Dark Matter, Dark Energy, and the Big Bang

1. Dark Matter, Dark Energy, and the Big Bang

2. The Father of Dark Matter • In 1933, Fritz Zwicky checked out the Coma Cluster (background image). The galaxies were flying around too fast (as measured by the Doppler effect) for their visible mass to keep them together, so he proposed dark matter was present.

3. The Mother of Dark Matter • A few decades later, Vera Rubin started to notice FLAT rotation curves in spiral galaxies.

4. Flat Rotation Curves – so what? • Following Rieke, images from Bennett and Pryke

5. Are we sure Dark Matter is real? Astronomers have argued that dark matter can explain the strange motions of galaxies in clusters and stars in galaxies, but that it can’t be normal stuff (baryons). Could we just have gotten gravity wrong on large scales? “MoND” = Modified Newtonian Dynamics Semi-viable alternative, until 2006…

6. The Bullet Cluster Given what we know about gravitational lensing (tracing the total mass in blue), hot X-ray gas in (the dominant baryonic mass, red), we can show that dark matter exists in at least one system: Images from Clowe et al. 2006 and the Chandra press release

7. The Bullet Cluster • Lensing of background galaxies seen in the optical images lets the mass distribution be mapped. • The X-rays trace the hot gas, the dominant source of baryons in this cluster merger. • They don’t line up! Why? Dark Matter seems to not interact with itself the way diffuse gas does during a cluster collision. • What is it?!

8. Order “Dark Matter” Online! HYPER-ANABOLIC POST-WORKOUT MUSCLE GROWTH ACCELERATOR FASTER ABSORPTION THAN WHEY STIMULATES PROTEIN SYNTHESIS SIGNIFICANTLY SPIKES INSULIN AND REPLENISHES GLYCOGEN 10:1:1 BCAA COMPLEX TRIGGERS PROTEIN SYNTHESIS

9. Or maybe…

10. The Nature of Dark Matter 0 Can dark matter be composed of normal matter? If just “bricks” or dark, regular matter, then its mass would mostly come from protons and neutrons = baryons. The density of baryons right after the big bang leaves a unique imprint in the abundances of deuterium and lithium. Density of baryonic matter is only ~ 4 % of critical density, total matter is 30%. Most dark matter must be non-baryonic!

11. WIMPs??? “WIMP” = Weakly interactive massive particle Neutrinos? Seem to have mass, but too small. Axions? From Wikipedia, “The axion is a hypothetical elementary particle postulated by Peccei-Quinn theory in 1977 to resolve the strong-CP problem in quantum chromodynamics (QCD).” As yet, not detected (axions are predicted to change to and from photons in the presence of strong magnetic fields, and this property is used for creating experiments to detect axions)

12. Direct Detections? Regular news stories the last several years. A lot of people think they’re seeing something, or may soon, from deep mines, to the ISS, to deep space. One story from 2014: Dark matter may have been detected – streaming from the sun’s core

13. 0 What of Dark Energy? On large scales, galaxies are moving apart, with velocity proportional to distance. It’s not the galaxies moving through space, it’s space expanding and carrying the galaxies along! The galaxies themselves are not expanding!!!

14. No one told Warlock…

15. Insert TCP 6e Figure 20.14 The brightness of distant white dwarf supernovae tells us how much the universe has expanded since they exploded.

16. 0 Measuring the “Deceleration” of the Universe … By observing type Ia supernovae, astronomers can measure the Hubble relation at large distances Distance  recession speed Size scale of the universe  rate of expansion It was expected that this would measure the deceleration of the universe, but …

17. 0 The Accelerating Universe Flat decelerating universe Apparent Magnitude of Type Ia Supernovae Flat accelerating universe Red Shift z In fact, SN Ia measurements showed that they were fainter than expected, and indicated that the universe is accelerating!

18. 0 The Cosmological Constant Cosmic acceleration can be explained with the cosmological constant, L (“Lambda”) L is a free parameter in Einstein’s fundamental equation of general relativity; previously believed to be 0. Energy corresponding to L can account for the missing mass/energy (E = m*c2) needed to produce a flat space-time. “dark energy”

19. 0 The Evolution and Ultimate Fate of the Universe Today, acceleration due to dark energy dominates. Until ~ 6 billion years ago, gravity of matter was stronger than acceleration

20. Summary of Unseen Influences Dark Matter: An undetected form of mass that emits little or no light, but whose existence we infer from its gravitational influence Dark Energy: An unknown form of energy that seems to be the source of a repulsive force causing the expansion of the universe to accelerate

21. Contents of Universe • “Ordinary” matter: ~ 4.4% • Ordinary matter inside stars: ~ 0.6% • Ordinary matter outside stars: ~ 3.8% • Dark matter: ~ 23% • Dark energy ~ 73%

22. Dark matter is still pulling things together. After correcting for Hubble’s law, we can see that galaxies are flowing toward the densest regions of space.

23. Maps of galaxy positions reveal extremely large structures: superclusters and voids.

24. Time in billions of years 2.2 5.9 8.6 13.7 0.5 35 70 93 140 13 Size of expanding box in millions of light-years Models show that gravity of dark matter pulls mass into denser regions—the universe grows lumpier with time.

25. Structures in galaxy maps look very similar to the ones found in models in which dark matter is WIMPs.

26. How about extrapolating back to the start of it all? The early universe must have been extremely hot and dense. Insert TCP 6e Figure 23.1

27. What is the history of the universe according to the Big Bang theory?

28. How do we observe the radiation left over from the Big Bang?

29. The cosmic microwave background— the radiation left over from the Big Bang—was detected by Penzias and Wilson in 1965. Insert TCP 6e Figure 23.6

30. Background radiation from Big Bang has been freely streaming across universe since atoms formed at temperature ~ 3000 K: visible/IR.

31. Background has perfect thermal radiation spectrum at temperature 2.73 K. Expansion of universe has redshifted thermal radiation from that time to ~1000 times longer wavelength: microwaves.

32. WMAP gives us detailed baby pictures of structure in the universe.

33. How do the abundances of elements support the Big Bang theory?

34. Protons and neutrons combined to make long-lasting helium nuclei when universe was ~ 3 minutes old.

35. Insert TCP 6e Figure 23.11 Big Bang theory prediction: 75% H, 25% He (by mass). This prediction matches observations of primordial gases.

36. Abundances of other light elements agree with Big Bang model having 4.4% normal matter—more evidence for WIMPs!

37. Mysteries Needing Explanation Where does structure come from? Why is the overall distribution of matter so uniform? Why is the density of the universe so close to the critical density? An early episode of rapid inflation can solve all three mysteries!

38. How does inflation explain these features of the universe?

39. Inflation can make all the structure by stretching tiny quantum ripples to enormous size. These ripples in density then become the seeds for all structures in the universe.

40. How can microwave temperature be nearly identical on opposite sides of the sky?

41. Regions now on opposite sides of the sky were close together before inflation pushed them far apart.

42. Density = Critical Overall geometry of the universe is closely related to total density of matter and energy. Density > Critical Density < Critical

43. Inflation of the universe flattens its overall geometry like the inflation of a balloon, causing the overall density of matter plus energy to be very close to the critical density.

44. Patterns observed by WMAP show us the “seeds” of structure in the universe.

45. Insert TCP 6e Figure 23.18 Observed patterns of structure in universe agree (so far) with the “seeds” that inflation would produce.

46. “Seeds” Inferred from CMB • Overall geometry is flat. • Total mass + energy has critical density. • Ordinary matter is ~ 4.4% of total. • Total matter is ~ 27% of total. • Dark matter is ~ 23% of total. • Dark energy is ~ 73% of total. • Age is 13.7 billion years. In excellent agreement with observations of present-day universe and models involving inflation and WIMPs!

47. Why is the darkness of the night sky evidence for the Big Bang?

48. Olbers’ Paradox If universe were 1) infinite 2) unchanging 3) everywhere the same then stars would cover the night sky.