more and more ability to accumulate
evidence - data -
the ability to handle that data is going
to get more and more complicated
um - who knows, perhaps AI will
solve the problem but there's going to
be a fascinating business,
already going on, of learning how to
accumulate
vast amounts of data and then how to
learn how to handle that data.
- There's actually a follow-up question
here from
[Jernesh?] who asks uh - finding new means
of propulsion that will take us deeper
into space
beyond that list - that you were just
talking about?
- Well yeah -
I don't know how far we're going to get
into space.
It would be so so wonderful to see  - to
see
the surface of a planet that's not too
far away maybe four
light years; getting there, though, that
would require that you
get let's say to attempt the speed of
light, it would take you 30 or 40 years,
that's not too bad;
would you volunteer to go? ... What's going
to happen, I think,
almost for sure is that they're going to
be
drones - sent out accelerated by light
pressure -
they'll get up to a tenth the speed of light
- they'll zip by
this  -this planet and take a snapshot,
send it back --
it's going to be - we're going to be  - so
tantalized
but you know, to discover what's
happening on a planet,
around another planetary system, is going
to be so moving...
it's - it's a goal that we just can't
resist. Humanity is going to
keep pushing for it - provided of course
if we don't blow ourselves up first.
--uh - Amir is asking what are your thoughts
on
alternative theories to inflation?
- So inflation is an answer to the
question
what was the universe doing before it
was expanding? It's a very attractive
answer.
On the other hand, uh - we should be
careful because it's been on the books
for - for
40 years, greeted with great excitement,
an elegant idea, so elegant that the[sum?]
it must be right.
The problem is we don't yet have a real
theory of inflation: we have a picture -
a great challenge remains: make a
definite prediction and then test it.
Until we have that I'll not be
advertising evidence that inflation
really happened.
It's- it's a wonderful idea -- people are
floating alternatives,
and we should keep an open mind and
again, I would caution you the next time
you see in the newspaper
"inflation proved" or else "inflation
disproved",
be very cautious, because almost
certainly
there'll be big arguments on whether
it's been really disproved --
maybe you simply used the wrong version
of inflation.
Again, a wonderful problem for the next
generation -
we'll add it to the list.     - Yeah - um -
there's a question here from Matt, I
think you touched on this a little bit,
but
uh - what are the most important areas of
further research
in cosmology?  --Luckily, there are lots of
areas for research
and lots of people are these days
fascinated in carrying out that research.
My favorite is the formation of galaxies
because they're complicated
but not so complicated that you can't
see lots of fascinating regularity
in their properties, and
maybe I'm only fooling myself but I
think
the standard theories of how the
galaxies form based on
this - the theory I wrote down in the
1980s -
has a name, "Lambda CDM",  it's worked
remarkably well so far,
far better than ever I expected but you
know, all the tests are of limited
accuracy
and there's certainly lots of room for a
better theory than Lambda CDM.
I'll be amazed if that's the last word;
in fact that's inconceivable.
There will be a better theory and
whether it changes our outlook on how
galaxies formed is
to be discovered, but meanwhile if we
find that Lambda CDM doesn't present us
with a
reasonably convincing picture of how the
galaxies formed,
then we have potentially a hint to how
to make a better theory. uh - So I'm
enjoying very much
studying progress in theories of galaxy
formation
and seeing what I hope are not germa -
not, will of the wisp, but actual hints of
how the theory can be improved
by improving the cosmology.
--All right - it's a big list to pull from
here  - um -
Alan Rubin is asking does the expansion
of the universe
increase the distances between the
electron shells and the nuclei of atoms?
No - the atoms are not expanding,
you and I are not expanding, the earth is
not expanding,
but maybe a little bit, the earth evolves;
the galaxies are not expanding very much
but the distance between galaxies is
increasing.
It's a complicated business, but
you must understand the expansion of the
universe
has no effect on you and me except for
that crazy cosmological constant dark
energy.
It's a constant understanding model a
constant that pushes a little bit on
everything
and so you and I are being slightly
pushed apart - don't worry, it's not a very
big effect;
the galaxy is being pushed apart very
slightly, it's still not a very big
effect,
but of course on still larger scales
become significant.
Anyway, apart from that, we are quite
isolated from the expansion of the
universe - it doesn't pull on us -
there's no answer except for tidal
fields;
that includes atoms, it includes you and
me, and it includes our -
our planet.
- um - Bill Bloomberg is asking you to
comment on the tension between different
measurements of the Hubble constant.
 - Right.  To me,
it's so impressive that the measurements
are as close as they are.
One measurement is based on what
astronomers observe about galaxies
relatively nearby, how far away are they,
how fast are they moving; the ratio is
held as constant, it's a measure of the
rate of expansion of the universe -
a very difficult measurement because
it's hard to get accurate distances to
galaxies but
but people are clever, [predictors?] are
better and better, they're making
spectacular progress
in getting that expansion rate. Meanwhile -
back at a redshift, back when the
universe was one- thousandth its present
size, when the temperature was three
thousand Kelvin
rather than three Kelvin,
the earlier universe was so hot that
matter was thermally ionized
with plasma. At a temperature of around
3000 Kelvin
the plasma combined with neutral atoms.
Before
the recombination of the plasma
interacted strongly with radiation  -[we
see?] radiation and thermal radiation
specifically free electrons scatter
radiation very well;
ions scatter electrons very well; the
result is that
the electrons, ions and radiation act
like a fluid,
with viscosity but also sound pressure.
The effects of those sound pressure can
be moved -can be observed -
as oscillations; from that you can infer
what the expansion rate of the universe
ought to be and there's a ten percent
discrepancy.
Through my entire career i've been
seldom being
being disturbed at ten percent
discrepancies. This field used to be
well, a factor of two in the Hubble
expansion
rate -- um- about 10 percent, though, is very
exciting
because if it's real, it's a hint
to how to make a better theory. I think
the best way to go forward, of course, is
to keep pursuing this apparent
discrepancy: is it really true,
or is there some deeply subtle
systematic error, but also to make many
other comparisons of a similar sort that
can be done?
So far, don't join an anomaly; it can be
done better,
and in the best of all possible worlds
we'll start to see a pattern of
anomalies of this sort.
That will be a really good clue on where
to go for still better theory.
I should emphasize I don't think the
standard Lambda CDN theory
is going to be shown to be radically far
off
uh -i think it's very clear from many
measurements that it's a good
approximation,
but it's by no means the final answer,
and surely there's a better theory,
and -uh- it'll be really exciting to have -
find - clues
that would guide us to that better
theory.
---I have a question here from Mel who asks
how does current science skepticism
affect reporting in a constantly
evolving
field like cosmology?  -- Well, of course --
we have it so good, uh, compared to
let us say, well, epidemiology --
people have known for a long time that
smoking is not good for you -
but you could - far beyond that - and you
shouldn't drink too much,
but you get into details it gets very
complicated - epidemiology is a
really complicated subject,  it's a
science,
an important one, really vitally
important, but
getting it right is so hard.
We, in cosmology, had a much easier task -
that of course is why we've arrived at a
compelling case that we have a good
approximation to what really happened,
because we could make pretty clean
predictions
to test and check out the theory.
Hard to make experiments in epidemiology
and in fact, well, the Covid virus -
so complicated - those people are really
working hard,
they're going to make progress - they are
making progress but -
that's a much harder field.
-um - I have a personal question for you -
Joe Blatt wants to know -um- a little bit
more about your formation as a scientist,
your education, early work, whatever you'd
like to talk about.
-- Well, I can tell you the story of my life -
um - briefly --
uh I - I realized i was a very
unsatisfactory student in high school,
not because i was in any way rebellious but
rather
I was a dreamer. I did the homework, but I
didn't pay much attention;
I skipped classes... I -- just a dreamer.
The result was I didn't know what I
wanted to do when I graduated from high
school,
so I I entered the University of
Manitoba in
engineering; that was all right, and I
guess I could have made my way through
life as a as a mediocre engineer,
but to my intense good fortune um I
started complaining about
the lack of physics courses and I got the
suggestion, well - why you should transfer
to physics.
I did - I immediately discovered that what
i love to do is physics.
i'm so deeply grateful to the faculty
and students at the University of
Manitoba in physics;
they showed me a lot about how to do
physics
and then they shipped me off to
Princeton for more work -
so i just followed directions. I got to
Princeton,
I fell into orbit around Bob Dicke and
here I am, still at Princeton - sort of
lack of imagination, isn't it? --ah, let's see--
Marty Glassman has a question: do you
have any comments on
Sir Roger Penrose's hypothesis of cyclic
cosmology?
Roger is a deeply impressive person;  I
love to talk to him,
um - but he's a theorist, and theorists will
have ideas,
and not all ideas can be right. I just do
not know how to judge the quality of his
theories -- but let him keep going --
you set your track record -
we must pay attention to him.
-- All right there are so many questions -
here I'm trying to pick one. Let's see:
Matt wants to know does Hubble's law
eventually have to level off so as not
to exceed the speed of light?
--It gets a little complicated, doesn't it?
We talk about
recession velocity; that's a good
approximation
when the velocities are small,
the nearby galaxies are moving away from
us,
and indeed the theory says, and I think
it's pretty convincing,
if you ran a tape measure between us and
the nearby galaxy
you would see that you have to pay out
the tape measure - the distance is
increasing -
that's okay and it's - it's a good thought
experiment if the galaxies are not too
far away,
but the further away they get the less
 possible that thought experiment is
and the less good it is to think of the
red shift which is what is observed, the
shift of radiation toward the red,
as a motion away because
that motion away, when computed at a
hypersurface of fixed time,
can exceed the velocity of light. Nothing
wrong with that -
special relativity is still a good
theory but it's a local theory,
and general relativity theory only
satisfies special relativity on local
scales,
so in fact as a
construct, yes, the universe is expanding
faster than the velocity of light
but of course it's not an observable
effect.
--um- I have another speed of light
question for you.
um - This -uh- audience member says
I have read that in the very early
seconds of the universe its expansion
rate was faster than the speed of light.
How can that be possible?  -- Well, there you
go, it still is
faster than the speed of light if you
look far enough away.
Again, this is what the theory - Einstein's
General Theory of Relativity - predicts.
Until recently you could take that
theory or leave it alone...
well, not so recently: when I began as a
graduate student,
there was one serious test of general
relativity - the procession of the
perihelion of Mercury -
the rest of the classical tests were
really very poor.
Since then -uh- the tests have improved
immensely,
so that by now we have a really
convincing story.
Of course general -that's not to say that
general relativity theory is the
absolute truth.
I'm not sure there is such a thing but
it is a wonderfully good approximation,
and that theory says yes, in the early
universe, the expansion rate was so great,
but should you be able to live in that
immense heat you would see
that objects not very far away are
moving away at immense speeds
so that you can't see very far away - 
everything is red shifted away because
it's
just - it was moving away faster than the
speed of light,
that's the way it is and we must live
with it.
--Okay -uh -this question: could you comment
on variable light theories
and how they might provide an
alternative explanation to cosmic
inflation theory?
--We should consider alternatives to the
standard
theory that's been so well tested it
hasn't so many tests.
I don't see hope for variable speed of
light theories there,
except in some subtle correction to be
discovered.
Uh  - inflation I deeply respect but I also
insist that it's not yet to be added
to our canon of established persuasively
established
physics. It's a beautiful idea but until
we
get experimental tests -- there was great
hope for that with polarization,
that didn't quite work out, so there's
still this big challenge -
find us tests of what happened before
the universe was expanded.
Will it be inflation? Many are betting so
but I wouldn't put any - I wouldn't
settle any bets until we have some
evidence.
--I don't even - I have not even heard of
some of these theories -- people are very
knowledgeable about
this subject.  - Yes- we gotta let a thousand
theories bloom, but
only a few are going to be on the right
track.   --Yeah --
um - I have another another question about
a theory is - um - what do you think of
multiverse theories?
um - from one of your former students!
Bob Zacker, Class of '80.
--Oh boy-  so multiverses - um -
I guess I am becoming a crusty old man...
i'm certainly getting old so crusty well -
um
Theory is a wonderful thing.I love
theories - I -I
it is so romantic to think of
multiverses - beautiful idea -
but is it to be admitted to the canon of
established physics?
well - in fact, it can't be, because
by construction, there's no way to check
on all those other universes,
and so to my way of thinking, multiverses
are a wonderful idea,
but to be put in there with fairy tales.
No I don't I don't know  it's [??] of
those,  no - but on the other hand --
yes - did you grow up with "Just-So Stories"?
"How the Leopard Got Its Spots"?
- I think so - well, I hope so!   Wonderful
stories -
but they're made up; and is inflation
made up?
Maybe not;  we'll see - lots of arguments
for it,
but let's see: are multiverses made up?
Yes.
--I don't think that you're crusty for
thinking that.  All right --
um - I think okay, I love this question -um-
and I'm gonna attach something of my own
onto it.
So someone says: Dear Professor, thank you
very much for your talk.
Could you please give some advice to the
new generation of cosmologists -
and I know that you've given us a hefty
list of things to tackle for the new
generation -
um - but I was also hoping that you could
comment something on women in cosmology,
um -I'm really interested in physics
myself,
and I know that the history of cosmology
has featured
so many male prominent figures, so I'm
excited to see what the future holds for
women in the field too.
--Did you notice the very last slide I
showed a bunch of
photographs of people - you noticed some
women in there?
--yes - um
I think women are less underrepresented
in the fields of cosmology
and astronomy than in say particle
physics -? am I wrong?
--I think so,  yeah - and 
we are making progress -um - I mean I've
- when I came to Princeton uh it was it
was
all male -- we all wore suits and ties--
it was pretty dreadful  --[??]
uh, we became co-ed,  what was it, around
1970?
- now I think there are more women
undergraduates than men.
We're working very hard on the graduate
school level
uh - but we have women on the faculty -
three? - but it's not enough -
but we're working on it... Well, I forgot
the original question - well
the role of physics, the role of women in
--in society - well - we are learning.
What was your question, I've
forgotten?      --um -- the
audience question, which I slightly
derailed, was advice you have for the
next generation of
cosmologists.    --Uh I can -
all first bear in mind that cosmology
is just one branch of natural science.
There are
so many branches for you to choose from
I -
would although I didn't -ever- pause to
consider what I wanted to do
it seemed to just fall to me. I try to
urge students
to look around and consider various
possibilities before becoming committed
to any one -
you may find this subject or that
particularly interesting for a while but
as you look into it,
you may notice that well, this other
subject is even more interesting.
You will do well if you can find a
subject that fascinates you -
so much that you're willing to spend a
lot of your career
working on that subject. I think
pretty clearly you don't make good
progress
in any subject - cosmology, anything else -
unless you become committed
to doing good things by working very
hard
at it. I feel moved at this point to also
throw in a caution: don't judge your
career
by prizes and awards,  judge your career
by what you did,
what you think of it. uh - I -I stress that
because
well, I've made mention of Nobel prizes,
wonderful things, but their award has got
to be capricious because there are so
many people who are doing so many things
that a lot of a lot of eventualities
have to occur
before the Nobel committee will deign to
look at you.
Ignore them. If you get these prizes,
wonderful,
wonderful, but if you don't, it's not a
negative reflection on you,
it's just the capricious nature of these
awards.
Cosmology still has a lot of open
questions -
um - but on the other hand,
look around you - you might find something
even more interesting.
-That sounds great... So do you think we
have time for one more question? There's
so many here, I'm just trying to pick -
oh my gosh, how will I choose? let's go to
the top
um
all right --let's see
oh, I'm being so indecisive -- okay. the
question that says:
cosmologists are always building new
instruments and telescopes designed to
measure
cosmological parameters with increasing
precision;
at what point do we decide to stop?
--Never.      - -Never.
People talk of final theories. I don't
know whether they exist,
maybe it's successive approximations all
the way down.
Bear in mind, none of our physics is
complete - it's approximations;
none of our measurements is  -is
infinitely precise,
they're approximations too. Working on
those approximations,
tuning them up ever better, better
theories, better experiments.
We made spectacular progress in so many
branches of physics,
we haven't approached completion in any
of them, and
I can't see completion of cosmology
anytime soon. I'm quite willing to
believe
that it's going to be successive
approximations all the way.
--So do you think there's a limit to what
we can learn in cosmology?
--Well, I suppose there's a limit to
everything,
but we come closer - no - we haven't.
--I like that, I think we should end there.
uh --
Thank you once again to Professor
Peebles for your presentation,
and for all of your outstanding work,  -uh
and thank you to all of you digital
audience for spending your evening with
us.
Please feel free to learn more about this
important book and purchase "Cosmology
Century" at the link below.
So on behalf of Harvard Book Store, the
Harvard Division of Science
and the Cabot Science Library, all here
in Cambridge, Massachusetts,
please have a good night, please keep
reading, and be well!
-well fare city - it was fun.
