When you look at the night sky what do
you see? You see a few stars, but you mostly see dark. What is in that dark? Is
it all a vacuum, like we've been taught in school? What you may not realize is
that the night sky really would look like this, if you could see all the
hidden matter that's invisible to our eyes. One of the biggest, if not the
biggest question in science right now is, "what is the universe made of?" Sure, I've
talked about the standard model and quantum field theory in several videos,
which try to explain what the universe is made of.
But these, so far, only attempt to explain what the visible matter in the universe
is made of. It turns out that visible matter - that's everything that you can
see on earth, and in space, with the most powerful telescopes, is only a minuscule
portion of all that exists. There's about six times more matter in the universe
than what we can see. This invisible matter is called "dark matter." This means
that what we're seeing are only the waves on top of the ocean. We're not
really seeing the ocean itself. Yet, without this ocean, we would likely not
exist. So if this dark matter is invisible, how the heck do we know that
it's even there? And if it exists, then what could it actually be? You'll be
surprised how much we know about it. Let's look at what our best theories in
science have to say about it, and that's coming up right now...
it turns out that we can detect invisible matter by its effect on the
visible matter that we can see and detect. The first evidence of dark matter
came from Swiss astronomer Fritz Zwicky in the 1930s, when he saw that the
galaxies in clusters, like the Coma Cluster, were moving at very high speed.
In fact, they were moving so fast that the galaxies could not stay
gravitationally bound, and should have gotten ripped apart, unless they had, he
figured, about 100 times the matter that he could see visibly. He coined the term
"dark matter" to describe this invisible mass. The problem was nobody believed him.
He was kind of an eccentric guy, and not very likable. Most scientists either
ignored him, or didn't believe him. It wasn't until almost 40 years later, in
the 1970s, that observations by Vera Rubin confirmed the idea of dark matter.
She measured that the velocity of stars on the outer edge of galaxies was about
the same as those closer to the center. But the mass at the edge of galaxies,
based on visible stars, gets smaller. This appeared to be impossible to explain
without the concept of unseen matter causing gravitational effects on the
outer edges of galaxies. If you compare the orbital speed of planets around the
Sun, you'll find that the speed slows down the further away from the Sun
you go. The curve of speed versus distance from the Sun follows a one
over the square root of distance relationship. However, when you look at
the orbital speed of stars in the Milky Way galaxy, you find that almost all
stars are orbiting at the same speed - about 200 kilometers per second. And most
other galaxies seem to show the same general trend. Near the center of the
galaxy there's enough visible matter to account for the speed. But as you get
further out, there's a greater and greater mismatch between the orbital
speed of stars and the visible matter. This is the reason it is thought that
most of the dark matter forms a kind of halo around the galaxy. There is likely
less of it near the center of the galaxy, but a lot more the
further out you go. Most other galaxies seem to show the same general trend. Now,
how do we know the dark matter is not just ordinary matter that is hard to see -
like dim stars, or black holes? This was thought to be a possibility decades ago,
and some of these were categorized as MACHOs or, MAssive Compact Halo Objects.
But today, we have so much observational data that this possibility has been all
but ruled out. The distribution of dark matter, based on the rotational curves,
gives us a clue. It appears to be distributed evenly, as well as like a
halo around the outer edges of galaxies, and not clumped up like visible matter
is. This can be explained if we theorize that this matter interacts very weakly
with itself. But ordinary matter does not interact weakly and thus tends to clump
together. Ordinary matter would likely not be so evenly distributed. And
research as recent as 2019, from a team in Japan, appears to have ruled out
primordial black holes as being the source of dark matter. In addition, there
are other more technical reasons, having to do with the rate at which new
elements were formed in the early universe, which indicate that only about
5% of the energy density of the universe can be explained by ordinary matter. The
rest has to be dark matter and dark energy. I'll be getting into details of
dark energy in a future video. By the way, most of the ordinary matter in galaxies
is not in the form of stars and planets, but interstellar gases. If you added up
all the mass of this gas, it would be about 3 times as much as the mass of all
the stars and planets of the universe put together. So a pie chart of the mass
of the universe looks like this - 4 percent stars and planets, 11 percent gas,
and 85 percent dark matter. The bullet cluster, is I think, the most interesting
evidence showing how the behavior of dark matter is not like ordinary matter.
This is an enhanced photograph of two clusters of galaxies passing through
each other. In a computer simulation of this, what you can clearly see is that
the gases of two galaxies are interacting with each other and heating
up, to emit x-rays, which are visible as bright pink areas. This is the ordinary
matter of the two clusters. The majority of the matter, however, as indicated by
the gravitational lensing, is in the blue areas of the photo. This
is the dark matter that simply did not interact with the ordinary matter, or
with itself, as the two clusters collided, and thus, passed right through. So you can
clearly see that most of the mass is not where most of the ordinary matter is.
There are some physicists who argue that dark matter doesn't really
exist, and the rotation curves can be explained if the laws of gravity were
different at larger distances. Again, this behavior in the bullet cluster would be
hard to explain if that's the case. Inventing a new particle is probably
less crazy than proving the laws of gravity wrong, given the mountains of
evidence that prove Einstein's general relativity is correct. So if it is not
dim ordinary matter, and not due to changes in gravity, then what the heck is
it? Let's look at what properties the potential Dark Matter particle would
need to have first. It obviously must be dark. This means it does not emit any
light and doesn't interact with anything to emit light, or any kind of
electromagnetic radiation. Two, it must of course interact through gravity, so it
cannot be a zero rest mass particle, like photons. Three, it must not interact with
itself, or interact very weakly with itself. Four, it must be cold. That is, it
is not moving at very high speeds, because if it was, then after the Big
Bang, it would have just kept on going, and not slowed down to form halos around
ordinary matter. And five, it must be stable. That is, it does not decay, because
if it decayed into other particles, it wouldn't still be here now in such large
quantities, 13.8 billion years after the Big Bang. if it's a particle, could it be
hiding in plain sight in the standard model of particle physics?
We need a particle that does not decay and is neutral. The only massive neutral
particle in the standard model that is stable is the neutrino. But because
neutrinos are so light, they're not cold. They move very fast, so they're like hot
dark matter. This would preclude neutrinos from
making the large structures in the universe, because the neutrinos would
want to move away while the ordinary matter wants to come together. So the net
effect would have been a smoothing out of
ordinary matter. If we invent a new particle that does not interact with
electromagnetism has no strong force nuclear interactions but does interact
through the weak nuclear force, and through gravity, then we have just
invented the dark matter particle. Scientists have a name for this particle
it's WIMP, or Weakly Interacting Massive Particle. Why would this particle need to
interact through the weak nuclear force? ...because it turns out that if we ask the
question, "what is the rate of interaction of dark matter particle and antiparticle
annihilation that you would need to have to give us the correct ratio of dark
matter that we observe today?" -- the answer is that they would need to interact with
the strength EXACTLY equal to the strength of the weak nuclear force. This
coincidence is called the "wimp miracle." And it so happens that there is a model
in particle physics that predicts exactly such a particle. This model is
called supersymmetry. Scientists like the WIMP because it was not invented for the
purpose of solving the dark matter problem, but comes about naturally in
string theory. And string theory requires supersymmetry to work. So what is
supersymmetry? supersymmetry is a special kind of symmetry between force particles,
the bosons, and matter particles the fermions.
Supersymmetry says that for every boson particle,
there would be a corresponding fermionic particle, and vice-versa. And it would
have the same charge, strong and weak nuclear force
interactions. It would have a different spin, and because of "symmetry breaking"
it could have a much higher mass. Now, supersymmetry has not been found to be
confirmed in nature. But if supersymmetry is true, then the Dark Matter mystery
could be solved, because there would be a perfect candidate for a Dark Matter
particle in supersymmetry. And it is called the Neutralino. The Neutralino would be
the lightest supersymmetric particle of the photon, the z boson, and the Higgs
bosons. So supersymmetry would double the particles in the current standard model.
Is this complication worth it? Well, Dark Matter is a benefit, but the
fact that it makes string theory work is also a benefit. And it explains some
mysteries as well, such as why the Higgs boson is lighter than it should be. But
the biggest problem with the neutralino theory, is that we should have been able
to detect it at the Large Hadron Collider. And so far, we have not. So this
puts a big damper on this theory. The Axion is the second viable candidate to
be the Dark Matter particle. The Axion was invented to solve a problem having
to do with something called the charge conjugation and parity symmetry problem
with the strong nuclear interaction. This is also called the strong CP problem. CP
symmetry basically means that the laws of physics should be the same if a
particle is replaced by its antiparticle - C symmetry, and its spatial coordinates
are inverted to its mirror image - that's the P symmetry. This is a technically
complicated issue for the strong force, but we can simplify it by calling it,
"the electric spin problem of neutrons." It so happens that the theorized existence of
a particle called the Axion solves this problem. Essentially, the problem can be
more simply shown in the following way: as you know, protons and neutrons make up
the nucleus of atoms. The neutron has a property called a magnetic moment, or
spin. This means that if you put the neutron in a magnetic field, the neutron
will have a spin. The reason for this spin, even though the neutron has no
charge, is because the neutron is made of three quarks that have spin. And all of
these combine to give the neutron a spin. And this is confirmed by
observation. But the neutron, according to theory, should also spin if exposed to an
electric field. However, for some reason, it does not. No one knows why. And this is
the problem. The lack of spin, it turns out, is dependent on one of the
fundamental constants of nature, called "theta." This theta has to be zero, or close
to zero for this observation of no spin of neutrons to happen. This is one of the
constants like the gravitational constant, or the Planck's constant. But
theta should vary anywhere from negative pi to pi. There's no reason that it
should just happen to be zero. This seems to be
a huge fine-tuning coincidence! And physicists don't like coincidences. In
1977 two physicists Roberto Peccei, and Helen Quinn proposed a theory which
solved this problem, by creating a new field for theta instead of it being just
a fundamental constant. And since most fields seek to maintain the lowest
possible energy state of zero, that's what theta would also seek to be - zero. But if
theta is a field, like the electromagnetic field, or the quark field,
it means that it will have a particle associated with an excitation of the
field. This particle of the theta field is called the Axion. But due to the nature
of the theta field, the Axion is predicted to have an extremely low mass.
There are a couple of reasons why Axions make an attractive Dark Matter
particle candidate. First, they do not require the existence of supersymmetry.
And they can be placed in the standard model as a much lighter cousin of the
higgs boson. But it's very low mass would be made up for in very large numbers. If
you had a cubic centimeter of space, it would contain roughly one WIMP particle,
if WIMPs were dark matter. But if Axions were dark matter, one cubic
centimeter of space would contain ten quadrillion, or roughly 10 to the power 16
Axions. Axion experiments are being done in Hamburg, Germany and in CERN. But
so far, nothing has been detected. So while scientists have placed big bets
that dark matter is likely the WIMP and/or the Axion, no proof, or smoking gun
evidence has been found. But this is not atypical of a story in science. Things
this important can take decades to resolve. It is possible that we just
don't have the right tools currently to find the particles this elusive. And we
might just have to be patient. Or, maybe, all this is wrong, and we need a
completely new theory to explain dark matter. Either way, I think this is a
really exciting area of research. And it's going to keep current and future
physicists busy for a really long time.
And if you like this video then
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questions, because I try to answer ALL of them. I'll see you in the next video my
friend!