So you've probably heard of dark matter
at some point, and about how it makes up
85% of all the matter in the universe.
But you might not know what exactly it is.
So today we're going to talk about
how it was discovered, what other
theories can explain the effects of dark
matter, and what exactly dark matter is.
The first to come up with the theory of
dark matter was the excellently named
Fritz Zwicky in 1933; when he was
performing observations on the Coma Cluster.
Which is the closest great
cluster to Earth.
Since it's such a large
cluster he decided to do something that
no one else had done before; he decided
to figure out how much force was holding
the cluster together.
Now sounds like a
pretty tricky task, but he was
actually pretty clever in how he went about it.
You see, there was already a
method of doing this for star clusters
called the Virial theorem.
Basically how it works is: you take the average velocity of
the stars in that cluster,
and then you figure out what the kinetic
energy is - and the energy required to
hold them all together is twice that.
But this is where he hit a bit of a snag in
doing this, because he figured out that
the average velocity of the galaxies in
this cluster was 1,000 km a second.
To put that into perspective: the
Andromeda galaxy is approaching the
Milky Way at a 110 km
 a second, so they're travelling
ten times there. And that's not even
taking into account the fact that
Andromeda and Milky Way are
approaching each other, so they
really have an average speed of 55
m a secon- 55 km a second.
Despite the fact that the galaxies in
the Coma Cluster are moving at such
ridiculously high speeds the cluster is
still staying together. Zwicky published
this in his 1937 paper "On the Masses of
Nebulae and Clusters of Nebulae". Which, if
you're a little bit confused at this
point why he's calling them the nebulae,
that's because it was only in 1925
that people actually figured out that
galaxies were separate things from the
Milky Way. So the term nebulae to refer
to them kind of stuck around for a while
after that. And if you want to watch a
video on that, I actually have one just
click on the card.
So what he explained in
this paper was that the mass, um no, yeah the
mass-to-light ratio of this cluster was 500:1.
Although more recent
calculations have put that down to 300:1
So what that means is that the Sun has a
ratio of 1:1, and within the
Milky Way you get some star clusters
that get up to like 3:1 so
assuming that these galaxies only have
stars - which they don't but let's just
pretend - then each star would have to
have 300 times the mass of the Sun, but
would only produce the same amount of
light as the Sun.
And even taking into
account the fact that most of the
galaxies and the comb cluster are
actually elliptical galaxies, which are
less bright than their spiral arm
counterparts, this difference just
couldn't be explained.
So the explanation that's what he came
up with for this discrepancy was that
there must be matter in the Coma
Cluster that just wasn't visible, and he
actually was the one who coined the term
"dark matter".
So in the same year as
Zwicky's paper (1937) another astronomer
Horace Babcock was finishing his PhD.
And he was performing observations on
the Andromeda galaxy. And he actually had
access to what was cutting edge
technology at the time, and Babcock found
that Andromeda actually violated
Kepler's second law of motion.
Which to put simply means that instead of stars
that were close to the centre of the
galaxy moving faster, and ones that were
further away moving slower; similar to how
Earth moves faster around the Sun than
say Saturn but slower than Mercury.
The velocity was pretty much constant
all the way across, and if anything it
got slightly faster towards each of the
galaxies. And he explains this in this
paper by saying
Despite the fact that Babcock came very
close to the truth, he never actually
came to the conclusion that dark matter
was responsible for this effect.
He, instead, thought that some of the light
simply wasn't leaving Andromeda and
was getting absorbed into it. However, his
argument became very important later in
the 1970s when a very similar experiment
was performed by Vera Kent in Rubin Ford.
And their experiment which was using
much more sophisticated technology they
found exactly the same results that
stars of any distance from the center of
Andromeda had similar velocities.
They, however,
did come to the conclusion that
the cause of this was dark matter, and
they were in part responsible for making
the theory so popular in the 70s. The
other influential pair of the 70s
who managed to make the Dark Matter
theory so popular were Jeremiah Ostriker
and Jim Peebles; they actually
performed no original calculations but
they managed to come to the conclusion
that the structure of the galaxy the
Milky Way
must be a flat disc of the visible
matter that were familiar with encased
within a sphere of dark matter.
How they came to this conclusion was they
would take a radius out from the centre
of the Milky Way,
and no matter how large or small that
was the stars within that radius would
behave proportionally to if they were in
a sphere of a million light-years across
with 1.5 trillion solar masses in it.
That's a pretty huge difference from if
you just look at the visible side
of what constitutes the Milky Way
because it's about a 30 light year
radius out from the centre, and
there's only 200 to 600 billion solar
masses. And that a very very striking
difference. And the other thing they
found was if this wasn't the case then
the Milky Way would be incredibly
unstable and it would form into a larger
bar, which it very clearly has not.
At this point we have two major arguments
for the existence of dark matter the
first being the fact that galaxies tend
to behave as though there is a large
amount of mass towards the outside of
them, but there is nothing visible there;
and the second being that galaxy
clusters can hold themselves together
despite the fact that they don't seem to
have enough mess to them to actually do that.
Now there's actually a little
addendum that we need to make to that
second argument; which is that even if
you don't think dark matter exists the
galaxies in galaxy clusters
don't make up with the majority of mass.
There's actually a
lot of hot gas in galaxy clusters which
radiates x-rays that can be seen by
x-ray telescopes and this constitutes
about four to five times the amount of
mass that the galaxies do. But the thing
is that galaxy clusters tend to require
somewhere between 10 and 50 times the
amount of mass of the galaxies in them
to hold them together. So even in the
best-case scenario the hot gas is only
going to make up about half of the extra
mass that you need to hold everything
together. So if that's not a possible
explanation for why galaxy clusters
don't separate. The next argument
for the existence of dark matter
comes from an effect called
gravitational lensing, which is
basically when there's an object of very
strong gravity which distorts any light
from behind it into a sort of arc shape.
And this was actually first predicted to
happen coming from galaxy clusters by
our friend Zwicky. And as you've probably
guessed when galaxy clusters do do
this
the light actually gets distorted a lot
more than you would predict based on the
visible matter of them. And it actually
agrees perfectly with the Virial theorem
in terms of how much extra mass is
needed to explain this difference. And
the final argument we're going to be
looking at is a kind of a weird one. It's
basically just the fact that there were
stars and galaxies in the early universe.
In the very early universe,
immediately after the Big Bang,
everything was light - there was light
everywhere in the universe - and that
meant that it was constantly interacting
with any of visible matter, and
consequently that would mean that the
visible matter would be pushed around
what too much to actually be able to
form any sort of structures; like stars
or galaxies. But obviously this doesn't
hold true for dark matter, because
dark matter isn't affected by visible light
in any possible way. So it can come
together, and it can fit of structures
and through that it could distort
gravity and pull in visible matter,
and cause it to form structures as well.
And this is kind of backed up by the
Cosmic Microwave Background, because
that's basically the leftover from all
the light that was present in the early
universe, and even though people say it's
the same everywhere, it's not actually
exactly the same everywhere; there's
fluctuations and and these fluctuations
are caused by gravity distorting the
light back then in the early universe.
So now we've looked at all the arguments
for the existence of dark matter that
brings us to the question of: does this
prove that Dark Matter exists? and the
answer is...
No, it doesn't.
There are still
scientists out there who are not
convinced by these arguments, and they
hold alternative theories. And these
alternative theories are generally
considered types of modified Newtonian
dynamics; which is a set of theories.
They're not all the same as one another,
but basically they all have the same
fundamental belief: that Einstein's general
theory of relativity - which is kind of
the big gravity theory - is either
incorrect or incomplete. And the aim of
modified Newtonian dynamics is to come
up with a new theory of gravity that is
complete and doesn't require this
explanation of ~oh there's just more matter
and we can't see it~ to explain these
effects. But none of these theories have
proven to be complete so far. The biggest
hurdle for them is the bullet cluster.
Which, despite its name, is actually two
separate clusters. It's a small one that
was shot through at a very high speed
through a large one, and traditional
theories of gravity should put the
centre of mass of this collective
cluster somewhere in the centre of the stars.
However, through observation for
gravitational lensing scientists have
found there actually each cluster has
its own separate centre of mass, which is
a lot easier to explain with dark matter
where each one would kind of have its
own body of dark matter, and its own
centre of gravity. So then comes the
question of: assuming Dark Matter
does exist, what is a main out of?
It's made out of some kind of matter,
but we don't really know what yet.
Because no one's observed it directly. We
do know that it is a form of a
nonbaryonic matter, and the reason we
know this is baryons (which are protons
and neutrons) when they form baryonic gas
can be backlit. And there's been no
observed backlighting of dark matter.
So that leaves nonbaryonic matter which is
more or less every other kind of
subatomic particle, but like I said no
one has observed them directly.
We've really only scratched the surface of
this topic today, I'd really encourage
you if you're interested in this to go
out and read more about it.
I have included some links in the description
below, but there's loads of information
out there you should definitely toward
your library look online have a good
read about this. Because there's a lot of
stuff to know. And, of course, if you have
any questions about anything that I have
explained today feel free to leave it in
the comments below. And I can answer that
for you and I'll see you next week
another video!
