How fast is the universe blowing up?

What is the hubble constant? Find out with Daniel and Jorge

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Episode Transcript

Speaker 1 (00:08):
Hey, Danny, does the universe just blow your mind? Oh?
My god. Constantly I feel like I can't crame enough
universe into my brain. Is just excluding what's the most
mind blowing thing about the universe in your opinion? I
think the thing that drives me the craziest is that
the universe itself seems to be blowing up. The universe
is blowing up. You mean like it's going viral on

(00:29):
social media? Probably hashtag universe, hashtag everything in the universe
Twitter account is lit. No, I mean that it's getting bigger,
and the speed at which the universe is getting bigger
is also getting bigger. Hi am pre hammack Tunis and

(01:00):
the creator of PhD Comics. Hi, I'm Daniel Whitson. I'm
a particle physicist, but my brain is filled with crazy
ideas about space, time and particles. So welcome to our
podcast Daniel and Jorge explain The Exploding Universe, a production
of My Heart Radio, in which we try to take
the entire universe, everything in it, and squeeze it down

(01:23):
into this audio connection to you, downloading it into your
brain so that it blows up your gray matter. And
so are we here to try to blow your mind
a little bit of physics at a time, you know,
just a little bit of physics each week, twice a week,
and hopefully your mind is getting maybe bigger, may be
more connected to this giant universe we have out there.

(01:44):
That's right, The universe is out there, and we think
it's for everybody understanding. This incredible place we live in
shouldn't just be the province of cutting edge scientists. It
belongs to everybody, and that wonderment, that amazement should be
accessible to everybody out there. And so our goal is
to make sure that you actually understand the way the
universe works and what science does and does not know.

(02:05):
Is the universe for everybody? Daniel, I don't know about that.
I mean, all solar systems matter, man, All solar systems
are made of matter. Yeah, that matter matters, no, But
I think that the universe is for everybody. You know,
you don't have to be a scientist to look up
at the stars and wonder how does this whole crazy
universe work? Or to look down at your feet and
wonder about the particles that you're made out of. And

(02:27):
everybody deserves an explanation. And you know, science is mostly
something that's publicly funded, it's put on by governments, it's
of the people, by the people, and for the people.
And so this is the for the people part where
we try to disseminate what science has learned to everybody
out there. Yeah, for sure, I think definitely the universe
itself is definitely of people and with people in it,

(02:47):
and for the people. Yeah, there's a lot of prepositions
you can go for there. I just hope it's not
through people. You know, we don't don't want to go
around people are over people, what do you mean? I
think the universe is all those things, the universe going
through me right now. Well, the goal of this podcast
is to get the universe into people. Well, we're trying
to um talk about the universe and just kind of

(03:07):
help everyone wrapped their heads around this incredible and complex
and really big universe, right and possibly getting bigger. Yeah,
And this whole idea of the size of the universe
is something which is very very modern and people have
been looking at the stars for a long time. People
have known about other planets, people had the idea that
there are other stars out there, But it's only been

(03:28):
a hundred years that we've known that there are other galaxies,
and that those galaxies are moving away from us, and
that the size of the universe itself might be expanding.
So it's very recent in human history that we really
have any understanding at all of the entire context of
our lives. Yeah, that's wild, Like a hundred only a
hundred years ago, we thought it was just us right,

(03:49):
like us and the stars around us, and maybe that's it. Yeah,
hundred years ago, people thought it was a bunch of
stars hanging in space, and it's just sort of been
like that forever. So most humans who ever lived had
the raw long understanding of the entire universe. Only the
people who were awake, alive and listening to this podcast
have any sense for the actual context of their lives. Yeah,

(04:09):
it's just a small error, you know, just a few
Brazilian whatever. And the thing I love about that is
it suggests that there might be other enormous contextual errors
that we're making, you know, basic assumptions we have about
the way the universe works that are just wrong. That
in a hundred years some future podcast will be smirking
at and twortling at our ignorance, being sarcastic about while

(04:32):
eating bananas precisely. Do you think people back then a
hundred years ago I thought the universe was a finite size,
or did they think it was infinite, or did they
think it had a size but just not as big
as we know to be right now. I think about
a hundred years ago, before Hubble, for example, people thought
it was just a bunch of stars and it was finite,
and it just sort of a bunch of stars hanging

(04:52):
in space. You know, imagine like a single galaxy, whether
or not space itself went on beyond the edge of
that galaxy. I think there was a lot of debate there,
but I don't think people ever imagine that there could
be like an infinite number of stars. Well, and so
that's the topic for today's podcast is it's about the
size of the universe and more specifically, how that size
is changing, because the size of the universe is changing, right,

(05:16):
that's right, And this is something that Hubble himself began.
Hubble is famous not because of the Hubble space telescope,
which was named after him, but because he's the guy
who discovered that the universe is expanding. The things that
are far away from us are moving away from us.
Really quickly, where like one raise in an expanding loof
of raisin bread. And the thing that's amazing is that

(05:36):
we're still learning about that. We're still learning about how
fast the universe is expanding, and we're still not sure
of it. We still don't really know the answer. Really.
We think it's expanding faster and faster, but we are
not quite sure how fast it's it's it's growing, or
kind of what's causing it. Right, that's exactly right. And
people measure this stuff and there's lots of different ways

(05:57):
to do it, and those measurements they make don't quite agree.
And so that's what we're gonna be talking about in
today's podcast. Yeah, So today on the podcast, we'll be
tackling the question how fast is the universe blowing up? Yeah?
And this is a really fun question and what I've

(06:18):
been tracking for a while because different teams of scientists
are trying to measure this expansion of the universe in
totally different ways, and for a while they sort of
agreed until recently their measurements be getting more and more precise,
and now they're not agreeing that well, And then I
got a question from a listener somebody who wrote in
to ask about it, and I thought, all right, it's
time to do a podcast on it. So here's a

(06:39):
question from Mike and Madison who wanted to know. Hi,
Daniel and Jorge. My name is Mike. I'm an engineer
from Madison, Wisconsin. Could you guys please explain what we're
doing to try and solve the unmatching hubble constant mystery? Also,
why does it have to be a single constant? Couldn't
the universe be expanding asymmetrically or at changing or different
rates depending on where you are in the universe. Also,

(07:01):
I'd like to give a shout out to my uncle
Jim McClain for introducing me to this amazing podcast. All right,
thank you Mike and Madison in medisine. I like how
it's alliterative. Yeah, and in a moment we will dig
into what the hubble constant is and how it's connected
to the expansion the universe and is it a constant
after all and all that kind of stuff. Yeah, because

(07:22):
it's kind of a very technical question. Like at first
I heard this question, but I didn't really even know
what he was asking about. Yeah, And the best way
to think about it is that the Hubble constant is
just one way to understand how fast the universe is accelerating.
It sort of helps determine it. But of course it's
confusing because it turns out the Hubble constant not actually
a constant, so it's an un constant constant. We are

(07:46):
constantly messing up the names of things in physics. We
are constantly throwing out the dictionary, it seems. You know,
if we just redefine the meaning of the word constant,
then it's a constant, right, and then we'll redefine the
meaning of the word redefine, in which case we'll be
do this all the time in physics right where we
have particles that spin but it's not really spin, you know,

(08:08):
we have particles with flavor, but they don't really taste
like anything. And now we have constants that are not
really constant. It's like a whole new language. It's like,
I feel like you're doing on a purpose, Daniel, just
to confuse us, and um makes us, UM makes us
wonder about this crazy No, no no, no, we I'm going
to use the Donald Trump defense. It's out of pure incompetence.

(08:30):
We're not trying to confuse anybody. We're just not capable
of doing any better. I see that's a defense name
after him, but not necessarily something he does. That's right,
But I was wondering are people paying attention to this?
Does everybody know what the Hubble constant? Is? Are they
aware this tempest and a teapot about how fast the
universe is expanding? Or is that something just scientists are

(08:50):
thinking about? Yeah, how many people out there even know
what the Hubble constant is? So, as usual, Daniel went
out there into the streets and as random strangers if
they knew what the Hubble constant is, think about it
for a second. Do you know what the Hubble constant is?
And if somebody asked you on the street to define it,
would you be able to give an answer? Here's what
people had to say. Something to do with the way

(09:13):
things around its space. I guess I don't know. Something
do the Hubble telescope? I don't know. It's the only
thing that I know that is Hubble esque, like a
mathematical equation or something. I feel like, something about how
the stretching of the universe that's do with gravity or something.
I mean, does it have to do with that one
Hubble and like a red blue shift or anything or not.

(09:36):
That's that w the extent that I would know, no idea.
Have you heard of Hubble? No? I guess it has
something to do with lights and the stars and space.
The scale you're getting there. Yeah, it's a scale constant
of lights through three dimensional space, and it's like a

(09:59):
cost bar jegal constant um. Doesn't have to do with
the size of the universe. I think that's all I
can get from my memory right now. I've heard of Hubble.
It's like the telescope, right, And I'm not sure what
the Hubble constant is, all right. I feel like some
people knew a lot about it, but a lot of

(10:19):
people didn't know anything about it or had heard of it. Yeah,
and some people were totally wrong. But I love these answers.
You know, some people think it has to do with
the Hubble space Telescope, which I guess indirectly it does,
because you know, the space telescope was named for Hubble,
who discovered this thing and and quantified. They've done a
lot of really good branding on the Hubble telescope, you know,

(10:40):
like it's a thing. People know what it is, and
because when most people associated with the name Hubble. Yeah,
the Hubble pr team has done a good job. Hey,
you know they produced these Instagram ready images all the time.
They're beautiful. You know, you just google Hubble and you've
got a lot of really gorgeous stuff to look at.
Let a Hubble bubble up. Yeah, you know, particle physics
service Umble doesn't produce as much like pretty pictures that

(11:02):
you can look at and say, oh wow, look at
that amazing thing out there in the universe, because it's
harder to visualize tiny particles. So from that point of view,
astronomy definitely has the lead over particle physics. Well, I
I am definitely in league with all of these people
on the street. I don't really know or have a
good idea of what the Hubble constant. Maybe. Up until
a few years ago, I never even heard it. I

(11:24):
mean I heard of the Hubble telescope, not the Hubble constant. Really,
do you remember the moment you learned about the Hubble constant,
probably like five minutes after meeting you. Then I do
bring it up pretty quickly in conversation. Hi, how's it going,
how's the weather. Let's talk about the Hubble constant. So
it's it's not related to the telescope. This telescope was

(11:47):
named after Edwin Hubble Um. But Hubble in his time
did a lot of amazing discoveries, and one of them
was this idea of a constant in the universe. Yeah, precisely.
The Hubble constant is related to the Hubble telescope, and
we actually do use the Hubble telescope now to help
nail down the Hubble constant, which is sort of a
fun little loop there. Yeah. Of course Hubble died well

(12:08):
before the space telescope launched, But you're right, he was
the one who figured out that the universe is expanding. Right.
Do you think he named the constant after himself or
was it named for him? Oh that's a great question. Um,
I have to go back and look at the paper.
Now we refer to it as H zero, you know,
H for Hubble and zero for constant. But I don't
know how the Santa Claus constant is. What if you

(12:31):
should have called it? Um No, But I don't know
if he called it H in his paper or if
he just observed this. The breakthrough the heat provided is
that he figured out a way to measure the distance
to really far away objects. You remember, we had a
whole podcast about how we measured the distance to stars.
It's tricky because you don't know when you look at

(12:52):
a star if it's really bright and far away or
really close and kind of dim. So you have to
know the distance um in other way. And he was
the first one to figure out a way to measure
the distance to far away stars because, as we talked
about in that podcast, it's really hard to tell that distance.
I mean from here from Earth, things just look like
little pinpoints of light and they can be really far,

(13:15):
they can be really closed. You don't really know, right,
So Hobbi used these really cool stars called Cepid's now
another astronomer. Hen we had a Levitt had earlier discovered
that there's a way to relate how fast these stars
pulsate to how bright they are. And we want to
say thank you very much to Marcus Pustle for raising
this issue and reminding us of Henrietta Levitt's work. Apologies

(13:38):
that we neglected to mention her contributions in an earlier version.
So if you know how bright these stars are supposed
to be because you can tell how fast they're pulsating,
then you know how far away they actually are by
measuring their brightness here on Earth. So building on Levitt's discovery,
this gave Hubble a way to estimate the distance to

(13:59):
those ours. And then that's a moment he made an
incredible discovery that some of these things were super duper
far away. He's like, Okay, now I have a way
to measure the distance to these stars. What are the numbers?
The calculation? That's what it sounded like. And they had
calculators back there were mechanically and it's probably turned cranker.
Maybe somebody was shoveling coal in the side of his calculator.

(14:22):
Um no, I guess I probably had a room full
of people doing math paper. Here's the sound of it.
There's my dramatic recreation of his calculation. But he had
this moment of discovery, developed this new tool, a way
to understand how far away things are. And the numbers
he got were crazy, Like the numbers he got were
like these can't even meet inside the galaxy. And that's

(14:44):
what made him realize that some of the little dust
he was seeing the guy weren't in our galaxy. There
were other galaxies far away. So he gave us this
ability to understand how far away things were and gave
us the first view outside of our galaxy into deep
deep space. That's the first he did, was he expanded
our idea of how big the universe was and how

(15:05):
far away things were. But at that time, I think
a lot of people, most people thought that the universe
was kind of fixed, right, like it wasn't. Maybe he
figured out how big it was, but at the time,
most people, I think thought the universe wasn't changing, like
it was fixed. Yeah, precisely. Once he was able to
know how far away stuff was, he could also measure
how fast it was moving away from us, and then

(15:27):
he made this plot. He's like, only maybe just plot
everything in terms of how far away it is versus
how fast it's moving away from us, And it just
sort of fell in a line. So the farther away
something is the faster it's moving away from us, and
that the slope of that line is the hubble constants
the ratio between how far something is from us and
how fast it's moving away from us. Because that's a

(15:49):
weird concept. I think. I think that can you imagine
a universe getting bigger? It's kind of not intuitive to
think that how fast things are moving and way from
us would change. Right, Like if you think of when
a grenade explodes out in space, you know, all the
bits are moving away from each other, but they're sort
of moving at the same rate. They're not moving faster

(16:11):
and faster the further out you go in the explosion,
right precisely. And the reason you shouldn't think of the
universe as a grenade is because of grenade, the explosion
comes just from the center. Is that one explosion, and
then everything is just getting pushed from there. But the
universe's expansion is totally different. It's much more like raisin
bread than like a grenade. When you cook a loaf
of raisin bread, it doesn't just expand from the center.

(16:33):
Every part of the bread is expanding, So all the
raisins are moving away from each other. Everything is stretching
at the same time. Even the stuff that's way out
there is also stretching. Yeah, if you wanted to mix
the metaphors, you'd have to, like have a grenade bread
loaf of bread that's grenades, you know, that's expanding all
the time, A bunch of tiny little grenades. I guess
in the end bread is expanding because of all the east.

(16:54):
You can think of the east is like microbial grenades.
It's like it's always exploding everywhere, and oh, the stuff
that's really far away has a lot of yeast between
here and there. And so it's the stretching and the
expanding compounds, you know, like it's it's getting bigger and
bigger and bigger, bigger and bigger and faster and faster.
The further away from you you go, precisely between us

(17:17):
and things that are far away, there's more space, and
so there's more space to be expanding, and so the
velocities are larger. And then you're saying that the Hubble constant,
it's is what tells us just how fast that's happening, Like,
is the raising bread crazy with it's some kind of
crazy yeast or was this some kind of you know, dull, old,
kind of mild, timid east which is expanding our raising

(17:40):
universe a little bit slower. Yeah, And so the Hubble
constant is expressed in terms of velocity per distance. For
every light year you go, how much faster are things
accelerating away from us? All right, let's get into the
details of this constant, and let's get into um these
apparent controversy about what that constant actually is and how

(18:02):
it's changing and why it's changing. But first let's take
a quick break, al right, Daniel. So the universe is
getting bigger, and the rate at which it is getting

(18:24):
bigger is getting bigger itself. And so this idea of
the Hubble constant, it's something that tells us how fast
it's happening. And the thing that would have blown Hubble's
mind is that this expansion is not constant. You know, Hubble, imagine, oh,
things are moving away from us at a certain rate,
and if you want more expansion, you just need a
larger space. And that's cool. But it's only twenty years

(18:44):
ago we realized that something else was happening as well,
that this expansion wasn't just continuing, but it was actually accelerating.
So the Hubble constant is not constant. In time. As
the universe is getting older and older, this expansion is
sort of picking up speed. Yeah, It's like Yeese is
going into overdrive. Yeah. And so that's why the Hubble
constant turns out to not be a constant. He thought

(19:05):
of a constant. He was just measuring it in one snapshot,
but it turns out that it's actually changing. Do you
think at some point maybe you'll consider changing the name
of it so that you don't have to caveat it
is the constant that's not a constant. There's a movement
now to call it the Hubble parameter, and I think
in most of general relativity they call it the Hubble parameter.

(19:26):
But there's also this, there's a Hubble constant which has
historical value to it. And so it's going to take
a while. You know, we're a hundred years in. Give
us another hundred years. Maybe you'll find the right name
for you. But it's maybe it's it's more like the
Hubble rate. Maybe would that be a better name, like
the Hubble rate of expansion of the universe. Yeah. Well,
in the end, really I think it's best to connect

(19:46):
it to the dark energy fraction of the universe because
the thing that's causing the expansion is this thing that's
dark energy. Right, it's only twenty years ago we discovered
that the universe is not expanding constantly. It's a spending
at an accelerating rate, which means that every year it's
getting bigger. Faster and faster. And we did this by
making another breakthrough by looking even further into the past

(20:09):
and into the far universe, by finding these supernove but
that we could use sort of in the same way
that Hubbo use those sthords to extend our distance ladder
even farther. And that told us that this acceleration of
the universe started about five billion years ago. And that's
what we call dark energy. We say dark energy is
some weird, mysterious thing which started dominating about five million

(20:29):
years ago, and it's causing the universe's expansion to accelerate.
You're saying that, um, the Hubble constant is not a
good name for it, and so the solution is not
to change the name, but to call it something else mysterious. Well,
I think it's to dig into the source of it
to understand why is the universe expanding? And I see,
let's not worry about the name. Let's focus on what's

(20:51):
making the universe get bigger and substance over style, right,
that's my motto, because I certainly don't have much style,
so I gotta go over a substance. There's I think
there's a physics style like it's a thing, isn't it.
You're either digging for compliments or you're baiting me into
a trap here. I can't tell which one maybe, But

(21:13):
the hubble constant, I think it's it's interesting to dig
into the units it has, because you were saying earlier
the hubble constant, which is not a constant, but I
guess that's it has a value right now, which is,
you know, kind of around seventy kilometers per second per
million light years, seventy kilometers per second per mega parsek

(21:36):
mega parsic, which is sort of like a distance, right, Yeah,
parsek is a distance, even though in Star Wars they
use it as a time like didn't Han Solo do
the kettle run in eleven parsecs or something, which makes
absolutely no sense. He ran tens and ten or something
like that. UM, and those units are sort of hard
to understand, so I transformed it to another set of

(21:56):
units that makes more sense to me. It's forty six
thou thousand miles per hour for every million light years,
so stuff around us is moving away from us at
forty six thousand miles per hour. For example, UM, if
you move a million light years away, things are moving
away from us another forty six thousand miles per hour.
Things around this are moving away from us at forty

(22:18):
six thousand miles per hour, but things a million light
years from here are moving at what is it, ninety
two thousand miles per hour, And if you go another
million years further out, you add another forty six thousand
miles per hour that it's moving away from us exactly.
And this is changing because as the universe expands, matter
and radiation and all that stuff gets diluted, right, it

(22:40):
gets thinned out. It's like fewer stars per cubic light year.
But dark energy, dark energy does and dark energy is
like a property of space. Every new chunk of space
that's made has its own dark energy. So dark energy,
we think, is probably constant in time while everything else
is getting diluted. And that's why the universe started excel
rating about five billion years ago, because it's about five

(23:02):
billion years ago that dark energy became the dominant thing.
It became of the energy density of any chunk of space.
The emptier space is, the I guess, the easier it
is for dark energy to expand. It is that kind
of what you're saying, that like, as it gets emptier
and emptier, it's easier for its expand and so it
expands fast. Precisely, there's some complicated general relativity there. The

(23:23):
expansion of the universe is controlled by how much matter
there is and how much radiation there is, which tends
to pull it together, and then also how much dark
energy there is, which tends to push it apart. And
so as matter and radiation get diluted away, dark energy
takes over. And and that's assuming that dark energy is constant.
That when you create new space, you get more dark energy,
and so that's what's causing this acceleration. There's less gravity,

(23:47):
I guess right, precisely. And so really what we're doing,
we're measuring the Hubble constants. We're trying to get a
handle on the dark energy. Like what fraction in the
universe is dark energy? We'd like to know about that now.
We'd also like you know about that in the future,
like is dark energy going to tear our universe apart?
And we're curious about it in the past, like the
very early universe, what fraction of the universe was dark energy?

(24:09):
How do these things all work? Because we just don't
understand dark energy like at all and so we know
that the Hubble constant or this kind of proportion of
dark energy is getting bigger, which means the universe is
getting bigger at a faster rate every second right now,
which is a little alarming. But I think what you
were you were saying is that there's some kind of

(24:31):
controversy about just how much dark energy there is because
we measure different ways, but they don't come out the
same number. That's kind of what Mike was asking about, right.
These two physical quantities, the amount of dark energy and
the Hubble parameter, they're connected, and so we measure them
together and lost to different ways. And when we do that,
we measure these using different techniques, we get different answers

(24:54):
for the Hubble constant. So that's what he calls the
unmatching Hubble constant mystery, precisely precisely, And we do this
a lot in science. We say, here's something we think
we understand. Let's measure it three different ways and see
if it agrees. If it doesn't agree, then that we
have to go back and question one of our assumptions.
It's like a clue that's something new is going on.

(25:14):
So it's a really valuable way to do things to
measure something in independent ways and look for a mistake,
because one of those ways could be like flawed, right,
And so you want to make sure that if you
look at it from different angles, it all looks the same. Yeah,
one of the techniques could have a problem with it, right,
and you don't want that bias to change the way
you look at the universe. But also your assumptions that

(25:34):
you make when you say, like, these two different techniques
should give the same answer, maybe one of those assumptions
is wrong. If you're watching a thunderstorm and you say, hey, well,
you know how far away was that flash? I'm going
to make an assumption about how far away it is
based on how long the difference between when the light
comes here and the sound comes here. You know, and
somebody else makes the same measurement somewhere else, do they

(25:56):
get the same answer? If not, that, you know, there's
something wrong with your base assumptions. And so you want
to make multiple measurements and that helps you check those
basic assumptions. You kind of want to double check if
you're going to make claims about the universe and the
future and how big it is. Yeah, I mean these
are grandios results. Yeah, absolutely, you definitely want to get
this stuff right. Okay, So there's two ways to measure

(26:18):
the Hubble constant or I guess the amount of dark
energy in the universe, and they don't agree. So so
what are these two ways? Well, the first one is
just looking at the distance sladders, like how far away
is stuff and what is its velocity? And we can
measure the velocity by looking at how much the light
from it is red shifted, meaning that if something is

(26:39):
moving away from you at a certain speed, it changes
the frequency of the light, like stretches the wavelength. And
so we can tell how fast something is moving away
from us by measuring its velocity directly, and we know
how far stuff away is. So this is a natural
extension of what Hubble did, and so we can use
that basically just to measure directly how fast is stuff
moving away from us? And that's I guess this is

(27:01):
the most straightforward. I mean, I know it's not simple,
but it's it's kind of the most direct way to
measure the expansion of the universe. Is you just look
at something really far away and you see how fast
it's moving, and you look at something really close by
and you measure how fast that move that's moving, and
so that gives you the whole picture of how the
the raising bread is expanded precisely. And the wrinkle there.

(27:21):
The thing that makes it not trivial is that the
stuff that's far away, we don't see what it's doing
right now. We see what it was doing a billion
years ago, for example, So we have to do some
back calculation to account for the fact that some of
the information we're getting is old. On the other hand,
that's also a cool clue because it tells you how
the expansion is changing over time. That's how we discovered

(27:42):
that it was accelerating. We saw stuff really far away
moving at a different speed than we expected. But isn't
it easy to confuse the two, Like, if something far
away is moving really fast, how do you know that
it's a factor of the time that's passed in between
or the factor of the distance it's from you. Well,
we measure those two things separately, right, We measured the

(28:03):
distance and the velocity totally separately. And once you know
the distance, then you can calculate how long the information
took to get here, and so we can sort of
triangulate all that stuff. I mean, the best thing would
be if we could get a complete snapshot of the
universe at every time, then we could get all this
crazy information and really triangulates stuff. You don't just get
to wish for the data you want, You work with
the data you have. I think the real triumph hear

(28:25):
of physics here is is the acronym for this project.
That's like such a great acronym. Yeah, this is called
the Shoes Experiment Supernova each zero for equations of state.
And I wish I'd been in that meeting where they
were like coming up with acronyms of the white board
to to explain this thing. I always wonder about that.

(28:46):
They're like did It's like do they try really hard?
Do they? You know what sacrifices must you make the
science to get the perfect acronym? I don't know what
kind of grammatical sacrifices must you mean? That's not even
the best slash worst acronym we're gonna talk about today.
Hang on for later on we'll be talking about all right.

(29:07):
So that's one way to measure the universe. It's just
measure things and how fast they're moving and how far
our way they are but we can also um do
something more interesting, right, Yeah, we can look back at
the very early universe. And we've talked about this on
the podcast about the surface of last scattering, the moment
that the universe went from a hot, opaque plasma and

(29:28):
cooled down and ionized and formed atoms that light could
fly through, and the light from that plasma, it's called
the cosmic microwave background, still flying around through the universe
because after that moment, the universe became transparent. And so
we get this light from the cosmic microwave background, and
we look at it and we look at all the
wiggles in it, and we can extract an incredible amount
of information from these wiggles, and the most important thing

(29:52):
that we pull out of that is we get the
fraction of the universe that is matter and the fraction
of that universe that was dark energy. But that's really
far back in time, at the time of the Big
Bank basically, right, Yeah, relatively speaking, it's three hundred thousand
years after the Big Bang, and we're getting a sense
for what was going on back then. And if you
look at it online, look for the cosmic microwave background,

(30:14):
it looks just like a massive it looks like a
giant soup, and and so and so. But you're saying,
you guys, have you know, special formulas that really look
into the what the soup looks like. And you can
firm that you can tell a lot of things like
how much dark energy that there was at the big
after the Big band, But how do you how did

(30:35):
you extrapolate that to now? Because didn't you tell me
that it's changing. Yeah, so we look at this bubbling
soup and precisely the arrangement of bubbles and the size
of the bubbles tells you a lot about the competing
forces on the soup. And some of those forces aren't matter.
They're pulling it together, and some of its dark energy
that's pushing it apart. And the important thing to understand
is we're not measuring the hubble constant itself back then

(30:56):
there weren't even stars back then. Or measuring is how
much dark energy there was. And you're right, we measured
dark energy how much there was back then. And then
what we do is we just assume that dark energy
hasn't changed. The dark energy is constant, that that every
unit of space has the same amount of dark energy
now as it did back then. Wasn't there less space
back there? So that means there's more dark energy now,

(31:18):
there's more dark energy now. Um, it's a bigger fraction
in the universe. Right now dark energy dominates the universe.
But in the early days it was a tiny irrelevant
bit player because most of the energy density was in
the form of matter and radiation. But then as the
universe expands, that dilutes and now matters like really spread thin.
Oh wait, you're saying that, like a cube of like

(31:38):
a cubic meter of space always has the same amount
of dark energy, no matter if it was now or
before when the universe was smaller. That is the key assumption.
We are assuming that we think that might be the case.
That's sort of the simplest idea. And what we're doing
by measuring the Hubble constant or the expansion of the
universe at different times, is trying to probe whether that

(31:59):
idea is wrecked. And so, assuming that dark energy is constant,
you measure what it was back in the time of
the Big Bang, you propagate that forward. You can get
a number for the Hubble constant, assuming of course, that
dark energy is constant and that radiation and matter have
just diluted. We don't know that dark energy is constant.
We're assuming that. And if you assume that, then you

(32:20):
get a number. And this number is different than the
number you get when you measure the velocity of the stars. Yeah,
if you look at the velocity of stars, you get
a number like seventy four with an uncertainty like one
and a half units of kilometers per second per megaparsic.
But if you look at the early universe, you get
a number like sixty seven point three with a smaller

(32:41):
uncertainty like half. And so those two numbers, you know,
they're different by you know, seven, and the uncertainty item
is pretty small. And both of those teams have been
working really hard to make their measurements more and more precise.
And as the measurements get more and more precise, the
numbers have not been getting close together. The errors have
been getting smaller, but the numbers have not been changing
because you know, as a as someone who's not a physicist,

(33:03):
I would look at these numbers and think, oh, that's
pretty good. Seventy four sixty seven, what's ten percent different?
Government work, you're not good enough to make for engineering.
But the key thing here is understanding your uncertainties, like
how well do you know these things? And people spend
a lot of time, like many many peach DTCs, understanding
what are the uncertainties on our distance measurements to supernova

(33:26):
or coming up with other ways to make these distance
measurements to cross check, or understanding the uncertainties in the
cosmic microwave background. And you've got to know those uncertainties
so you know how well do I know this thing?
Because if you don't know how well you know it,
you can't answer the question are these two numbers in
agreement or not? So a lot of the work goes
into nailing down the size of these uncertainties to knowing

(33:47):
how well you know something. So that's the mystery, then,
is that we're trying to measure how much dark energy
there is in the universe, which is making it grow bigger.
And if we measure look at it one way, it's
it says there should be seventy four of this dark energy.
If you look at it another way, it says it
should be sixty seven. And that's that bothers a lot.
It bothers them because it seems really unlikely to be

(34:10):
an accident. Like, if there really is one Hubble constant
and both of these things are measuring the same number,
then what are the chances of getting two numbers that
are this far apart. It's like we've done into calculation,
we do the statistics, and it's like one in ten thousand.
So it seems really unlikely. A much more likely explanation
is that there's something wrong, either something wrong with our

(34:30):
assumptions or something wrong with one of these measurements. All right, well,
let's dig into what could explain this mystery and what
it means for the future of the universe and for
you and for me, and but and for the people
for whom the universe is for, but not the people
for for whom the universe is not for. But first,

(34:51):
let's take a quick break. All right, we have a
disagreement in physics in the measurement of how much dark
energy there is in the universe. And so how do

(35:12):
you guys decide? Do you just fight it out? Do
you get into a boxing ring or cage or something
and you throw a couple of pencils in and see
what happens? Are Yeah, it's too physicist grappling with whiteboard
markers and calling each other's faces and stuff. Um, well,
maybe one side as a chalk on the other side
as a marker. Now, it's all actually very friendly, incongenial,

(35:35):
and everybody wants to understand it, and it's sort of
a good situation. You know, when you make one of
these discoveries that two measurements you make of the same
thing don't agree, it's a clue, it's a sign. And
that's what we're looking for. We are trying to understand
the universe, not just confirm what we thought. And so
when the universe tells you that you're understanding is wrong,
that's it's the first clue to getting new understanding. And

(35:59):
so they're getting room and they try to think, like, well,
what could explain this? Is one of us doing it
wrong or is one of our assumptions wrong, And that's
I think is the most exciting explanation. Right. Well, I
have a favorite, Daniel, I don't know if you have
a favorite. I like seventy four more than I like.
I mean, like one of one of these measurements seems

(36:21):
more direct to me, Like if you're measuring the speed
of the stars directly through my telescopes. That seems a
lot more direct than like looking at a picture of
the universe fourteen billion years ago, and then it's replating it. Like,
do you guys have a favorite? Do you think one
of them in particular is probably wrong? Or what's the
general feeling? Well, I like the one from the early

(36:44):
universe because it's just so clean and precise, Like you
don't need to know how far away anything is, or
make some extrapolation from this kind of star and the
other kind of star and sort of walk up the ladder.
There's a lot of assumptions involved in those distance measurements.
Whereas the cosmic microwave background so pure and clean and
so much about it works. It's predicted and been confirmed

(37:05):
in so many other ways that we have this model
of the universe that just really holds together. It's hard
to imagine how that is wrong. And so I like
that measurement. I'm not sure why. It's maybe just an
aesthetic thing. You do have a favorite. I do have
a favor. I've just admitted on areas well. So there's
some possible explanations that for what could be wrong, right,

(37:28):
that's because something must be wrong if these measurements are
not matching up. So what's what's a possible explanation. I
think one of the favorite explanations of cosmologists is this
thing called dynamical dark energy, the idea that dark energy
isn't maybe just like a property of space and constant
that for every cubic meter is basically the same dark energy,
but maybe it's changing in time as the age of

(37:50):
the universe. I have to say, holy cow, that's amazing,
and that would help resolve because remember, we have this
measurement from the early universe that's measuring one dark energy
for action that gives you a Hubble constant, and these
more recent measurements from nearby stars and supernovas that give
you a more recent measurement. So one way to make
those two things agrees to say you're not actually measuring

(38:11):
the same thing. The thing you're measuring, it's is itself changing.
So you're saying, um, one possible explanation is that the
Hubble constant, which is not a constant, is actually measuring
something that is not a constant. You constantly amaze with
your understanding. That's kind of what you say. I feel
like that's kind of what you're saying it's not only
not a constant, but what you're it's measuring is not

(38:33):
a constant. That's right. And to shroud our previous mistakes,
we slap a cool label on it and call it dynamical, right, yeah,
d D. And then, of course, you know another thing
we do is to try to like get an unbiased
third estimate, Like, let's come up with a third way
to measure this and see if it agrees with one
or the other two. Let's do this democratically, let's take
a vote. You're saying, there's a third way now to

(38:56):
measure this dark energy in the universe, and I think
it deserves a Nobel prize right away, just in its
awesome acronym. Well, it's an acronym that contains acronyms. So
they call it the Holy Cow experiment, uh aged zero
lenses in Cosmo Girl well spring, right, and so Cosmo

(39:18):
Girl is the name of another experiment that stands for
something else. And so these guys have used data from
the Cosmo Girl experiment to try to measure the expansion
rid of the universe totally independently. Wow, I mean that's
just genius in acronym. It's like not only are you
betting an acronym in an acronym, that you're betting a

(39:39):
whole different project in this project acronym. And then if
they discover something awesome, they get to shout, holy cow,
we discovered it, Holy cow, holy cow did it. And
this is another way essentially to measure how far things
are away, and it uses gravitational lenses and says, let's
say you have a really bright source of light, like

(40:00):
a quasar, and then between you and that source of
light is a is a big lens, like a big galaxy.
Because remember, galaxies have a lot of gravity, and gravity
bend space, so it can act like a lens. And
what happens is then that quasar gets distorted and you
get multiple versions of it arriving here at Earth because
the galaxy between you and the quasar has lensed it.
You know, sometimes you get like weird and duplication effects

(40:23):
and a lens and so somehow that tells you something
about how it's expanding. The university expanding, Yeah, because the
different images take different amounts of time to get here,
and these quasars flicker as you can watch these different
images flicker, and by how much time there is between
the flickering in one image and the other image, you
can tell essentially how much space it's gone through. And

(40:45):
so the delay between the two different images gives you
a sense for how far away the original quasar was.
Is it kind of like lightning and thunder like precisely
you see it and you hear it, and you use
those two things to figure out how far away the
lightning was and how bright it was. Precisely, that's exactly
the way we do it. And so this is a
totally different way because it doesn't rely on supernova, doesn't

(41:06):
rely on sephids or the other stuff. It's another way
to make the distance measurement. And their measurement agrees with
the supernova measurement, really with my favorite measurement, not you.
I should have said it agrees with you. That was
their announcement. Actually, holy cow agrees with cartoon. Holy Cow

(41:26):
cartoon is nailed it. The only person surprised was the cartoonist.
So so it's agreeing with one of the measurements, which
is measuring measuring the stars themselves. And so then, um,
doesn't that close the argument doesn't that you know, and
the mystery it doesn't because remember they're measuring things at

(41:46):
different times in the universe, and so this would have
been problematic for the supernova measurement if it had disagreed,
because they're measuring the same thing and sort of the
same epic of the universe, and they really should This
is like confirmation of the supernova measument. And but the
early universe one from the cosmic microwave background is measuring
something older, and so it could still be that they're

(42:07):
both right. And the explanation is that dark energy is changing. Oh,
I see, it's it's like there you could say that
it's not wrong. It's just that a change between when
I measured it and now. Yeah, it's like I didn't
get the answer wrong on the test. I was just
answering a different question. Maybe this tells us that this
dark energy constant is changing or has changed since the
beginning of time. Yeah, it could be. Um, there's a

(42:30):
lot of things we can do to check the cosmic
microwave background radiation measurement, and they've done all those checks
and it all works out and it really seems very convincing.
It's hard to imagine how they would get that number wrong.
On the other hand, the supernova measurement now has independent
verification from a completely different way to measure these distances.
So it's hard to understand how that one could be wrong.
So I think we're going to have to rethink our

(42:52):
fundamental understanding of what's going on with dark energy. Right,
Maybe dark energy not a constant after all. Maybe it's cool.
Maybe we should never be assuming things are constant. You know,
that's just sort of like the physics things like, don't
call constants constance. We're constantly making that mistake. Well, um,
it sounds then, though, that this mystery is getting resolved

(43:15):
as we speak right now, So Mike and Madison stay tuned.
It sounds like, as we speak, we're resolving this mystery. Yeah,
and other stuff is gonna come online to sort of
give us more pictures of this. We can use things
like gravitational waves from neutron stars collisions to try to
measure the distance to things. So that's gonna give us
another measurement and hopefully that can peer further back in

(43:37):
time than the quasars are. The supernova so we really
need to do is get another measurement of dark energy
in the very early universe. And so people have ideas
for how we might do that, and gravitational waves might help,
and so stay tuned. This cosmic mystery might eventually get resolved,
and it might get resolved in a way that totally
up ends our understanding of the entire universe. But I

(43:58):
think one thing is here, which is the mind blowing part,
which is that it's pretty clear. And now I guess
three measurements that the universe is expanding, and it's expanding
faster and faster, like this is not a theory anymore. No,
that's for sure. Nobody, no reasonable scientists disagrees with that.
It's even more well understood than climate change. Of scientists,

(44:20):
that's right. All the scientists except the ones that go
on Fox News, believe the universe is expanding and that
expansion is accelerating. So I guess, yeah, the next time
you can go out there and look at the night sky,
just think about maybe the future. You know, in the future,
things are going to be even bigger. The future is big.
It's looking big, and it's also uncertain because if dark

(44:40):
energy is changing, we don't know what's changing it, why
it's changing, and how it's planning to change in the future.
Is dark energy gonna get stronger and stronger. Is it
gonna stop dissipate? Turn around, go the other direction. We
really just can't predict the future because we have no
understanding of this dominant source of energy in the universe. Alright,
so stay two and thank you Mike for sending us

(45:02):
this question. If you have a question about the universe
or about something that you've always wondered about or read about,
send it to us and we will try to answer it.
Thanks to everybody who sends in their questions. Remember questions
at Daniel and Jorge dot com is your fastest route
to an answer about the universe. I hope you enjoyed that.
Thanks for joining us. See you next time. Before you

(45:32):
still have a question after listening to all these explanations,
please drop us a line. We'd love to hear from you.
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Feedback at Daniel and Jorge dot com. Thanks for listening,
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(45:54):
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