Hello!
In this lecture we'll define dark matter and
dark energy and then talk about what evidence
astronomers have for the existence of dark
matter, as well as some possibilities of what
it could be made of.
We need to first define what we mean by dark
matter and dark energy.
Even though they both have the word "dark"
in their names, dark matter and dark energy
are not related.
Dark matter is an undetected form of mass
that emits little or no light, but whose existence
we infer from its gravitational influence.
Dark energy is an unknown form of energy that
seems to be the source of a repulsive force
causing the expansion of the universe to accelerate.
It's a little crazy to think about, but the
matter we're used to, the stuff all around
us, makes up about 5% of all the matter in
the universe.
About 27% of the universe is in the form of
dark matter, and about 68% is in the form
of dark energy.
There is currently ample scientific evidence
to support the existence of dark matter.
I'm going to describe the observations that
provide this evidence, and what these observations
can tell us about the nature of dark matter.
We'll see that we have evidence for dark matter
in our own Milky Way, as well as in other
spiral galaxies.
Astronomers also have data from elliptical
galaxies that is consistent with dark matter.
On a larger scale, observations of galaxy
clusters imply there are large amounts of
dark matter within clusters.
Kepler's laws tell us that for any astronomical
system that has its mass concentrated at its
center, objects near the center should be
moving faster than objects far away.
We saw this in our solar system- the inner
planets move faster, as well in the accretion
disks.
We therefore expect that stars near the center
of our galaxy should move faster than stars
at the outer edges.
But this is not what we observe.
If we were to make a rotation curve of our
galaxy- that is a plot of the velocity of
the stars in our galaxy versus their distance
from the center- we would expect to see something
like this.
Fast moving stars near the central bulge,
and slower ones farther out.
But instead, this is what we see.
The rotation curve is flat.
Stars near the center are moving at the same
velocity as stars on the edges
The flat rotation curve implies most of the
galaxy's mass is not at the center.
Where, then, is the mass?
Well, it's in an enormous spherical halo of
dark matter with 10 times the mass of all
the stars in the disk.
Our Milky Way is not unique when it comes
to this distribution of dark matter.
Astronomers also observe flat rotation curves
in other spiral galaxies, implying that dark
matter is common.
We also see evidence for dark matter in elliptical
galaxies.
By studying absorption lines of elliptical
galaxies, astronomers can determine how fast
the stars in these galaxies are moving.
It turns out that the speeds of stars remain
fairly constant from the center of the elliptical
galaxies to the edges, just as with the spirals.
It appears ellipticals also have dark matter.
There is observational evidence for dark matter
on even larger scales.
In the 1930s, astronomer Fritz Zwicky argued
that galaxies clusters had enormous amounts
of dark matter, but his idea wasn't taken
seriously at the time.
Zwicky measured the motions of galaxies within
the clusters.
Using Newton's law of universal gravitation
he used the velocities of the galaxies to
estimate the cluster's total mass.
The total mass you expect is the mass from
all the stars in all the galaxies.
One can estimate this mass by looking at the
luminosities of the galaxies.
If the galaxy has the luminosity of 100 billion
suns, it's reasonable to say that galaxy has
a mass of a 100 billion solar masses.
What Zwicky found was that the mass obtained
using the motions of stars was much larger
than the mass of luminous matter in the galaxy.
The mass we find from galaxy motions in a
cluster is about 50 times larger than the
mass in stars!
Hot gas within galaxy clusters offers more
evidence for the existence of dark matter.
Within galaxies in a cluster, there is often,
hot x-ray emitting gas.
There can be a lot of it ? up to seven times
as much mass in the form of gas compared to
stars.
We can measure the temperature of the gas,
and since temperature is related to the movement
of the gas particles, we can calculate the
velocities of the particles.
From the velocities we can calculate the total
mass of a cluster.
The results obtained from hot gas agrees with
results found by studying orbital motions.
Dark matter in clusters of galaxies is up
to 50 times the combined mass of the stars
in the cluster's galaxies.
The effect of gravitational lensing also provides
evidence for dark matter.
Gravitational lensing occurs because massive
objects can actually bend light.
Einstein predicted this in his general theory
of relativity.
Here's how it works.
Imagine a large galaxy cluster.
We're observing the cluster from Earth.
Behind the cluster is a large spiral galaxy.
The light from the spiral galaxy passes through
the cluster on its way to Earth, but the light
bends due to the mass of the cluster.
We see ghost images of the galaxy in the directions
from which the light appears to be coming.
This is a Hubble Space Telescope image of
a galaxy cluster acting as a gravitational
lens.
The yellow elliptical galaxies are cluster
members.
The small blue ovals are multiple images of
a single galaxy that lies almost directly
behind cluster's center.
The thing is, if you estimate the amount of
mass in the cluster based on the luminous
stars, there is not nearly enough to cause
this sort of lensing.
When astronomers calculate the mass using
gravitational lensing, it agrees with the
masses obtained from the orbital motions of
the galaxies and the velocities of hot gas
particles.
Gravitational lensing supports dark matter.
It seems there's ample observational evidence
for dark matter, both in individual galaxies
and in galaxy clusters, but perhaps there's
another solution.
What if our understanding of gravity is wrong?
Does dark matter really exist?
We therefore have two options.
Either dark matter really exists, and we are
observing the effects of its gravitational
attraction or something is wrong with our
understanding of gravity, causing us to mistakenly
infer the existence of dark matter.
Physicists are seriously looking into the
latter.
It's called MOND- Modified Newtonian Dynamics.
Nevertheless, gravity has been so well tested
that most astronomers prefer option one.
If we agree dark matter exists, then what
is it made out of?
There are two basic possibilities.
Dark matter could be ordinary matter- the
matter we're used to, only we can't see it.
A possibility astronomers thought of early
on was celestial objects like brown dwarfs,
white dwarfs, and black holes.
We may not be able to see these objects, but
they certainly have mass.
Astronomers call these objects MACHOS, which
stands for Massive Compact Halo Objects.
The second option is that dark matter is some
sort of exotic matter.
Astronomers often call this possibility for
Dark matter, WIMPS, which stands for Weakly
Interacting Massive Particles.
The name implies the particles have mass,
but we're talking about very small subatomic
particles.
Astronomers believe WIMPS are the best bet
for a variety of reasons.
Why are WIMPS the leading hypothesis?
First, there simply does not appear to be
enough ordinary matter.
The amount of deuterium- heavy hydrogen- left
over from the Big Bang, as well as observations
in the cosmic microwave background, indicate
that ordinary matter adds up to only about
one-seventh of the total amount of matter.
The rest of the matter is hypothesized to
be exotic dark matter, or WIMPS.
Second, dark matter as WIMPS would also explain
why dark matter seems to be distributed throughout
spiral galaxy halos rather than concentrated
in the disks.
We learned earlier that galaxies are thought
to form as gravity pulls together matter in
regions of slightly enhanced density.
This matter would have consisted mostly of
dark matter mixed with some ordinary hydrogen
and helium gas.
The ordinary gas could collapse to form a
rotating disk because individual gas particles
could lose orbital energy.
But WIMPS cannot produce photons and rarely
interact and exchange energy with other particles.
As the regular gas collapsed to form a disk,
WIMPS would therefore have remained stuck
in orbits far out in the galactic halo- just
where most dark matter seems to be located.
The case for the existence of WIMPS is fairly
strong, but not a sure thing.
Detecting the particles directly is an enormous
challenge.
Astronomers are working on this in two different
ways.
The first and most direct way is with detectors
than can potentially capture WIMPs from space.
Because these particles are thought to interact
only very weakly, the search requires building
large, sensitive detectors deep underground
where they are shielded from other particles
from space.
So far work in this area has provided some
potential signals for dark matter, but no
definitive proof.
The second way scientists are currently searching
for dark matter particles is with high energy
particle accelerators.
Particle collisions in accelerators produce
a variety of subatomic particles.
None of the particles found so far has the
characteristics of a WIMP, but scientists
are optimistic that the Large Hadron Collider
will soon reach collision energies great enough
to produce dark matter particles.
I'll leave you with this 2011 quote from a
scientist searching for dark matter: "If I
were to make a bet, I would put my money on
the first unambiguous evidence for particle
dark matter appearing within the next few
years.
Once those detections start taking place,
we will begin to shed light on dark matter's
properties in detail.
If 2011 is an embarrassing time to be a cosmologist,
it is an exciting one too."
Take care, I'll talk to you soon.
