1. Dark Matter, Dark Energy
Tuesday, February 12

2. Version 1.0: “Superdome” model
Recap of the history of cosmology:
Version 1.0: “Superdome” model

3. Version 2.0: Geocentric model
spherical Earth at center

4. Version 3.0: Heliocentric model
Sun at center

5. v. 3.1: Infinite heliocentric model

6. v. 4.0: Big Bang model

7. v. 4.1: Hot Big Bang model
Cosmic Microwave Background

8. v. 4.1.1: Hot Big Bang model with space-time curvature.
Mass & energy cause space to curve. This curvature causes an observed bending of the path of light.

9. “Gravitational lensing” by a massive cluster of galaxies

10. Observation: on large scales, space is close to flat
Observation: on large scales, space is close to flat Implication: density is close to the critical density.

11. Positively curved spaces are finite, but without a boundary.
Flat & negatively curved spaces are infinite (unless you add a boundary).
Positively curved spaces are finite, but without a boundary.

12. The part I can see looks flat!
Suggestion: space is positively curved, but with a radius of curvature much larger than the Hubble distance (4300 Mpc).
The part I can see looks flat!
This gives the universe a huge (but finite) volume.

13. The critical density required for space to be flat is
E = mc2 implies m = E/c2: this critical density can be contributed by mass or by energy.

14. If Einstein’s theory is correct, the electrons, protons, neutrons, neutrinos, photons, etc. in the universe must sum to (nearly) the critical density.
1 m of the universe
9×10-27 kg

15. The accounts must balance!
Let’s do an inventory of the universe:
How much mass/energy is contributed by electrons, protons, neutrons, photons, neutrinos, et cetera to the 9×10-27 kilograms per cubic meter.
The accounts must balance!

16. Inventory: photons provide 0.01% of the critical density. Pffft.
First: PHOTONS
Lots of photons from the cosmic microwave background, fewer photons from starlight.
Photons are easily detected!
Inventory: photons provide 0.01% of the critical density. Pffft.

17. Next: ELECTRONS, PROTONS, & NEUTRONS
Ordinary matter (stars, planets, gas clouds, people) is made of electrons, protons, & neutrons.
These objects are easily detected because they emit photons.
Electrons, protons, & neutrons provide 4% of the critical density.

18. Where’s the rest of the mass & energy?
Ordinary light & ordinary matter make up only 4% of the universe.
Where’s the rest of the mass & energy?
To answer that question, we must turn to the Dark Side of the universe.

19. Light, and objects that emit light, are easy to detect.
Light, and objects that emit light, contribute only 4% of the mass/energy content of the universe.
The remaining 96% of the universe must consist of mass or energy that does not emit (or absorb) light.

20. Dark matter = massive particles that do not emit, absorb, or otherwise interact with photons.
Dark matter could also be called “invisible matter”: If it’s invisible, how do you know that it’s there??

21. In the Solar System, 99.8% of the mass is contained in the Sun.
Dark matter can be detected by its gravitational effect on ordinary matter.
In the Solar System, 99.8% of the mass is contained in the Sun.
For planets farther from the Sun, the orbital speed is smaller.

22. In the Milky Way Galaxy, the orbital speed of stars is nearly constant with distance from the Galaxy’s center.

23. But… the glowing stars of the Galaxy are concentrated near its center.
Conclusion: the mass of the Galaxy is not concentrated near its center.
But… the glowing stars of the Galaxy are concentrated near its center.
The Galaxy must have an extended “halo” of dark matter, to prevent the high-speed stars from escaping.

24. bright stars
dark “halo”: nearly spherical distribution of invisible massive particles

25. Clusters of galaxies contain lots of dark matter.
How do we know? ) Galaxies in clusters move very rapidly: if there weren’t dark matter to anchor them, they’d fly away ) Gravitational lensing requires lots of mass.

26. Dark matter provides 23% of the critical density.
How Much Dark Matter?
Adding together the dark matter around galaxies, in clusters, and on larger scales, we find there is more dark matter than ordinary matter!
Dark matter provides 23% of the critical density.

27. What’s the dark matter made of?
Neutrinos make up part of the dark matter.
Although detecting neutrinos is difficult, it has been done!

28. Neutrinos provide < 2% of the critical density.
Although we don’t know the mass of neutrinos exactly, we know it’s tiny…
neutrinos
electron
Neutrinos provide < 2% of the critical density.

29. WIMP = Weakly Interacting Massive Particle
Most of the dark matter must be particles other than neutrinos.
One candidate for the office of “dark matter”: the WIMP.
WIMP = Weakly Interacting Massive Particle

30. I predict a Nobel Prize for the 1st to succeed!
According to particle physics theory, WIMPs should be much like neutrinos only more massive.
Neutrinos have already been detected: particle physicists are still trying to detect WIMPs.
I predict a Nobel Prize for the 1st to succeed!

31. What is the “something else”?
Inventory of the universe:
Light = diddly-squat
Ordinary matter = 4%
Dark matter = 23%
Something else = 73%
What is the “something else”?

32. Let’s call the “something else” dark energy.
The “something else” isn’t ordinary (luminous) matter, dark matter, or energy in the form of photons.
Let’s call the “something else” dark energy.

33. Dark energy is even less well understood than dark matter.
Dark energy is a uniform energy field that permeates the universe (unlike dark matter, it doesn’t “clump up”).
Since its energy density is so low everywhere, how do we know the dark energy’s there?

34. One reason for thinking that dark energy exists:
The universe is flat on large scales; there isn’t enough mass to do the flattening, so there must be energy.
If the energy emitted light, we’d have seen it by now, so it must be dark energy.

35. The weird reason for thinking that dark energy exists:
Einstein found that a component of the universe whose energy density was constant in time and space would provide a repulsive force.
Yes, this is an unexpected result: Newton would not approve!

36. Dark & luminous matter make the expansion of the universe slow down.
Einstein called this component of the universe the “cosmological constant”: we call it “dark energy”.
Dark & luminous matter make the expansion of the universe slow down.
Dark energy makes the expansion of the universe speed up!

37. Testing for dark energy:
●Look at a supernova (an exploding star as bright as 109 Suns) ●Measure its redshift and flux ●If the expansion of the universe is speeding up, then a supernova with large redshift will be overly faint.

38. The result of the test:
The expansion is speeding up, implying the presence of dark energy.
Science magazine’s “Breakthrough of the Year” for 1999!

39. How old are stars & planets?
Thursday’s Lecture:
How old are stars & planets?
Reading:
Chapters 7 & 8