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Dark Matter
Inquiring Minds
Want to Know
Prof. David Toback
Texas A&M University
Inquiring Minds Want to Know
I keep hearing about “Dark
Matter” in newspapers, on TV,
etc
• What are people talking about?
• Why is it called Dark Matter?
• What do we know about it?
• What’s the evidence for it?
• So what?
Quick Answer
Stuff out in
space we
can’t “see”
directly
What is Dark Matter and why do we
call it Dark Matter?
What do we see when
we look at the
Heavens?
• Easy to see light
from things like
stars
• Can see the light
from stars AFTER it
has left the star and
interacted with
whatever is between
us and the star
Anything else out
there?
What is Matter?
• Lots of ways to understand the word “matter”
• Examples:
– Rocks
– You
– The Earth
– The Sun
– Atoms
• Our Sun and Earth are attracted to each
other by gravity
• They are attracted to each other because
they are made of “matter”
– They have “mass”
Dark Matter
• There is evidence that
there is “stuff” in space
that we can’t “see”
directly
• Call this stuff “Dark
Matter”
– Stuff that is “like atoms” in
that “has gravity”
6
Evidence for Dark Matter
1.The Rotation of
Galaxies
2.Gravitational
“Lensing”
3.Colliding Galaxies
7
Evidence 1
Watch the way
stars move around
the center of a
Galaxy
8
The Planets and the Sun
Gravity does
a good job
of
predicting
the planets
path around
the Sun
9
How Quickly do the Planets go
Around the Sun?
•Inner planets
like Mercury,
Venus and
Earth go
quickly around
the Sun
•Planets move
MUCH slower
As the Solar System Turns
What about Stars and Galaxies?
•Can again use Gravity to
predict the orbits of stars
as they move around the
center of the galaxy
•Should look “like” the
planets as they go around
the Sun
12
Is that what our galaxy, the
Milky Way, looks like?
Does it spin
like our solar
system?
Closest stars
go around
quickest?
The Data
Data doesn’t look
anything like
expectations!
Difference
due to Dark
Matter?
14
How the Galaxy Turns
Dark Matter?
Data well explained by lots of “Dark
Matter” we can’t see
About five times as much Dark
Matter as stars  Forms a “halo”
around the entire galaxy
16
Evidence 2
Look at the effect
gravity has on light as
it travels through the
Universe towards us
17
What happens to light as it goes past the Sun?
Path
doesn’t curve
Path
does curve
• Newton: only things with “real” mass “feel”
the force of gravity. Light has no mass!
• Einstein: Objects with “Energy” move
according to the curve of space-time,
regardless of whether they have mass or not
18
More Detail
Can see the
star directly
Now the Sun
curves the
path of the
starlight
Star
looks like
it’s here
The Great Experiment of 1915
• Look at a star’s position “behind” the sun as it “passes” in
between us and the star
• Result: The apparent position of a star is different when the
sun is close to the path
• In other words, we observe that the gravity of the sun “bent”
the path of the light coming from the star
Dark Matter and Lensing
• Dark Matter
“Lenses” the
galaxies behind
them like a
prism
• Evidence that
the light coming
to us is passing
through lots of
matter we can’t
see directly
Looking at Light from the Universe
Galaxy
Light from a Galaxy
Hydrogen
Another
Galaxy
Cloud
Looks like a Galaxy
Looks
Lookslike
likea aGalaxy
Galaxy
behind a cloud of heavy
behind a cloud of
“stuff” that isn’t just
hydrogen
stars and hydrogen
22
More on Lensing
Sometimes
we can
even see
more
than one
image of
the same
galaxy!
23
Evidence 3
Colliding
Clusters of
Galaxies
24
Look at Colliding Clusters of
Galaxies
Atoms in the Galaxies interact and slow down as
they pass through each other
Atoms
Atoms
Dark Matter doesn’t interact much so it isn’t
slowed down much
Dark Matter
Dark Matter
25
Colliding Galaxy Clusters
The atom part and the Dark Matter
part of Galaxies interact differently
as they pass through each other
Atoms and
Dark Matter
Atoms and
Dark Matter
26
Colliding Galaxy Clusters
Galaxy
Galaxy
Galaxy
Light from a Galaxy
Atoms and
Dark Matter
Atoms and
Dark Matter
27
Evidence for This in Nature?
Colliding Clusters of Galaxies
Blue is the part from
lensing only
“Fast  Dark Matter”
Red part from
observing the light
“Slow Atoms”
28
What IS Dark
Matter?
We don’t know…
Still working on it…
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Is that the whole story?
What’s the
Matter in the
Universe?
No real
understanding of
what this stuff is…
Concluding Remarks
• The case that there is lots of Dark
Matter in the Universe is very
compelling
• The fate of our Universe depends
on Dark Matter
• Might be able to discover it using
the Large Hadron Collider (LHC)
• If scientists are right, might be
able to tie together Astronomy,
Particle Physics and Cosmology