# Physics Lecture 28/29/30 - the Critical density of the universe

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1. the strength of the universes gravitational pull depends on what?
the density of matter in the universe. ^ density, ^ gravitational pull
2. critical density (definition)
precise density marking the dividing line btw eternal expansion and eventual collapse
3. Is there enough mass from visible objects to halt the expansion of the universe (what percent)

what does this mean for dark matter
No. contribute .5% of matter density needed to halt expansion.

means dark matter needs to have at least 200 times the mass in stars
4. Is the expansion of the universe expanding? If so what is pushing galaxies apart?
• Yes.
• Dark energy - repulsive force.
5. Recollapsing universe
if there is no dark energy and the matter density of the universe is larger than the critical density, the universe will collapse back in on itself
6. Critical Universe

also called what?
If there is no dark energy and the matter density of the universe equals the critical density, then the collective gravity of all its matter is exactly the amount needed to balance expansion

stops expanding after infinite time, and overall geometry is "flat" - also called "flat universe"
7. Coasting Universe
If there is no dark energy and the matter density of the universe is smaller than the critical density, then the collective gravity cannot halt expansion --> the universe will keep expanding forever

aka open universe
8. Accelerating universe
If dark energy exerts a repulsive force that causes the expansion of the universe to accelerate with time --> could be flat open or closed

*supernova data fit this curve best*
9. What would happen if at a time close to the Big Bang, Omega < 1?
The universe expands, and omega gets smaller very very quickly

How quickly? - to have a value of .005, Omega would have to be 1 x 10-25 about 100,000 yrs after the big bang
10. Dark Matter
- does not interact with light (reflect, create), but does create gravity
11. The spectra of different locations will have different ...
redshifts

(parts which come towards you --> bluer, away from you --> redder)
12. How to measure the rotation of a galaxy
• use a slit to capture light across the galaxy
• make a spectrum at each position
13. Rotation Curve of planets around the sun - "Keplerian Rotation"

- sun has all the mass, so GMcentral is _____
- velocity drops with __________
14. So in the Solar System orbital speed ...

But in galaxies, orbital speed ...
• - falls with radius because all the mass is at the center
• - no new mass at large radii

• - does not decrease with radius
• - there must be more mass at large radii
15. Two proposals to explain the flat rotation curves of galaxies
• 1. we dont understand gravity
• 2. the mass distribution is galaxies does not trace the stars - instead in a large halo that we cant see

--> DARK MATTER (mass implied is huge - 10x the mass we see from starlight)
16. Gravitational Lensing
light bends around objects with mass (black holes)
17. How much dark matter is there?
90% of the mass in galaxies
18. Why cant it be
- gas
- very faint stars
- dust
• - gas always glows
• - they would still emit too much light, and we would see them as MACHOS
• - they would be warm enough that they'd still glow in infrared, and we dont detect enough to account for the mass in dark matter
19. dark matter only interacts with normal matter ______________
gravitationally
20. Observational evidence for dark matter in galaxies is ....
flat rotation curves (Once you get out “beyond” where the mass is,the rotation velocity should decrease, like in the solar system. but in galaxies we never see this)
21. Why must there be dark matter (accelerating universe)
there must be "extra" mass between the galaxies to accelerate them gravitationally
22. gravitational lensing permits us to measure what?
the total mass in a cluster
23. how to find gravitational lensed galaxies in an image
look for long arched streaks
24. the further away a galaxy is, the higher its __________
doppler shift (redshift)
25. Dark matter?

aka?
• whatever it is thats causing expansion to accelerate
• - aka quintessence
• - aka cosmological constant
26. What are MACHOs
Massive compact halo objects

## Card Set Information

 Author: asnodgrass ID: 51870 Filename: Physics Lecture 28/29/30 - the Critical density of the universe Updated: 2010-11-27 20:51:37 Tags: physics critical density Folders: Description: critical density of the universe and other options Show Answers:

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