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Language: en
I don't think there are any physicists
in the world who are satisfied with our
explanations of dark energy and dark
matter
>> which is we have no idea what they are
that's explanation I know
>> right right so that's why it puts you in
a realm where experiments even things
where you think you know the answer are
important to do
>> yeah [music]
[music]
Adam Reese welcome Welcome to Particles
of Thought.
>> Nice to be here.
>> Oh man, have I been waiting to talk to
you for years. You are a cosmologist.
You've have a Nobel Prize. You're our
first Nobel Prize winner here. Yeah. And
um man, you discovered dark energy.
You're a co-discoverer of one of the
biggest paradigm shifts in the history
of astrophysics, right? Since Hubble
discovering that the universe is
expanding, discovering that it is
accelerating completely unexpected. And
even crazier, I hear that a big part of
your work was done on your honeymoon.
So, how do you win a Nobel? How do you
do Nobel Prize winning work on your
honeymoon?
>> Well, you have to do a little work ahead
of time, it turns [laughter] out.
But in our case, you know, it was just a
multi-year process of a research
experiment and everything came to a head
right at that time. Um, and so, you
know, I left for my honeymoon, but I had
the results. I checked the results. Um,
I shared the results with colleagues,
they had checked the results, and it was
just that moment to have the
conversation, uh, which started over
email. And uh and because I was away at
the actual wedding, a bunch of my
colleagues were saying, "Well, this is
what it looks like. This is what I think
we're seeing." And I had done a lot of
the work. So, it was I came back from
the wedding packing my bags for the
honeymoon and uh checked the email and I
was like, "Whoa, I think I got to answer
some of this." So, I started responding
and uh got some icy stairs from my wife
who was like, "I don't get it. This is
our honeymoon. [laughter] Like, are you
is this the way our our life is going to
be that you're always working?" I was
like, "No, this is a really special
email." Like, [laughter] "I really got
to get back to these guys and and but
yeah."
>> Did you believe it?
>> Um, you know, I don't think you ever
believe something in the beginning. Um,
you know, in science, we all have the
experience of, I'll say, making so many
mistakes. I mean, you know, science is
really, really hard. You're always doing
something that, you know, hopefully,
ideally, you're getting to a point that
nobody's done before. And, uh, there's
just a million ways to do something
wrong. Um, and so you know, all your
research career you you've been like,
"That doesn't make sense. Oh, I got a
bug here. This doesn't make sense. Oh, a
negative sign there." I mean, how many
times just even writing a computer
program, it tells you this doesn't run,
you know? And so you're constantly
living that, right? And then so when you
actually if you actually ever do see
something that you can't find out why it
doesn't why it's wrong, but yet it seems
surprising, you know, it takes a long
time to build trust in that. Yeah, I bet
it I bet it does. So, dark energy could
be stuff in spaceime. It could be the
intrinsic energy density of spaceime,
but whatever it is is causing spaceime
on the largest scales to expand ever
more rapidly, like a repulse of gravity,
right? Is that correct?
>> That's correct.
>> All right, let's get into this
discovery, but I want to unpack it a
piece at a time. Sure. So you basically
use a special type of exploding star to
measure the size of the universe versus
time. So that type of star is a type 1A
supernova. Right. So tell us about that
process of you know what those stars are
and what leads to them exploding.
>> Sure. So uh in order to gauge and
measure the universe we need to be able
to measure as you say how far away
things are. And
>> so we have to look out in the universe
and see things that we can recognize.
Okay? Just like here on Earth, you know,
you cross the street, you see the
headlights of a car and you can gauge
how far away that car is by how bright
the headlights appear. That requires
that you actually understand that
headlights are pretty luminous things,
right? So we look out into space and we
don't have any of these humanmade
objects where you can go, "Oh, there's
a, you know, a Toyota Corolla with its
headlights and stuff." So instead, you
have all these lights and you're like,
"What are these things?" And so it took
astronomers really a century I would say
to get to the point where they
understood them well enough and in in
the case of what we're discussing there
are certain kinds of stars that explode
at the end of their life called a
supernova and uh it's important because
by being so luminous they're billions of
times the luminosity of the sun you
could see them very very far away. I got
I got to interject here. When you say it
explodes at the end of its life,
>> by definition [laughter]
>> that is the end. Not more is going to
happen. That's that's absolutely true.
But there are different ways that that
can happen. And so this is really the
essence of it is there are different
flavors of supernova just like there's
different flavors of stars and they're
not all the same. And so you would, you
know, make a a terrible misestimate of
distance if you confused. It's like
confusing, you know, a motorcycle
headlamp for, you know, a Mac truck or
something, you know, they're just very
different. So, um, in the 1930s, it's
not far back, um, Saman Chandra Seekar,
the famous Indian astrophysicist,
explained that a certain kind of star
could not exist, could not be stable,
couldn't hold itself up against gravity
if its mass exceeded the Chandra Seekar
limit, his limit, which is about 1.4
times the mass of the sun. And so a star
just below that will be holding itself
up by gravity but will be so close to
the conditions of fusion throughout the
entire star like a like a fusion bomb.
And so we think a friend and with
friends like these who needs enemies,
right? uh will be orbiting that star and
somehow right so two stars orbiting at
somehow mass will transfer or move over
and so whether it's you know teaspoon by
teaspoon we're not sure but at some
point it crosses that Chandra Sikar
limit whether that happens cuz the two
stars merge or whether material spilled
over somehow it crosses that limit and
then you get a runaway thermonuclear
explosion and the beautiful thing for a
cosmologist is that because they always
blow at or around 1.4 times the mass of
the sun, they're going to be very
uniform. And so you see one far away and
if you can recognize it's that kind now
you can how figure out how far away it
is.
>> Oh, so essentially you know how bright
they are. So based on how bright they
appear and this particular star is
always around the same brightness.
Correct. Sort of thing. Right. Right. So
identifying an exploding star sounds
easy, but you know, how often does a
star like that explode in a galaxy like
ours or any galaxy? And then
>> how do you find them if you don't know
where they're going to explode?
>> Yeah. Yeah, that's a great question. Um,
so a supernova explosion is very rare
and we're grateful about that because if
they were blowing up all around us, you
know, we wouldn't last very long. Um,
and so, you know, we get a nice long
window here, uh, before that happens.
So, about once a century in a galaxy
like ours, a supernova of this type will
explode. So, if you just picked your
favorite galaxy and just stared at it,
right, it's going to take about 100
years. This would not be a good thesis
project for a graduate student, right?
>> Not not one from this planet.
>> That's right. That's right. So, what
what you want to do then is you want to
stare at many galaxies at the same time.
And so this is like the question, how do
you win the lottery? And the answer is
you buy all the lottery tickets, right?
And so you buy a lot of lottery tickets.
And so what changed the game was around
the 1990s was the development of new uh
cameras on telescopes that had very wide
fields of view. They used electronic
detectors, CCDs, wide fields of view.
And so for the first time, you could
take a single image that might have
10,000 or 100,000 galaxies in one image.
And then you take an image like that a
month later, you digitally subtract one
from another, and you don't know which
galaxy's going to have a supernova, but
certainly some of them will because
there's just so many galaxies you've
been monitoring.
>> Yeah. Yeah. And so when you do the
subtraction, everything that didn't
change stays there. Correct. And then
the stuff that changed pops out, right?
And then you have to figure out was that
>> what we're looking for?
>> Yes, that's right. That's right. And
then um a type 1A supernova will have a
certain um spectral fingerprint. So when
the star explodes uh it's mostly made of
carbon and oxygen and it will also burn
or fuse elements to higher up on the
periodic table. You'll get a lot of
silicon and sulfur. And so when you take
a spectrum, when you take the light from
the distant supernova and you pass it
through a prism like we split the colors
of light, um you will get a spectrum and
you will look for features in that
spectrum which tell you, oh there's
silicon, there's sulfur. More
importantly, the whole fingerprint looks
exactly almost the same every time one
of these type 1A supernovi goes off. So
not only are they all the same
luminosity, they have the same
fingerprint. So you look for that and
then you know you have one and then you
want to measure how bright it is tells
you how far away it is and you want to
measure how much its light has
redshifted by the expansion of space.
And so because space is expanding some
wavelength of light is traveling to us
from the supernova and as it travels
space expands and it stretches those
wavelengths of light and longer
wavelength light is redder light. So we
call it the red shift. And so you know
the red shift is telling us how much the
universe expanded
>> where where the distance measurement is
telling us how long ago that supernova
exploded because knowing the speed of
light and distance tells you time.
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>> Tell me what you expected to see versus
what you actually saw.
>> Right. Right. So in the 1990s uh
astronomers were using these type 1A
supernovi to measure how fast the
universe is expanding today. So that
meant observing nearby type 1A
supernovi. So there's an extra wrinkle
to this story which is really what makes
this all possible is it takes light a
long time to reach us from these distant
supernovi. And so when we look further
and further away with these techniques,
we're not actually measuring how fast
the universe is expanding today, but how
fast it expanded in the past. Okay? So
there's this kind of built-in, you know,
time delay, if you will, that actually
is our superpower because it allows us
to look at the past expansion history of
the universe just by looking at more and
more distant supernovi. So by the 1990s
we had well measured the nearby rate.
The new game was to look for these
ultraistant ones which were very faint.
So they were just at the edge of what
you could find with telescopes. But they
would tell us how fast the universe was
expanding many billions of years ago.
>> Just as a quick insert uh cuz I want to
get back to that story.
>> What size telescopes were you using?
>> Yeah, so we were using uh 4 meter
telescopes. Yeah. So they're they're
good size. I mean by today's standards
they're not the biggest but by those
standards they were just about the
biggest you know it was around the time
the first 10 m telescope became
available KEK in Hawaii the Hubble Space
Telescope had first become available so
while Hubble doesn't have the field of
view to find the supernovi it can follow
them up
>> oh to get that spectrum
>> to the spectrum where the light curve
the rise and fall of the light so it was
a kind of all hands on deck all the best
instruments around the world operating
at the same time we looked for these
ultraistant ones, we would find them,
get their spectrum, and use it to
measure how fast the universe was
expanding then.
>> So, so a quick question about that. Now
that you it it turned out to be a Nobel
Prize winning discovery, and we're still
going to get back to what you what you
saw and what you expected, but you know,
now we realize how important it was, but
time on these big telescopes is
competitive. So, did they really
appreciate were you getting told a lot
of nos? No, sorry.
>> Right. Um, you know, I think at the time
people recognized that this was a very
important experiment to do. And so
something very unusual happened actually
at the time. This was the mid 1990s. For
example, the Hubble Space Telescope was
the hardest telescope to get time on. It
was still pretty new at that time and it
was oversubscribed. And the director at
the time, Bob Williams, thought this was
such an important experiment that he
gave his own special pot of high-risk
director's discretionary time. Yeah. and
he gave it to both teams. And so again,
this is very unusual. Usually you
compete with other astronomers to do the
experiment on [clears throat] something.
And he said, "Let's have two experiments
doing this just to be able to cross-ch
checkck the results."
>> So that was that was
>> in his for Yeah. The Hubble Deep Field.
He did time for these experiments. So
anyway, when we were looking at the
data, the prevailing wisdom was that we
would see the expansion of the universe
slowing down, decelerating. And why is
that? Because all after the big bang,
the universe is expanding, but there's
all the matter in the universe that has
attractive gravity that is going to pull
back on the expansion, right? It wants
to be back together. And so, you know,
like like tossing a rock up in the air,
you know, you give it that initial
throw, but then the pull of the earth is
going to pull the rock back. And if you
could measure how much that rock is
decelerating, you essentially weigh the
Earth. Wow.
>> Right. And so we thought, oh, we're
going to weigh the universe by measuring
how much the expansion was slowing down.
And that'll even tell us whether the
universe had escape velocity from
itself. Would it expand forever or would
it recolapse? Like that rock, if you
toss it enough, high enough and far
enough and and fast enough and the earth
weighs little enough, it will escape the
gravitational pole. So that was the
game. Yeah.
>> And so, uh, just before my honeymoon,
um, I had done the final analysis of the
data and the
>> I had what looked like a sign error
because, uh, it seemed to be showing me
that the universe was not decelerating,
it was accelerating. And [snorts] that's
like, you know, doesn't make any sense.
Like, if you toss that rock up in the
air and then it went up like a rocket,
um, you know, you'd go, what is doing
that? Cuz attractive gravity doesn't do
that.
>> Right. Right. So when you get a result
like that
>> Yes.
>> I mean man I mean I would be sweating
because I because I know how our
colleagues are. Yes. If if
>> they're rough. They're rough. They're
rough. And chances are if you've gone
through everything
>> and you're like I'm sure this is right
>> because we have so many and everybody's
so smart. Somebody's going to be like,
"No, you're in it. It's that." And they
know immediately, right? So, how did
your team receive this result and how
did you feel reporting this result to
your team?
>> Yeah, very nervous. Um, you know, at
first you find something like this and
as I said, you're sure you're wrong and
but you know, you know the process to go
through. I'm going to go over all the
steps, check everything, do this, do
that, right? Cuz you want to find your
own error first, right? Absolutely. In
fact, you want to find your error and
not even admit to anybody that you made
an error, right? Yeah,
>> your former adviser, your colleagues,
you're just like, I'll just find
>> Not to mention, you were pretty young.
>> Uh, correct. Correct. I was just fresh
out of graduate school. And so, you
know, I hadn't had any significant
results. And so, yeah, you want to find
your own error. And then, um, when I
couldn't and I checked everything, then
I began working with people on the team
saying, "Look, I'm seeing something
funny. It's probably gonna go away, but
can you just check step B to C and can
you check step C to D? Can you
reproduce?" And it was just a series of
farming out. And then Brian Schmidt was
who was my colleague, he was leading the
team, did the final check on the last
step. So I remember he had moved to
Australia and I was in California. So
when I would send him an email, it would
take like 12 hours before I'd get a
response. So I had a very sweaty night
one night when I was like, I've checked
everything. This is the final
calculation is do you see what it
favors? And he wrote me back the next
morning an email that said, "Well, hello
lambda." Okay. And now lambda is, as you
know, the Greek symbol for what we call
the cosmological constant or dark
energy. And so
>> he saw the exact same thing.
>> Wow. That that is well, Brian seems to
have had the belief in the result to uh
make such a bold statement,
>> right? Yeah. It was really interesting
when I look back on the emails of our
teams uh talking about this, they almost
skew I'm going to say at some level with
age because you know the older you are
in science the more you've seen and
you've seen things come and go and be
wrong and you probably become more
conservative. I think for me it was an
advantage. This was like my first rodeo,
right? So I was like, "Hey, maybe we
just discovered something about the
universe. Isn't that cool?" Yeah.
>> Well, what was really surprising is how
everybody else accepted it so quickly.
Like the the the field accepted your
result as if it were true. Never
happens.
>> This is very important. So there were a
couple of reasons why that was. I mean
there was another competing team that
was doing a similar experiment and they
were reporting the same results. That
was important. But also sometimes when
you find something
>> it fits in the sense that when once you
show it you say actually this solves
multiple problems. And so one of the big
problems at the time was the age crisis
if you remember
>> the stars versus the universe.
>> Correct. Correct. Right. So it looked
like there were stars that were much
older than the universe. And when we say
that we think as astrophysicists we can
calculate the lifetime of a star from
how much energy it puts out and how much
energy it has. It's like you know
driving a car how much gas do I have in
the tank and what's you know how how
many miles per gallon do I get and and
when will I run out? Right? There's that
side of the calculation, but the other
was the universe itself. Depending on
how fast it's expanding, you can run
this movie backwards in time and say
when was the big bang then in that case,
right? And so based on an expectation
that the expansion was slowing down,
right? We thought, oh, this is a slow
rate for the universe right now. So
therefore, when we run the universe back
in time, it would be relatively young.
Okay? But if instead the universe is
accelerating, we go, "Oh, this is
actually a faster rate than it normally
would have." This means the universe was
suddenly older, older than the the
oldest objects in it. So there's kind of
a a breath of relief from other parts of
the cosmology community. Another element
was there was this uh puzzle that about
70% of the universe was missing. Yeah.
>> So there's a deep universe called
inflation, you know, that says how the
universe
>> propagated after the big bang and it
expects that there's a certain amount of
matter and energy in the universe and
astronomers had only found about 30% of
the matter
>> and we didn't even know there was going
to be energy.
>> So yeah, so what we found actually fit
in the it was the missing uh you know
puzzle piece if you will there too. So,
so although there was a lot of
skepticism, there was also a lot of
well, you know, this does fit a lot of
things.
>> This is pretty convenient here, even
though it's crazy,
>> right? But even after our results came
out, it still took years of additional
confirmation. So um from the standpoint
of supernova observations, we continued
to push back further in time and we were
eventually able to see the universe
>> uh change over from decelerating to
accelerating which is a very important
signal because I mean if the universe is
really made of dark matter and dark
energy when it's compact it will feel
its matter more and it will decelerate.
It's only once it dilutes and thins out
by expansion that dark energy is waiting
in the wings and pushes it apart. So we
could eventually by the early 2000s we
were seeing that with supernovi and then
the cosmic microwave background
radiation measurements came along and
they confirmed this picture
>> first with uh W map
>> right some groundbased stuff then W map
then plank and so you know by the mid
2000s um my confidence went way up in
this and my confidence peaked in 2011 uh
because we won the Nobel Prize
>> and I I thought that's this is probably
>> validated.
>> Probably true.
>> Validated probably.
>> I wonder if there have been any uh Nobel
Prize discoveries where they discover
later Oh, actually not.
>> Right. [laughter]
>> There are a few things that don't hold
up that well. Let's put it that way.
>> Right. Right. Your discovery goes back
to your 1998.
>> Yes.
>> Is now 2025.
>> Yeah.
>> Did you think that the answer to what
dark energy is would be known by now?
>> Wow. That's a good question. Uh I don't
know that I ever contemplated at the
time, you know. I I think it was mostly
a focus of is this right? Is this right?
>> Um and then after that uh I think that I
I do think these missions that are
coming up have a good chance to tell us
what it is. I'll be totally honest with
you. I think the way science like this
works is observers give clues which
comes from doing experiments. So nature
is showing you how things go. But I
would say the ultimate answers generally
come from theorists. You know, people
like an Einstein who sit there and
synthesize this and say, "Wait, here's
what's really going on." And so I don't
ultimately think we come to understand
dark energy just through experiments,
but rather theorists synthesizing
experiments. And unfortunately, you
know, it's hard to predict when a
theorist will make a breakthrough, you
know. And I mean I've asked people this
before. When Einstein did his
development of general relativity, did
people see that coming? Was that like on
the horizon? And the answer is usually
no.
>> Um you know there were some clues but
nobody would have derived general
relativity because the orbit of Mercury
was processing around the sun which is
one of the anomalies of of Newton's
physics. So it's hard to predict uh
theoretical breakthroughs.
>> Okay. So let's let's go back then to um
you mentioned lambda.
>> Yeah.
>> So lambda shows up in Einstein's
equations as a constant that he
inserted. And there's that historical
story that I hear people uh misquote.
You know that you know people often say
Einstein said that was his biggest
blunder. But I saw another um science
communicator Sabina Hassenfeder I think
who said no he didn't say that about
that. That's not what that was. But I
don't know the the the truth
historically. Um but I do know that when
I look at that equation and I attempt to
interpret what it means, it can be read
in one of two ways. One way is that dark
energy is some stuff in spaceime. And
the other way is that dark energy is the
intrinsic energy density of spaceime.
What is your interpretation?
>> Yeah. Um I mean you know as it depends
on in this discussion which side of the
equation you put it on because basically
what you're trying to balance what sorry
what Einstein was trying to balance was
that he thought that the universe was
static that it wasn't expanding or
contracting. That's what astronomers of
the day told him because astronomers of
the day didn't know that what we call
galaxies were actually outside the Milky
Way. So they thought everything was in
the Milky way. Nothing is really
expanding in the Milky Way.
>> A universe of one galaxy.
>> Right. Right. So they said, "Hey,
nothing's really expanding or
contracting." And he was like, "Wow,
that's a puzzle because this term in the
equation, which looks like kind of like
Newton's gravity, will cause things to
pull together, there must be something
pushing the other way." So he made an
amazing discovery, which is that
although the gravity of stuff of matter
is attractive, that the gravity of empty
space could be repulsive, could go the
other way, and that these two could be
in balance. So he called this the
cosmological constant and he saw that it
could exist as as an extra term in his
equation and this is important. There
was a place for it. I mean this isn't
like if if if Newton had had this
problem he would be stuck because you
know in his gravity there is no option
for something to be repulsive. But the
the curious thing is Einstein's gravity
recognizes different forms of
>> matter and energy as having different
gravity. And so energy itself can be
repulsive. It has a curious property
that we call negative pressure.
>> Exactly. That's what I was going to say.
You could have positive energy and
positive pressure which would be
attractive or positive energy and
negative pressure. Right.
>> Which would be repulsive.
>> Correct. Correct. And so so that is how
we we attempt to understand it today as
though the universe is filled with this
kind of energy um that has this property
of negative pressure. And so it has
repulsive gravity. I think Einstein
didn't even go that far. I think it was
a term in his equation that
>> with no physical interpretation.
>> Yeah. With with initially not a lot of
physical interpretation. It was just uh
what we would like to call a boundary
condition. Well, the universe is static.
Therefore, a term is there.
>> Wow.
>> Um but uh and of course once he learned
about a decade later from Hubble and
others that the universe was expanding,
he certainly thought this was a mistake
and removed it. you know this question
of if whether he called it his biggest
blunder or not is more you know
anecdotal apocryphal you know who is he
talking to did they remember it
correctly but the sentiment is certainly
true that he thought
>> well if the universe is expanding I
don't even need this why did I come up
with it but you know once that once that
toothpaste was out of the tube right
there was no putting it back I mean he
demonstrated it could exist it may exist
there's even a physical interpretation
of it could be the energy of the vacuum
that quantum theorists uh wouldn't know
how to get rid of if they wanted to. And
so we have always been in this situation
of this ambiguity. Is it going to be
there or is it not? You know, and every
couple of decades somebody says, I think
I see it. And then others were like, no,
I don't. But it wasn't really until 1998
that we saw the direct consequence of
it. That is irrefutable.
>> Irrefutable. Now in 2025 is even become
stronger and stronger. Right. So, I'm
going to ask you a question, man, that
you know, we're going to get back to
dark energy, but I got to take a ad
ajacent detour because there are things
we say all the time and we're
comfortable saying them. And for me,
when I see it in mathematics, I
understand the math, but the physical
manifestation
doesn't I I don't it it isn't always
intuitive to me.
>> And one that does that for me is the
statement spaceime is expanding. LIKE
WHAT? LIKE I I I I see the math. We
can't explain it in math, but
intuitively like why do you deal with
that? Like what does that mean?
>> Right. There's a there's a third way
besides the math and the intuition,
which is the observation really. And so
as an observer, you know, it's a fact
that you look at things and every aspect
of them tells you that the universe is
expanding. You go, well,
>> what do you think is happening in space
microscopically?
>> Yeah. I mean I think that at um I think
on every scale things are expanding.
However, there is also the opposition to
expansion which can be attractive
forces. So you get to the scale of you
know the atom and it's not expanding
because the electromagnetic forces or
the strong nuclear force are holding
things together. You get to the scale of
the earth and the earth is not
expanding. There's electromagnetic force
and there's gravity that are holding
things together. So you know
>> and the galaxies aren't expanding. the
it gs aren't spinning because there's
attractive gravity holding things
together. It's sort of like I don't know
imagine, you know, skaters on ice where
they want to be the ice wants to be
pulling them apart, but they're holding
together. They're holding hands. So, you
can overwhelm the the expansion which is
going on because it's not all that
strong. It's just that when you get out
into intergalactic space, everything is
so diluted at that point that you are
now experiencing the expansion of space
that's being kicked into higher gear by
dark energy.
>> Got it. Got it.
>> So, it's really, you know, it's really
the competition of two titans, you know,
going in opposite directions and who
wins where depends on how diluted one of
them is relative to the other. So deep
extragalactic space is really more and
more the playground of dark energy and
you know galaxies and clusters and
that's really the playground of dark
matter now.
>> Ah ah that's really good. So back to
dark energy precisely. So I haven't been
deep into dark energy for about a decade
>> and you know I remember there were
competing models you know just like with
dark matter it had it competing models
and then a lot of them got kicked out.
>> Right.
>> What remains for dark energy? What are
the possibilities for right
>> describing what dark energy is? Um and I
remember everyone was looking to to to
measure this w parameter the the correct
>> yeah yeah yeah the equation of state of
the universe to to identify it. So has
things change
>> right? So remarkably you know Einstein's
cosmological constant remains
the sort of in pole position uh for the
last 25 years or so because in a way
it's the simplest and uh you know
physicists love simple and elegant and
so if you tell them two stories one of
them is simple and one of them has extra
you know features and and and uh chance
occurrences going on they don't like
that on nearly as much.
>> But this one has a chance occurrence
that we call the cosmological
coincidence.
>> It does. And I'm glad you brought that
up because to me, I'm one of those
people who often pushes back on my own
community and says this is not a contest
of something simple and something
complex. This is a contest of something
complex and something else complex that
we, you know, and I don't really see a
big advantage of one or the other. But I
will say this, this is what's important
is for dark energy, we have models that
are static where we say dark energy is
just it's always been in the universe.
It's like a constant of nature. Uh it
will always be there. It acts like this
cosmological constant and the universe
will accelerate forever. Okay. And then
an alternative is that
>> dark energy is instead temporary energy
due to a field in space. So you could
think of any field you know of like the
electric field or the magnetic field.
This would be a different field and a
field has energy. I mean just take a you
know take a compass and play with it in
a magnetic field and you'll see there's
some energy. It's able to shove that
needle around right? So there's energy
there and that energy could be the dark
energy. Um and in if that's the case
then it is probably temporary. It
probably changes over time. And so the
important test right now is to see has
our dark energy been changing over time
or not. A third possibility we have to
keep in mind is um that we've broken
Einstein's theory of gravity and that it
only kind of looks like there's dark
energy. But it's really because we've if
you go to the scale of the whole
universe, right, that Einstein's uh
gravity doesn't continue to operate
correctly. It's sort of like Newton's
theory of gravity broke down. uh as you
get close to the sun because gravity is
strong there. And so, you know, you
could try to invent structures and and
epicycles and things that will work. And
so, you know, we have to keep in mind
that, you know, it's possible we broken
Einstein's theory of gravity. And so,
the test there would be that you
wouldn't find a story about dark energy
that actually worked at all scales.
You'd say, "Oh, I need one kind of dark
energy to explain the accelerating
expansion of the universe." but a
different kind to explain how structures
are growing in the universe.
>> By structures you mean galaxies,
galaxies clusters.
>> Correct. Correct. Right. So that's
gravity on a different scale. And so
right now we're doing a lot of new
experiments with new facilities. Uh you
know the new Reuben telescope, the new
uh Roman NASA Roman telescope due to
launch next year. Uklid uh new desi
experiment. So that's the desert result
that how so the desert result shows that
dark energy maybe have been observed to
be changing with time
>> right
>> how big are those error bars?
>> Yes. So that was a great surprise to
people when this first came out now
almost 2 years ago that all the
experiments up till now had been kind of
zeroing in on well it looks pretty
static it looks kind of like the
cosmological constant and then Desi
which was the best experiment up till
that time was like wait not so fast we
see evidence that it has changed the
dark energy over the last several
billion years and if that is true that
would be the biggest clue
>> we have about the nature of dark energy
I would say since it was discovered.
>> Yeah. Yeah. The expansion rate of or or
dark energy becoming weaker,
>> right? Is that what it suggests? Is dark
energy affecting weakening?
>> That is that is a face value
interpretation of it. I think another
interpretation of it is also that our
best cosmological observations don't all
fit together that there's some uh
conflict between them. And so it's
unclear whether the conflict you are
solving it correctly by allowing dark
energy to vary or if it's that model is
breaking in some other way.
>> What does DESI stand for? A dark energy
survey
>> spectroscopic
instrument.
>> Instrument. Oh yeah. Yeah.
>> Yeah.
>> Um yeah. So it was very interesting when
it came out. Desi is the best
measurement of uh a feature in the the
distribution of galaxies called the
baron acoustic oscillation which is kind
of a standard measuring length. So we
talked earlier about
>> Oh what does he use?
>> Yeah. Yeah. So we we talked earlier
about using the brightness of a
supernova to measure distances. You
could also in principle if you look far
away and you see a big object looking
tiny, right? That tells you how far away
it is. Like I I look out on the highway
and I see little bitty cars and I know
those aren't really little bitty cars.
They're really regular [clears throat]
cars far away, right? And so likewise um
Desi looks at this feature called
Barryon acoustic oscillation which was
supposed to be originally 150 megapars
across but it looks itty bitty tiny and
they can use it to measure how far away
it is. So we use Barryon acoustic
oscillations, we use supernovi, we use
the radiation left over from the cosmic
microwave background. We bring them all
together in the context of something
called lambda CDM which is our story.
It's our standard model. It's everything
we know that how it fits all the physics
and the inventory of the universe and we
apply it to this newest data and it
doesn't all fit.
>> It doesn't all fit together nicely.
>> When we look at these different
cosmological techniques, okay, you have
the cosmic microwave background
radiation measurements which are
sampling the universe 13 12 billion
years ago. You have your supernova
cosmology uh measurements which are
sampling the universe over some numbers
of hundreds of millions of years up to
today. Then you have your barriion
acoustic oscillation measurements which
are probably over a similar time scale
but I would guess have bigger error
bars.
>> Yep.
>> And the three of these are giving you
different answers. So the question is
two two parts. How do you go from data
to physical model? And when these
physical models in disagree cuz you know
we're going to get to this other thing
called the Hubble tension in a second
>> and that's real disagreement.
>> Yeah. So how do you get to go from
measurement to to to to to physical
model theory and theory and how do you
when they disagree how do you resolve
these disagreements?
>> Right. Um I mean first of all this is
wonderful because this is the process of
science is you say I look at a lot of
phenomenon uh I understand some things I
develop an understanding based on what I
have seen and then I use that
understanding which we will call a model
okay um to predict things I haven't yet
seen and I will test whether my
understanding is right and a you know
good robust
>> closer to correct model has power beyond
where it was learned. It has power to
predict the future and other
experiments. Okay? But sometimes the
things don't the the theory doesn't
match the uh experiment. I think they
said in the movie Oppenheimer over and
over, right? Theory will only get you so
far. Right. Right.
>> You know what, man? I've not been able
to get into Oppenheimer. I've started it
like three times and I've not been able
to push on it.
>> Push on. [laughter]
>> Push on. Okay.
>> But uh anyway, and so uh so this is our
process. It's the science process and
sometimes the data doesn't match the
theory and if it's good reproducible
data then you have to revisit the
theory. So here's the question. Is it
just fitting the equation to the data?
Does that how you get to the theoretical
>> pretty much pretty much I mean you know
you you start with this model today
called lambda CDM which is the the model
that really after that late 1990s work
that we talked about became the standard
model and it literally says lambda CDM
literally says there's dark energy
that's what the lambda stands for.
There's cold dark matter that's what the
CDM stands for. That's a lot of the
stuff but there are more elements in
there. Usually people are saying the
universe is flat geometrically. So
that's sort of in there. They're saying
how many nutrinos there are and what
their properties may be. Um there are a
number of onzots in it. So it's a really
it's really a package which is really
like the it's the physics of the
universe and it's the inventory of the
universe and then given that you could
predict the outcome of an experiment
okay like desi um or of multiple
experiments and then generally these
theories these models actually have free
parameters in them in the sense that um
you say it's still the same model but
there could have been more matter and
less dark energy or vice versa. how much
do we have? And so you actually allow
the model to best fit the data, okay, to
learn those parameters. Now,
>> that might work out well or you might
find, hey, when I when I compare the
model to this data, I get a different
set of parameters than when I compare it
to this other one. That means they're
not agreeing very well. Or you might
even say, I can't get this model in any
form to fit. I have to add some new
physics. And so what most recently with
these new results from Desi was
combining Desi Kio microwave background
and supernovi and saying you know the
model fits great any one of these data
sets but once I start to require it to
fit two or three at the same time
>> it's starting to fail now why is that
these experiments are probing different
points in the history of the universe
and so just like if you I don't know
draw some complex curve uh you know on a
piece of paper I could fit that curve
with the line over little portions of
it, right? But if I try to make that
line work for the whole thing, now I'm
going to start to see some
discrepancies. And so that's kind of a
perspective of when things break down,
it may be that there's more wrinkles to
the model that's not traveling the
straight line, the story is more
complicated. you know,
>> you know, when when one of the most
amazing results I've ever seen uh come
from our friend who just passed away, uh
George Smoot and um JWST PI, who was
also on Kobe, what's his name?
>> Uh John Mather.
>> John Mather, right? Smoot and Mather
when they showed their Kobe black body
curve and they had the error bars on the
on the line that was the you know so the
data the data was was dots with error
bars and the theory was the black body
curve and then you read and said oh the
error bars have been multiplied by 400
so that they are
>> so you can see them yes I know visible
right that is actually the best as I
understand it the best uh measure of the
energy coming from a hot object that
exists in nature that's ever been
measured and it's actually of the big
bang.
>> Yeah. So, here's what I'm getting.
Here's where I'm going. When you made
this measurement in 98, there were 42
supernovas. The the the error bars were
visible. Now, you know, a quarter
century on, how many more data points do
you have? And how small have the error
bars gotten? And when you compare it to
these other observations like Barryon
acoustic oscillation and CNB I know CNB
has smaller bars.
>> Sure.
>> Is it?
>> Yeah. So compared to you know in the
1990s we had dozens of supernovi and now
we have thousands. Um and Reuben this
new facility is just about to turn that
thousands to tens hundreds thousands and
millions. Okay. So we're even now at
still just taking off. But um to answer
your question u what was your question?
So the question is we're comparing these
three different Yes.
>> data sets, but I'm assuming they're not
all of similar quality.
>> That's right. But what's I'll say
ingenious is that as long as you can
state what that quality is, then you
could still use it to test. Yeah. You
weigh it correctly. So we're factoring
that in, I would say. Now that isn't to
say you'll meet some cosmologists who
will say well I prefer this data and I'm
less confident about that data but you
know I would say in terms of the actual
quality of the data like do I have a lot
or a little we're pretty good at
understanding how that propagates into
our knowledge about the model.
>> Okay. Okay. So, in terms of of
certainty, I would love to hear you say
three statements [laughter]
clearly and and be because
I think that you know the average person
who may be listening to this and I've
heard people say this in in casual
conversation that think that oh this big
bang model is just a model. We don't
know that that's reality.
>> Dark energy. How certain are the
existences and the happenings of the
universe expanding, the big bang
happening, dark energy existing, dark
matter existing.
>> So, so the important thing and I sort of
teach this in my Astro 101 course is to
distinguish between the observation and
the conclusion uh or explanation of the
observation. So, for example, in our
case, we saw that the universe is not
just expanding but accelerating. Okay,
that's closer to
>> uh the reality of the situation. It's
what we actually see. Okay, and it's
also a statement about an action that
the universe is doing. The
interpretation then becomes, well, what
would do that? Well, if Einstein's
theory of gravity is true, then it
requires this term to be active and that
term represents the background energy.
So, we'll call that dark energy. But
I've now walked down a road where I'm
using theory now to explain and I'm not
maybe completely comfortable with all
the theory. Right? We know that that
general relativity doesn't work or is
not compatible with quantum theory. So
you have to keep in the back of your
mind if this is the right theory.
>> So now let's get into the Hubble
tension.
>> The Hubble tension. Now when I think
about the Hubble tension, you know,
sometimes man, you know, I'm going to be
honest. I love me some nerds.
>> Yeah. But I don't always vibe with the
nerds. And why do I not vibe with the
nerds? Because sometimes I feel like
they're making too much out of a a
topic. And and so the classic examples
to me is what? Whether or not black
holes have hair,
>> right? I'm just like, is it really that
deep, you know? So the Hubble tension,
where does that fall?
>> Much bigger deal to me. Much bigger
deal.
>> Well, okay. So going back to this idea,
>> define it. What is it? Yeah.
>> Yeah. Yeah. Yeah. Well, so what is the
Hubble tension? Okay. So we reach a
point today where we say we kind of
think we have a pretty good model of the
universe but you know has big areas of
ignorance. 95 96% is in the form of dark
matter and dark energy that we have kind
of cartoonike explanations for but
that's fine. Okay. And then we say all
right let's really test this model.
Let's see if it's really right. Right.
So the what I've been calling the best
end to end test of the model. Right? You
do an endto-end test when you really
want to know does my thing, you know,
operate as I expect. The best endto-end
test of the universe is to look at the
cosmic microwave background which tells
you the state of the universe shortly
after the big bang. Okay? And it allows
you to predict how fast the universe
should be expanding today. It's like if
you had a kid who was 2 years old, you
could predict what height they will grow
to based on growth charts and your
understanding of human physiology. Okay?
But the end test of that story is to
actually measure today how fast is the
universe expanding. A number called the
Hubble constant. That would be like
measuring that kid's height when they
are fully grown.
>> Right?
>> If you really understand things, the two
will match within the error bars, within
the uncertainties. And so over the last
decade we've seen this mismatch growing
and growing in significance. First it
was one or two times the error bars
away. Then it was three. Then it was
four.
>> Then it passed five. Now in physics five
is considered the kind of gold standard
for like going from uh don't bother me
with that uh to like this doesn't make
sense. And now we're probably up to six
or six and a half. Okay. And the reason
that it has grown is because the data is
getting so much better. We've had the
Hubble Space Telescope. Now we have the
James Web Space Telescope. Previously we
had crude parallaxes. Uh now we have the
European Space Agency Gaia mission
measuring parallaxes. Um you know the
cosmic microwave background data has
gotten better. First it was groundbased,
then it was WMAP, then it was plank, and
now it's also these groundbased
experiments with high resolution like
ACT and SPT. So, and then we have many
techniques for making these
measurements. So, when we measure the
Hubble constant locally, we build what
is called a distance ladder.
>> Right. So, speaking of that, I just want
to define a term. You mentioned parallax
which is a geometrical way of
determining distances and it works best
for nearby objects but you can crossc
calibrate more distant objects where
they overlap. Correct.
>> Right. Something of that nature.
>> Yeah. Yeah. Yeah. And that process we
call the distance ladder. I mean ideally
you would look at some distant galaxy
and you would measure its distance from
us geometrically by looking at parallax.
the the parallax is when the earth goes
around the sun and your perspective on a
nearby object changes with respect to
something distant, you form a triangle
in space and you can measure how far
away it is. The problem is things are
far away. They're really far away. So
they that that shift in position becomes
imperceptibly small. And so like you
said, you can only measure it for stars
in the Milky Way, but you calibrate the
luminosity of a certain kind of