1. Dark Matter Background Possible causes Dark Matter Candidates
Non Dark Matter Theories
11th March 2009
Terry Evans

2. Background Galaxy Rotation Velocities Galaxy Cluster velocities
Gravitational Lensing
Flatness of the Universe
11th March 2009
Terry Evans

3. Galaxy Rotation Velocities
We can (reasonably easily) get the total mass of stars, gas and dust in a galaxy
The speed of stars is too high to stay bound by gravity alone
The rotation at different distances from the centre of the galaxy doesn’t follow the simple inverse square law
11th March 2009
Terry Evans

4. Galaxy Rotation curve
11th March 2009
Terry Evans

5. Galaxy Cluster Velocities
Historically first “problem” identified (1933)
Speed of galaxies is too high to stay bound by gravity alone
11th March 2009
Terry Evans

6. Gravitational Lensing
A nearby mass will deflect a ray of light from a distant source (General Relativity)
11th March 2009
Terry Evans

7. Gravitational Lensing
11th March 2009
Terry Evans

8. Gravitational Lensing
11th March 2009
Terry Evans

9. Gravitational Lensing
A nearby mass will deflect a ray of light from a distant source (General Relativity)
The deflections and distortions measured indicate the mass deflects more than predicted by General Relativity
11th March 2009
Terry Evans

10. Strong Gravitational Lensing
11th March 2009
Terry Evans

11. Weak Gravitational Lensing
Done as a statistical process
Makes a number of assumptions on distribution and shape of distant galaxies
Most studies done using Weak Lensing
11th March 2009
Terry Evans

12. Flatness of the Universe
Observations of the CMB indicate that the Universe is “Flat”
Measured and estimated mass is only 5% of that need to make the Universe flat
11th March 2009
Terry Evans

13. Possible Causes Unseen (Dark) Matter Theories of Gravity incorrect
Something even more exotic
11th March 2009
Terry Evans

14. What is Ordinary Matter
Atoms (Protons, Neutrons, Electrons)
Free Protons (Hydrogen Ions) and Electrons
Mesons
i.e. Everything we can see, touch and measure
11th March 2009
Terry Evans

15. Standard Model
11th March 2009
Terry Evans

16. Dark Matter Candidates
Unseen Ordinary Matter
Hot Dark Matter
Warm Dark Matter
Cold Dark Matter (CDM)
11th March 2009
Terry Evans

17. Unseen Ordinary Matter
Could be ordinary matter we can’t see
Hot gas. Would emit X-rays in collisions
Cold gas. Would be able to see it in absorption lines against distant sources (Lyman Alpha “forest”)
Faint condensed objects - MACHOs
Black Holes
11th March 2009
Terry Evans

18. Unseen Ordinary Matter
Can estimate the total amount in the universe
Still not enough !!
11th March 2009
Terry Evans

19. Hot Dark Matter Particles moving near speed of light
Neutrinos with (some) mass
Doesn’t explain overall structure of the universe or galaxy formation (the universe would be much smoother)
Fast neutrinos certainly exist but not a large component
11th March 2009
Terry Evans

20. Warm Dark Matter Particles moving at sub-light speeds
Postulates a new particle (Sterile Neutrino)
Only reacts with the standard model by gravity
Heavy particle (1012 GeV)
Not popular
11th March 2009
Terry Evans

21. Cold Dark Matter By far the most promising Dark Matter theory
Postulates massive (unknown!) particles
Particles moving slowly (i.e. normal speeds)
11th March 2009
Terry Evans

22. CDM Successes Galaxy Rotation by a large CDM “halo” around the galaxy
11th March 2009
Terry Evans

23. Galaxy Rotation with CDM
11th March 2009
Terry Evans

24. CDM Successes Galaxy Rotation by a large CDM “halo” around the galaxy
Cluster dynamics by CDM within the cluster (the galaxy halos)
Galaxy Cluster distribution by clumping of CDM in the early universe
Cosmic Microwave background variations
11th March 2009
Terry Evans

25. CDM Issues What is CDM exactly??!!??
Galaxy formation model predicts lots of satellite galaxies that haven’t been observed
Predicted rotation curves for gas poor dwarf galaxies is wrong
11th March 2009
Terry Evans

26. What is CDM
Could be Super-Symmetric particles predicted by String Theory
Massive and don’t interact with Standard Model except by Gravity (WIMPs)
Nothing detected in experiments (yet)
11th March 2009
Terry Evans

27. Alternatives to Dark Matter
Modified Gravity – MOND
Exotic alternatives
11th March 2009
Terry Evans

28. MOND MOdified Newtonian Dynamics Modifies Newton’s 2nd Law (F=ma)
Predicts the flatness of galaxy rotation curves
Predicts dynamics of gas poor dwarf galaxies much better than CDM
11th March 2009
Terry Evans

29. Exotic Alternatives Magnetic Fields
Quantum Gravity (what is Gravity??)
Variations of the Gravitational constant
Inertial Mass <> Gravitational Mass
(what is Mass??)
11th March 2009
Terry Evans

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Terry Evans

31. Tests Test CDM MOND Galaxy Rotation Significant Issues Good Agreement
Galaxy Clusters
Needs Hot Dark Matter
Structure of the Universe
No real test
11th March 2009
Terry Evans

32. Rotational Velocities NGC1560
11th March 2009
Terry Evans

33. Bullet Cluster
11th March 2009
Terry Evans

34. Bullet Cluster Two galaxy clusters that have collided
Gravitational lensing (and mass) has followed the galaxies
The gas has been left behind
Seen as “proof” of CDM but MOND proponents disagree (unsurprisingly!)
11th March 2009
Terry Evans

35. Where are we? Nothing fully describes all of what we see
CDM comes closest but what is it?
Theoreticians looking for a GUT
Probably where we were in the late C19th
Luminiferous Ether
Length and time contraction separate
Indivisible atoms
11th March 2009
Terry Evans

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Terry Evans

37. Newton’s Gravity F=GMm r2
Gravity acts as if proportional to the masses and the inverse square of the distance
F=GMm r2
Would not deflect light (massless)
11th March 2009
Terry Evans

38. Einstein’s Gravity Gravity is a distortion of Space-Time
Light is deflected
Explains phenomena like precession of Mercury’s orbit
Predicts Gravitational Redshift
Nothing has contradicted it (yet)
Fails in black holes and start of Big Bang
11th March 2009
Terry Evans

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Terry Evans