Okay, so welcome to the deep dive everybody.
Today we are going to be tackling. Sounds fun.
A really interesting topic that I think a lot
of people are probably familiar with to some
degree. But hopefully we'll be able to cover
it in a new and exciting way today. All right.
So that is the topic. Of dark matter and dark
energy. Okay. And for people who aren't already
aware, this stuff basically makes up. 95 % of
the universe. Wow. Which is kind of a crazy thing
to think about. That's a lot we don't know, then.
It is. So everything that we can see, the stars,
the planets, the galaxies, all the stuff that
we're made of, makes up a measly 5%. Really,
just 5 %? Of the known universe. Wow. So, yeah.
Kind of crazy. That's more than a little humbling,
isn't it? It is it is to think everything we've
ever observed is just a tiny fraction Yeah, and
it really puts into perspective Just how much
we still don't know exactly and how much there
still is out there to learn So I guess you could
say that we're kind of like explorers going on
this journey together Okay, like that to make
sense of these really mind -bending ideas. Yeah,
let's dive in. We'll do our best to kind of Explain
them in a way that Hopefully is approachable.
Yeah and easy to understand but yeah, it's gonna
be a wild ride I think it's incredible that 95
% isn't just empty space. It's not like we're
missing a few puzzle pieces here It's this huge
unknown presence that's actually shaping the
universe. Yeah. Yeah, that has a real effect
Yeah on the things that we can see absolutely
and that's what makes it so fascinating to study
exactly So when we talk about dark matter You
know, we're talking about this really elusive
thing. You could almost think of it as like the
universe's shy, introverted cousin. OK. You know,
it barely interacts with anything. It's like
the cosmic wallflower. Exactly. You know, it
doesn't even interact with light. That's why
it's so hard to even detect. Yeah, that's why
we can't see it. So let's unpack how we even
got to the point of knowing that this stuff exists.
Yeah. How did we figure out something was there
if we can't see it? Right. So the first clues
for dark matter actually came from astronomers
looking at how galaxies spin. Galaxy spin. Yeah.
So think about our solar system. All right. So
you have the sun at the center. Right. And then
the planets orbit around it and the planets further
away from the sun orbit much slower. That makes
sense. The sun's gravity is weaker further out.
Exactly. And that's because the force of gravity
decreases. with distance. Right, so it's a most
predictable system. Right, it's like a cosmic
ballet. Exactly, and so scientists initially
thought that the same would apply to stars within
galaxies. Stars closer to the center would be
zipping around much faster than those further
out. That makes sense, yeah, because the center
of the galaxy would have the strongest gravitational
pull. Right, all that mass concentrated in the
middle. Right, then they started looking at these...
distant spiral galaxies. Okay. And they found
something really weird. What was that? The stars
at the edges of the galaxies were moving way
faster than they should have been based on the
amount of visible matter that was there. So like
the visible stars and gas weren't enough to explain
those speeds. Exactly. It was almost as if there
was way more mass. Holding those galaxies together
interesting. So like there was some invisible
source of gravity right then what we could actually
see that's wild and so this mismatch between
The observed spees the stars and the amount of
visible matter was a really strong indication
That there had to be some kind of invisible matter
that was providing this extra gravitational pull
dark matter. Yeah dark matter It's like imagine
a figure skater spinning really fast. Okay. You
know, like so fast that their arms should just
fly off. Yeah. But something invisible is keeping
them glued on. I like that analogy. It really
captures the idea. So that invisible force is
like the dark matter. And it's not just preventing
galaxies from flying apart. Right. It also played
a key role in how galaxies formed in the first
place. Okay, so we know it's there because of
its effects. Yeah. But the big question is...
What is it? What is this stuff? So scientists
have come up with all sorts of different theories.
about what dark matter could be. And one of the
most popular ones for a long time has been the
idea of... Weekly Interacting Massive Particles.
Weekly Interacting Massive Particles. Which is
a mouthful. It is, but luckily we have the acronym...
Soap Knees. Wings. And the name kind of gives
you some hints about what scientists thought
these things might be. I break it down for me.
So massive implies that they have a lot of mass.
OK, that makes sense. Massive things have a lot
of mass. Right. And in fact, the source material
actually mentions that they could have masses
of several to V. Several to V, that's pretty
heavy. Yeah. So for anyone who's not a particle
physicist, which is probably most of us, can
you put that into perspective? Sure. So a TeV
is a tera electron volt, which is a unit of energy.
But in particle physics, we often use it to describe
mass because of that famous equation, EMCO. Right.
Energy and mass are basically two sides of the
same coin. Exactly. So to put that into perspective,
a proton, which is one of the particles that
makes up an atom's nucleus, has a mass of about
one GV or giga electron volt. Okay. So a Tevi
is a thousand times larger than that. So we're
talking about particles that are potentially...
thousands of times more massive than a proton.
That's right. These are heavy hitters in the
particle world. Wow. And because they are so
massive, it's thought that they wouldn't be zipping
around at the speed of light. OK. Instead, they'd
be moving much slower, what we call cold dark
matter. Cold dark matter. Yeah. So their average
speed would be well below the speed of light.
And why is that important, the speed? I mean.
Well, it turns out that the speed of dark matter
particles would have had a major impact on how
galaxies formed. Oh, really? Yeah. If they were
moving too fast, they wouldn't clump together
gravitationally. OK. And they wouldn't be able
to provide the scaffolding for galaxies to form.
Oh, I see. They need to be slow enough to kind
of hang out in one place and exert their gravitational
influence. Like the cosmic glue that holds everything
together. Exactly. OK. So we've got massive.
Let's talk about the weakly interacting part.
Right. Because that's the other half of the name.
Right. So this means that they're predicted to
only interact with other matter. through the
weak nuclear force. The weak nuclear force. Okay,
so that's one of the four fundamental forces
of nature. Right. But as the name suggests, it's
very weak. And it only acts over extremely short
distances. Yeah, it's kind of like the shy introvert
of the force world. So because of this, the chances
of a wimp actually interacting with a regular
particle are incredibly low. Yeah, like astronomically
low. Which makes them... extremely difficult
to detect. Is it trying to find a needle in a
cosmic haystack? Yeah, and WIMP, the source mentions
that WIMPs are predicted to exist in theories
like supersymmetry and extra dimensions. Right,
so those are some pretty mind -bending concepts.
Yeah, so for those of us who aren't theoretical
physicists, can you kind of give us a simplified
explanation of what those are? Sure, so supersymmetry
is essentially a theory that proposes that. For
every known particle in the universe, there's
a corresponding partner particle. OK, like a
mirror image. Exactly. And some of these hypothetical
partner particles could have the properties of
WIMPs. So it's like a whole new set of particles
that we haven't discovered yet. Right, and extra
dimensions is the idea that our universe might
have more than the three spatial dimensions that
we're familiar with. Right, length, width, and
height. Exactly. So imagine like a fourth spatial
dimension. that we can't perceive directly. And
these extra dimensions could contain particles
that behave like dark matter? OK, so it's like
there's this whole hidden realm that we can't
see. Kind of like a shadow universe. Where dark
matter might be hanging out. Yeah, it's a fascinating
idea. It is. So there have been a lot of experiments
to try and detect WIMPs. One of those famous
ones was the Large Underground Xenon Detector,
or LUX. LUX, yeah, I've heard of that one. And
this thing was built deep underground. Why underground?
To shield it from all the other radiation. Oh,
that makes sense. You don't want interference
from other particles. Right, you want to make
sure that if you detect something, it's actually
a wimp, and not just some other random particle.
Smart. So they ran this experiment for several
years, but unfortunately, they didn't find...
any conclusive evidence of wimps. Oh, that's
a shame. It is. But the search continues with
even more sensitive experiments being planned.
That's the spirit of science. Never give up.
Exactly. So wimps are still a possibility, but
the jury's still out. Fair enough. So let's move
on to another popular dark matter candidate.
OK. And that is axions. Axions, all right. And
these are very different from wimps. How so?
Well, for starters, they're predicted to be incredibly
light. Okay, so instead of massive, we have super
light. Right. The source mentions that they could
have masses on the order of microelectron volts.
Microelectron volts? Wow, that's tiny! Yeah,
so compared to wimps, they're like feathers.
So what about their interactions? Are they also
weakly interacting? Even more so. Axions are
not predicted to interact through the electromagnetic
force. Yeah. Or the strong nuclear force. So
they're practically invisible to us, then. Yeah.
And not affected by the force that holds atomic
nuclei together. Right. So they're incredibly
difficult to detect. So if they're so light and
barely interact with anything, how could they
possibly be dark matter? Well, that's a good
question. And it turns out that they're very
weak interactions. Yeah. And incredibly light
mass actually make them very stable. Stable?
Yeah. They could essentially exist. Yeah. for
the entire age of the universe. OK, so they're
like the cosmic Methuselahs of particles. Exactly.
And that fits in really nicely with the fact
that we know dark matter has been around for
billions of years. That makes sense. And despite
the challenges, there are actually experiments
trying to detect axions. Oh, wow, really? Yeah,
like the ADMX experiment. ADMX, what does that
stand for? It stands for the Axion Dark Matter
Experiment. Catchy. And there's also the Casper
experiment. Casper? OK. I don't even want to
know what that stands for. Yeah, it's a bit of
a mouthful. But basically, these experiments
are trying to observe the really subtle effects
that axions might have on things like electromagnetism
and the spins of atomic nuclei. So it's like
they're looking for the faintest whisper against
the roar of the universe. Exactly. It's a really
difficult thing to do. I bet. OK, so we've talked
about massive and weakly interacting particles.
Limpies. We've talked about. super light, and
even more weakly interacting particles. Axions.
So what's next? Well, there's another interesting
candidate, sterile neutrinos. Sterile neutrinos.
OK, so we already know that neutrinos exist.
Right, those tiny little ghosts of the particle
world. Yeah, they're these incredibly tiny particles
that interact through the weak nuclear force.
and they're incredibly difficult to detect. But
we have managed to detect them, right? We have,
with these massive underground detectors. Yeah,
those huge golden chamber detectors that look
like something out of a sci -fi movie. Right,
so how are sterile neutrinos different? Well,
the key difference is that they're theorized
to interact solely through gravity. Just gravity?
Yeah, no weak force, no electromagnetic force,
nothing else. Wow, so they're even more isolated
than whips. True cosmic loners. Yeah, it's like
they're practically ghosts. So how could we ever
hope to find something that only interacts through
gravity? Ah, that's a great question. And the
truth is, it would be incredibly difficult. But
the source does mention a couple of possible
detection methods. One is by looking for x -ray
emissions from galaxy clusters, which might be
produced if sterile neutrinos decay. Interesting.
And another possibility is to search for subtle
imprints they might have left. on the cosmic
microwave background radiation. The CMB. Exactly
that afterglow of the Big Bang. So it's like,
we're trying to look for the faintest shadows
that they might have cast on the universe. Pretty
much. But as with all these candidates, we haven't
actually found any direct evidence of their existence.
So for now, they remain hypothetical. OK, so
moving on, we have another interesting idea,
and that is dark photons. Dark photons. So. Are
these like regular photons? The dark. That's
a good way to think about it. Ah. But instead
of carrying the electromagnetic force, they would
carry a new fundamental force. OK. A dark force,
if you will. A dark force. That would only interact
between dark matter particles. OK. It's like
dark matter has its own. separate communication
network. You got it. That we're not privy to.
Exactly. Interesting. So if dark matter is made
up of particles that can talk to each other through
this dark force, it could potentially explain
some of the things we observe about its distribution
in the universe. Like what? Well for example,
the distribution of dark matter in dwarf galaxies
doesn't quite match. what our simulations predict.
If dark matter particles were only interacting
through gravity, we'd expect to see a much denser
concentration of dark matter in the centers of
these galaxies. But that's not what we observe.
So the dark force could smooth things out a bit.
Exactly. If these particles can interact with
each other through this dark force, it could
lead to a more even distribution of dark matter.
And the source mentions that. These dark photons
might even be able to mix. with regular photons.
Mix. Yeah. It's a theoretical possibility. And
if it happens, it could lead to some really subtle
signals that we might be able to detect. OK,
so that would be a way of indirectly detecting
them. Exactly. But so far, no such luck. OK,
so no evidence yet. Not yet, but the search continues.
So we've gone through this whole list of potential
dark matter particles. But what if dark matter
isn't a particle at all? Now you're talking.
Yeah. And that brings us to the idea of emergent
gravity. Emergent gravity, that sounds pretty
radical. It is. It's a completely different way
of thinking about dark matter. All right. I'm
intrigued. Lay it on me. So instead of saying
that there's this new type of particle that we
haven't discovered yet, emergent gravity proposes.
That gravity itself isn't what we think it is.
Whoa, hold on. You're saying gravity isn't fundamental.
Well. Not in the way we currently understand
it. Emergent gravity suggests that. Gravity might
actually emerge from a more fundamental phenomenon.
And what phenomenon would that be? Quantum entanglement.
Quantum entanglement, you mean that spoopy action
at a distance? The one and only. So as you know,
quantum entanglement is this really strange connection
where two particles become linked no matter how
far apart they are. Right, it's like they're
communicating instantaneously even across vast
distances. And so the idea with emergent gravity
is that the effects that we attribute to dark
matter, like the faster than expected rotation
of galaxies, might actually be caused by... this
intricate web of quantum entanglement that permeates
the universe. So it's like gravity is an emergent
property of this underlying quantum entanglement.
Exactly. It's a really mind -blowing concept.
It really challenges our understanding of what
gravity is. Yeah, and it's important to point
out that this is still a very theoretical idea.
There's no direct experimental evidence. to support
it yet but it's a fascinating possibility it
is okay so we've covered a lot of ground we have
on dark matter now let's shift gears okay talk
about the other big mysterious component hmm
of the universe dark energy dark energy the cosmic
accelerator yeah so this is the force that's
thought to be responsible for the accelerating
expansion of the universe. Right. So for billions
of years, the universe has been expanding. Since
the Big Bang. Exactly. Right. And if gravity
was the only force at play, we'd expect that
expansion to slow down. Right. Because gravity
pulls things together. But in the late 1990s,
astronomers made a shocking discovery. What was
that? The expansion of the universe wasn't slowing
down. It was speeding up. Speeding up. Yeah,
like someone had stepped on the cosmic gas pedal.
OK, so what could possibly be causing that? Well,
that's the million dollar question. And the leading
explanation is dark energy. Dark energy. It's
mysterious force that's pushing everything apart.
And it's not just pushing things apart. It's
pushing them apart at an accelerating rate. Right.
So the expansion is getting faster and faster.
OK. So what are some of the leading theories
about what dark energy could be? All right. Well,
one of the simplest explanations. is the cosmological
constant. And this idea actually goes all the
way back to Einstein. Oh, wow. Yeah, he originally
introduced it as a way to keep the universe static.
Static? Because back then, people thought the
universe was eternal and unchanging. Right, the
steady state model. But later when it was discovered
that the universe was actually expanding, he
abandoned the cosmological constant, calling
it his biggest blunder. Oh, the irony. Right.
But now the cosmological constant is back. And
it's being used to explain the accelerating expansion.
Exactly. So what is it exactly? It's basically
the idea that empty space itself has energy.
Empty space having energy, that's pretty mind
-blowing. Yeah, and this energy is thought to
have a negative pressure. Negative pressure.
What does that mean? Well, it means that instead
of pulling things together, like gravity does,
it pushes them apart. Oh, I see. And so this
energy of empty space is what's thought to be
driving the accelerating expansion. OK, but where
does this energy come from? Well, that's where
things get a little tricky. One way to think
about it. Is in terms of quantum fluctuations
quantum fluctuations. So in the quantum world
Empty space. It's actually empty. It's full of
these Virtual particles that are constantly popping
into and out of existence So like a constant
sea of particles being created and destroyed
exactly and these particles Have energy. Okay,
so that's the source of the energy. Yeah, and
so the energy associated with these quantum fluctuations
Is what's thought to contribute? To the cosmological
constant that makes sense, but there's a problem
a problem. What is it a big one? Oh when physicists
try to calculate The value of the cosmological
constant based on quantum field theory they get
an answer. That's way too big way too big How
big are you talking? We're talking 120 orders
of magnitude 120 orders magnitude. That's a one
with 120 zeros after it Yeah, so it's like trying
to measure the distance to the moon and being
off by the size of the entire solar system. Wow,
that's not just a little off that's astronomically
off. It is. And so this is a huge problem for
the cosmological constant idea. Yeah, it's a
major discrepancy. Because it suggests that there's
something fundamental that we're missing. About
how quantum mechanics and gravity work together.
Exactly. So while the cosmological constant is
the simplest explanation for dark energy, it
also comes with this huge baggage. Yeah, a lot
of unanswered questions.