November 2, 2023 • 44 mins
Daniel and Jorge dig into the history of matter, dark matter and dark energy, explaining how they used to have very different ratios.
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Speaker 1 (00:08):
Hey, Daniel, what's your favorite kind of pie?
Speaker 2 (00:10):
Oh? Boy, what's not my favorite? I mean, they're all
so good.
Speaker 1 (00:14):
Are you a fan of chocolate pie?
Speaker 3 (00:15):
You know?
Speaker 2 (00:15):
I am? I mean who isn't?
Speaker 1 (00:17):
What about white chocolate pie?
Speaker 2 (00:19):
Hold on? Is that a thing? I mean, who would
do that to a pie?
Speaker 1 (00:23):
I'm sure there's a whole universe of pie out there.
I imagine there's a you know, someone in this infinite
universe that thinks white chocolate pie is the best.
Speaker 2 (00:30):
If so, then that's definitely proof that aliens exist.
Speaker 1 (00:33):
But I thought your favorite might be the number pie
pie are squared? Give you the area of the pie,
the actual pie you want to eat.
Speaker 3 (00:43):
I guess pie are around pie for everyone.
Speaker 1 (01:01):
Hi am Jorge Made, cartoonist and the author of Oliver's
Great Big Universe.
Speaker 2 (01:05):
Hi. I'm Daniel. I'm a particle physicist and a professor
at UC Irvine, and I'll try almost any kind.
Speaker 1 (01:10):
Of pie, really, any kind of pie. What about Durian pie?
Have you tried that one?
Speaker 2 (01:15):
I might have to hold my nose, but I hear
Durian's delicious.
Speaker 1 (01:18):
Yeah, I've come across Durian pie, and you kind of
do have to hold your nose. If it's not something
you're used to, it's pretty powerful stuff.
Speaker 2 (01:26):
Sounds like an exotic adventure.
Speaker 1 (01:27):
But anyways, welcome to our podcast, Daniel and Jorge Explain
the Universe, a production of iHeartRadio.
Speaker 2 (01:32):
In which we take you on an exotic tour through
the craziness and mysteries of our universe. We hope to
explain to you everything that makes sense to us and
everything that doesn't make sense to us, about the way
this amazing, beautiful, sometimes tasty, and sometimes stinky universe works.
Speaker 1 (01:48):
That's right. It is a delicious universe and fragrant as well,
full of amazing facts and incredible processes going on, all
of which we take together, mix it up, put it
inside of a crust, and bake it for you here
on the podcast.
Speaker 2 (02:00):
One of the deepest but most concrete mysteries of the
universe is what's it all made out of? Anyway? What's
out there in the universe? How much of it is
this stuff? How much of it is that stuff? And
why Daniel, would you eat mystery pie?
Speaker 1 (02:13):
Like, if somebody gave you mystery pie, would you still
eat it?
Speaker 2 (02:17):
Depends on the somebody.
Speaker 1 (02:18):
I gotta say, Yeah, I guess if it's like a
ten year old, you don't want to try a mystery pie.
But if it's like a famous chef, maybe.
Speaker 2 (02:25):
Yeahah, sure, absolutely, yeah, I'll eat a mystery pie and
no problem.
Speaker 1 (02:29):
What if it's like a world famous ten year old chef.
Speaker 2 (02:32):
Is this some reality prank show? Then probably no.
Speaker 1 (02:38):
That sounds like a great idea for the next YouTube
pit mystery pie.
Speaker 2 (02:43):
But one reason you might take a bite of mystery
pie is just your curiosity. You want to know what
kind of pies are out there in the universe, what's
possible to make out of the basic building blocks? What
kinds of stuff are there in the universe? In the end,
that provides some crucial context to the nature of our existence.
Are we made out of the most common stuff? What
other kinds of stuff is out there? How does it
(03:04):
all come together to make this glorious cosmos?
Speaker 1 (03:07):
I guess the universe is kind of like a deep
dish pie of mystery, isn't it, Because there's so much
we don't know about it. Ninety five percent of the
universe is a total mystery.
Speaker 2 (03:16):
To us exactly. And we can take this pie metaphor
one step further and describe what the universe is made
out of In terms of a pie chart, Basically, some
fraction of the universe is made out of our kind
of matter, another fraction is made out of dark matter,
A much bigger fraction is dark energy, mysterious stuff we
don't even know. So understanding the universe pie is a
(03:36):
basic goal of physics.
Speaker 1 (03:38):
Yeah, that's a very basic and deep question about the
universe that you can ask, what is the universe made
out of? What is it all about? And so we've
discovered some pretty eye opening facts about the universe, and
one of those facts is the idea that the kind
of stuff we're made out of, You and I, apples
and chocolate and white chocolate. Also, they're all made out
of kind of stuff that only represents five percent of
(04:00):
the entire universe.
Speaker 2 (04:01):
Some of the most delicious moments in physics are those
times when you realize that everything you've understood is just
a tiny fraction of reality, that you've been looking at
it like an unrepresentative sample of everything, and that the
wider context is very different from what you imagined. It's
like you're opening a third eye to understanding what the
universe is really like. And in the last few decades.
(04:23):
That's been our experience discovering that most of the universe
is not the kind of stuff that makes up me
and you and hamsters and lamas and pies.
Speaker 1 (04:31):
And stars and galaxies. It must have been pretty incredible
to think that all of these thousands of years of
science and knowledge seeking as humans have all been just
for the five percent of the universe that we now
know we represent.
Speaker 2 (04:44):
Yeah, and it's sort of a special moment in science
when we know very well how little we know. We're
always prepared for the fact that there's stuff we don't
know out there. But now we can measure very precisely
what fraction of the universe is the stuff we know
anything about, which tells us very accurately how little we
know about the universe.
Speaker 1 (05:02):
Yeah, And so today on the podcast, we'll be asking
the question, what's the matter with the matter in the universe?
Something wrong with the universe? Or is it more like,
what's the matter with you? Like it's doing something it
shouldn't be doing.
Speaker 2 (05:19):
Well, there's lots of really interesting questions you can ask
about this universe. Pie the fractions of the universe that
are made up of the various components. One of them
is like why is it this way? Another another way?
But you can also ask questions like has it always
been this way? Has the universe had different distributions of
matter and dark matter and dark energy at other points
in its history? And what does the future hold?
Speaker 1 (05:41):
Like how long has this pie been sitting out there?
And has it changed or decayed or rotted or changed
to another kind of pie In the meantime?
Speaker 2 (05:50):
Does it matter when you get a slice of the
universe pie, You might get a very different serving. It
might be more chocolatey or less chocolity, depending on when
you got your slice.
Speaker 1 (06:00):
Oh, like not just a mystery pie, but like an
ever changing mystery pie.
Speaker 2 (06:04):
Exactly because the universe is a dynamic place. It's not static.
It's expanding and it's cooling, and it's flashing around and frothing,
it's making galaxies and black holes. All sorts of stuff
is happening, and so we have no reason to believe
that the Universe pie today is the Universe pie that's
always been.
Speaker 1 (06:21):
Do you think the universe pie is getting more delicious
or less delicious?
Speaker 2 (06:25):
I'm an optimist, so I like to think that the
Universe pies most delicious days are ahead of us.
Speaker 1 (06:30):
Oh wait, so that means you're never going to eat
some are you until the day before you die or something?
Speaker 2 (06:36):
No, it means I'll continuously eat it and every bite
will be more tasty than the last.
Speaker 1 (06:40):
Oh but what if it's not infinite? What if it's finite?
Then you got to play some sort of like you know,
optimization game here? How much of the pie should you
eat every day?
Speaker 2 (06:49):
You think there's some scenario where I eat the last
bite of the universe and use it up.
Speaker 1 (06:55):
I don't know how hungry are you for knowledge about
the universe?
Speaker 2 (06:59):
Famished, But you know what they say that last bite
is always the most delicious, like the last cookie in
the bag.
Speaker 1 (07:04):
Yeah yeah, but although if you eat it right before
you talk, you're not going to have a very long
lasting memory of it.
Speaker 2 (07:10):
But I guess if I eat the entire universe, then
I'm the last bite of the universe. I'd have to
like eat myself.
Speaker 1 (07:15):
Wait, are you saying you're the most delicious thing in
the universe?
Speaker 2 (07:20):
If I eat everything else, then I'm the only thing
left in the universe, and I'm both the most and
least delicious thing in this very realistic example.
Speaker 1 (07:29):
Right right, Yeah, yeah, I don't know that. I think
our whole analogy just just broke down.
Speaker 2 (07:34):
Yeah, Daniel contains super massive black holes.
Speaker 1 (07:37):
But anyways, we have found out that the pie of
the universe is made out of only five percent of
the stuff that we're familiar with. The rest of the
stuff is a big mystery. And as you said, Daniel,
these percentages have been changing exactly.
Speaker 2 (07:49):
You might think that five percent is a minuscule fraction
of the universe, but wait till you hear about the
fraction that it's been in the past.
Speaker 1 (07:57):
Well, it's usually what we were wondering how many people
out there had thought about the pighness of the universe
and its composition, And so Daniel went out there and
asked folks, has normal matter always been five percent of
the universe?
Speaker 2 (08:09):
Thanks very much to everybody who answers these questions. I
really appreciate it, and we love hearing your voices on
the podcast. Gives us the sense that the podcast is
not one directional. It's an interactive learning experience. So if
you'd like to participate in the future, please write to
me two questions at Danielandjorge dot com.
Speaker 1 (08:26):
So think about it for a second. Do you think
normal matter has always been five percent of the universe.
Here's what people had to say.
Speaker 4 (08:36):
Well, I guess you've got normal matter, and you've got
dark matter, and you've got dark energy. I know that
dark energy is getting bigger as the universe expands, so
I would guess that when the universe was really small,
just after the Big Bang, there wasn't any dark energy,
so therefore normal matter would have been a much greater proportion.
Speaker 5 (09:00):
I don't think that there is any reason for that
percentage to be constant, but to assume that is also
to assume that regular matter, dark matter, or dark energy
to be created spontaneously.
Speaker 6 (09:13):
No, norm matter has not always been five percent of
the universe dark or the Big Bang. As far as
I know, normal matter an inche matter almost out of
one ratio, but no matter one out is there for
a part of our own universe. As for dark round
and dark energy, I don't know about that part.
Speaker 1 (09:26):
All right, awesome answers here. Most people just assume that
it hasn't always been five percent.
Speaker 2 (09:31):
Yeah, I think people are ready to be surprised.
Speaker 1 (09:34):
I guess maybe people are used to having a universe
be always changing, so like, why would it be the same.
Speaker 2 (09:39):
Yeah, that's true, although the universe is fourteen billion years old,
so you might speculate that it's kind of grown up
already and not going to change that much anymore.
Speaker 1 (09:47):
M all right, well let's dig into it, Daniel. Let's
first of all talk about this pie and where it
came from, and what we know about the pie of
the universe, how much normal matter is there now, and
is it really that normal?
Speaker 2 (10:00):
Normal is really a very inappropriate label for our kind
of matter, because we're not the biggest chunk of the
pie at all. Really, we should call that like familiar matter,
or just our kind of matter, or.
Speaker 1 (10:10):
Something delicious matter. Apparently we are the most delicious things
in the universe.
Speaker 2 (10:14):
And so on the podcast, we often talk about the
universe pie in three different categories normal matter, dark matter,
and dark energy. Physicists usually break this down into four
different categories, where they distinguish normal matter and radiation into
two different categories. So let's break that down. Normal matter
(10:34):
is the kind of stuff that you and I are
made out of, things built out of quarks and electrons,
which usually go to making protons and neutrons, and then
electrons make atoms, etc. So all the atomic matter, but
it also includes things made out of exotic quarks, top
quarks and down corks and bottom quarks, stuff that doesn't
make up the proton and the neutron but can be
created in the universe. All this kind of stuff we
(10:56):
call baryonic matter or normal matter.
Speaker 1 (11:00):
Guys, all that stuff is stuff that has mass. Like,
those are all particles with mass.
Speaker 2 (11:05):
Those are all particles with mass, but not all particles
with mass are in that category. Dark matter, for example,
we think has mass, but is not in that.
Speaker 1 (11:13):
Category assuming it's a particle though.
Speaker 2 (11:15):
Assuming it's a particle or some kind of matter with mass.
So yeah, all the normal matter has mass. That's one
of the distinctions, is this draw between matter and radiation.
Radiation is things that are effectively massless, things moving at
or very near the speed of light. So photons, for example,
are radiation. That's obvious. They have always been radiation, they
always will be radiation. Neutrinos are very very low mass
(11:38):
particles that move at almost to the speed of light,
so we count them as radiation even though technically they're
made out of stuff. They have mass to them.
Speaker 1 (11:46):
Wait, neutrinas we don't count as normal matter. Why not.
Speaker 2 (11:49):
We count them in the sort of normal slice, but
we don't call them matter. We call them radiation if
they're moving near the speed of light. And you'll see
why this distinction is important when we talk about how
the universe expand, because the expansion of the universe affects
radiation and matter differently.
Speaker 1 (12:04):
All right, then what are the other slices of the pie.
Speaker 2 (12:07):
One more caveat on the matter radiation distinction is that
the same particle can be in a different category depending
on its speed. If you take an electron, for example,
and it's just sitting there, it's not moving, it has mass,
it's not moving near the speed of light, you call
it matter. Speed up that electron to nearly the speed
of light, physicists now call that radiation because it's effectively massless,
(12:29):
its energy is so much bigger than its mass. It's
going near the speed of light, now you'd call that radiation.
So the same kind of particles can move from one
category to the other as the universe cools or heats, So,
as usual, physicists have given things kind of confusing.
Speaker 1 (12:44):
Names, also kind of arbitrary. Is it's like when it
goes kind of fast, it's totally categorized it as one thing,
and when it goes kind of slow, it's totally something
totally different. Doesn't seem very scientific, Daniel.
Speaker 2 (12:59):
Yeah, well, there are differences in the way the universe
treats things moving near the speed of light or at
the speed of light, and things moving slower. It's not
a bright line between them. There's a transition there. But
that's true for basically all of science. You know, it
was it different between water vapor and water liquid. There
is a difference, right, It's not a bright line, there's
a transition there between them. But we notice these trends
(13:21):
and we draw a dotted line and we treat them
as different stuff. So here physicists notice that as the
universe expands, matter and radiation get treated differently, so they've
drawn this line between them. Though you're right, it's really
part of a continuum.
Speaker 1 (13:34):
All right. So then what are some of the other
pieces of the pie?
Speaker 2 (13:37):
So the really big pieces of the pie are dark matter?
Speaker 3 (13:40):
Right.
Speaker 2 (13:40):
Dark matter is something that's out there in the universe.
We know. It's not made out of quarks and electrons
or photons or neutrinos or any kind of particle that
we know about. It's some kind of matter that's out there.
We see its effect gravitationally, so we know that it
has some kind of mass because it bends space in
the universe. It causes things to move, it hold together,
galaxies that spind changes the structure of the universe, all
(14:04):
this kind of stuff. So we know that it's out there,
but we don't know what it.
Speaker 1 (14:07):
Is, right, And so far we only categorize it as
something different because we don't know what it is. It
could be like, if we know what it is, we
would maybe categorize it as normal matter too, right, Or
if a physicist looks at it and squints, maybe it'll
say it's radiation too well.
Speaker 2 (14:21):
There could be dark matter and dark radiation, right. If
you have dark matter particles moving near the speed of light,
there would be dark radiation. We are very confident, though
nobody can ever be certain, that dark matter is not
made out of normal matter. It's not like a novel
rearrangement of quarks that hides in dark blobs, or some
weird combination of neutrinos, or just zillions and zillions of neutrinos.
(14:43):
And if you're curious about more details about that, check
out all of our episodes on dark matter. Briefly, we
are pretty sure that dark matter is not made of quarks,
because we know a lot about the number of quarks
in their universe that controls the amount of helium and
hygien made very early in the universe. So we're pretty
sure we can account for the number of quarks and
there's not enough to explain the dark matter. We know
(15:04):
that dark matter is not neutrinos because neutrinos move at
nearly the speed of light, and for dark matter to
have affected the shape and the structure of the universe
that we see today, the galaxies and their distributions, it
can't be moving very fast. It's low speed, and so
we're pretty sure dark matter is not made of neutrinos.
All of which to say, we're pretty sure that dark
matter is a different kind of stuff. It's not made
(15:25):
out of normal matter. It really is a different slice
of the pie.
Speaker 1 (15:27):
All right. Well, let's get into maybe the deeper parts
of our mystery pie, which is dark energy. So let's
dig into that. But first let's take a quick break.
(15:48):
All right, we're eating up the pie of the universe
here today. We're serving folks a big slice or a
little slide.
Speaker 2 (15:54):
We are serving up a big fat slice of universe pie,
and it's mostly a dark pie. I'm hoping it's chocolate.
Speaker 1 (16:01):
You're hoping. What else could something dark in a pie case?
Speaker 2 (16:06):
Yeah, mud pie is the most innocent example.
Speaker 1 (16:10):
You go, boison berry.
Speaker 5 (16:12):
Go.
Speaker 1 (16:13):
Well, the pie of the universe, as we talked about,
is about five percent the normal stuff that you and
I are made out of, that hamsters and stars and
planets are made out of. About twenty seven percent is
dark matter, at least matter. We so far that we
don't know whether where to categorize it or we don't
know much about it. But there is the big piece
of the pie. Basically, the filling of the pie is
something totally different.
Speaker 2 (16:34):
All the stuff we talked about today, dark matter, normal matter,
radiation only adds up to be like thirty two percent
of the stuff in the universe. Normal matter is five percent,
dark matter is twenty seven percent. Radiation is almost zero.
It's like ten to the minus four fraction. Dark energy
is sixty eight percent of the energy of the universe,
(16:56):
and we don't know what it is. All we know
is that dark energy is the mysterious stuff that's making
the universe expand faster and faster.
Speaker 1 (17:04):
Yeah, I guess. One day we notice that the universe
is expanding, it's accelerating in its expansion, and so we
gave that acceleration or whatever it might be causing that
acceleration a name. That's the name dark energy.
Speaker 2 (17:15):
Yeah, exactly. And we measure that expansion and we think
how much energy does it take to make that accelerating
and expansion happen. And that's what we call dark energy.
And we have a couple different ways to measure dark energy.
One is just by measuring the expansion of the universe,
and the other is by looking at the overall curvature
of the universe. All these pieces of the pie have energy,
so they all contribute energy density to the universe. And
(17:38):
Einstein tells us that energy density, not just mass, is
what curves space and causes the effect we call gravity.
So all of these things contribute to the overall energy
density of the universe, which affects its curvature. As we
recently talked about on the podcast, they all add up
to make space nice and flat. So they all add
up to what we call the critical density. We can
(18:00):
measure the sum of all of these components by measuring
the overall curvature of the universe.
Speaker 1 (18:05):
It's a flat pie universe. It's more like a pizza pie.
Speaker 2 (18:08):
You're saying, it's a thin and crispy pie, not a
deep dish.
Speaker 1 (18:12):
Well, I guess I have a question, which is, like,
what form is dark energy present in the universe? Like
is it actually there or is it kind of like
hidden behind the scenes that it only comes out when
to expand the universe.
Speaker 2 (18:24):
We don't know what dark energy is like. We don't
have a microscopic picture of what it looks like, or
what it's doing, or how it works. We know really
very little about it. It fits into our model of
the expansion of the universe and the accelerating expansion of
the universe if it does a couple of things. If
it's some kind of field the way that light is
a wiggle in the electromagnetic field, we imagine some other
(18:47):
new kind of field, some kind of field that has
high potential energy the way, for example, the Higgs field
has high potential energy. It's stuck in its little local minimum,
which is why it has so much energy stored in it.
So if you have some field with a bunch of
potential energy stored. It has this accelerating expansion effect according
to general relativity. So that's basically all we know about it.
(19:07):
We don't know what field it is. We don't know
why it exists. It's a big mystery. If you try
to ask, like, well, do the fields we know about
can they provide that potential energy? Do to calculation, that
turns out the answer is know and the fields we
know about are off by about a factor of ten
to the one hundred. So we really very clueless about
what it is that's creating this accelerating expansion. And when
(19:29):
you hear dark energy, you should really just think about
it as a description of our observation of the expansion,
not any sort of understanding of what's causing it.
Speaker 1 (19:38):
I wonder if it's like if you take two bowling
balls and you connect them with an invisible spring, and
then you bring the bowling balls together, you're creating like
some stored potential energy between the two boiling balls. It's
like you can't see it, but it's there. There's that
potential energy hitting there is and dark energy sort of
like that, like there's something about space that has this
potential energy stored in it. That's making everything accelerate then
(20:00):
get bigger.
Speaker 2 (20:00):
Yeah, that's a great description. Potential energy can be sort
of invisible, right, it's just the configuration of that field.
That's what potential energy is. Like you put that bowling
ball up on a shelf, It's got energy stored in
its configuration, the fact that it's up on the shelf
and not on the floor. Now you take a field
and you displace it from zero, you say, okay, the
field has some value. And fields can have all sorts
(20:20):
of different potential energies depending on their values. And so
this particular field, whatever it is, seems to have a
lot of potential energy stored in it, in whatever configuration
it happens to be in. It's like an infinite number
of invisible bowling balls stored on the shelves everywhere.
Speaker 1 (20:35):
In space, or like in an infinite number of invisible
springs tied between different everything together. Right.
Speaker 2 (20:41):
Yeah, that are precompressed exactly. And there's a seeming contradiction
here in dark energy. Like on one hand, we say
dark energy is sixty eight percent of the energy in
the universe. You might think, wow, it's overwhelming, it's dominant,
it controls everything, But in our lives and in our
experience and in our Solar system. Dark energy is basically negligible,
like we didn't discover it until we looked at the
(21:02):
expansion of the universe on large scales. You can't see
it in the Solar System. You can't feel it with
gravitational experiences here on Earth. Because dark energy gets stronger
over greater distances. It's the opposite of gravity. Gravity gets
weaker over greater distances. Like we have two bowling balls
next to each other, they feel each other's gravity. You
put one on Jupiter, you can basically ignore the gravity
(21:23):
the other bowling ball. But dark energy because it's a
feature of space itself. As you get more space between stuff,
it gets stronger and stronger. So you don't see dark
energy unless you're looking over really really large distance scales,
And so it has no effect on my life, or
door life, or our life in the Solar System or
super high precision measurements of the orbit of Jupiter can't
(21:43):
detect any dark energy. But it is the dominant fraction
of energy in the universe.
Speaker 1 (21:48):
I guess it's the reminder of how big the universe is.
Right like places like Earth where you have a lot
of normal matter clumped together, or even our Solar System,
they're pretty rare in the universe, right Like, outside the Earth,
it's mostly empty space. Outside of the Solar System, it's
even more empty space.
Speaker 2 (22:04):
In the same way that dark matter overwhelms the kind
of matter in the universe that's also not in our experience, right, Like,
you can't detect dark matter in our Solar System by
looking at like how the voyager probe moves in its gravity,
and that's because it's spread out through all that empty space.
Dark matter is just not nearly as clumped as normal matter.
So all the space between us and other stars, where
there's almost no normal matter, is smoothly filled with dark matter. So,
(22:27):
as you say, the bigness of the universe means that
that adds up to a big chunk of the pie.
Speaker 1 (22:32):
All right, Well, let's summarize the pie. Then we know
that the pie of the universe is about five percent
normal matter, twenty seven percent dark matter, almost zero radiation,
and about sixty eight percent dark energy. That's the pie
of the universe as we know it today. And so
the big question of the episode today is has that
always been the case? Has it always been those percentages
(22:53):
and will it change in the future. So what do
we know of that new.
Speaker 2 (22:56):
So we know that these fractions can change and have changed,
and we're radically different earlier in the history of the
universe and very likely will be radically different in the
future of the universe. And it turns out we live
in a very peculiar time in the history of the
universe when these fractions are at all similar to each other.
Most times in the universe, one of these fractions totally dominated,
(23:19):
and in the deep future of the universe, we expect
one of these fractions to totally dominate. Sort of weird
that things are kind of at all in balance right now.
Speaker 1 (23:28):
Well, I guess there are different ways that the universe
can change its composition, right, Like maybe a normal matter
turns into dark matter, or dark matter turns into dark energy,
or something like pies is there, but the ingredients change
from one to the other. Or you could have like
the pies growing and just like every day there's more
dark energy, there's more dark matter, and so the percentages
(23:50):
change in that way, right, So in which way is
the universe changing?
Speaker 2 (23:54):
So in all of those ways? Number one, a certain
kind of stuff can turn into other kinds of stuff.
For example, matter can emit radiation. An electron hanging out
in the universe can give off a photon that increases
the radiation fraction of the universe. Photons can turn into matter,
a photon can convert into an electron, and a positron
that's converting radiation into matter. Very straightforward, and these kind
(24:15):
of processes happen. Also, we think between normal matter and
dark matter that we can dig into that in a minute. Also,
the universe is expanding, and as it expands, the various
fractions get treated differently. They dilute differently as the universe
gets bigger. Finally, the universe is cooling. It's lowering in temperature,
and as the temperature gets colder, some of these processes
(24:38):
that convert one thing to another turn on or off
at various times. So all of these things change through
the history of the universe because of all of those reasons.
Speaker 1 (24:47):
Interesting. So there are many ways in which the universe
is changing. Although I feel like you forgot the third
way in which the universe can change.
Speaker 2 (24:53):
What's that puberty?
Speaker 1 (24:56):
No, when a physicist you know, looks at it differently
or wakes up differently, I think that's radiation. No, you
know today that looks like normal matter to it.
Speaker 2 (25:06):
There's always an arbitrariness to how we call these things,
and you know, I look forward to arguing with alien
physicists about the meaning of radiation.
Speaker 1 (25:14):
But as you said, the universe has gone through a
lot of changes in its body. I guess it's gone
through puberty, or is going through puberty. And so Daniel
maybe steps through what some of these changes have been
and what are they like, what happened in them.
Speaker 2 (25:26):
So let's start at the very beginning and zoom back
to the earliest point we know, which is when the
universe was very dense and very hot, right, filled with
some kind of plasma. We don't know what happened before this.
We don't know what came before this to create this hot,
dense plasma. Their theories about inflation and infulton particles and
all that is very very speculative. What we are very
(25:47):
certain about is that about thirteen point eight billion years ago,
the universe was very hot and very dense and everything
was sloshing around. And from that point on we can
model very precisely how that expands and how it cools,
and everything changes and that's about as far back as
we can go and be confident. We can speculate deeper
and talk about like crazy theories about before that. But
(26:07):
what we know very well precision cosmology takes us back
to that moment when the universe was very hot and
very dense, and at that moment we think the universe
was ninety nine point ninety nine a bunch more nine
percent radiation, that it was essentially all radiation and everything
else was a tiny fraction.
Speaker 1 (26:25):
Like it was all basically light in the Trino's right,
because that's the only thing you count as radiation is
light in theatrinas.
Speaker 2 (26:31):
Oh, I'm going to disappoint you. Actually, if you had
electrons back then, they counted as radiation because the universe
was so hot, electrons were moving at the speed of light,
and basically everything was moving it nearly the speed of light.
Because the universe was so hot, everything was so fast
that it didn't really matter how much mass it had,
even like a top cork if it existed back then.
(26:52):
The top cork is the most massive fundamental particle we
know about. But back then, in the very early universe,
it was moving it basically the speed of light because
the universe was that hot, and so everything gets counted
as radiation.
Speaker 1 (27:08):
I feel like you're basically just making stuff up that.
Speaker 2 (27:13):
I tried to warn you.
Speaker 1 (27:15):
It's been taken to it for a second here, like,
why is it important that if it's moving it closest
to the speed of flight you call electron radiation. Isn't
it still an electron?
Speaker 2 (27:23):
It's still an electron. Absolutely, but it's going to be
important as soon as the universe starts expanding and cooling.
I just want to add that there are other reasons
to think that the very early universe was filled with radiation,
not just because we call it radiation, but also because frankly,
it was very hot and filled with charge particles. And
what do hot charge particles do? They give off photons.
(27:43):
So there really were a lot of photons in the
early universe, many more than we have today. It's not
just a slippery naming.
Speaker 1 (27:50):
Scheme, but it's partly a slippery naming scheme.
Speaker 2 (27:54):
Yes, absolutely, it's partly a slippery naming scheme, but not entirely. Also,
in the early universe there was matter and antimatter, and
that annihilates and forms photons, So there really was a
lot of photons in the early universe. So it's partially
a slippery naming scheme calling stuff radiation that today we wouldn't.
But also there really was a lot of radiation stuff
(28:14):
that we would call radiation today.
Speaker 1 (28:16):
Well, then let's maybe be helpful to people and break
it down out of this is what you call radiation,
how much of it was actually light and how much
of it was electrons or quarks or you know things
with that today we would call normal matter.
Speaker 2 (28:31):
Thanks to the CMB and bearing on acoustic oscillation, we
can actually make measurements of like the photon to quark ratios.
So we do have those numbers. They change sort of
rapidly as the universe is cooling.
Speaker 1 (28:44):
Okay, So then so then what was causing these changes
between like electrons and photons.
Speaker 2 (28:50):
So what happens very early on is the universe starts
to expand, Right. What's happening is more space is being created,
but you're not creating more stuff, right, and so what
how if you have like ten particles in a tiny
box and then you make the box bigger, all now
the density decreases, right, and it gets more dilute, And
that makes sense. So the energy density is dropping, dropping, dropping,
(29:12):
So that's how it works for matter. Right, you increase
the box, you get a drop in energy density. For radiation,
the rules are a little bit different. For radiation. The
energy density actually drops faster as you increase the size
of the box because the particles get red shifted. The
expansion of space stretches the wavelengths of these particles.
Speaker 1 (29:32):
Of the photons, but doesn't it stretch also the electron.
Speaker 2 (29:36):
If they're moving very fast, if they're treated as radiation,
And that's the distinction. If you're considered radiation, it's because
effectively you have no mass, which makes your wavelength expand.
As space gets expanded, that means that your energy density
is dropping more quickly, because not only are you getting
more dilute, you're also getting red shifted. So as time
(29:56):
goes on, the energy density of radiation drops fast faster
than the energy density of matter, which does two things. One,
now some of that radiation slides over into the matter category,
and also the radiation slice of the pie starts to
decrease relative to the matter slice of the pie.
Speaker 1 (30:13):
Okay, I think what you're maybe saying is that as
the universe expands, the amount of energy that's stored in
momentum is changing. That's really kind of what's happening, right,
And so at some point, for example, the electrons were
mostly momentum because we're going so fast, but at some
point they slowed down so much that they were mostly
just the mass of the electron.
Speaker 2 (30:34):
Yeah, that's exactly right. But remember that there's something else
also happening for photons, right. Photons do get red shifted,
and people write and ask about this all the time,
like what happens to the energy of a red shifted photon?
Where did it go? Because we've had the concept of
conservation of energy banged into our head for so many years. Well,
expanding space does not respect the conservation of energy. If
(30:55):
you have a photon in a chunk of space and
then you expand that space, the wavelength of that phot
quoton also gets expanded because space itself is expanding, and
so now it has less energy. And that energy didn't
go anywhere. There's just is less of it, because the
universe doesn't respect conservation of energy unless space is fixed.
Speaker 1 (31:13):
But I guess the question is, like, it sounds like
our classification of what you call radiation and matter was
changing very rapidly during those times. But were the actual
like number of electrons changing, Were there more electrons being
created or destroyed? Was there more light being created or destroyed?
Or did that mostly stay the same.
Speaker 2 (31:30):
In the very beginning that mostly stayed the same, like
you had a bunch of processes. These things are sloshing
back and forth. Photons can turn into electron positron pairs.
Electron positron pairs can annihilate back into photons. The very
early universe, we think was in thermal equilibrium. They have
all these things going in both directions and they basically equalized.
That's what happens at a hot plasma if you give
it enough time. Then the universe is expanding and some
(31:52):
stuff gets slowed down, so it falls out of the
radiation category into matter, and stuff that got left in
the radiation category is losing energy density compared to the
matter category. So now the matter category is growing and
the radiation category is dropping, and so eventually we get
to a matter dominated universe. The universe started out dominated
by radiation, but as it expands and cools, it becomes
(32:14):
a matter dominated universe.
Speaker 1 (32:17):
And so in that way, you would say that the
percentages of matter and radiation in the universe were changing.
Speaker 2 (32:22):
That's exactly right, and that happened about fifty thousand years
after the Big Bang. At that point, the matter and
radiation sort of hit a crossover point. Radiation is dropping
more quickly, but it started out higher, and that's the
point where they cross each other. Meanwhile, dark energy is
humming along like a tiny fraction of the universe like
zeros or zerz are one this whole time, playing the
(32:43):
long game, waiting for its turn to dominate. But at
around fifty thousand years the universe became matter dominated. It
was cooled enough that a lot of stuff slowed down
and became essentially called matter, and the photon energies got
decreased because of this radiation stretching.
Speaker 1 (33:00):
And let's get into then the long game of dark
matter and the newcomer dark energy and how it took
over the entire pie of the universe. Let's dig into that,
but first let's take another quick break. All right, we're
(33:23):
talking about the universe pie, the chocolate pie, the white
chocolate pies, all the pies in the universe and what
they're made out of, and how that's been changing since
the birth of the universe and throughout all of its
history and maybe into the future. And so if we've
learned that at the beginning of the universe, physicists would
qualify most of the stuff in the universe as radiation
because it was going so fast. But as it cooled
(33:44):
down and expanded, things sort of slowed down enough that
they started to be called more a regular matter, which
is the kind of matter that we're made out of.
And so you were saying that dark matter was sort
of waiting in the wings to make an appearance here
in the history of the universe.
Speaker 2 (33:56):
Well, it was more thinking about dark energy as waiting
in the wings. Playing the very very very long the
billion year game, the first few tens of thousands of
years were a battle between radiation and matter, including normal
matter and dark matter. Both of those contributions combined were
still smaller than radiation in the very early times. Than
after about fifty thousand years, matter together overwhelmed the radiation category,
(34:19):
and that includes dark matter and normal matter. But there's
an interesting mix there between the matter and the dark
matter fractions, Like there were a bunch of electrons and
there were a bunch of protons, but we also think
there was a lot of dark matter in the early universe,
and we suspect very strongly that in the early universe,
matter and dark matter were turning into each other, that
there was some kind of force that allowed one to
(34:40):
turn into the other or back.
Speaker 1 (34:42):
Wait. And so in the point zero zero zero zero
zero zero one percent of stuff in the early universe
that you call matter, you're also including dark matter in there.
Speaker 2 (34:49):
Also including dark matter in there exactly. And we think
that there was a higher percentage of that that was
dark matter than there is today. That at the matter
section of the pie, dark matter plus normal matter, that
there was more dark matter as a fraction than there
is today.
Speaker 1 (35:04):
And you're saying that dark matter can turn and was
turning into regular matter and vice versa because it was
so hot.
Speaker 2 (35:10):
Yes, And we don't understand how this works, because we
don't know what dark matter is and whether it's a
particle and what forces it feels. But in order to
tell the story and be consistent with everything we understand,
there needs to be some mechanism for dark matter and
normal matter to slosh into each other to explain how
we got from having more dark matter in the early
universe to having less of it. Today. The story goes
something like this dark matter we think is some massive particle,
(35:33):
some very heavy particle, And as the universe is cooling,
matter and dark matter are turning back and forth into
each other. But as things get colder and colder, matter
can no longer turn into dark matter because dark matter
is too heavy. No longer is there enough energy around
to smash together normal matter particles and turn it into
dark matter. So dark matter creation stops, but dark matter
(35:54):
annihilation doesn't. Dark matter is still turning into normal matter
right in that one direction, but the reverse this process
is no longer happening. So instead of being in balance,
now dark matter is turning into normal matter, but it's
not happening the reverse, and so the dark matter fraction shrinks.
Speaker 1 (36:10):
Whoa. So like the regular matter was growing.
Speaker 2 (36:13):
In the early universe, some dark matter got turned into
normal matter and then it got frozen out. It didn't
get turned back into dark matter like it did when
things were hot and slashing around freely.
Speaker 1 (36:23):
Then to pie bake exactly.
Speaker 2 (36:26):
Somebody baked the pie. And then as the universe expands,
it cools further and there's less and less dark matter.
Then there's not enough dark matter around for there to
be much annihilation, So dark matter stops turning into normal
matter and it gets freezes in or baked in. I
guess you could call it in our analogy, but in
physics terms they call it dark matter freeze out.
Speaker 1 (36:46):
So then when when did this freeze out or baking happen?
Speaker 2 (36:48):
It happened about ten to the miners eight seconds after
the Big Bang?
Speaker 1 (36:52):
WHOA what? And since then it's been the same proportion
of regular matter and dark matter.
Speaker 2 (36:58):
Yeah, the dark matter normal matter fraction got frozen in
very early on as the universe expanded and cooled, and
then the radiation normal matter fraction changes as things expand further.
And that turnover point was like ten thousand years. So
there's lots of really fascinating time scales here.
Speaker 1 (37:14):
Okay, So then since ten to the minus second, since
the Big Bang, and we've had the same amount of
regular matter and dark matter. It sounds like what you're saying,
and so those proportions stayed the same, right, it would
be about five to one, right, m M. But then
waiting in the wings, you're saying there was dark energy.
So back then there was no dark energy.
Speaker 2 (37:32):
There was not no dark energy, but there was less
universe than there is today, and dark energy is built
into space. Every chunk of space comes with dark energy.
So you have less universe, you have less dark energy,
but it's constant in density, right, more universe, less universe,
You don't change the density of dark energy. But as
you expand the universe, you do change the density of
matter and radiation. As we talked about, you expand the box,
(37:55):
you have a smaller density of matter, and radiation drops
even faster. As the universe is expanding, both matter and
radiation are having their energy density drop very very quickly.
But dark energy isn't. It's a constant density. You make
more space, you get more dark energy. That doesn't happen
for electrons. So as the universe expands, dark energy starts
(38:15):
to creep up in its fraction.
Speaker 1 (38:17):
Although I wonder if you can make the case that
dark energy was always there, like if we don't know
what it is, and it's just like a hidden, invisible
potential energy that could may move things, wouldn't you say
it was already built into the universe from the beginning.
Speaker 2 (38:31):
Yeah, that's exactly the model, right. We think it's an
inherent part of space. As long as you had space,
you had dark energy, and it was there during this
first ten to the minus eight seconds, but the other
stuff had such a high energy density that it swamped
the whole pie. The dark energy was always there with
its same energy density, it was just small compared to
the energy density of the other components, which then faded
(38:52):
as the universe expanded.
Speaker 1 (38:54):
I guess what I mean is like, if you're closing
the universe off at a certain point, and you're saying
that the energy of the universe is not conserve, there's
more energy being pumped into it. But what if you
count the pump and where this energy is coming from.
Then maybe I wonder if you could say that dark
energy was always there at the same percentage.
Speaker 2 (39:11):
Yeah, perhaps when there's lots of the theories of dark
energy and what it might be is it's some kind
of weird field, is it some kind of other stuff?
And so those various theories would upset these fractions if
you included them.
Speaker 1 (39:22):
Yeah, And so that's how the universe pie has been changing.
But it seems like it hasn't really changed much since
ten to the minus eight seconds, since the Big Bang,
except just that dark energy has been growing.
Speaker 2 (39:32):
Yeah, dark energy has been growing, which makes for a
fascinating tug of war because dark energy is growing. But
in the early universe, like a billion years in, we're
still matter dominated, right, Radiation is faded away. We're in
a matter dominated universe. The universe is expanding, but that
expansion is now decelerating. It's slowing down because the universe
is dominated by matter, and what does matter tend to do?
(39:54):
Pulls things together, right, It slows down the expansion of
the universe. But because it was expanding still even though decelerating,
dark energy is creeping up and up and up, and
dark energy makes the universe expand faster, and eventually the
universe expanded enough so that dark energy just took over.
And around eight billion years after the beginning of the universe,
(40:14):
dark energy was the dominant fraction. And you'll see that
the universe start to accelerate its expansion because dark energy
takes over. And that's basically the future. Dark energy is
a runaway process. If nothing else changes, dark energy will
continue to grow as a fraction of the energy density
of the universe, making the universe accelerate faster and faster,
increasing the dark energy fraction faster and faster.
Speaker 1 (40:36):
All right, So then in like a billion years from now,
what's going to be the percentage or a pie breakdown
of the universe if you had to get well.
Speaker 2 (40:43):
It's like sixty eight percent dark energy. Now it's just
going to increase. It's going to go to ninety percent,
ninety five percent, ninety nine percent. But this is over
billions and billions and trillions of years into the future.
But it's only going to crank up. But you know,
we don't know that, right. Remember, we don't know what
dark energy is. We don't know for sure what its
behavior is going to be in the future. This model
describes very very well the history of these energy fractions
(41:06):
and how the universe pie has changed, with a couple
of caveats, like the measurements of the dark energy in
the early universe don't one hundred percent agree with our
measurements in the late universe. There is this hubble tension.
But mostly this picture of the sloshing pieces of the
pie holds together very well and matches all of our data.
Speaker 1 (41:23):
All right. So then in the future, normal matter, dark matter,
and radiation, they're all going to stay the same amount,
but the percentage is going to go down because dark
energy is growing, and so in the far future it's
going to be like ninety nine nine nine nine percent
dark energy then the universe.
Speaker 2 (41:38):
Yeah, exactly. And the universe started out dominated by one
fraction of the pie, it's going to end up dominated
by another fraction of the pie. And there's this little
window in the middle where multiple fractions are not zero.
It's a very unusual time to be in the universe
when you have a bunch of different kinds of stuff around.
You got dark matter, you've got normal matter, you've got
dark energy, all at the same time.
Speaker 1 (41:58):
But according to you, we're still in the undelicious part
of the universe. Apparently, apparently, thanks, things can't get worse,
but they can only get better.
Speaker 2 (42:09):
I wouldn't say undelicious, I'd say less delicious than the future.
That's the optimistic way to think about it.
Speaker 1 (42:14):
Oh, there you go. And let's hope that dark energy
is the most delicious thing in the universe, because that
seems to be the only thing that's going to be
around in the future percentage wise.
Speaker 2 (42:22):
And caveats for those of you who they like to
think a lot about dark matter. This assumes a fairly
simple model of dark matter. That it was in thermal
equilibrium with everything else early in the universe. That doesn't
have to be the case. You can have other theories
of dark matter axions, et cetera that are so weakly
coupled that hardly interacts, so they're not thermally mixed with
the stuff in the universe. There's lots of other ways
you could change this picture. This is like the simplest
(42:44):
model we can make that describes everything we see, and
it works really pretty well.
Speaker 1 (42:49):
All right, Well, it's another big reminder that the universe
is a big, mysterious piece of pastry out there. There's
still a lot to learn, a lot to explore, a
lot to taste, and a lot to bake as well.
Speaker 2 (43:00):
And as you take a big bite of the universe,
remember that not only is our kind of stuff unusual
in the universe, it's getting more and more unusual. And
we live in an unusual time in the universe when
our kind of stuff and dark matter is a really
significant fraction of the stuff out there in the universe,
alien civilizations in the year three trillion, that we'll have
a very different kind of physics to deal with.
Speaker 1 (43:21):
All right, Well, we hope you go out there and
grab your piece of the pie of the curiosity and
mystery of the universe. You hope you enjoyed that. Thanks
for joining us, See you next time.
Speaker 2 (43:40):
Thanks for listening, and remember that Daniel and Jorge Explain
the Universe is a production of iHeartRadio. For more podcasts
from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever
you listen to your favorite shows.
Hey, Daniel, what's your favorite kind of pie?
Speaker 2 (00:10):
Oh? Boy, what's not my favorite? I mean, they're all
so good.
Speaker 1 (00:14):
Are you a fan of chocolate pie?
Speaker 3 (00:15):
You know?
Speaker 2 (00:15):
I am? I mean who isn't?
Speaker 1 (00:17):
What about white chocolate pie?
Speaker 2 (00:19):
Hold on? Is that a thing? I mean, who would
do that to a pie?
Speaker 1 (00:23):
I'm sure there's a whole universe of pie out there.
I imagine there's a you know, someone in this infinite
universe that thinks white chocolate pie is the best.
Speaker 2 (00:30):
If so, then that's definitely proof that aliens exist.
Speaker 1 (00:33):
But I thought your favorite might be the number pie
pie are squared? Give you the area of the pie,
the actual pie you want to eat.
Speaker 3 (00:43):
I guess pie are around pie for everyone.
Speaker 1 (01:01):
Hi am Jorge Made, cartoonist and the author of Oliver's
Great Big Universe.
Speaker 2 (01:05):
Hi. I'm Daniel. I'm a particle physicist and a professor
at UC Irvine, and I'll try almost any kind.
Speaker 1 (01:10):
Of pie, really, any kind of pie. What about Durian pie?
Have you tried that one?
Speaker 2 (01:15):
I might have to hold my nose, but I hear
Durian's delicious.
Speaker 1 (01:18):
Yeah, I've come across Durian pie, and you kind of
do have to hold your nose. If it's not something
you're used to, it's pretty powerful stuff.
Speaker 2 (01:26):
Sounds like an exotic adventure.
Speaker 1 (01:27):
But anyways, welcome to our podcast, Daniel and Jorge Explain
the Universe, a production of iHeartRadio.
Speaker 2 (01:32):
In which we take you on an exotic tour through
the craziness and mysteries of our universe. We hope to
explain to you everything that makes sense to us and
everything that doesn't make sense to us, about the way
this amazing, beautiful, sometimes tasty, and sometimes stinky universe works.
Speaker 1 (01:48):
That's right. It is a delicious universe and fragrant as well,
full of amazing facts and incredible processes going on, all
of which we take together, mix it up, put it
inside of a crust, and bake it for you here
on the podcast.
Speaker 2 (02:00):
One of the deepest but most concrete mysteries of the
universe is what's it all made out of? Anyway? What's
out there in the universe? How much of it is
this stuff? How much of it is that stuff? And
why Daniel, would you eat mystery pie?
Speaker 1 (02:13):
Like, if somebody gave you mystery pie, would you still
eat it?
Speaker 2 (02:17):
Depends on the somebody.
Speaker 1 (02:18):
I gotta say, Yeah, I guess if it's like a
ten year old, you don't want to try a mystery pie.
But if it's like a famous chef, maybe.
Speaker 2 (02:25):
Yeahah, sure, absolutely, yeah, I'll eat a mystery pie and
no problem.
Speaker 1 (02:29):
What if it's like a world famous ten year old chef.
Speaker 2 (02:32):
Is this some reality prank show? Then probably no.
Speaker 1 (02:38):
That sounds like a great idea for the next YouTube
pit mystery pie.
Speaker 2 (02:43):
But one reason you might take a bite of mystery
pie is just your curiosity. You want to know what
kind of pies are out there in the universe, what's
possible to make out of the basic building blocks? What
kinds of stuff are there in the universe? In the end,
that provides some crucial context to the nature of our existence.
Are we made out of the most common stuff? What
other kinds of stuff is out there? How does it
(03:04):
all come together to make this glorious cosmos?
Speaker 1 (03:07):
I guess the universe is kind of like a deep
dish pie of mystery, isn't it, Because there's so much
we don't know about it. Ninety five percent of the
universe is a total mystery.
Speaker 2 (03:16):
To us exactly. And we can take this pie metaphor
one step further and describe what the universe is made
out of In terms of a pie chart, Basically, some
fraction of the universe is made out of our kind
of matter, another fraction is made out of dark matter,
A much bigger fraction is dark energy, mysterious stuff we
don't even know. So understanding the universe pie is a
(03:36):
basic goal of physics.
Speaker 1 (03:38):
Yeah, that's a very basic and deep question about the
universe that you can ask, what is the universe made
out of? What is it all about? And so we've
discovered some pretty eye opening facts about the universe, and
one of those facts is the idea that the kind
of stuff we're made out of, You and I, apples
and chocolate and white chocolate. Also, they're all made out
of kind of stuff that only represents five percent of
(04:00):
the entire universe.
Speaker 2 (04:01):
Some of the most delicious moments in physics are those
times when you realize that everything you've understood is just
a tiny fraction of reality, that you've been looking at
it like an unrepresentative sample of everything, and that the
wider context is very different from what you imagined. It's
like you're opening a third eye to understanding what the
universe is really like. And in the last few decades.
(04:23):
That's been our experience discovering that most of the universe
is not the kind of stuff that makes up me
and you and hamsters and lamas and pies.
Speaker 1 (04:31):
And stars and galaxies. It must have been pretty incredible
to think that all of these thousands of years of
science and knowledge seeking as humans have all been just
for the five percent of the universe that we now
know we represent.
Speaker 2 (04:44):
Yeah, and it's sort of a special moment in science
when we know very well how little we know. We're
always prepared for the fact that there's stuff we don't
know out there. But now we can measure very precisely
what fraction of the universe is the stuff we know
anything about, which tells us very accurately how little we
know about the universe.
Speaker 1 (05:02):
Yeah, And so today on the podcast, we'll be asking
the question, what's the matter with the matter in the universe?
Something wrong with the universe? Or is it more like,
what's the matter with you? Like it's doing something it
shouldn't be doing.
Speaker 2 (05:19):
Well, there's lots of really interesting questions you can ask
about this universe. Pie the fractions of the universe that
are made up of the various components. One of them
is like why is it this way? Another another way?
But you can also ask questions like has it always
been this way? Has the universe had different distributions of
matter and dark matter and dark energy at other points
in its history? And what does the future hold?
Speaker 1 (05:41):
Like how long has this pie been sitting out there?
And has it changed or decayed or rotted or changed
to another kind of pie In the meantime?
Speaker 2 (05:50):
Does it matter when you get a slice of the
universe pie, You might get a very different serving. It
might be more chocolatey or less chocolity, depending on when
you got your slice.
Speaker 1 (06:00):
Oh, like not just a mystery pie, but like an
ever changing mystery pie.
Speaker 2 (06:04):
Exactly because the universe is a dynamic place. It's not static.
It's expanding and it's cooling, and it's flashing around and frothing,
it's making galaxies and black holes. All sorts of stuff
is happening, and so we have no reason to believe
that the Universe pie today is the Universe pie that's
always been.
Speaker 1 (06:21):
Do you think the universe pie is getting more delicious
or less delicious?
Speaker 2 (06:25):
I'm an optimist, so I like to think that the
Universe pies most delicious days are ahead of us.
Speaker 1 (06:30):
Oh wait, so that means you're never going to eat
some are you until the day before you die or something?
Speaker 2 (06:36):
No, it means I'll continuously eat it and every bite
will be more tasty than the last.
Speaker 1 (06:40):
Oh but what if it's not infinite? What if it's finite?
Then you got to play some sort of like you know,
optimization game here? How much of the pie should you
eat every day?
Speaker 2 (06:49):
You think there's some scenario where I eat the last
bite of the universe and use it up.
Speaker 1 (06:55):
I don't know how hungry are you for knowledge about
the universe?
Speaker 2 (06:59):
Famished, But you know what they say that last bite
is always the most delicious, like the last cookie in
the bag.
Speaker 1 (07:04):
Yeah yeah, but although if you eat it right before
you talk, you're not going to have a very long
lasting memory of it.
Speaker 2 (07:10):
But I guess if I eat the entire universe, then
I'm the last bite of the universe. I'd have to
like eat myself.
Speaker 1 (07:15):
Wait, are you saying you're the most delicious thing in
the universe?
Speaker 2 (07:20):
If I eat everything else, then I'm the only thing
left in the universe, and I'm both the most and
least delicious thing in this very realistic example.
Speaker 1 (07:29):
Right right, Yeah, yeah, I don't know that. I think
our whole analogy just just broke down.
Speaker 2 (07:34):
Yeah, Daniel contains super massive black holes.
Speaker 1 (07:37):
But anyways, we have found out that the pie of
the universe is made out of only five percent of
the stuff that we're familiar with. The rest of the
stuff is a big mystery. And as you said, Daniel,
these percentages have been changing exactly.
Speaker 2 (07:49):
You might think that five percent is a minuscule fraction
of the universe, but wait till you hear about the
fraction that it's been in the past.
Speaker 1 (07:57):
Well, it's usually what we were wondering how many people
out there had thought about the pighness of the universe
and its composition, And so Daniel went out there and
asked folks, has normal matter always been five percent of
the universe?
Speaker 2 (08:09):
Thanks very much to everybody who answers these questions. I
really appreciate it, and we love hearing your voices on
the podcast. Gives us the sense that the podcast is
not one directional. It's an interactive learning experience. So if
you'd like to participate in the future, please write to
me two questions at Danielandjorge dot com.
Speaker 1 (08:26):
So think about it for a second. Do you think
normal matter has always been five percent of the universe.
Here's what people had to say.
Speaker 4 (08:36):
Well, I guess you've got normal matter, and you've got
dark matter, and you've got dark energy. I know that
dark energy is getting bigger as the universe expands, so
I would guess that when the universe was really small,
just after the Big Bang, there wasn't any dark energy,
so therefore normal matter would have been a much greater proportion.
Speaker 5 (09:00):
I don't think that there is any reason for that
percentage to be constant, but to assume that is also
to assume that regular matter, dark matter, or dark energy
to be created spontaneously.
Speaker 6 (09:13):
No, norm matter has not always been five percent of
the universe dark or the Big Bang. As far as
I know, normal matter an inche matter almost out of
one ratio, but no matter one out is there for
a part of our own universe. As for dark round
and dark energy, I don't know about that part.
Speaker 1 (09:26):
All right, awesome answers here. Most people just assume that
it hasn't always been five percent.
Speaker 2 (09:31):
Yeah, I think people are ready to be surprised.
Speaker 1 (09:34):
I guess maybe people are used to having a universe
be always changing, so like, why would it be the same.
Speaker 2 (09:39):
Yeah, that's true, although the universe is fourteen billion years old,
so you might speculate that it's kind of grown up
already and not going to change that much anymore.
Speaker 1 (09:47):
M all right, well let's dig into it, Daniel. Let's
first of all talk about this pie and where it
came from, and what we know about the pie of
the universe, how much normal matter is there now, and
is it really that normal?
Speaker 2 (10:00):
Normal is really a very inappropriate label for our kind
of matter, because we're not the biggest chunk of the
pie at all. Really, we should call that like familiar matter,
or just our kind of matter, or.
Speaker 1 (10:10):
Something delicious matter. Apparently we are the most delicious things
in the universe.
Speaker 2 (10:14):
And so on the podcast, we often talk about the
universe pie in three different categories normal matter, dark matter,
and dark energy. Physicists usually break this down into four
different categories, where they distinguish normal matter and radiation into
two different categories. So let's break that down. Normal matter
(10:34):
is the kind of stuff that you and I are
made out of, things built out of quarks and electrons,
which usually go to making protons and neutrons, and then
electrons make atoms, etc. So all the atomic matter, but
it also includes things made out of exotic quarks, top
quarks and down corks and bottom quarks, stuff that doesn't
make up the proton and the neutron but can be
created in the universe. All this kind of stuff we
(10:56):
call baryonic matter or normal matter.
Speaker 1 (11:00):
Guys, all that stuff is stuff that has mass. Like,
those are all particles with mass.
Speaker 2 (11:05):
Those are all particles with mass, but not all particles
with mass are in that category. Dark matter, for example,
we think has mass, but is not in that.
Speaker 1 (11:13):
Category assuming it's a particle though.
Speaker 2 (11:15):
Assuming it's a particle or some kind of matter with mass.
So yeah, all the normal matter has mass. That's one
of the distinctions, is this draw between matter and radiation.
Radiation is things that are effectively massless, things moving at
or very near the speed of light. So photons, for example,
are radiation. That's obvious. They have always been radiation, they
always will be radiation. Neutrinos are very very low mass
(11:38):
particles that move at almost to the speed of light,
so we count them as radiation even though technically they're
made out of stuff. They have mass to them.
Speaker 1 (11:46):
Wait, neutrinas we don't count as normal matter. Why not.
Speaker 2 (11:49):
We count them in the sort of normal slice, but
we don't call them matter. We call them radiation if
they're moving near the speed of light. And you'll see
why this distinction is important when we talk about how
the universe expand, because the expansion of the universe affects
radiation and matter differently.
Speaker 1 (12:04):
All right, then what are the other slices of the pie.
Speaker 2 (12:07):
One more caveat on the matter radiation distinction is that
the same particle can be in a different category depending
on its speed. If you take an electron, for example,
and it's just sitting there, it's not moving, it has mass,
it's not moving near the speed of light, you call
it matter. Speed up that electron to nearly the speed
of light, physicists now call that radiation because it's effectively massless,
(12:29):
its energy is so much bigger than its mass. It's
going near the speed of light, now you'd call that radiation.
So the same kind of particles can move from one
category to the other as the universe cools or heats, So,
as usual, physicists have given things kind of confusing.
Speaker 1 (12:44):
Names, also kind of arbitrary. Is it's like when it
goes kind of fast, it's totally categorized it as one thing,
and when it goes kind of slow, it's totally something
totally different. Doesn't seem very scientific, Daniel.
Speaker 2 (12:59):
Yeah, well, there are differences in the way the universe
treats things moving near the speed of light or at
the speed of light, and things moving slower. It's not
a bright line between them. There's a transition there. But
that's true for basically all of science. You know, it
was it different between water vapor and water liquid. There
is a difference, right, It's not a bright line, there's
a transition there between them. But we notice these trends
(13:21):
and we draw a dotted line and we treat them
as different stuff. So here physicists notice that as the
universe expands, matter and radiation get treated differently, so they've
drawn this line between them. Though you're right, it's really
part of a continuum.
Speaker 1 (13:34):
All right. So then what are some of the other
pieces of the pie?
Speaker 2 (13:37):
So the really big pieces of the pie are dark matter?
Speaker 3 (13:40):
Right.
Speaker 2 (13:40):
Dark matter is something that's out there in the universe.
We know. It's not made out of quarks and electrons
or photons or neutrinos or any kind of particle that
we know about. It's some kind of matter that's out there.
We see its effect gravitationally, so we know that it
has some kind of mass because it bends space in
the universe. It causes things to move, it hold together,
galaxies that spind changes the structure of the universe, all
(14:04):
this kind of stuff. So we know that it's out there,
but we don't know what it.
Speaker 1 (14:07):
Is, right, And so far we only categorize it as
something different because we don't know what it is. It
could be like, if we know what it is, we
would maybe categorize it as normal matter too, right, Or
if a physicist looks at it and squints, maybe it'll
say it's radiation too well.
Speaker 2 (14:21):
There could be dark matter and dark radiation, right. If
you have dark matter particles moving near the speed of light,
there would be dark radiation. We are very confident, though
nobody can ever be certain, that dark matter is not
made out of normal matter. It's not like a novel
rearrangement of quarks that hides in dark blobs, or some
weird combination of neutrinos, or just zillions and zillions of neutrinos.
(14:43):
And if you're curious about more details about that, check
out all of our episodes on dark matter. Briefly, we
are pretty sure that dark matter is not made of quarks,
because we know a lot about the number of quarks
in their universe that controls the amount of helium and
hygien made very early in the universe. So we're pretty
sure we can account for the number of quarks and
there's not enough to explain the dark matter. We know
(15:04):
that dark matter is not neutrinos because neutrinos move at
nearly the speed of light, and for dark matter to
have affected the shape and the structure of the universe
that we see today, the galaxies and their distributions, it
can't be moving very fast. It's low speed, and so
we're pretty sure dark matter is not made of neutrinos.
All of which to say, we're pretty sure that dark
matter is a different kind of stuff. It's not made
(15:25):
out of normal matter. It really is a different slice
of the pie.
Speaker 1 (15:27):
All right. Well, let's get into maybe the deeper parts
of our mystery pie, which is dark energy. So let's
dig into that. But first let's take a quick break.
(15:48):
All right, we're eating up the pie of the universe
here today. We're serving folks a big slice or a
little slide.
Speaker 2 (15:54):
We are serving up a big fat slice of universe pie,
and it's mostly a dark pie. I'm hoping it's chocolate.
Speaker 1 (16:01):
You're hoping. What else could something dark in a pie case?
Speaker 2 (16:06):
Yeah, mud pie is the most innocent example.
Speaker 1 (16:10):
You go, boison berry.
Speaker 5 (16:12):
Go.
Speaker 1 (16:13):
Well, the pie of the universe, as we talked about,
is about five percent the normal stuff that you and
I are made out of, that hamsters and stars and
planets are made out of. About twenty seven percent is
dark matter, at least matter. We so far that we
don't know whether where to categorize it or we don't
know much about it. But there is the big piece
of the pie. Basically, the filling of the pie is
something totally different.
Speaker 2 (16:34):
All the stuff we talked about today, dark matter, normal matter,
radiation only adds up to be like thirty two percent
of the stuff in the universe. Normal matter is five percent,
dark matter is twenty seven percent. Radiation is almost zero.
It's like ten to the minus four fraction. Dark energy
is sixty eight percent of the energy of the universe,
(16:56):
and we don't know what it is. All we know
is that dark energy is the mysterious stuff that's making
the universe expand faster and faster.
Speaker 1 (17:04):
Yeah, I guess. One day we notice that the universe
is expanding, it's accelerating in its expansion, and so we
gave that acceleration or whatever it might be causing that
acceleration a name. That's the name dark energy.
Speaker 2 (17:15):
Yeah, exactly. And we measure that expansion and we think
how much energy does it take to make that accelerating
and expansion happen. And that's what we call dark energy.
And we have a couple different ways to measure dark energy.
One is just by measuring the expansion of the universe,
and the other is by looking at the overall curvature
of the universe. All these pieces of the pie have energy,
so they all contribute energy density to the universe. And
(17:38):
Einstein tells us that energy density, not just mass, is
what curves space and causes the effect we call gravity.
So all of these things contribute to the overall energy
density of the universe, which affects its curvature. As we
recently talked about on the podcast, they all add up
to make space nice and flat. So they all add
up to what we call the critical density. We can
(18:00):
measure the sum of all of these components by measuring
the overall curvature of the universe.
Speaker 1 (18:05):
It's a flat pie universe. It's more like a pizza pie.
Speaker 2 (18:08):
You're saying, it's a thin and crispy pie, not a
deep dish.
Speaker 1 (18:12):
Well, I guess I have a question, which is, like,
what form is dark energy present in the universe? Like
is it actually there or is it kind of like
hidden behind the scenes that it only comes out when
to expand the universe.
Speaker 2 (18:24):
We don't know what dark energy is like. We don't
have a microscopic picture of what it looks like, or
what it's doing, or how it works. We know really
very little about it. It fits into our model of
the expansion of the universe and the accelerating expansion of
the universe if it does a couple of things. If
it's some kind of field the way that light is
a wiggle in the electromagnetic field, we imagine some other
(18:47):
new kind of field, some kind of field that has
high potential energy the way, for example, the Higgs field
has high potential energy. It's stuck in its little local minimum,
which is why it has so much energy stored in it.
So if you have some field with a bunch of
potential energy stored. It has this accelerating expansion effect according
to general relativity. So that's basically all we know about it.
(19:07):
We don't know what field it is. We don't know
why it exists. It's a big mystery. If you try
to ask, like, well, do the fields we know about
can they provide that potential energy? Do to calculation, that
turns out the answer is know and the fields we
know about are off by about a factor of ten
to the one hundred. So we really very clueless about
what it is that's creating this accelerating expansion. And when
(19:29):
you hear dark energy, you should really just think about
it as a description of our observation of the expansion,
not any sort of understanding of what's causing it.
Speaker 1 (19:38):
I wonder if it's like if you take two bowling
balls and you connect them with an invisible spring, and
then you bring the bowling balls together, you're creating like
some stored potential energy between the two boiling balls. It's
like you can't see it, but it's there. There's that
potential energy hitting there is and dark energy sort of
like that, like there's something about space that has this
potential energy stored in it. That's making everything accelerate then
(20:00):
get bigger.
Speaker 2 (20:00):
Yeah, that's a great description. Potential energy can be sort
of invisible, right, it's just the configuration of that field.
That's what potential energy is. Like you put that bowling
ball up on a shelf, It's got energy stored in
its configuration, the fact that it's up on the shelf
and not on the floor. Now you take a field
and you displace it from zero, you say, okay, the
field has some value. And fields can have all sorts
(20:20):
of different potential energies depending on their values. And so
this particular field, whatever it is, seems to have a
lot of potential energy stored in it, in whatever configuration
it happens to be in. It's like an infinite number
of invisible bowling balls stored on the shelves everywhere.
Speaker 1 (20:35):
In space, or like in an infinite number of invisible
springs tied between different everything together. Right.
Speaker 2 (20:41):
Yeah, that are precompressed exactly. And there's a seeming contradiction
here in dark energy. Like on one hand, we say
dark energy is sixty eight percent of the energy in
the universe. You might think, wow, it's overwhelming, it's dominant,
it controls everything, But in our lives and in our
experience and in our Solar system. Dark energy is basically negligible,
like we didn't discover it until we looked at the
(21:02):
expansion of the universe on large scales. You can't see
it in the Solar System. You can't feel it with
gravitational experiences here on Earth. Because dark energy gets stronger
over greater distances. It's the opposite of gravity. Gravity gets
weaker over greater distances. Like we have two bowling balls
next to each other, they feel each other's gravity. You
put one on Jupiter, you can basically ignore the gravity
(21:23):
the other bowling ball. But dark energy because it's a
feature of space itself. As you get more space between stuff,
it gets stronger and stronger. So you don't see dark
energy unless you're looking over really really large distance scales,
And so it has no effect on my life, or
door life, or our life in the Solar System or
super high precision measurements of the orbit of Jupiter can't
(21:43):
detect any dark energy. But it is the dominant fraction
of energy in the universe.
Speaker 1 (21:48):
I guess it's the reminder of how big the universe is.
Right like places like Earth where you have a lot
of normal matter clumped together, or even our Solar System,
they're pretty rare in the universe, right Like, outside the Earth,
it's mostly empty space. Outside of the Solar System, it's
even more empty space.
Speaker 2 (22:04):
In the same way that dark matter overwhelms the kind
of matter in the universe that's also not in our experience, right, Like,
you can't detect dark matter in our Solar System by
looking at like how the voyager probe moves in its gravity,
and that's because it's spread out through all that empty space.
Dark matter is just not nearly as clumped as normal matter.
So all the space between us and other stars, where
there's almost no normal matter, is smoothly filled with dark matter. So,
(22:27):
as you say, the bigness of the universe means that
that adds up to a big chunk of the pie.
Speaker 1 (22:32):
All right, Well, let's summarize the pie. Then we know
that the pie of the universe is about five percent
normal matter, twenty seven percent dark matter, almost zero radiation,
and about sixty eight percent dark energy. That's the pie
of the universe as we know it today. And so
the big question of the episode today is has that
always been the case? Has it always been those percentages
(22:53):
and will it change in the future. So what do
we know of that new.
Speaker 2 (22:56):
So we know that these fractions can change and have changed,
and we're radically different earlier in the history of the
universe and very likely will be radically different in the
future of the universe. And it turns out we live
in a very peculiar time in the history of the
universe when these fractions are at all similar to each other.
Most times in the universe, one of these fractions totally dominated,
(23:19):
and in the deep future of the universe, we expect
one of these fractions to totally dominate. Sort of weird
that things are kind of at all in balance right now.
Speaker 1 (23:28):
Well, I guess there are different ways that the universe
can change its composition, right, Like maybe a normal matter
turns into dark matter, or dark matter turns into dark energy,
or something like pies is there, but the ingredients change
from one to the other. Or you could have like
the pies growing and just like every day there's more
dark energy, there's more dark matter, and so the percentages
(23:50):
change in that way, right, So in which way is
the universe changing?
Speaker 2 (23:54):
So in all of those ways? Number one, a certain
kind of stuff can turn into other kinds of stuff.
For example, matter can emit radiation. An electron hanging out
in the universe can give off a photon that increases
the radiation fraction of the universe. Photons can turn into matter,
a photon can convert into an electron, and a positron
that's converting radiation into matter. Very straightforward, and these kind
(24:15):
of processes happen. Also, we think between normal matter and
dark matter that we can dig into that in a minute. Also,
the universe is expanding, and as it expands, the various
fractions get treated differently. They dilute differently as the universe
gets bigger. Finally, the universe is cooling. It's lowering in temperature,
and as the temperature gets colder, some of these processes
(24:38):
that convert one thing to another turn on or off
at various times. So all of these things change through
the history of the universe because of all of those reasons.
Speaker 1 (24:47):
Interesting. So there are many ways in which the universe
is changing. Although I feel like you forgot the third
way in which the universe can change.
Speaker 2 (24:53):
What's that puberty?
Speaker 1 (24:56):
No, when a physicist you know, looks at it differently
or wakes up differently, I think that's radiation. No, you
know today that looks like normal matter to it.
Speaker 2 (25:06):
There's always an arbitrariness to how we call these things,
and you know, I look forward to arguing with alien
physicists about the meaning of radiation.
Speaker 1 (25:14):
But as you said, the universe has gone through a
lot of changes in its body. I guess it's gone
through puberty, or is going through puberty. And so Daniel
maybe steps through what some of these changes have been
and what are they like, what happened in them.
Speaker 2 (25:26):
So let's start at the very beginning and zoom back
to the earliest point we know, which is when the
universe was very dense and very hot, right, filled with
some kind of plasma. We don't know what happened before this.
We don't know what came before this to create this hot,
dense plasma. Their theories about inflation and infulton particles and
all that is very very speculative. What we are very
(25:47):
certain about is that about thirteen point eight billion years ago,
the universe was very hot and very dense and everything
was sloshing around. And from that point on we can
model very precisely how that expands and how it cools,
and everything changes and that's about as far back as
we can go and be confident. We can speculate deeper
and talk about like crazy theories about before that. But
(26:07):
what we know very well precision cosmology takes us back
to that moment when the universe was very hot and
very dense, and at that moment we think the universe
was ninety nine point ninety nine a bunch more nine
percent radiation, that it was essentially all radiation and everything
else was a tiny fraction.
Speaker 1 (26:25):
Like it was all basically light in the Trino's right,
because that's the only thing you count as radiation is
light in theatrinas.
Speaker 2 (26:31):
Oh, I'm going to disappoint you. Actually, if you had
electrons back then, they counted as radiation because the universe
was so hot, electrons were moving at the speed of light,
and basically everything was moving it nearly the speed of light.
Because the universe was so hot, everything was so fast
that it didn't really matter how much mass it had,
even like a top cork if it existed back then.
(26:52):
The top cork is the most massive fundamental particle we
know about. But back then, in the very early universe,
it was moving it basically the speed of light because
the universe was that hot, and so everything gets counted
as radiation.
Speaker 1 (27:08):
I feel like you're basically just making stuff up that.
Speaker 2 (27:13):
I tried to warn you.
Speaker 1 (27:15):
It's been taken to it for a second here, like,
why is it important that if it's moving it closest
to the speed of flight you call electron radiation. Isn't
it still an electron?
Speaker 2 (27:23):
It's still an electron. Absolutely, but it's going to be
important as soon as the universe starts expanding and cooling.
I just want to add that there are other reasons
to think that the very early universe was filled with radiation,
not just because we call it radiation, but also because frankly,
it was very hot and filled with charge particles. And
what do hot charge particles do? They give off photons.
(27:43):
So there really were a lot of photons in the
early universe, many more than we have today. It's not
just a slippery naming.
Speaker 1 (27:50):
Scheme, but it's partly a slippery naming scheme.
Speaker 2 (27:54):
Yes, absolutely, it's partly a slippery naming scheme, but not entirely. Also,
in the early universe there was matter and antimatter, and
that annihilates and forms photons, So there really was a
lot of photons in the early universe. So it's partially
a slippery naming scheme calling stuff radiation that today we wouldn't.
But also there really was a lot of radiation stuff
(28:14):
that we would call radiation today.
Speaker 1 (28:16):
Well, then let's maybe be helpful to people and break
it down out of this is what you call radiation,
how much of it was actually light and how much
of it was electrons or quarks or you know things
with that today we would call normal matter.
Speaker 2 (28:31):
Thanks to the CMB and bearing on acoustic oscillation, we
can actually make measurements of like the photon to quark ratios.
So we do have those numbers. They change sort of
rapidly as the universe is cooling.
Speaker 1 (28:44):
Okay, So then so then what was causing these changes
between like electrons and photons.
Speaker 2 (28:50):
So what happens very early on is the universe starts
to expand, Right. What's happening is more space is being created,
but you're not creating more stuff, right, and so what
how if you have like ten particles in a tiny
box and then you make the box bigger, all now
the density decreases, right, and it gets more dilute, And
that makes sense. So the energy density is dropping, dropping, dropping,
(29:12):
So that's how it works for matter. Right, you increase
the box, you get a drop in energy density. For radiation,
the rules are a little bit different. For radiation. The
energy density actually drops faster as you increase the size
of the box because the particles get red shifted. The
expansion of space stretches the wavelengths of these particles.
Speaker 1 (29:32):
Of the photons, but doesn't it stretch also the electron.
Speaker 2 (29:36):
If they're moving very fast, if they're treated as radiation,
And that's the distinction. If you're considered radiation, it's because
effectively you have no mass, which makes your wavelength expand.
As space gets expanded, that means that your energy density
is dropping more quickly, because not only are you getting
more dilute, you're also getting red shifted. So as time
(29:56):
goes on, the energy density of radiation drops fast faster
than the energy density of matter, which does two things. One,
now some of that radiation slides over into the matter category,
and also the radiation slice of the pie starts to
decrease relative to the matter slice of the pie.
Speaker 1 (30:13):
Okay, I think what you're maybe saying is that as
the universe expands, the amount of energy that's stored in
momentum is changing. That's really kind of what's happening, right,
And so at some point, for example, the electrons were
mostly momentum because we're going so fast, but at some
point they slowed down so much that they were mostly
just the mass of the electron.
Speaker 2 (30:34):
Yeah, that's exactly right. But remember that there's something else
also happening for photons, right. Photons do get red shifted,
and people write and ask about this all the time,
like what happens to the energy of a red shifted photon?
Where did it go? Because we've had the concept of
conservation of energy banged into our head for so many years. Well,
expanding space does not respect the conservation of energy. If
(30:55):
you have a photon in a chunk of space and
then you expand that space, the wavelength of that phot
quoton also gets expanded because space itself is expanding, and
so now it has less energy. And that energy didn't
go anywhere. There's just is less of it, because the
universe doesn't respect conservation of energy unless space is fixed.
Speaker 1 (31:13):
But I guess the question is, like, it sounds like
our classification of what you call radiation and matter was
changing very rapidly during those times. But were the actual
like number of electrons changing, Were there more electrons being
created or destroyed? Was there more light being created or destroyed?
Or did that mostly stay the same.
Speaker 2 (31:30):
In the very beginning that mostly stayed the same, like
you had a bunch of processes. These things are sloshing
back and forth. Photons can turn into electron positron pairs.
Electron positron pairs can annihilate back into photons. The very
early universe, we think was in thermal equilibrium. They have
all these things going in both directions and they basically equalized.
That's what happens at a hot plasma if you give
it enough time. Then the universe is expanding and some
(31:52):
stuff gets slowed down, so it falls out of the
radiation category into matter, and stuff that got left in
the radiation category is losing energy density compared to the
matter category. So now the matter category is growing and
the radiation category is dropping, and so eventually we get
to a matter dominated universe. The universe started out dominated
by radiation, but as it expands and cools, it becomes
(32:14):
a matter dominated universe.
Speaker 1 (32:17):
And so in that way, you would say that the
percentages of matter and radiation in the universe were changing.
Speaker 2 (32:22):
That's exactly right, and that happened about fifty thousand years
after the Big Bang. At that point, the matter and
radiation sort of hit a crossover point. Radiation is dropping
more quickly, but it started out higher, and that's the
point where they cross each other. Meanwhile, dark energy is
humming along like a tiny fraction of the universe like
zeros or zerz are one this whole time, playing the
(32:43):
long game, waiting for its turn to dominate. But at
around fifty thousand years the universe became matter dominated. It
was cooled enough that a lot of stuff slowed down
and became essentially called matter, and the photon energies got
decreased because of this radiation stretching.
Speaker 1 (33:00):
And let's get into then the long game of dark
matter and the newcomer dark energy and how it took
over the entire pie of the universe. Let's dig into that,
but first let's take another quick break. All right, we're
(33:23):
talking about the universe pie, the chocolate pie, the white
chocolate pies, all the pies in the universe and what
they're made out of, and how that's been changing since
the birth of the universe and throughout all of its
history and maybe into the future. And so if we've
learned that at the beginning of the universe, physicists would
qualify most of the stuff in the universe as radiation
because it was going so fast. But as it cooled
(33:44):
down and expanded, things sort of slowed down enough that
they started to be called more a regular matter, which
is the kind of matter that we're made out of.
And so you were saying that dark matter was sort
of waiting in the wings to make an appearance here
in the history of the universe.
Speaker 2 (33:56):
Well, it was more thinking about dark energy as waiting
in the wings. Playing the very very very long the
billion year game, the first few tens of thousands of
years were a battle between radiation and matter, including normal
matter and dark matter. Both of those contributions combined were
still smaller than radiation in the very early times. Than
after about fifty thousand years, matter together overwhelmed the radiation category,
(34:19):
and that includes dark matter and normal matter. But there's
an interesting mix there between the matter and the dark
matter fractions, Like there were a bunch of electrons and
there were a bunch of protons, but we also think
there was a lot of dark matter in the early universe,
and we suspect very strongly that in the early universe,
matter and dark matter were turning into each other, that
there was some kind of force that allowed one to
(34:40):
turn into the other or back.
Speaker 1 (34:42):
Wait. And so in the point zero zero zero zero
zero zero one percent of stuff in the early universe
that you call matter, you're also including dark matter in there.
Speaker 2 (34:49):
Also including dark matter in there exactly. And we think
that there was a higher percentage of that that was
dark matter than there is today. That at the matter
section of the pie, dark matter plus normal matter, that
there was more dark matter as a fraction than there
is today.
Speaker 1 (35:04):
And you're saying that dark matter can turn and was
turning into regular matter and vice versa because it was
so hot.
Speaker 2 (35:10):
Yes, And we don't understand how this works, because we
don't know what dark matter is and whether it's a
particle and what forces it feels. But in order to
tell the story and be consistent with everything we understand,
there needs to be some mechanism for dark matter and
normal matter to slosh into each other to explain how
we got from having more dark matter in the early
universe to having less of it. Today. The story goes
something like this dark matter we think is some massive particle,
(35:33):
some very heavy particle, And as the universe is cooling,
matter and dark matter are turning back and forth into
each other. But as things get colder and colder, matter
can no longer turn into dark matter because dark matter
is too heavy. No longer is there enough energy around
to smash together normal matter particles and turn it into
dark matter. So dark matter creation stops, but dark matter
(35:54):
annihilation doesn't. Dark matter is still turning into normal matter
right in that one direction, but the reverse this process
is no longer happening. So instead of being in balance,
now dark matter is turning into normal matter, but it's
not happening the reverse, and so the dark matter fraction shrinks.
Speaker 1 (36:10):
Whoa. So like the regular matter was growing.
Speaker 2 (36:13):
In the early universe, some dark matter got turned into
normal matter and then it got frozen out. It didn't
get turned back into dark matter like it did when
things were hot and slashing around freely.
Speaker 1 (36:23):
Then to pie bake exactly.
Speaker 2 (36:26):
Somebody baked the pie. And then as the universe expands,
it cools further and there's less and less dark matter.
Then there's not enough dark matter around for there to
be much annihilation, So dark matter stops turning into normal
matter and it gets freezes in or baked in. I
guess you could call it in our analogy, but in
physics terms they call it dark matter freeze out.
Speaker 1 (36:46):
So then when when did this freeze out or baking happen?
Speaker 2 (36:48):
It happened about ten to the miners eight seconds after
the Big Bang?
Speaker 1 (36:52):
WHOA what? And since then it's been the same proportion
of regular matter and dark matter.
Speaker 2 (36:58):
Yeah, the dark matter normal matter fraction got frozen in
very early on as the universe expanded and cooled, and
then the radiation normal matter fraction changes as things expand further.
And that turnover point was like ten thousand years. So
there's lots of really fascinating time scales here.
Speaker 1 (37:14):
Okay, So then since ten to the minus second, since
the Big Bang, and we've had the same amount of
regular matter and dark matter. It sounds like what you're saying,
and so those proportions stayed the same, right, it would
be about five to one, right, m M. But then
waiting in the wings, you're saying there was dark energy.
So back then there was no dark energy.
Speaker 2 (37:32):
There was not no dark energy, but there was less
universe than there is today, and dark energy is built
into space. Every chunk of space comes with dark energy.
So you have less universe, you have less dark energy,
but it's constant in density, right, more universe, less universe,
You don't change the density of dark energy. But as
you expand the universe, you do change the density of
matter and radiation. As we talked about, you expand the box,
(37:55):
you have a smaller density of matter, and radiation drops
even faster. As the universe is expanding, both matter and
radiation are having their energy density drop very very quickly.
But dark energy isn't. It's a constant density. You make
more space, you get more dark energy. That doesn't happen
for electrons. So as the universe expands, dark energy starts
(38:15):
to creep up in its fraction.
Speaker 1 (38:17):
Although I wonder if you can make the case that
dark energy was always there, like if we don't know
what it is, and it's just like a hidden, invisible
potential energy that could may move things, wouldn't you say
it was already built into the universe from the beginning.
Speaker 2 (38:31):
Yeah, that's exactly the model, right. We think it's an
inherent part of space. As long as you had space,
you had dark energy, and it was there during this
first ten to the minus eight seconds, but the other
stuff had such a high energy density that it swamped
the whole pie. The dark energy was always there with
its same energy density, it was just small compared to
the energy density of the other components, which then faded
(38:52):
as the universe expanded.
Speaker 1 (38:54):
I guess what I mean is like, if you're closing
the universe off at a certain point, and you're saying
that the energy of the universe is not conserve, there's
more energy being pumped into it. But what if you
count the pump and where this energy is coming from.
Then maybe I wonder if you could say that dark
energy was always there at the same percentage.
Speaker 2 (39:11):
Yeah, perhaps when there's lots of the theories of dark
energy and what it might be is it's some kind
of weird field, is it some kind of other stuff?
And so those various theories would upset these fractions if
you included them.
Speaker 1 (39:22):
Yeah, And so that's how the universe pie has been changing.
But it seems like it hasn't really changed much since
ten to the minus eight seconds, since the Big Bang,
except just that dark energy has been growing.
Speaker 2 (39:32):
Yeah, dark energy has been growing, which makes for a
fascinating tug of war because dark energy is growing. But
in the early universe, like a billion years in, we're
still matter dominated, right, Radiation is faded away. We're in
a matter dominated universe. The universe is expanding, but that
expansion is now decelerating. It's slowing down because the universe
is dominated by matter, and what does matter tend to do?
(39:54):
Pulls things together, right, It slows down the expansion of
the universe. But because it was expanding still even though decelerating,
dark energy is creeping up and up and up, and
dark energy makes the universe expand faster, and eventually the
universe expanded enough so that dark energy just took over.
And around eight billion years after the beginning of the universe,
(40:14):
dark energy was the dominant fraction. And you'll see that
the universe start to accelerate its expansion because dark energy
takes over. And that's basically the future. Dark energy is
a runaway process. If nothing else changes, dark energy will
continue to grow as a fraction of the energy density
of the universe, making the universe accelerate faster and faster,
increasing the dark energy fraction faster and faster.
Speaker 1 (40:36):
All right, So then in like a billion years from now,
what's going to be the percentage or a pie breakdown
of the universe if you had to get well.
Speaker 2 (40:43):
It's like sixty eight percent dark energy. Now it's just
going to increase. It's going to go to ninety percent,
ninety five percent, ninety nine percent. But this is over
billions and billions and trillions of years into the future.
But it's only going to crank up. But you know,
we don't know that, right. Remember, we don't know what
dark energy is. We don't know for sure what its
behavior is going to be in the future. This model
describes very very well the history of these energy fractions
(41:06):
and how the universe pie has changed, with a couple
of caveats, like the measurements of the dark energy in
the early universe don't one hundred percent agree with our
measurements in the late universe. There is this hubble tension.
But mostly this picture of the sloshing pieces of the
pie holds together very well and matches all of our data.
Speaker 1 (41:23):
All right. So then in the future, normal matter, dark matter,
and radiation, they're all going to stay the same amount,
but the percentage is going to go down because dark
energy is growing, and so in the far future it's
going to be like ninety nine nine nine nine percent
dark energy then the universe.
Speaker 2 (41:38):
Yeah, exactly. And the universe started out dominated by one
fraction of the pie, it's going to end up dominated
by another fraction of the pie. And there's this little
window in the middle where multiple fractions are not zero.
It's a very unusual time to be in the universe
when you have a bunch of different kinds of stuff around.
You got dark matter, you've got normal matter, you've got
dark energy, all at the same time.
Speaker 1 (41:58):
But according to you, we're still in the undelicious part
of the universe. Apparently, apparently, thanks, things can't get worse,
but they can only get better.
Speaker 2 (42:09):
I wouldn't say undelicious, I'd say less delicious than the future.
That's the optimistic way to think about it.
Speaker 1 (42:14):
Oh, there you go. And let's hope that dark energy
is the most delicious thing in the universe, because that
seems to be the only thing that's going to be
around in the future percentage wise.
Speaker 2 (42:22):
And caveats for those of you who they like to
think a lot about dark matter. This assumes a fairly
simple model of dark matter. That it was in thermal
equilibrium with everything else early in the universe. That doesn't
have to be the case. You can have other theories
of dark matter axions, et cetera that are so weakly
coupled that hardly interacts, so they're not thermally mixed with
the stuff in the universe. There's lots of other ways
you could change this picture. This is like the simplest
(42:44):
model we can make that describes everything we see, and
it works really pretty well.
Speaker 1 (42:49):
All right, Well, it's another big reminder that the universe
is a big, mysterious piece of pastry out there. There's
still a lot to learn, a lot to explore, a
lot to taste, and a lot to bake as well.
Speaker 2 (43:00):
And as you take a big bite of the universe,
remember that not only is our kind of stuff unusual
in the universe, it's getting more and more unusual. And
we live in an unusual time in the universe when
our kind of stuff and dark matter is a really
significant fraction of the stuff out there in the universe,
alien civilizations in the year three trillion, that we'll have
a very different kind of physics to deal with.
Speaker 1 (43:21):
All right, Well, we hope you go out there and
grab your piece of the pie of the curiosity and
mystery of the universe. You hope you enjoyed that. Thanks
for joining us, See you next time.
Speaker 2 (43:40):
Thanks for listening, and remember that Daniel and Jorge Explain
the Universe is a production of iHeartRadio. For more podcasts
from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever
you listen to your favorite shows.
Hosts And Creators
Daniel Whiteson
Kelly Weinersmith
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