What would happen if dark matter and dark energy disappeared?

Dark matter and dark energy are 95% of the energy in the Universe. But do we need them?

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Episode Transcript

Speaker 1 (00:08):
Hey, or hey, let's do a physics thought experiment. Do
I have to think about it? You have to have thoughts? Well,
does it involve you know, Einstein on a moving train
looking through a prison or something. No, No, we're going
bigger than that, Bigger than like a supermassive black hole,
even bigger. What could be bigger than billions of suns? Well,

(00:30):
imagine that you could visit the universe's control room and
just like turn something off. What would you remove from
our universe? I imagine that all the time, Daniel waited,
did you say what would I remove? Or who would
I remove? Let's not make it personal. What would you
remove from the universe? In that case? Probably Twitter? Twitter?

(00:53):
I would probably remove immediately. I thought you were going
to say, alarm clocks or emails? Yeah, those are pretty
bad too, deadlines if I can flip multiple switches, yeah,
they're all being flipped. Well, if that sounds good to you.
The next time we have an election, vote for Jorge
for Universe Control Room operating m Hi am Jorgemmay, cartoonists

(01:28):
and the creator of PhD comics. Hi, I'm Daniel, I'm
a particle of physicist, and I'm terrified to let Jorge
into the control room of the universe. Are you worried
I'm gonna do like a Homer Simpson and eat my
doughnuts and excellently press the red switch. No, I'm terrified
because you ask really good questions and so you'd be like, hmmm,
what happens Is I pushed this all the way up?

(01:50):
Or if I just flip all those knobs and the
physicists would be like, what, don't even think about that?
That's impossible. What if I flipped the switch on that
everyone that gets a physics agree, I would love that.
That is actually our goal, that's one of the missions
of this podcast, in fact, is to educate everybody out
there with a little bit of physics, to turn the
switch on in your head, to make you think about

(02:11):
the universe and to do a look out into the
cosmos and maybe a whole different way. One of my
proudest moments in this podcast was seeing somebody tweet that
they had learned more physics from our podcast than in
eight years as a PhD student in what But it
was on Twitter, so who knows if it was really Also,
could it have been one of your grad students or

(02:33):
my stock puppet account on Twitter. One of your students
is trying to tell you something, Daniel like, I hear
more from my professor through this podcast than in actual life.
I pay plenty of attention to my students, but thank
you for standing up for them. Well. Yeah, so, welcome
to our podcast, Daniel and Jorge Explained the Universe, a
production of I Heart Radio, in which we talk about
everything that's amazing in the universe, everything that's mysterious, everything

(02:55):
that we do know, everything that we don't know, everything
that we would like to know, and everything that Daniel
students want to ask him but can't seem to find
him in his office. For Yeah, I think they know
by now to pretend to be somebody else and then
send you a question. That's right because we think that
everything that you wonder about, everything that we wonder about,
everything that scientists are trying to figure out, We think

(03:15):
that those questions belong to everybody, and then everybody deserves
to know what's going on and what we know and
what we don't know. So our goal is to transport
you to the very forefront of human knowledge, and to
do it without using any fancy words, because there are
still a lot of questions out there in the universe.
There are a lot of big mysteries that scientists are
actively wondering about, and so scientists, for everyone, we should

(03:39):
all be involved in this quest to understand where we
are and how it is that we're here. Yeah, and
one of the biggest questions we have about the universe
is just why is it the way it is? Did
it have to be this way? Is there some rule
that tells us that the universe could only have been
this way or could it have been totally different? So
sometimes we look around with think, well, could the universe

(04:01):
work without this bit or that bit? Or what if
we flip these knobs with things still operate? Yeah, you mean, Daniel,
we can't just say it is what it is. It
definitely is what it is, But the question is why
is it what it is and not something else? Yeah?
Because I think you know, pondering these fundamental almost philosophical
questions is also a big part of physics, you know,

(04:22):
asking what if questions like why is it like that
and not something else? What if the universe was totally different?
Why isn't it what it Isn't that would be on
my tombstone? Right? Why you just made me think they're Daniel,
there's too many negatives in that one. Yeah, And we
are curious about that, and so we play these mind

(04:42):
games sometimes like do we need this bit or do
we need that bit? Or how do these two things
work together? And that's very helpful because it helps you
understand why we have the various pieces of the universe
that we have. We look out into the universe and
we see the way it works, and we try to
explain it, and we build up the minimal set of
bits that we need to explain everything that we see

(05:04):
in the universe. And so it's helpful to understand what
would happen if you yanked at one part of the
universe or yanked out another or turned something off. Ye.
And you know, it's not just kind of a fun
game to play, but it's also, you know, kind of
a fundamental way in which science is done, right. I mean,
when you ask yourself why something is like that, you
in science, you kind of have to ask yourself, what

(05:25):
if it wasn't like that? You know, I'm thinking about
the null hypothesis, Yeah, exactly. Sometimes you have to ask,
like do we really need this bit? Because the simpler
explanation is always preferred, and so if you can explain
the universe without some little extra you know, whiz bang
bit in your theory, then great. Like if you have
the current theory universe plus you have one with like

(05:47):
invisible space kittens, well, you don't really need the invisible
space kittens. They don't add anything to your theory. You
like the invisible space kittens, you don't need the invisible
space kitten. I would totally flip that swhich and that's
why we're not letting you into the control room. But
the point is we want the simplest theory, the one
that explains everything we see with a minimal set of ideas,

(06:10):
with the minimal set of numbers and details and complicated bits,
because we think, we hope the universe is simple, and
we can have long philosophical disagreements about whether the universe
should be beautiful and should be pretty, but the goal
of looking for simplicity has succeeded for a long time. Right,
I'm thinking simple space kittens. Then then then you can't

(06:31):
argue so single colors. For example, you don't have any
calico invisible space kittens. Alright, Well, on today's episode, we'll
be asking a really big what if question, maybe about
as big as you can ask about the universe, right, yeah,
I mean we're talking about the universe. What if it disappeared?
That's right. We are gonna let Jorge delete of the
universe Thanos could only do half the universe. Je has

(06:55):
even more power. That's right. I am even more inevitable.
I'm inevitably inevitable. Everybody out there, tremble in your headphones
that I'll just snap my fingers limping on my infinity
glove here, actually need to glove to get that's right. Yeah,
that's right. That's a whole second arc of the Avenger sequence, right, Avengers,

(07:16):
return of the Invisible Space Kittens, Jorges Army to destroy
the universe or maybe destroyed our Avengers after game post game,
still gaming anyways, So to be on the program, we'll
be asking the question what if dark matter and dark

(07:37):
energy disappear? Bom bom bomb or yes, the sound effect
would be like yow, I like that. You should do
that all the time here on the podcast. Due oh,
I make those sound effects when I talked to my
students in their meetings, and that's why they prefer to
listen to the podcast No No, But it's a really

(08:00):
fun question because, as you know, dark matter and dark
energy are huge for actions of the universe. They play
an enormous role, not just in how the universe look today,
but how it looked in its early stages when it
was a baby. Yeah, dark matter represents about of all
the mass and energy in the universe, and dark energy
represents about sixte right, Daniel, of all everything that there is. Yeah,

(08:23):
that's right. And we often get questions from listeners about
what that really means, like what's the denominator? You know,
if you don't know how big the universe is, how
can you say that dark matter is of what? And
it's a great question of the whole thing. You don't
need a denominate the whole thing. You don't need anominator
because we usually think about it in terms of density,

(08:44):
Like take any side blob of the universe. We're talking
about of that and not just of the matter, but
of the energy density, because remember that matter, that mass,
the stuff that makes me and you, it's just another
form of energy. And we talk go at the energy
density because that's really what controls the curvature of space.
In general, relativity it's not just stuff, but it's energy density.

(09:08):
So when we talk about this pie that five of
the universe is the kind of matter we're made out of,
in twenty seven is dark matter and sixty is dark energy.
We're really talking about the fractions of the energy density
of any volume of the universe, right, And so the
universe is really big or really small or infinite, it's
kind of still means the same thing. Yeah, let's just

(09:29):
talk about it without knowing the answer to that other
super massive, incredibly important question that we're amazingly totally clueless about.
And so we're asking the question, what would happen if
you take it away, like if you went to the
control room of the universe and just like flip the
off switch on those two things, that's right, and made
the dramatic sucking sound. And Daniel, do we mean like

(09:50):
if we trend them off now, or if we like
went back in time and took them out of the
birth of the universe. Yeah, well, immediately when you ask
that question, then I want to say, let's do all
of them, because they're all really fascinating different experiments, and
if you're going to understand the impact of these things
on the universe. You want to play different games, and
so yeah, we'll talk about if there had never been

(10:11):
dark matter and dark energy in the universe, what would
our universe look like? And also what would happen if
you just deleted them from the universe, like instantaneously. Right now,
we're going to cancel dark matter and dark energy. Right.
Have you heard what dark matter dark energy said a
hundred years ago? Boy, in modern context, it's embarrassing. We
heard of their workplace policies, oh man, exactly, they were

(10:35):
not very tolerant. But first, I was curious what our
listeners knew about this subject, and so, as usual, Daniel
went out there into the wilds of the Internet to
ask people what they thought would happen if dark matter
and dark energy disappear. So, if you're curious to play
intellectual thought experiments for future episodes, please write to us
two questions at Daniel and Jorge dot com. We'd love

(10:57):
to get your voice on the podcast. So think about
it for a second. What do you think would happen
if both of those big things disappeared. Here's what people
had to say if dark matter disappeared, assuming that it
did exist before and now it disappeared, then I would
assume that the galaxies would start to fall apart. The

(11:18):
solar systems will probably be okay because the gravity there
is enough to hold it together, but there wouldn't be
enough matter to hold the galaxies together. If dark energy disappeared,
then the universe would continue to expand it would stop accelerating.
I believe I'm gonna go ahead and assume that the

(11:38):
universe is is functioning the way it is now. I
would imagine if dark energy disappeared, there'd be enough momentum
of the expanding universe that it would continue expanding, and
that if dark matter disappeared, I don't know if there
would be enough regular matter to stop that expansion. Galaxies
would start flying apart, and possibly even and solar systems

(12:00):
might start flying apart. So would be kind of a
chaotic experience there. If we're saying dark energy is somehow
involved with the expansion of the universe, then I'm assuming
that that's not going to happen anymore. Plus, listening to
the latest episode on Black Holes, are not sure what's
actually going to happen there. But I'm not sure that
it actually interacts with us in any other way. That

(12:23):
we know of right now, So that's a total unknown
for me. Probably disaster. First of all, dark matter. Dark
matter is between twenty of the universe. That would be
pretty pretty bade. Everything would go crazy for sure if
dark matter disappears. We know that that is the thing

(12:45):
keeping us inside of our galaxy, and I did it,
and that galaxy is spinning, all the planets and all
the star starts with shootout and dark kind of is
the thing that's expanding University, So maybe Huntin stops or
maybe you know, to start to implode or something. All right,
it sounds like the main message here is disaster. That's right.

(13:09):
Nothing Good's right. The message is basically, keep Jorge out
of the control room called the Avengers, called the Avengers back,
knowing should have that much power, even the Avengers. I
think that was not thinking big enough, right, Yeah, exactly,
that was his problem. Yeah, he was not ambitious enough.
It actually wasn't one of the infinity stones of dark

(13:30):
matter or dark energy that they had, like a dark name.
I don't remember the physics of it, but one of
the cubes is supposed to be powered by dark energy,
if I remember, isn't one of the infinity stones, the
same cube that Thor was fighting over and that was
powered by dark energy. You're getting me excited here, Daniel.
We can spend the rest of the podcast going back

(13:50):
through the test rack and the power that's it is
powered by dark energy. That's the one. Yeah, but that
turned out to have a stone inside of it. Woe. Anyways,
back to physics. So not a lot of people have
predicted good things would happen. Most people seem to think
that the universe would sort of fall apart. Nobody is
in favor of this proposal. Everyone wants to keep those

(14:10):
things around as much as possible, which is why we're
doing it as a thought experiment and not in reality
that you know of, Daniel, And you know, there's a
long and great history of thought experiments. Einstein's development of
relativity was aided by thought experiments, just thinking what would
happen if and sometimes they're just very useful to get
you to think about the extreme situations. You know, you

(14:31):
can't really do the experiment of flying in a spaceship
at the speed of light and turning on a laser,
but you can think about what the rules that we
have suggest. You can think about what it would happen
if you did that experiment. Now, we'd love to actually
do it and to make those measurements, but even when
we can't, we can still do the thought experiment to
test the boundaries of the idea. Right, And you also

(14:52):
kind of do thought experiments before you do actual experiment, hope.
So like you probably work through all the possible ways
that it could go and then you that's how you
know to look for Yeah, we definitely thought about it
before we built the large change on collider. For example,
good will this destroy the world or not? We thought
about that, and we also thought about what we could
discover and how to build it, and so yeah, there's
a lot of four planning involved in all of these experiments.

(15:15):
All right, Well, just for those of you who might
not be familiar, let's recap really quickly what dark matter
and dark energy is. So let's start with dark matter.
Explain dark matter in three minutes before we delete it
from the universe. Let's remind people what it is. So
dark matter we know is matter. It's some invisible stuff
that's out there, and we know that it's matter and

(15:35):
that it's out there because we see its gravitational effects.
It's holding galaxies together that are otherwise spinning way too
fast to be held together by the gravity of the
stars that we can see. It controls the whole structure
of the universe, seeding the formation of galaxies and all
kinds of stuff. And we've measured that it's about twenty
seven percent of the energy budget of the universe, which

(15:58):
is about five times as as the normal matter, the
stuff that makes up stars and gas and planets and
bananas and all that good stuff, right and dark bananas obviously,
But it's a big deal. I mean, it's not just
like some small thing out there in space. It's it's
literally five times more than the kind of stuff that
we're made out of in stars and black and you know,

(16:20):
black holes and galaxies. Yeah, we are literally the tip
of the matter iceberg. And it's a wonderful lesson that
there's so much going on out there that we weren't
aware of until very recently, which means that there are
big surprises coming around the bend in the future. And
so we know that it's out there, we know that
it's matter. It's definitely made of stuff because it has gravity.

(16:40):
We don't know what it is. We don't know if
it's a particle. We don't know if it's something else.
We don't know, if it's seventies seven kinds of particles.
I don't know a lot about it other than that
it's dark and that it's matter. Hence the name dark matter, right,
and so that's dark matter, and so dark energy also,
how would you describe it? So dark energy is similar
to dark matter in the sense that it's important, it's
a big in the universe, and we don't know very

(17:01):
much about it. Otherwise, that's about all it has in
common with dark matter. Just basically the word dark dark
because we can't see it with visual light, right, Yeah,
we can't see it with visual light. And also we
just don't really understand it. Dark energy is not really
a thing. It's not something we understand, it's not something
we have a theory about. It's just the observation that

(17:21):
the universe is expanding and that that expansion is accelerating.
So space is getting bigger and bigger, but it's getting bigger,
faster and faster every year, and that takes energy. It
takes a lot of energy. And we only recently, like
years ago, discovered that this expansion is accelerating. So dark
energy is a description of that acceleration of the expansion.

(17:45):
We don't have a theory that describes that. We don't
have a model that makes it work. We've just seen
that this happens and we know that it's important. Right.
It's kind of like you're five year old when they
don't want to go to sleep and they're jumping in
the bed. You're like, where do they get this energy? Like,
it's not like they're glowing, they're just acting in a
funny way. That's right. But they both contribute to this pie. Right.
Dark energy takes a lot of energy. We can calculate

(18:07):
how much energy it takes to expand the universe and
to accelerate that expansion, and it's just under seventy percent
of the energy density of the universe. And dark matter
is and they fit together in this pie. And the
pie is important because various elements of the pie work
together to control whether the universe is expanding or contracting,
and whether stuff is being formed like galaxies and stars,

(18:30):
or whether everything is just sort of floating out their
diffuse and so as we'll learn, these two elements are
very important for the formation of our universe and to
keep it blowing up, or to make a contract, or
all of those things. So these are the most important
parameters into like determining the whole structure and the shape
and the size of the universe. Right, it's almost in
a way like if you like the universe way, it

(18:52):
is now like we are more expendable than dark matter
or dark energy. It's like you can take away all
the galaxies and stars and humans and bananas, and the
universe would be less affective than if you took out
one of these two things. That's right, If dark Corey
gets into the universe control room, Dark Daniel will say, yeah, sure,
turn off all the bananas another matter, It doesn't make
any difference. Let's just delete ourselves. But then who built

(19:17):
the control room Daniel? Well, you know, we don't know.
But we are a tiny, maybe irrelevant fraction of the universe,
and so we can't expect that everything is made in
our image. Right, All right, let's get into what would
happen to the universe if these two things suddenly disappeared.
But first let's take a quick break. All right, Daniel,

(19:48):
you've led me into the control room of the universe,
and I see a big switch here that says dark
matter with two options on or off. I'm gonna go
with off. Here, I'm gonna flip the switch. Are you
ready hold onto something? Click? All right, I've just made
dark matters. I hear. What's gonna happen? All right? Well,
it's important to know when we are in the universe simulation, like,

(20:12):
are we going to remove dark matter from the very
very beginning so there never was any, or are we
going to delete it? We're going to cancel dark matter
in the universe today. I guess that two different things
might happen, right, all right, So let's let's start with
the early universe. Let's say we pulled it back to
the future, went back to the early universe, and then

(20:32):
got rid of dark matter, but the universe be very different.
The universe would be really different fourteen billion years later
if there had never been any dark matter in it,
because dark matter is really the thing that makes the
universe interesting. Briefly, it's gravity is the reason that we
have formed stars and galaxies and all sorts of interesting stuff.

(20:53):
Like the gravity from our kind of matter is not
sufficient to pull together into interesting structures this early in
the universe. Without the gravitational assist of dark matter, because
I guess without the gravity that dark matter brings, things
wouldn't have clumped together as much or as interestingly, which

(21:13):
one are you saying, yeah, both of them. I mean,
the brief history of the universe is that you have
very small over densities, little places where stuff is denser
than other stuff that came out of quantum fluctuations in
the very early universe, and then gravity takes over and
stuff that's denser has more gravity attracts more stuff, which
makes more gravity, which attracts more stuff. It's a feedback cycle,

(21:36):
and what you need is stuff for that to happen.
And so if you have dark matter and it's five
times as much stuff as there is normal matter, then
that happens a lot faster. And something that people don't
typically understand is that dark matter is not just like
everywhere in the universe. It's clumped, are big blobs of it,
and those blobs are centered everywhere there is normal matter,

(21:58):
and it's not coincidence. Right. Think about dark matters forming
like depressions in the shape of the universe to collect
normal matter, so that gas and hydrogen and helium falls
into those depressions and then it squeezed together into stars
and galaxies, etcetera. Without them, it would be much smoother.
It take a lot longer for gravity to pull the
hydrogen together and to form stars and galaxies. That was

(22:21):
dark matter clumped initially, or was it out there in
the Big Bang pretty smooth or were there quantum fluctuations
in the dark matter as well? It was the quantum
fluctuations that caused any structure at all, Like everything else,
the universe would have started out perfectly smooth if not
for these quantum fluctuations, and then we would have dark matter,

(22:41):
even dark matter, Yeah, because dark matter and normal matter
were made in the Big Bang, right when we went
from really really hot and dense so that all the
fields were crazy, and then they broke down and created
the particles as the universe started to expand and cooled.
So they were all made essentially during this initial expansion
of the universe. But as you say, it would have

(23:02):
been perfectly smooth without those quantum fluctuations, and so dark
matter like accentuates it, like exaggerates the effect of these fluctuations,
and it speeds things up. If you do a simulation
of the universe without dark matter, we've literally done this.
Then you just don't get stars and galaxies fourteen billion
years into the universe, but you get them eventually, Like
it's the effect of dark matter just speeding things up,

(23:25):
or is it like we would never clump together as
a galaxy. Well, it depends a little bit on what's
going on with dark energy. It speeds things up, right,
And so given enough time without dark energy expanding the universe,
then yeah, it would form these gravitational structures and you
would get stars and galaxies. But there's a bit of
a race here, right. Dark energy is coming on the
scene and it's pulling the universe apart. It's making it

(23:48):
harder for gravity to form structures. So we got a
little bit of a window there right before dark energy
yanks the universe apart. It makes everything too far apart
to do anything interesting. You've got a window there to
form interest structures. So you don't have infinite time to
get stuff done. You've got a deadline. I mean you
have fourteen billion years, but you still got a deadline, right, Well,
assuming that dark energy keeps going, that's right, Assuming that

(24:10):
dark energy keeps going and that you know, you don't
stumble into the control room and turn it off or
accidentally double it or something. Stay tuned, Stay tuned. That's
the last part of the podcast. So we wouldn't get
interesting structures. So what would the universe look like today?
It would just look sparser, like there wouldn't be like
galaxies and stars will be more spread out apart, or
there'll be less stars. Yeah, we would essentially just be

(24:33):
earlier in the development of structure. And so whereas in
our universe we had the first population of stars form
in less than a billion years after the Big Bang,
that would have happened much later, and galaxies formed, you know,
in the first billion years or so. That would also
happen much much later. And so at this point in
the universe. It depends a little bit on the details
of how are you modeling dark energy, But we would

(24:54):
probably have some stars, but not big complicated galaxies like
we have today. So dark matters like this, steroids kind
of of the growth of the universe. All right, I
thought the universe was organic and you know, hormone free,
but I guess even the universe dopes a little. That's right.
Market pressures, man, market pressures, alright, So then the other
scenario that's interesting is like, what have you turned off

(25:15):
dark matter? Now? Like we are where we are now,
it is what it is, but then suddenly something happened
to dark matter. That's right. Say you have had dark
matter that super charged the acceleration of structure in the universe,
gave us stars and galaxies, and then you deleted it
Santos style from the universe. Well, you know, the structures
that we have now depend on dark matter. It's not

(25:36):
just like, well, we needed dark matter to get where
we are, but now that we're rich and famous, we
could forget about our friends from our hometown. We still
need dark matter because it's holding our galaxy together. Like
our galaxy is spinning, right, and things that are spinning
tend to fly apart if there's not something holding them together.
Like we say often in the podcast, you put ping

(25:56):
pong balls on a merry go round and spin it,
ping pong balls fly out. The reason that the stars
in our galaxy, which is spinning, don't fly out into
intergalactic space is because gravity is holding them together, and
mostly that's dark matter. So if you deleted dark matter
from the galaxy, then it'd be like as if you

(26:16):
suddenly turned the merry go around on five times as fast,
and you know, the stuff in the outer ring of
the galaxy would get tossed out into intergalactic space. There
wouldn't be enough gravity to hold it today. Would it
just fly off or would it just you know, the
galaxy would just get bigger in terms of things being
more spread out. It would get bigger. But the stuff
on the outside would essentially have escape velocity suddenly, so

(26:38):
they would get tossed into intergalactic space. We would lose
all those stars in the sort of exerbs of the galaxy. Well,
we don't really need those stars to be there's a
lot of crucial votes in those districts, man, so be careful.
Which is it's a lot of politics here today, Daniel.
But your fate depends sort of on your distance from
the center of the galaxy. If you're the very edge

(27:00):
of the galaxy, you're getting tossed out into space. You're
becoming a rogue star, which is, hey, maybe not that bad. Like,
what do you really need the galaxy for anyway? You know,
if you believe in in limited galaxies, then maybe that's
good for your philosophy. Well, what would happen to us
in particular, like would our solar system be affected at all?
Like would life go on the same for us, or
would we also fall apart. Well, we're in so of

(27:21):
that middle region and we would stay still gravitationally bound
to the main galaxy. But we get a little further
from the center, and probably what would happen is that
all the stars in our vicinity would clump together to
form basically like a little dwarf galaxy. You may not
be aware, but our galaxy has little satellites, like you
know our planet has a moon, right, our galaxy has

(27:43):
its own satellites, things that orbited called dwarf galaxies, little
clusters of stars that form many galaxies. So probably our
son would end up in one of these little clumps
that would be orbiting the old core of the Milky Way, really,
but maybe further out or something further out, So instead
of being like in the suburbs, would be a little
town out deep in the wilderness. So you're saying, without

(28:05):
dark matter, you would get more of these satellite galaxies.
Why would we get more Well, because the middle part
of the galaxy, this middle band where we live, wouldn't
be held into the galaxy anymore sort of get spread
out and instead of having being tossed all the way
into space, we don't have enough velocity for that, We
don't have escape velocity. We become gravitationally bound to the galaxy,

(28:25):
but in sort of a clump, and so this middle
part of the galaxy we sort of break up into
a bunch of little galaxies that are then orbiting the
old poor All right, Well, it sounds like if we
took it out early in the universe, we wouldn't be here.
The universe would look really different. But if he took
it out today, you know, daily life, We mean, we
we would sort of orbit around things differently, but it

(28:46):
maybe wouldn't affect us in our daily lives. Yeah, we're
almost not affected at all by the structure of the galaxy.
Doesn't really matter to us at all whether those other
stars are there. You know, we're dominated by the experience
of the Sun, but whether there are other stellar n
verse is not really a big deal. And you know,
it affects maybe our prospects for galaxy spanning civilizations once
in the future if we end up in a little

(29:08):
dwarf galaxy further from the main core. But you know,
if our ambitions are closer to home, then it doesn't
really change our lives if dark matter is deleted today, right?
Or I wonder how long before we would even notice
dark matter was gone? Oh my gosh, are you Are
you trying to admit? Are you trying to confess to
having done something last night? Where are they going to notice?
I'm like the bad guy and Watchman. It's like I

(29:28):
flipped the switch twenty four hours ago, Daniel. This is
the modern version of the tell tale heart, right, can
anybody tell? They're all looking at me? Yeah, Like, like
how long ago could I have flipped the switch before
we even notice? And that's a good question, and it
goes to like how well we know the structure of
our galaxy, which is a whole fascinating podcast topic. We

(29:49):
have schedules for a couple of weeks from now, but
we know that the structure of the galaxy pretty well,
and we can tell the distance to nearby stars, and
so if that's suddenly changed, we we would be able
to tell. But not for a few years, right, because
the information we're getting is years old, because the closest
stars are a few light years away and most of
the rest of the galaxy is five ten a hundred

(30:10):
thousand light years away, so it would be a while
before we actually saw hundreds of years or something, right, Yeah,
it could be hundreds or thousands of years before we
figure that out. Before we're like, holl hey, what did
you do? Or is great great great great great grandchildren
or to be held to account? So what would it
look like? Would we see less stars out in space
when we looked at the nice guy? Yeah, we would
see less stars out in space, and we'd be separated

(30:31):
from the main core of the galaxy. Now, the core
of the galaxy wouldn't change very much because it's got
so much gravity already in the supermassive black hole would
hang on to that. Instead of seeing the Milky Way
as a band across the sky, we would see the
core of the galaxy in our sky. Be pretty spectacular. Actually,
it would get more intense. Maybe, yeah, I might get
more intense. All right, Well that's um dark Matter. Thank

(30:53):
you for playing dark Matter. We like what you're doing.
We'll keep it. I guess I vote yes on dark
You don't even know what it is, Daniel, I don't
know what it is, but I want to keep it.
Wrong I like it, I fear changed. Right, let's talk
about what would happen if we took dark energy away?
But first let's take another quick break. All right, Dan,

(31:24):
we're talking about what would happen if we took out
these big chunks with the universe way dark matter and
dark energy, And so let's talk about what would happen
if I made dark energy disappear? And again, I guess
you can talk about taking it away at the beginning
of the universe and today, So what would happen if
I took it out when the universe started. It's fascinating

(31:44):
because these two different components, dark matter and dark energy
sort of dominate the stage at different part of the
history of the universe, Like dark matter controls the structure
of the universe from very early days, whereas dark energy
really only takes over in the last few billion years,
and so they really play a different role here, which
is fast. Yeah. To understand this, we have to understand

(32:06):
a little bit about the history of the expansion of
the universe. So go all the way back to the
very beginning and the universe somehow is created. We don't
understand that at all. And then there's inflation when the
universe goes from very hot and very dense to suddenly
much larger, and again we don't really understand that at all.
But from that going forward, we think we have an

(32:28):
understanding of how the universe expands and how that expansion
accelerates or decelerates, and it's controlled by how much matter
and radiation there is, how much energy there is in
the universe, and also in this dark energy, and the
two forces are usually working at odds with each other,
which so dark energy wasn't very instrumental in the Big Bang,
like it was an inflation sort of partly due to

(32:50):
dark energy, So we don't know. Inflation has a very
similar effect to dark energy, and that expands the universe
very rapidly, But we don't know if the mechanism of
inflation is the same as the mechanism of dark energy,
Like they both have the same effect, but we don't
know if they're the same thing. They could be, and
in some theories they are. In some theories they're totally different,
but we're really just at the very beginning stages of

(33:12):
understanding these things at all, and so we can't really
say that inflation is the same as dark energy. So
let's put inflation aside and think about what happens post inflation,
right basically after the Big Bang, if there was no
dark energy, I see, all right, So that the universe
just exploded or expanded rapidly after inflation, and now everything's

(33:33):
kind of hanging out there. So what would happen if
I turned off dark energy? So really not very much
like in the early universe, dark energy plays almost no role.
And the reason is that things are still really very
hot and dense. They have a lot of radiation. In
the first thousand years of the universe. Mostly the universe
is just radiation. We're talking like photons, like crazy, bouncing
around everywhere for the first forty seven thousand years. And

(33:56):
it's a little counterintuitive, but what happened in the very
beginning of the universe is that you expansion of the universe,
even though it's mostly just filled with stuff, right, And
typically think of stuff causing gravity, which causes you know, contraction,
But the matter in the radiation and the universe actually
caused some expansion early on. It made the universe get bigger. Now,

(34:16):
the effect of dark energy is to accelerate that expansion,
is to make that expansion happen faster and faster and
faster gravity works the other way, it makes the expansion
happen slower. Are you saying that dark energy didn't play
a role, Like it wasn't important or it was just
turned down in the early moments of the universe. It
didn't really play a role at all. Dark energy gets

(34:38):
more important as the universe gets bigger because it has
one really weird property is that it doesn't get diluted.
Like every chunk of space has the same amount of
dark energy, and if you double the space, you make
space bigger, you get more dark energy, whereas with matter
and radiation, like if you double the size of space,
then the matter gets less dense because you have the

(35:01):
same amount of matter, Like you have two protons in
a cubic meter of space, and then you expand the
universe to be twice as big. Now you have two
protons in two cubic meters of space, so you have
less matter density. But dark energy just doubles when you
double the space. So as space gets bigger, dark energy
gets more and more important. So dark energy, you could

(35:22):
have deleted it from the very early universe, it would
have had no role. But as the universe gets bigger
and bigger, it starts to take over and become dominant. Right,
I guess it's kind of like if you took it
out before it grew, then it wouldn't make a difference.
But now, in as the universe grew, dark energy also grew,
and so it just became more consequence. That's right. And
so if you had deleted it from the early universe

(35:43):
and it never had happened, right, we never had dark energy,
then the history of the universe would be like, you
have the Big Bang, you have all this matter, you
have all this radiation. You still have expansion of the
universe in the very early universe. But then gravity is
taking over. It's slowing down that expansion. Eventually it's stops
the expansion and it turns things around. We have a
contraction and the universe reverses and comes back into a

(36:05):
big crunch. So dark energy is the reason we're still around.
While we're still around, is the reason why we haven't
had a big crunch. But you wouldn't have noticed it
in the very early universe because it didn't really take
over until a little bit later. So wait, so I
took out dark energy at the beginning, the universe would
have expanded. And are you saying that by now, like
fourteen years into it, we wouldn't be here, like the

(36:27):
universe would have crunched already. Absolutely. Yeah. Without dark energy
in the universe, gravity would have won. It would have
slowed down the expansion. It would have stopped and turned
it around and crunched us. We wouldn't have this universe
without dark energy, Like none of the starts would have formed,
or they would have been starting to form. But then
the universe would crunch. Because you still have dark matter,

(36:47):
you would have formed stars and galaxies. But then without
dark energy, all that gravity would have gone into a
runaway effect and collapse the universe back into a very
very dense state. Okay, so you took it out early.
What have you waited a little bit before taking out
dark energy? So there's a bit of a dial there, right.
If you take it out too early, the universe crunches.

(37:09):
If you leave it in a little bit longer, then
allows the universe to expand more, It fights gravity longer,
and it gives you a longer and longer window at
some point. If you turn off dark energy late enough,
it's too late, like gravity can no longer win Like
in our universe today with dark energy, we have the
acceleration of the universe happening right now, like space is expanding,

(37:31):
and it's expanding faster and faster. So, for example, if
he turns it off today, gravity could not pull the
universe back into a dot. It's already too late for gravity.
Really is, because if you take out dark energy, the
universe stops expanding. It's like it's fixed in size. Wouldn't
gravity eventually bring everything together? Yes, But if you take

(37:52):
out dark energy, you don't stop the expansion. You stop
the acceleration of the expansion. You stop adding to the velocity.
Things still have a velocity. Remember Newton's lass tell us
that things and motions stay in motion unless you apply
a force to them, and so dark energy is that force.
If you stop pushing on things, they're still flying out.
Their space is still expanding. And so dark energy is

(38:14):
the acceleration of the expansion, not the expansion itself. So
if you take out dark energy, you stop the acceleration,
but things are still expanding and gravity does not have
the power to turn that around. Wait, what what is
still expanding? Like the distance between stars and galaxies or
with space itself still be you know, multiplying and growing both.

(38:34):
We're talking about the expansion of space itself, right, and
that expansion is accelerated by dark energy. But you still
get expansion even without dark energy. Really, so if you
take out dark energy, space would still be growing. What
is making it grow? Don't you need energy for that? Well,
the expansion can continue without dark energy. But matter itself
causes the expansion of the universe, like we were talking

(38:56):
about earlier in the very early university was just matter
causes the universe to expand. Gravity gives you a negative
derivative on that expansion. It slows down that expansion, and
dark energy increases that expansion. So sort of two things
to talk about there, like the velocity of space and
its acceleration. So matter gives space velocity to expand outwards,

(39:20):
gravity gives you a negative acceleration on that, and dark
energy gives you positive accelerat Dark energy is kind of
like the booster rocket or like the turbo yeah, and
the universe. Yeah, So if you take away dark energy
in the very very early universe, so it never happened,
then gravity would win and things would crunch. If you
take it away today, right then it's sort of too late.

(39:41):
He's already won the game. And gravity can't turn things around,
like things are moving out too fast and too far
away for things to turn around. And there's like a
middle time there where if you deleted dark energy just
the right moment, you would find like a balance there.
And there's some moment there there's a tipping point, and
I'm not sure you could actually walk that and stay
on that tipping point, but there's some point at which

(40:03):
if you don't delete dark energy before that moment, it's
gonna win. So I guess you're saying that dark energy
could retire right now. It's done its job, it's put
in its time, and now the universe will never crunch
back down into right, I mean without even if you
took out dark energy, that the universe will never crunch
back down into an inverse big bang. That's right. To

(40:25):
make an inverse big bang, now you need dark energy
to reverse, not just to disappear. You needed to turn
around and somehow cause the contraction of the universe. And
you might think, well, but it's only ever caused the
expansion of the universe. Why would it cause contraction, which
is a great question. But remember that we don't know
anything about dark energy. We have speculations that it might

(40:46):
be the cosmological constant or something else, so the energy
of empty space, but we really have no understanding of
what it is, which means we can't predict its future.
And it could do something bonkers like turn around and
change direction and cause the contract of the universe, like
it could start taking energy away from space almost and
we don't understand it, so we can't predict its future.

(41:07):
If dark energy disappeared today, then yes, there's already enough
expansion that gravity can't win. And if you kept dark
energy doing what it's doing, which is causing the acceleration,
then you know it's already game over. And we're accelerating
towards having more and more distance between galaxies, having these
little blobs of stuff being more and more separated and
more and more isolated, and each one gravity is strong

(41:29):
enough to hold together. Right, dark energy is not going
to rip you from the Earth or rip the Solar
System apart, because gravity has won these little micro battles
to hold these little structures together. More likely will collapse
into a bunch of black holes than separated with vast
distances between other black holes. Not a happy outlook black

(41:50):
holes to the end of time. White's you have a
dark choice here of people between lightness and dark that's right,
reality or fantasy. All right, Well, I'm getting the sense
of that dark energy and dark matter were super important

(42:11):
for us to be here. We needed them to get
to where we are. But now that the universe is
kind of chugging along, we really don't kind of need
either of them right now. Well, that's true, Like you
could turn them off now and the humanity would probably
go on the same trajectory as it would have been. Yeah, well,
you know that's not very grateful. You know, it's just like,
what can I remove from my life that I don't
need anymore now that I've made it. That's a very

(42:33):
generous attitude. We should remember how we got here and
all the little people necessary along the way. But you know,
as Mary consas is, you know, think about it and
does it brings you joy? Daniel, And if not, just
throw it out. Dark matter brings me joy. But you're right,
we don't need it. We could delete dark matter from
our universe and our solar system would be fine. We

(42:54):
can delete dark energy from our universe, and it wouldn't
really change the fate of our solar system all. So
you're right, there were instrumental in getting us here, but
we could cut them out of our enter. I guess.
I mean like in a way almost like if you
get rid of dark energy, it almost makes the universe
more accessible, right, because if things aren't accelerating further apart,
then it increases our chances of you know, reaching other

(43:17):
places in the universe. Yeah, that's true. It's a bit
of a grim picture to imagine that dark energy could
be creating these vast distances between superclusters of galaxies, making
it literally impossible to explore the universe. And so yeah,
I would actually like to turn off dark energy or
reverse it a little bit, bring some of that stuff
a little closer. We have our human rocket engines a
chance to explore that. You mean you would like me

(43:39):
to go into the control room and fiddle with the knob,
be like off on off sufficient adult supervision. Yes, so
I'd be like flipping the switch and then just so
that the eighties are close enough for you to say
hi to them, and then switching it back on. That
sounds great, Yeah, do that all right, let me, let

(43:59):
me put on my infinity glossier and uh twiddle my thumbs.
All right. Well, it sounds like these are a really
super fun thought experiments, but I feel like they're also
a big part of how you know that these things exist, meaning,
like you know, you play these experiments in your head,
like what if dark matter never existed, Well, the universe

(44:20):
wouldn't be the way it is, which sort of tells
you that dark matter does exist. Yeah, exactly. And it's
not just these large scale structures the organization and the
size the universe. There are a lot of other details
that tell us how much dark matter there is, how
much dark energy there is, how they interact, and that
come from lots of different observations, you know, from looking
at the early light of the universe at all sorts

(44:42):
of other crazy detailed measurements. So it really seems like
dark matter and dark energy are a thing and that
they are necessary things to make the universe the way
it is. And of course the next question is like
why are they at those values? Could there have been
more dark matter or more dark energy? Why these numbers
and not other number? Right, But we almost kind of
don't want to ask those questions, because then we wouldn't

(45:04):
be here. I want to ask those questions because I
want to know is it an accident that we're here?
Or are we inevitable as the only possible universe one
that has this kind of structure, or are we totally
lucky one in a trillion chance. To me, those are
the deepest questions of physics. Those are the reasons I
got into physics, is to try to answer these massive

(45:25):
philosophical questions about the context of our very lives. Right, yeah, See,
it's not just Sanna's who's inevitable or wants to be inevitable,
it's Daniel. It's questions. Really, it's wondering about the universe
that's ineviable. All right, Well, I think we've reached the
endgame here of our episode. We hope that you look

(45:46):
out into the universe tonight or tomorrow or the next
day and think about what are some of the things
that led to us being here, and how important they are,
and what would happen if things were totally different, and
share with us you're wondering in your thoughts and your questions.
Please write to us two questions at Daniel and Jorge
dot com or interact with us on Twitter at Daniel

(46:06):
and Jorge, where we make bad jokes and answer questions.
Thanks for joining us, see you next time. Thanks for listening,
and remember that Daniel and Jorge Explain the Universe is
a production of I Heart Radio. For more podcast from

(46:27):
my heart Radio, visit the i heart Radio app, Apple Podcasts,
or wherever you listen to your favorite shows. Yea