Greetings and welcome to the Introduction to Astronomy.
In this lecture, we are going to talk about dark matter.
And that is some of the unseen matter
that makes up a lot of the material in the universe.
In fact, what we see in terms of stars and galaxies
and nebulae and planets and everything that we normally
study in astronomy is found to only
be a very tiny portion of the amount of matter
in the universe.
Dark matter is just that it is completely dark,
and we cannot see anything of it except for its gravitational
effects.
So it gives off no specific types
of electromagnetic radiation that we can see.
So how do we know that it exists if we can't see it?
Well, what we can do is make some various kinds
of measurements to try to better understand dark matter.
So normally we look at what we call ordinary matter that
is the stuff that you and I are made up
of as are all the galaxies and stars and planets.
This gives of electromagnetic energy.
And if you remember that includes
things like visible light and it also
includes the rest of the electromagnetic spectrum,
whether it be x-rays and gamma rays or radio waves.
But dark matter is different and that turns out to be about 90%
of the mass of many galaxies is located in a dark matter halo.
How can we see that that exists?
Well, we look at galaxies like this
and we measure the rotation of their stars and the gas
that we see in them.
So as we move out, we watch and we
see what the rotation what the velocity of those stars is.
And we expect that it should increase
as you start from the interior and work out
where it is should increase.
But then we expect when you get out here
towards the edge of the galaxy where you're not
seeing anything else beyond it.
It should expect to decline and this is what
we see within our solar system.
The objects further away from the sun
once they are outside of the mass in the solar system, which
is essentially all of the planets
then the velocities decrease as you get further away.
But what we observe from stars and galaxies
is that it continues to increase out
to the edge of the visible galaxy
and then even looking at hydrogen clouds from 21
centimeter lines it still continues to increase.
There is no sign that it is decreasing down to the level
that we would expect.
And what that means is that there must be a lot more
matter in the galaxy in order to explain this rotation curve,
the only way we can explain that is through a lot more matter
within the galaxy.
And we see this in most galaxies in most or all galaxies.
So what can we learn about dark matter in galaxies
and how can we use this to determine clusters.
How about in clusters of galaxies.
That was just in one galaxy.
We can use the motions of the galaxy.
There must be enough mass within the galaxy cluster
to keep the galaxies from escaping.
Otherwise, things like clusters and super clusters
would not remain.
They would disappear over time.
So we would not be able to see it
galaxy clusters galaxies would just be spread
out all over the universe.
So we can use their motions and doing that, we can then
estimate what the dark matter distribution has to be like
in order to keep the clusters together
to keep them the galaxies from spreading apart.
Because if we go just by the galaxies that
and add up their mass there is nowhere near enough mass.
Those galaxies would eventually end very quickly
on astronomical frames spread out over the universe.
Now, another way we can see this is
through gravitational lensing and what we see
is that gravitational lens is a bending of light due to ion
doing too due to the predictions of Einstein's relativity
and the cluster gravitational field will bend the light
from more distant galaxies.
So distant galaxies behind this will give us
multiple and distorted images.
So we see some here and here and all these images
have been distorted by the light not only of one big galaxy
here.
But of all the other galaxies around it plus any dark matter
that is present.
So the dark matter while it does not
contribute to the light of the galaxy cluster at all
does contribute to its mass and will cause the bending
to be more significant.
We can use Einstein's model of gravity
to be able to explain figure out how much mass there
must be to explain the distortions that we see.
And when we do that, we find out that there is many times
the amount of matter that we need with that must be present
that we simply cannot see.
So what about this dark matter and how much of it is there?
Well, we can take a look at that and see
that if we want to go back and recall,
we had looked at the thing called a mass to light ratio.
And if you recall the sun had a mass to light ratio of one
it just meant that it had a mass of one solar mass
and a luminosity of one solar luminosity.
Galaxies are showing a mass to light ratio of 10
meaning that they have 10 times the amount of mass
for each luminosity unit.
So there is a significant amount of dark matter
as we get further and further up into small clusters and then
into large clusters.
The mass to light ratio increases.
And what that tells us as we see the increase here in the table
is that there must be substantial amounts
of dark matter present in these clusters.
Otherwise, we would not get these kinds of large number
300 is an incredibly large mass to light ratio.
And that's what we see for some of these really big galaxy
clusters.
So not only is there a lot of galaxies there.
But for each galaxy there could be 10 or hundred or a couple
of hundred galaxies worth of matter
that is completely invisible.
And that's what we mean by dark matter.
It's not just a few extra dim stars or a few black holes it
means that for each galaxy that you see in an image you have
to imagine that there can be even 100 galaxies worth
of matter scattered around and is in some form that we cannot
see and what could it be what could this dark matter be
composed of.
Well, there are a couple different possibilities
that we talk about.
And those are the MACHOs and the WIMPs.
The MACHOs are massive compact halo objects and that
is essentially ordinary matter.
Things like black holes, brown dwarfs white dwarfs.
These are all things that are very hard to see.
And would be incredibly faint.
So we wouldn't see them directly,
but we would see their gravitational effects.
We wouldn't see the effect of their gravity measurements
have been done and showed that there are simply not
enough of these.
So there are not enough of them to account for them
out of dark matter needed in the halo of our galaxy.
Meaning that if they don't work for our galaxy
they're probably not going to work for other galaxies
as well.
The WIMPs on the other hand, are weakly
interacting massive particles.
These are exotic particles that do not
emit electromagnetic radiation.
So these are unusual subatomic particles
not things like protons, neutrons, and electrons
that make up ordinary matter.
But other more exotic types and those
are what we think may make up the dark matter now
because it does not seem like any type of ordinary matter
can possibly work.
Now, there are two, we can look at dark matter in two models
and those are hot and cold dark matter.
And if we remember hot and cold temperatures
refers to the speed of the particles.
Hot dark matter would be things that are moving very quickly.
Cold dark matter would be things that are moving slowly now
hot dark matter would not do a whole lot because it would
smear out the clumps and would inhibit the growth of clusters
of galaxies.
On the other hand cold dark matter
would not be moving very quickly and it
would grow clumps and give us something
consistent with our current observations.
The model here running time forward as we go across.
And then down is a symbol is a cold, dark matter simulation
and it starts off with the material relatively uniformly
spread out and over time it condenses down and gives us
a structure relatively similar to what we see today.
So these simulations can give us things
that match the structure of our universe.
Again, not precisely it's not going to match up
exactly where the galaxies are.
But the overall pattern is what we
are looking for now, the other thing that we have
and that we want to mention is what we call dark energy.
Dark energy is another mysterious substance quite
different than dark matter.
Now dark matter as we said makes up
the majority of the matter of the universe far more
than any of the ordinary matter that we see.
So this chart kind of breaks it down.
What is the universe made up of.
Well heavy elements that make up the earth
are about 3 onehundredths of a percent - things like stars
are about half of a percent of the mass of the mass
and energy of the universe.
Hydrogen and helium get us up to about 4% So about four to 4
and 1/2 percent of the material that we see
is in ordinary matter.
What we would call ordinary material.
Everything we study in an astronomy class
that would be the nebulae that would be stars that
would be galaxies that would all be included in these three
here.
Everything else is this dark matter,
which we have been looking at making up 25% vastly
outnumbering by five times the amount of ordinary matter
and even more so is this dark energy and dark energy we will
talk about in a coming chapter.
But dark energy is and another mysterious substance
that seems to permeate the universe
and actually makes up most of the mass energy
of the universe.
Now as a comparison, we can look at here
as if we imagine one gram of luminous ordinary matter.
That's the ordinary stuff that we
see every day for every one gram of that there
would be four grams of non luminous ordinary matter.
So that would be things that we can't see.
Hydrogen gas that would not be glowing things that would not
be glowing most of the material in the universe
is invisible to us.
But for those five grams of what we call ordinary matter
there would be 27 grams of dark matter
and 68 grams of dark energy.
So in reality only about 4% to 5% of the universe
is the ordinary matter.
That is the stuff that makes up everything with us
here on Earth that makes up all of the stars
and the nebulae and the galaxies and everything
that we've studied in astronomy to this point on.
That's only about 4% to 5% of the universe and the rest of it
is in this mysterious dark matter, which
we've looked at in this lesson and in dark energy, which
we will come and look at coming up.
So let's finish up this lesson on dark energy with our summary
and what we find and have talked about
is that we know that there are vast amounts
of dark matter in galaxies and in clusters of galaxies.
The dark matter is likely composed of cold meaning slowly
moving exotic particles.
What we call cold dark matter.
And as we looked at the end luminous
ordinary matter again stars, galaxies planets
makes up only a tiny fraction of the mass of the universe.
So that concludes our lecture on dark matter.
We'll be back again next time for another topic in astronomy.
So until then, have a great day, everyone.
And I will see you in class.
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