Dark MatterInquiring Minds Want to KnowProf. 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”

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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”

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Evidence for Dark Matter

1.The Rotation of Galaxies

2.Gravitational “Lensing”

3.Colliding Galaxies

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Evidence 1

Watch the way stars move around the center of a

Galaxy

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The Planets and the Sun

Gravity does a good job of predicting the planets path around the Sun

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

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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

Is that what our galaxy, the Milky Way, looks like? Does it

spin like our solar system? Closest stars go around

quickest?

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Data doesn’t look anything

like expectations!

The Data

Difference due to Dark

Matter?

How the Galaxy Turns

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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

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Evidence 2

Look at the effect gravity has on light as it travels through

the Universe towards us

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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

More Detail

Star looks

like it’s here

Can see the star directlyNow the

Sun curves the path of

the starlight

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

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Looking at Light from the UniverseGalaxy

Light from a Galaxy

Looks like a Galaxy

HydrogenCloud

Looks like a Galaxy behind a cloud of

hydrogen

Looks like a Galaxy behind a cloud of heavy “stuff” that isn’t just stars and

hydrogen

Another Galaxy

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More on LensingSometim

es we can even see more than one image of the same galaxy!

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Evidence 3

Colliding Clusters of Galaxies

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Look at Colliding Clusters of Galaxies

Atoms

Dark Matter

Atoms

Dark Matter

Atoms in the Galaxies interact and slow down as they pass through each other

Dark Matter doesn’t interact much so it isn’t slowed down much

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Colliding Galaxy Clusters

Atoms and Dark Matter

Atoms andDark Matter

The atom part and the Dark Matter part of Galaxies interact differently as they pass through

each other

Atoms Atoms

Dark Matter Dark Matter

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Colliding Galaxy Clusters

Atoms and Dark Matter

Atoms andDark Matter

AtomsAtoms

Dark Matter Dark Matter

Light from a Galaxy

GalaxyGalaxy Galaxy

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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”

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What IS Dark Matter?

We don’t know… Still working on

it…

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