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Inquiring Minds Want to Know Prof. David Toback Texas A&M University
Dark Matter Inquiring Minds Want to Know Prof. David Toback Texas A&M University
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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?
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Stuff out in space we can’t “see” directly
Quick Answer Stuff out in space we can’t “see” directly
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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?
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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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Call this stuff “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
The Rotation of Galaxies Gravitational “Lensing” Colliding Galaxies 7
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Watch the way stars move around the center of a Galaxy
Evidence 1 Watch the way stars move around the center of a Galaxy
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Gravity does a good job of predicting the planets path around the Sun
The Planets and the Sun Gravity does a good job of predicting the planets path around the Sun
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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
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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 12
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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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Difference due to Dark Matter?
The Data Difference due to Dark Matter? Data doesn’t look anything like expectations! 14
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How the Galaxy Turns
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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 does curve Path doesn’t 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
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More Detail Can see the star directly Star looks like it’s here
Now the Sun curves the path of the starlight
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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
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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 Universe
Galaxy Light from a Galaxy Another Galaxy Hydrogen Cloud Looks like a Galaxy behind a cloud of heavy “stuff” that isn’t just stars and hydrogen Looks like a Galaxy behind a cloud of hydrogen Looks like a Galaxy
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More on Lensing Sometimes we can even see more than one image of the same galaxy!
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Colliding Clusters of Galaxies
Evidence 3 Colliding Clusters of Galaxies
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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
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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 Atoms Atoms Dark Matter Dark Matter
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Colliding Galaxy Clusters
Light from a Galaxy Atoms and Dark Matter Atoms and Dark Matter Atoms Atoms Dark Matter Dark Matter
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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…
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Is that the whole story? What’s the Matter in the Universe?
No real understanding of what this stuff is…
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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
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