Bruce Partridge: What Happened After the Big Bang?
Bruce Partridge, Emeritus professor of astronomy in the science
department at Haverford College is a true pioneer and hero in the study of the
cosmic microwave background. He’s one of the OGs.
But it sure helped establish the cosmic nature of the radiation
that And Williamson found.
He was involved in the 1st measurements of the CMB spectrum to
confirm its true cosmic origin. That result decimated the steady state theory.
He was also one of the 1st scientists to look for the small scale temperature
fluctuations which provided us with detailed insights into the distribution of
matter in the early universe. He’s made major contributions in both theory and
experiment, helping us understand the cosmos, turning cosmology into a
precision science. Join us for an exciting episode as we explore the early
Welcome everybody to another exciting episode of the Into the
Impossible podcast featuring a friend, a colleague, a collaborator, And most
importantly, a mentor in the space of education of my field, cosmologists,
generations of them, And that’s Bruce Partridge, who’s an emeritus professor at
Haverford College in Pennsylvania. How are
you today, Bruce? Doing well. They’re here to talk to you. Yes.
It’s about Maybe even at you.
Yes. The Internet is quite is Quite amazing. It allows us to do
these things. And I am talking to Bruce because of many things. First of all,
he’s an incredible scientist and amazing, knowledge about the field, its its
past, present, and maybe even its future, having been involved with some of the
greatest experiments of all time, including, Perhaps, you know, one of the 1st
or second experiments to really go after the detection of the CMB and its
properties. And Bruce was involved with, with my grand advisor. So my grand
advisor was David Wilkinson, and he advised Peter Timby. And I’m Philip Peter
Timbe is soon to be hopefully collaborating closely with us on the Simons
projects as well.
So as I talked to you earlier in the week, We always love to do
a segment on this podcast that represents something you’re not allowed to do,
you’re not supposed to do, which is to judge a book by its cover. And you have
2 wonderful books, one of which I read 30 years ago, which is called three k.
So I’ve always been eager to ask you how you came up with the title,
and the cover design because it depicts the Horsehead Nebula,
which to my knowledge has nothing to do with the 3 Kelvin background, but maybe
it does. So Bruce, the clever title three k was mine. I figured a nice
abbreviation. The damn cover was designed by Cambridge University Press. It’s
part of a series, And they all show the Horsehead Nebula, which you’re right,
has nothing to do with the micro background at all.
I was thinking we could talk about the nebulae just for a minute
in that It’s often said that, I think it was McKellar, had detected properties
of cyanide, in the interstellar medium, and that Supposedly, that was, you
know, revelatory of a 3 Kelvin background. What do you make of that? Did you
know about those measurements? What do you think about those measurements in
the early days 1940?
The situation is following. These little cyanogen molecules, c
n, that float around in space, But they appear to be excited as though they
were bathed in a roughly 3 Kelvin field of radiation. They’re not at zero
temperature, they’re 3 kelvins, 3 degrees above absolute zero, or roughly 5
degrees Fahrenheit above absolute zero. This is written down Back in the
thirties and forties, and it was described by the discoverer as a being of some
interest. But George Field, among other peoples, remembered reading that paper.
And then when Enzys and Wilson found the 3 Kelvin radiation, he he recognized
that that 3 k Three Kelvin radiation might be responsible for the excitation of
cyanogen, and that gave us a measurement at a particular wavelength of 2.6
millimeters. Wasn’t very precise, but it sure helped establish the cosmic
nature of the radiation that Enzius and Wilson found.
That’s right. Now I look at a lot of your research and you have,
you have an h index. I think it’s the, you know, the cube of mine or something
like that or Number of papers and citations that number close to a100000, which
is which is just astounding. And I I’ve gone through many of them because
they’re all treasures and little Diamonds, and they’re not so rough. Many of
them are incredibly readable. But I want to ask you about when I think of the
Bruce Partridge brand, I think very high quality theory, but always attached
and never divorced from experiments from the very beginning. Can you talk about
your philosophy as a scientist? To couple together theory and experiment is
very hard to do, But you managed to do it. Can you give us tips to mortals like
me? How did you manage to cultivate that? Is that, like, intention by
intentionality? Experiments like Planck, ACT, Now the Simons Observatory.
What is that philosophy that says guided?
In in terms of the theory side, I was interested in a field that
was interesting but fairly simple. And if you go back to the sixties, cosmology
was simple. We we didn’t know anything, so it was a very simple subject to get
into. And that extended also to my abilities and interest in in the
experimental side. When I showed up at Princeton as a postdoc, there were 2
experiments Going on, one was the most famous one, namely looking at the
microwave background and trying to establish that it was cosmic, And the other
was measuring the shape of the Sun, because Bob Dicke had a theory that would
call on the Sun to be somewhat oblate, Switched in its in its properties. So I
went down to look at the solar abladeness experiment, a whole room full of
electronics, too complicated. I went to look at the microwave background. There
was a horn, went into waveguide, I knew about waveguides, went into the
detector, I knew about detectors, So I signed up for that.
So it was this sort of search for simplicity and stuff that I
thought I could do.
And those early experiments, when when Reputed. What what year
did you arrive at at Princeton
as a as a 65.
It’s right around the time of the
3 months after the paper was published that that Established the
microwave background. So it would it was early days.
Now I’ve read that paper, you know, many times, not not and And
the companion pay I always call the Penzies and Wilson paper the companion
paper because the companion the Penzies and Wilson paper is only I think it’s
less than a full page in the AppJ. It’s very short,
I mean they were being very careful. They said, yeah, we found
this signal. It’s as though everywhere we look, We’re looking at a surface of 3
degrees above absolute zero, not 0, but 3 degrees above, and they didn’t
interpret it. The Crucial moment, as you just pointed out, was the
interpretation that this is the heat cool down heat left over from the Big
Bang, and that was in the to keep people rolling in Wilkinson paper that you
probably read a million times as I have.
That one is incredible. And in, in many times when I read it, I
Point out it it doesn’t nowhere do the words Big Bang appear, but instead the
the collapse from a previous epoch appears almost as if they Kind of thought
that it might be more likely that there there was, obviously, formation of the
nuclei, but they didn’t necessarily believe that it was the origin of time or
Perhaps something like that. Take us back to those, to that year, that magical
year in cosmology. We’re coming up on the, what, 60th anniversary. I can’t
It’s Close to. Yeah. And
Yeah. So tell me, what
was that roughly speaking.
What was going on the zeitgeist, the spirit of the times back
So going back to the sixties, there were basically 2 competing
theories. Other speakers in this series may have mentioned that, but one was
steady state, In which the universe was always the same, everywhere the same,
and always the same, and in order to keep the density the same, Electrons and
protons up out of nothing, and competing with that was the Big Bang Theory, the
universe had a finite beginning, it just started and the suggestion was, most
proponents to Big Bang Theory accepted this, There would also be a hot big
bang. So those 2 things were in the air. And Dicke and his colleagues were, in
a sense, trying to Push them together a little bit by imagining an infinite
universe in time that simply cycled. It expanded, then it contracted, then it
expanded, then it contracted. And that universe had to be hot for the following
reason. In any one of these phases of the universe, stars make heavy elements.
And after many cycles, you’d have nothing left but heavy elements.
The universe would consist of nothing but iron and nickel, and
it doesn’t. So to get rid of the heavy elements, you have to have a hot big
bang, which boils them away, turns them back into their constituent neutrons
and protons. So there’s a built in, you had to have heat in this model, and
what’s interesting was that the Brinson guys are actually setting out to find
this. They had Built a piece of equipment specifically designed to look for
heat left over from the Big Bang when the Faithful telephone call came from
these 2 guys. Excuse me. I’m just gonna be informal, hold up a picture here.
Of course. You know? There they are.
These are the 2 guys, Penzias and Wilson, and behind them the
Horn radio telescope that 1st noticeably detected the radiation, so they got in
touch with the Princeton group and the story repeated in your book, Brian, Is
that Dickie was meeting with his young colleagues, this is before just before I
got there, put his hand over the phone and said, well boys, we’ve been scooped.
Engeus and Wilson had found a signal that looked like It might be heat left
over from the Big Bang as Dicke and company were predicting. Important is that
the Princeton experiment was specifically designed to look for this heat, so
very quickly within a year it had produced better results than Penzias and
Wilson And consistent with the original discovery. But again, Brian, as you point
out in your book the Nobel Prize went to Penzias and Wilson And not the Dickie,
you know, Wilson Wilkinson and Peebles. Well, Peebles got his lair. But
Yeah. That’s right. Recently. And, Peebles, of course, is the
coauthor on one of your early papers. We’ll talk about that in a minute. But
I’ve always found the saddest story, the saddest person in the whole affair was
this guy Ed Ohm. Oh, I I don’t know much about him. But, but other than that,
he used the very same Porn antenna at Holmdel, which is a national historic
landmark, and also, did several of the same types of experiments that use
similar types of radiometers.
But the one thing he was missing was a was an internal
calibrator that could check at rapid rates to get rid of the one over f signal.
And, of course, we know that by the name of your adviser, as a Dickey switch.
And so they employed that. So the Dickey switch really is the thing that but Ed
Ohm is not blameless in this whole affair Because if you go over his technical
report in the Bell Labs telephone systems journal, which I read many times many
years ago, He does a thorough error analysis. And then there’s a 3 Kelvin term,
and he accounts that to the to the atmosphere or to the antenna temperature.
It’s really not Clear to me. I think Novikov and others later point also saw
that term even before Penzies and Wilson. So they’re kind of A lesson that I
teach to my undergraduates that error analysis seems annoying,
but it’s very important you get it right. And that you account
for all those errors. And that’s that’s one of the things that Penzias and
Wilson did very very well. They saw the signal, they didn’t say we’ve seen a
signal, I’m gonna publish it in the Astrophysical Journal. They busted their
butts trying to show that it was not radio stations from New York or pigeon
poop in that horn, Which would also radiate and cause a signal. They they were
very careful. Ohm, I think, didn’t make those same steps, And the Russians who
read Ohm’s paper, unlike the Princeton group, made the mistake of assuming That
what Ohlm was seeing was the atmosphere. Where it wasn’t, it was in addition to
So 19 thirties, Cyanogens, photogram in space, kept warm by
something. Nobody paid any attention. Ohm, Jasper Wall Did a radio astronomy
experiment in which looking at the spectrum of emission from our galaxy seemed
to show that there was a sort of additional term that he couldn’t explain equal
to 5 Kelvin, so there were hints. And I I like to Make the point that what
changed from hints and so on to a prediction was that paper by by the Princeton
group. They went out on a limb and said, this stuff is cosmic in origin. And
then the next couple of years, Dave and I and other people worked to try to pin
that down by doing 2 tests, if I may go on for a moment. 1st was the spectrum,
that is, if there’s heat left over from the Big Bang, you should get the same temperature,
3 Kelvin or whatever, At every single wavelength you measure, and we set up a
series of experiments to prove that you did, and incidentally got a better
value for the Temperature is now 2.7 is closer. And the other is that if this
stuff is left over from the beginning of the universe, The beginning of the
universe is everywhere around you.
In any direction which you look out, you’re seeing back in time.
So this radiation, if it’s cosmic in origin, ought to be isotropic. That is the
same in all directions. Pindes and Wilson had checked to see if it wasn’t
coming from New York City, or Philadelphia, or the center of our galaxy, But
the limits they set on it, isotropy, whether it’s the same in all directions,
were pretty poor, And Dave and I recognize it with fairly simple equipment. We
can improve it by a couple of orders of magnitude, and we did.
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hit that subscribe button. Now back to my chat with Bruce.
And One piece of advice I’d love to get, we have a lot of young
listeners, PhD students, even graduate students. Working in the field of CMV b
mode polarization for, you know, 25 years almost by now. And we have yet to see
we we well, we saw signal in 2014, but, of course, we had to recamp that signal
now or the interpretation of it. Still accurately measured, highly accurate,
And dominated by astrophysical, not man made or earth bound systematic. So it’s
an incredible accomplishment. Still is the most sensitive measurement. How did
you go through the years? So this is partially a question from one of your
former students by the name of professor Stefan Alexander. And I asked him for
a question for you today.
And he asked me, essentially, how did you have the the sort of
courage or or how did you have the patience To do the 1st sorts of anisotropy
measurements as you did to see if it was isotropic and not really see fruition
until David Wilkinson and and George Smoot and and others, measured the
anisotropy convincingly. I I would say the spectrum was Known by you to be very
close and others, to black body or or was very easy. But the anisotropy was
completely in upper limits after upper limits for almost 30 years. How did you
have the courage and patience to deal with that? Give me advice to kind of,
keep patient. Because it’s been longer since the between the detection of the
CMB and its first anisotropy than it has been from its 1st anisotropy to
measuring b mode polarization. How can I have patience or what can you advise
my students and I To do in terms of coping with decade after decade, perhaps of
upper limits? When do we give up?
Don’t. This is this is the one word advice. But remember, back
in the sixties and even into the seventies, the idea was not to find
anisotropies because, Frankly, we weren’t listening to the theorists and didn’t
really understand how rich the field of anisotropies could be. Instead it was a
different aim and that is to show that the radiation was in fact isotropic.
It’s easy to imagine, Let’s say starlight being thermalized by dust and
emitting at the 3 Kelvin level. It’s easy to imagine, indeed, it was suggested
by many people, but that would tend to be brighter towards the plane of the
galaxy than perpendicular to the plane of the galaxy. So our aim was not
initially to find anisotropies, but not to find them, to set better and better
upper limits. And If I can hold up another thing here, here’s a plot.
This is, again, this is rather informal, But a plot of
measurements of temperature, and you notice the scale over here is in 1,000 of
a Kelvin. Yeah, and this was a very crude experiment with lousy dicky
switching, which we Set up first on a tall building of the Princeton University
campus and then realizing that New Jersey weather is not ideal, we took it out
to Arizona And ran it it was run remotely for a couple of years. The Arizona
location involves some interesting stories. Day Wilkinson discovered that Yuma,
Arizona is the sunniest place in the United States. I discovered through my
Military thought that there was an army base there. That’s right. Proving
grounds, the Yuma proving ground and further research Showed that there was a
very secure area where we could put this piece of equipment about the size of a
small hut out in the desert and not have it bothered, And it was secure because
the army was busy testing nerve gas shells. The idea was you build Big wooden
racks, put a bunch of nerve gas shells, and then wait to see if they leak.
Needless to say, that area was fenced and patrolled.
So I I shouldn’t feel bad about sending my graduate students,
you know, to Chile for 3 weeks where there are active minefields.
We were issued gas masks and we were told that there were
certain physiological signs and if you notice those, it was probably too late
to put on the gas mask. So instead the monitoring system was rabbits. I
promised you to talk about the rabbits. Yes. So the rabbits were Stationed in
hutches around these nerve gas shells and the idea was that it started the
nerve gas started to leak the rabbits would die. Well, about halfway through
our time there, the rabbit started to die. So there was a big fuss, all kinds
of tests, we weren’t allowed in for a while, got quite complicated. And it
eventually turned out that the nerve gas wasn’t leaking, but the army in its
wisdom had bought 3 dozen rabbits all the same age.
So we happen to to have been there at the time when the rabbits
reached three score in 10 years and we’re beginning to cry Of natural language.
Well, what we were able to show is there’s some scatter back and forth, But
there are no excursions that are bigger than about a third of a percent of the
microwave background, so no evidence That the radiation was coming from a
particular place, strong support for the cosmic interpretation By 1968 or so.
And the original paper, Nupendzius and Wilson and subsequent
ones by by you guys at at Princeton, the topic of polarization was broached,
Actually, as early as, 1960 well, in ‘sixty 5, Penzies and Wilson set a limit
of 10% or unpolarized, A little below 10%. And then in 1967, I believe it was,
Martin Rees, Lord Martin Rees now is a Three time guest on this podcast. He
came out with a paper, that suggested the CMB could be polarized. And in fact,
it could be highly polarized. And that was based in the model that the, that
the Big Bang or the Hubble expansion could be anisotropic. It could have an,
you know, An anisotropy to it that would generate a quadripole moment in the
photons. So it’d be a huge quadripole moment in the CMB’s anisotropy, And that
would generate a large amount of polarization. And I like to point out when I
talk to him that, you know, he was right, but for the wrong reasons.
But that that actually has a lot in common with a lot of what
science is about. For example, Galileo believe that the Earth orbited around
the sun. We know that’s true. But he used the title pattern of the Earth’s
Tides as a piece of evidence for sloshing and swirling of tides, and it has
nothing to do with that. So can you comment on, You know how sometimes
incorrect theories can generate useful scientific tests and sometimes
detections. And, obviously, I’m gonna pivot to inflation and dark matter and string
theory in just a bit, but talk about how how much should an experimentalist
listen to a theorist?
My first answer would be, in this field at least, not enough,
and I’ll come back to that. But first, let me talk about the polarization
business. The detector that Dave Wilkinson I used was intrinsically polarized,
so had there been a huge signal in polarization, we would have seen it. In
addition, In Rees’ paper, in order to make a big polarized signal, not not a
tiny one, you have to have an anisotropic universe. And that would have
resulted in a sort of dipole potato shape, sausage shaped temperature distribution
in the sky, which we didn’t see. The polarization that we do see now is from a
much The subtler effect. There is a small quadruple moment, which we do expect,
and that produces the roughly tenths of a percent type polarization that we
detect in the microarray background. I said earlier that we should have
listened to the theorists a little bit more Because so many of the early people
in this field came out of Bob Dicke’s group, and Bob Dicke was an absolute
master at null experiments, Showing that such and such was smaller than certain
The time rate of change of the gravitational constant was below
1 part 10 to 12 and so on and so forth. A lot of our experiments were designed
to be null experiments, and we weren’t particularly interested in Finding
something and we didn’t particularly pay attention to the theorists who were
telling us if you’re looking for Small variations in the temperature, you ought
to be looking on scales of the order of a degree or below. And that’s reflected
in the design of the COBE satellite, which you know about, the experiment that
won George Smoot his Nobel Prize, Was consciously designed to look for small
changes in temperature, different parts of the sky, but at the wrong scale. A
10 degree scale instead of 1 degree scale. We could have done much better, might
even have found the anisotropies Earlier had the experiment been designed
better and had George and the rest of us listen to the theorists. We were busy
Trying to get lower and lower limits.
Yes. It’s natural, to do that. I guess the question, you know,
that comes up all the time with me, and and there seems to be, Obviously, there
is groupthink in any organization of individuals just because they have,
incentives to Maybe, you know, combine or be be related to those in their field
that are setting the trends and so forth. So nowadays, you see a lot of more
string theorists than people looking at things like, loop quantum gravity or
some other alternative that could be plausible. So too, I worry that There are
an awful lot of people invested in inflation. And I wonder, are there parallels
that you see as an observer both then and now Between the dominance of a
theoretical paradigm and the, cultural pressure for young people to go into
that field, Either as experimentalist in my case or as theorist in in in
I don’t buy the argument about the social pressure. There’s a
sort of standard view, yeah, inflation is important and of course it is
consistent with a lot of things we’re finding, including isotropy. But When I
read of a a new experimental result, my very first reaction is, how can I show
this is wrong? Can I do a simple experiment to show that it’s wrong? I’ll give
you an example. I will send a paper to review, which claimed that because of
some complicated theory, There had to be a strong anisotropy in the gamma ray
background. So I did the calculation and discovered That if I simply held up a
plateful of raw eggs, the Gamma rays that this guy was predicting would be
present, would cook it instantly. I think a lot of us have that sort of
skeptical, how can I show this is wrong attitude And are not terribly hide
bound by the sort of prevailing orthodoxy, let’s call it?
And you look at your advisor, Dicke, I don’t think he gets
enough credit for comp, you know, the contributions. First of all, he was an
exemplar of of what I think of as the paradigm of a physicist, like, Fermi or
Galileo in that he knew the theory and he could do experiments too. And that’s
extremely rare. We’ll talk about later your philosophy on pedagogy. But one of
the things that Dicke contributed to the theory of inflation was, I think he
pointed out, and correct me if I’m wrong, Bruce, To a young Alan Guth that
there was this, you know, kind of cosmic coincidence of the flatness of the
universe, not Knowing that it was exactly flat but knowing knowing it was
within an order of magnitude of being flat, can you talk about that, that kind
of, those notions and why it wouldn’t it be inflation that I would first think
to sort of want to Explain. It would be probably the missing, you know, Baryons
or missing matter. What what made that stand out so much to to your adviser, to
Bob Dicke, if you can speculate? Obviously, he’s not here anymore.
I will have to speculate, but it’s the following: take 2 parts
of the universe That are very far apart. Suppose one part of the universe wants
to tell another part of the universe what to do. For instance, point A wants to
tell point B what temperature to be. That information can’t travel faster than
the speed of light. So on very large scales, there’s no way that one part of
the universe knew to be 3 Kelvin and another part of the universe knew to be 3
Kelvin Unless it was an initial condition, and you don’t like initial
conditions. You don’t like to have to say, well, the universe started out in
just such a way That is now 3 degrees Kelvin. So how do you explain that? And
that worried Dickey. He was worried about that.
He was worried about Mach’s principle and some other things that
have to do with these large scale properties and puzzles in the universe. And
what Alan Guth and company did was basically to explain it and that is to say
that at some earlier time the universe was so small That regions that are now
too far apart to talk to each other were perfectly happy in confabulation early
on, And then the universe expanded exponentially, which we call inflation. So
the fact that the, Back to this picture which I keep showing because Yeah. This
this picture shows you that the universe is pretty much the same in all
directions. Well, how did it know On a large scale, pretty the same in all
directions. It had to be in contact, causal contact, to use a technical word,
At some early time, and that’s that’s what’s behind inflation is to get that
Going back to the same year in 1967, I pulled up A paper which
is still getting citations, from a young Bruce Partridge and, young, Jim
Peebles. And it’s called Are Young Galaxies Visible. And you talk about the
purpose of this paper is to assess the general population Possibility of
observing distant newly formed galaxies. To this end, a simple model of galaxy
formation is introduced. And you talk about star forming and their luminosity.
And then you say they’re they these bright phases would correspond to an epoch
of a few, tens or hundreds of millions of years corresponding to a redshift
between 10/30. I wanna talk about recent, so called claims or discoveries or
controversies, as our British friends might say, regarding The seeming
observation of very mature galaxies at very high redshift, much higher than
ever anticipated. And For this, I I am old enough that I can actually remember
the controversies that similarly seem to erupt After the Hubble Deep Field was
released in 95 or so.
That was in the middle of my graduate student career, beginning
of it. So I ask you, what do you make of these of these findings and
controversies? I mean, are they just not reading your paper? This paper that
you wrote, in other words, these scientists are saying that the universe must
be much older than we previously thought, possibly even eternal, because the
galaxies that we see are 2 Highly organized grand designs and spirals and so
forth. You showed almost again 60 years ago, I said 54 years ago, that this was
possible. What’s going on here?
Okay. Well, this goes back to the previous discussion where we
talk about a sort of orthodoxy. The orthodoxy these days on galaxy formation,
it’s probably correct but let’s question it, is that galaxies are built up by
mergers of little things. So you start with a bunch of little things, they
merge together, that blob merges with another blob, and pretty soon You’ve got
a galaxy. The problem is that you’ve got to do that really, really fast to
explain mature galaxies very early in the history of the universe. The approach
that Peebles and I took was very different, and that is a blob of gas of
galactic mass Collapsed in on itself and started to form stars. So you went
from nothing or relatively smoothly distributed gas to a galaxy, not by merger.
Maybe there was some something in that after all.
Maybe that’s how these mature systems do form. It’s always
interested me that the way you find that a galaxy Is it large redshift and is
star forming is to look for a sharp discontinuity in the spectrum introduced by
the Lyman Lyman Bray, Which is in that paper. What may be going on is that
galaxies form in Special places, more like the Partridge Peebles model, but not
universally, and that mergers are responsible for most galaxies, but not the
Ones that the Webb is finding or that these galaxies look bigger and more
massive than they actually are, And that is a possibility, but it is sort of
fun to argue with the with the conventional orthodoxy Because if you start with
things that are, let’s say, a 1000000 times the mass of the sun, and you merge
2 of them, you get something that’s 2,000,000 times the mass of the sun. And
then you have to merge again and get something that’s 4,000,000 times the mass
of the sun, and then 8,000,000, and how the hell do you get something that is
Pushing a 1000000000 times the mass of the sun in the time that’s allowed.
You’re, More on the iconoclastic or maverick side of things,
which is Yeah.
I enjoy it.
Yeah. Which is which is fun to do, but it’s it’s tough to First
of all, you have to have the prerequisites to get to that level. And I feel
like these guys that are criticizing or saying the Big Bang never happened,
Like, Eric Lerner and even, Rajendesh Gupta at University of Ottawa, you know,
published the universe is 26,000,000,000 years old. They all sort of rely on
on, you know, kind of a series of just so stories, but they can always point to
flaws in the in the Big Bang And especially in the early universe cosmology
because every model will have its lacunae. And I think obviously, you know, the
Big Bang does too. I would say of all the ones that are pointed out by Lerner
and Gupta and others, the one that still is sort of in question that I’d like
to get your take on. Although, you know, I don’t think this has been a field of
study for you. But it’s the lithium abundance problem that there seems to be
The largest gap between predicted abundances in the BBN, you know, kind of
taxonomy and observed, the biggest one extent is in lithium.
Can you talk about that? Is that something we we should be
concerned about? Or is this just messy nuclear physics that The guys that
operate, you know, giant Van de Graaffs will someday figure out. What what do
you make of the lithium problem as this proves the Big Bang never happened?
I think it’s a very weak read in which on which to claim that
you’ve disproven the Big Bang. Just to set some context, the early universe
starts out it’s it’s Hydrogen, neutrons, a little bit of helium, and a tiny
tiny amount of lithium is produced as a sort of byproduct. But lithium is a
fragile nucleus. It can be made in cosmic ray interactions, so it’s not all
that Convincing as a as a proof or disprove for the Big Bang. The deuterium
abundance on the other hand is very important And what’s interesting to me is
that nuclear physicists, guided by people like Jim Peebles and Bob Waggoner,
We’re predicting how much deuterium and how much helium should emerge from the
Big Bang, and we discover exactly that amount, And the amount of deuterium is
consistent with a very small amount of ordinary matter in the universe, Which
the microwave background also emphasizes. So lithium is sort of, to me, a side
issue. It can be made, it can be destroyed in stars. So it’s it’s a little
again, just a week read on which to undermine so much other observational
On your website, which we’ll link to in the video description
below, you, have a nice discussion of, you know, the research interests that
you have had and maintained. And it concludes with one statement that careful
measurements of the CMB fluctuations, both from space and the ground, Have
turned cosmology into a precision science. I had, Mike Turner on, who I know
you know. And he said, not about you, but he’s he’s famous for saying, you
know, precision cosmology is nice but accurate cosmology is better. I wonder if
you can, talk about, are there things that are still outstanding, maybe not
mysteries, but There’s further research in areas of the CMB that might be
considered to have been a closed book. And I’m thinking about distortions. I’m
talking about Compton Y. I’m talking about, chemical potentials.
Are there still things Of interest that a young student might be
able to contribute to in that particular field?
Yes. I think so. Both in terms of spectral distortions, which is
what you’re mentioning. But to me much much more interesting in a sense is is
pushing harder and harder and harder on the Visible anisotropies, if I can just
scoot aside for a while. Here’s here’s a map of the sky, and these Regions of
red and blue are slightly higher and slightly cooler regions of the microwave
background. Studying those has proven to be an absolute bonanza in terms of
refining cosmological theories. As an instance, if you look at that picture
carefully, you’ll see that there’s quite a lot of structure at roughly the 1
degree scale. Yeah.
Less at 2 degrees and less at half a degree. Well, why should
that be? Well, it turns out that that is exactly what you expect From the size
of the universe at the time we’re mapping it, provided the universe has a flat
spatial curvature, And not otherwise. So simply finding where the peak is in
terms of the distribution of fluctuations tells you about the curvature of the
universe. You already mentioned the b modes, we’re looking for those. At the
small scale end, the way structure forms in the universe can distort The
microwave background fluctuations a little bit, not much. Mhmm. But because the
measurements are now so good, we can put constraints on things like the speed
at which gravity pulls matter together to make galaxies and so on. And there
are, Fortunately for young people in the field, still some some interesting
You’ve probably talked about this before, but if you ask how
fast the universe is expanding, If you’d asked that question in 1960, you’d get
2 answers from 2 warring groups, one claiming it was expanding twice as fast as
the other. It got to the point where if you ever had a meeting dealing with
cosmology, you had to invite 1 person from each group or they’d get PO’d. Now
the debate is between the supernova guys We’re claiming, a number that’s about
10% different from the number that Brian and the CMB Folks are claiming, I
don’t think we’re at the point where you have to have 1 from each school at
each cosmology, but there’s a real tension there. The difference between 67 in
the standard units and 73 in the standard units is 3 or 4 times the error. So
something isn’t right somewhere And that needs to be sorted out. And the rate
of growth of structure, how fast gravity pulls things together, is also
somewhat in dispute.
Or Mark to Brian, this is the s eight, Sigma eight, Tommy
Briscoe. So there’s still work to be done.
Yeah. And I see that as, yeah, one of many tensions. And I I
just had the opportunity to to, visit my alma mater, which Case Western last
week, and I met with a friend of mine who wasn’t a professor until a couple
years after I graduated, and that was Glenn Starkman. And, he is is making a
very convincing case that based on that image behind you that you helped to
make, I believe that’s from Planck. And
you were WMAP, I was saying. But Oh, that’s WMAP. Okay. Well,
Plank did a very, very upgraded and beautiful Entry and also in
in strong agreement with the WMAP map. So there’s Planck. Yeah.
I’m I’m partial to WMAP because it has my grand advisor’s name
as the first and I’m sure your friend David Wilkinson is, nice to be located in
L two. I pointed out he’ll probably be Orbiting the universe, forever, his
namesake instrument. But Glenn has pointed out that if you take that image
behind you And you put it that’s in galactic coordinates, I assume. But if you
kinda rotate it 45 degrees, it turns out it’s in ecliptic coordinates. And he
says that if you take, and you make a power spectrum of, or correlation
function, not a power spectrum, but of the data in the northern ecliptic
hemisphere And you compare it to the Southern ecliptic hemisphere. And this was
pointed out by our good friend, David Spergel, back in 2003 From those data
behind you, so I know that for a fact, that there’s an asymmetry. There’s a big
asymmetry between the statistical properties of the north and south hemisphere.
Barely visible here, but this was looks a little redder than up
That’s exactly right. And there are things like the cold spot.
Interestingly enough, the axis of evil, so called, we’ll talk about that in
just a second. But the cold spot is in the Southern hem ecliptic hemisphere,
and that seems to agree with what you’d get from a Gaussian random simulation
based on Lambda CDM. But the North isn’t. The North, you only get in if you
account for everything, I think with the latest Planck data, Glenn, Glenn
student, Joanne was showing me this last week. There’s only a 0.02% chance that
arises at random. If you’ve been around this
a long time. What do
you make of these kind of, these asymmetries? Are we trying to
demand Too much of this big bang bonanza that is the CMB. Is it really fair to
act to ask for it to be Accurate and precise at the 2 hundredth of a percent
That may be pushing it. And again as you well know the sort of
standard explanation for the hot spots and the hemispheric asymmetry It
basically goes back to playing poker. Any given hand that you’re dealt is
highly unlikely. I mean, not just the Royal Straight flush, but You know 2 of
spades, 3 of clubs, blah blah blah. Any particular thing is unlikely, but it
has to be something. Yeah. And so some of that 0.02% is covered by cosmic
variance, just the universe happens to be that way. So it doesn’t keep me up at
Oh, I would love to see someone actually Improve the
measurements or figure out how to beat cosmic variants to see if there’s
something that’s statistically odd about the microwave background At large
scales, again, I’m a kind of classic, but not losing sleep over that. Unlike
losing sleep over The rate of expansion of the universe and the growth of
structure, those worry me.
Are there other explanations that you prefer For, say,
explaining the Hubble tension, I’m quite partial and I wanna explain my
reasoning because I’d love to get your opinion as one of the heroes and and
legends in the field. But My my philosophy of experiments and even theories is
that you should always do very extremely risky science On the one hand with one
of your hands, but the other hand, you should do something that’s known to
exist and known to be there such that the whole thing is on empty, pursuit. I
always laughed When my colleagues in high energy physics down the hall would
say the most exciting thing we could discover with the Large Hadron Collider is
nothing. And I would say, yes. And then you will discover unemployment. But, so
it was a big gamble, but it was also safe on the other hand that We sort of had
a good idea. I I like to do searching for the the cosmic microwave background’s
b modes because, they may be there. It’s Extremely high risk science.
They may not be there even if inflation took place. And then on
the other hand, I like to do very low risk stuff. And one of the Topics that
you’ve worked a lot on when you were, with the ACT team folks and you still are
a contributing member of many other papers, Is look for the mass of neutrinos.
I wanna ask you I’m sorry to keep saying I I was gonna say as a legend, I’m
gonna keep stop saying that. I’m just gonna say as someone who’s been in the
field for a long time
I’m not sure
yet. As an observer of this field for a very long time, let’s
say we are successful, we meaning us on the Simons Observatory, and we measure
The mass of cosmic neutrinos. For the first time ever, we have lower limit. We
have an upper limit. We don’t have a detection. Will our colleagues in high
energy physics Department circles, will they believe us, Bruce, based on your
knowledge of history and thought of the philosophy of this field?
I think, frankly, it’s Touch and go unless it’s a really clear
measurement. Going back in the history one of the things that you can use both
Cosmic nucleus synthesis, say, the formation of helium and deuterium in the
early universe, to tell you is the number of neutrino species, And it was not
clear whether that was 3 or 4 or 5 back, let’s say, in in early seventies. Dave
Schramm and others Interpreted the astrophysical data that data and said no
it’s got to be 3. Yeah, wasn’t clear that anybody believed that. We now know
that it’s 3.05 roughly speaking, so it’s not 4, it’s not 5. A little bit later,
I use the astrophysical data, just pure CMB and some other stuff, to point out
that the the lab measurement Of the lifetime of the free neutron was not right.
Didn’t didn’t agree with the astrophysical data. To say that was a Stone that
sank without a ripple would be an understatement, but later the lab experiment
showed that I was right and so on.
So It it takes a lot of effort. If we publish, let’s say, a 3
sigma measurement that the neutrino mass is 0.057 plus or minus 0.02. Will
people get up in arms about it and Try to improve the lab experiments to to
justify this or to confirm it, I I don’t know. If people like Mike Turner who
worked, And many others who work at the interface between cosmology and
particle physics, so I think would take it seriously. But Whether your
colleagues down the hall who are big accelerator guys will, I don’t know. I hope
we do the experiment, that is I hope we Come up with this number and publish it
and say, there it is, guys.
Bruce, before I have to go and teach myself, I wanna ask you
about pedagogy. You are one of the best educators, not just at cosmology, but
but you’re you’re dedicated your life, at a predominantly undergraduate no, at
an undergraduate serving institution that’s had a host of eminent scientists
come out of it, out of your classes. And you have sort of revolutionized and
enhanced the teaching of cosmology at the undergraduate level. I I wanna ask
you about your theory of pedagogy and in the following, sense. I teach a lot of
undergraduates, and I teach graduate students experimentalists. I’ve had a few
theoretical, you know, graduate graduate students who are only doing theory,
not experiment. What is The theoretical minimum, or shall I say, the
experimental minimum. What should an experiment, a theoretical graduate student
know about experimental physics or experiment let’s just stick to cosmology.
What should he or she know? She’s starting off she’s really
excited to come up with an explanation for the Hemispherical asymmetry, the,
some external neutrino signature in the CMB, a large extra dimension. But what
should she know about experimental astrophysics before she goes and does
everything she’s going to do in theory?
I would say that the main lesson to take away, and I harp on
this when I talk to to theorists, is just how damn difficult these experiments
are. They’re not easy. You have a big piece of equipment the size of your lab
or bigger, sitting at a temperature of 300 Kelvin, And you’re trying to make
measurements of the order of 10 microkelvin with that piece of equipment. It’s
not easy guys. Given that, Theorists pay real attention to the issue of
systematics and instrumental effects. Don’t leave that to the to the
instrumentalists. Think about it yourself, if I see a signal, suppose I
suddenly discover a hemispheric asymmetry. Are you really sure that doesn’t
have to do with the way your instrument is designed and the fact that it
happens to be in the southern hemisphere and not the northern? It may be that
the answer is okay, but think about it.
That’s what I would tell theorists.
We always end with a comment from the guest On a quote from sir
Arthur c. Clark, who was the namesake of the foundation that endowed the center
that I’m, affiliated with here in San Diego, the Arthur c. Clark Center for
Human Imagination. And Arthur, good old Arthur, I don’t know if you ever knew
him or met him.
I didn’t. I’ve read his books.
Yeah. So he had, many, many quips and sayings, one of which is
the only way to know the limits of the possible is to go beyond them into the
impossible. And that’s Origin of the name of this podcast. Another one that I
like to use on my colleagues who think too highly of themselves is he used to
say, for every expert, There’s an equal and opposite expert. But the question I
want you to comment on, is the following. He said, when a distinguished older
scientist So something is possible. He or she is very likely to be right. But
when he or she says something is impossible, They are very much likely to be
But I wanna ask you not if you, you know, if you agree, but
rather What have you changed your mind on in this field of cosmology? As our
final question, have you been wrong or have you changed your mind about
something that you held very firmly In your youth and and yet change came to
The business of galaxy formation. We had Jim and I had a
particular Model in mind and most people don’t believe that so I’ve been sort
of forced to change my mind, although I haven’t really given up on it. The
other is dark matter. When when people started talking about dark matter it
seemed to be absolutely and totally ridiculous. Stuff that we know nothing
about somehow is important in the universe, and I simply refuse To believe that
despite all the good work of Vera Rubin and and people like that showing that
it probably there is something out there. And what finally got me to believe to
believe in it was a the bibulous conversation with Jerry Osterreich, drinking
beer on a boat on Lake Ontario. And he said, Bruce, goddamn it. If if you don’t
see Fluctuations at a level of at least 10 to the minus 3 in the microwave
background, it’s all over.
Well, I’d already done an experiment Throwing the upper limit
was less than that. So what was going on? Well, what’s going on is that the
fluctuations in the microwave background They’re produced by the baryons and
the gravity is mediated by the dark matter, so the dark matter can be happily
gravitating away And not make the fluctuations that Osterreich insisted we
should be seeing. So in a sense, my own work came up behind me and kicked me in
the butt. Maybe that’s a good way to say end this. Right?
There is. Well, I wanna thank you for many things, not the least
of which is Inspiring me as a as a young graduate student with that wonderful
book, three k, with that horrible cover. But, you made up for it with your book
with Jim Peebles and Lyman Page, past guest on the podcast, Finding the Big
Bang. We’ll put a link to those books in the show notes. And, Bruce, I just
wanna thank you, from the bottom of my heart for being an exemplar of what a
good cosmologist should be And being a mentor and through all your years of
service, which we also didn’t get to talk about, maybe we’ll do this in person
someday, part 2. But, Bruce, thank you so much And I hope you enjoy the rest of
Thank you. And let’s join in thanking the Simons Foundation.
Absolutely. Thank you, Bruce.
Thanks for watching this interview with Bruce. I hope you
enjoyed it. I had so many great takeaways and memories from this episode. I
know you will too. Now on my YouTube channel, I try to give you as much Each
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