The Universe, Dark Matter, Dark Energy

1. The Universe, Dark Matter, Dark Energy

2. What do we actually know about the Universe? • 3° K background • Age = 13.799±0.021 Billion Years • Pattern of the galaxies is “strange”

3. The cosmic microwave background – the radiation left over from the Big Bang – was detected by Penzias & Wilson in 1965

4. The Big Bang: Evidence • We have detected the leftover radiation from the Big Bang. • The Big Bang theory correctly predicts the abundance of helium and other light elements.

5. The early universe must have been extremely hot and dense So hot that the distinction between Matter and Energy was meaningless Photons could transform into matter and back to energy

6. Photons converted into particle-antiparticle pairs and vice-versa E = mc2 Every particle has a reaction path like this…. Early universe was full of particles and radiation because of its high temperature

7. Planck Era Before Planck time (~10-43 sec) No theory of quantum gravity No link between the very big and very small Universe is the size of a nucleus

9. Do forces unify at high temperatures? Four known forces in universe: Strong Force Electromagnetism Weak Force Gravity

10. Big Bang theory prediction: 75% H, 25% He (by mass) Matches observations of nearly primordial gases

11. Abundances of other light elements agree with Big Bang model having 5% normal matter – more evidence for WIMPS!

12. Observed patterns of structure in universe agree (so far) with what inflation should produce

13. “Genetic Code” Inferred from CMB • Overall geometry is flat • Total mass+energy has critical density • Ordinary matter ~ 5% of total • Total matter is ~ 32% of total • Dark matter is ~ 27% of total • Dark energy is ~ 68% of total • Age of 13.8 billion years In excellent agreement with observations of present-day universe and models involving inflation and WIMPs!

14. 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

15. Contents of the Universe • “Normal” Matter: ~ 5% • Normal Matter inside stars: ~ 0.5% • Normal Matter outside stars: ~ 4.5% • Dark Matter: ~ 27% • Dark Energy ~ 68%

16. Evidence for Dark Matter

17. Rotation curve A plot of orbital velocity versus orbital radius Solar system’s rotation curve declines because Sun has almost all the mass

18. Rotation curve of Milky Way stays flat with distance Mass must be more spread out than in solar system

19. This was actually discovered in the 1930’s • When researchers talk about neutron stars, dark matter, and gravitational lenses, they all start the same way: "Zwicky noticed this problem in the 1930s. Back then, nobody listened . . .“ • His father-in-law was a US Senator….he got his wife to fund the Palomar Observatory • A pioneer in rocket and jet propulsion • Presidential Medal of Freedom • Another pioneer was Vera Rubin • Studied physics under 3 Nobel prize winners. • The first woman to observe at the Palomar Observatory.

20. From the motions of Milky way stars: Mass within Sun’s orbit: 1.0 x 1011MSun Total Galaxy mass: ~1012MSun That is 10 TIMES the mass within the Sun’s orbit!

21. The visible portion of a galaxy lies deep in the heart of a large halo of dark matter

22. What about other galaxies? We can measure the rotation velocities of galaxies in a cluster from their Doppler shifts

23. Same story as ours…..Spiral galaxies all tend to have flat rotation curves indicating large amounts of dark matter

24. The mass we find from galaxy motions in a cluster is about 50 times larger than the mass in stars!

25. Thought Question What would you conclude about a galaxy whose rotational velocity rises steadily with distance beyond the visible part of its disk? A. Its mass is concentrated at the center B. It rotates like the solar system C. It’s especially rich in dark matter D. It’s just like the Milky Way

26. Thought Question What would you conclude about a galaxy whose rotational velocity rises steadily with distance beyond the visible part of its disk? A. Its mass is concentrated at the center B. It rotates like the solar system C. It’s especially rich in dark matter D. It’s just like the Milky Way

27. More Evidence Clusters contain large amounts of X-ray emitting hot gas Temperature of hot gas (particle motions) depends on mass….higher temps, more mass 85% dark matter 13% hot gas 2% stars

28. More Evidence Gravitational lensing, the bending of light rays by gravity, can also tell us a cluster’s mass

29. All three methods of measuring cluster mass indicate similar amounts of dark matter

30. And still more evidence • Angular Cosmic Background Radiation fluctuations • Baryon acoustic oscillations • Redshift-space distortions • Type Ia supernova distance measurements • Lyman-alpha forest • This is pretty typical of science…..once an observation is thought possible we see it everywhere.

31. What is Dark Matter..…?Two Options • Ordinary Dark Matter (MACHOS) • Ordinary matter (baryons…protons and neutrons) • Massive Compact Halo Objects: dead or failed stars in halos of galaxies • Extraordinary Dark Matter (WIMPS) • Weakly Interacting Massive Particles: mysterious neutrino-like particles

32. MACHOs occasionally make other stars appear brighter through lensing … but not enough lensing events to explain dark matter

33. What are WIMPs? • They “weakly” interact with normal matter, like neutrinos • Neutrinos: Travel close to the speed of light, lack electric charge, have a VERY small mass, and are able to pass through ordinary matter almost undisturbed, they are extremely difficult to detect. • First suggested to exist in 1930 (from conservation of energy arguments), first detected in 1956. • They don’t radiate photons • They have lots of mass (Higgs boson) • Interact through the weak nuclear force and gravity

34. Why WIMPs? • There’s not enough ordinary matter…we have to find something…. • WIMPs could be left over from Big Bang • Models involving WIMPs explain how galaxy formation works • But…..doesn’t it bother you that we haven’t noticed these things until now…..?

35. What is the role of dark matter in galaxy formation?  • Gravity of dark matter (maybe) is what caused protogalactic clouds to contract early in time • Dark matter may hold small galaxies together

36. WIMPs can’t contract to center because they can’t radiate away their orbital energy

37. 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 lt-yrs Models show that gravity of dark matter pulls mass into denser regions – universe grows lumpier with time

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

39. What is the evidence for Dark Energy?

40. Brightness of distant white-dwarf supernovae tell us how much universe has expanded since they exploded

41. #1: White Dwarf Supernova Accelerating universe is best fit to supernova data

42. #2: The Size of the Cosmic Background Radiation • The size of “space” and its density seems to exceed the total of normal and dark matter. • The pattern “seems” to require the influence of dark energy

43. 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 lt-yrs #3: Galaxy Configuration The pattern of galaxies “seems” to require both dark energy and dark matter

44. What is Dark Energy? • …..we don’t really know • And it may not actually exist….

45. What is Dark Energy? • Hypothetical form of energy that permeates all space and has strong negative pressure that counteracts gravity • Homogeneous • Not very dense (10-29 grams per cubic centimeter) • Does not interact with any force except gravity!

46. The "cost of having space": that is, a volume of space has some intrinsic, fundamental energy • The energy density of empty vacuum • As space expands the density of dark energy remains constant, but the total amount grows • At about 5 billion years the total dark energy = total matter

47. BUT…..maybe the behavior of dark energy changes over time? • If the field strength increases the “negative gravity” pull would rip structures (like the Earth) apart. • OR the field strength could drop and the Universe would contract into a “big crunch” that may start another big band cycle?

50. old older oldest Estimated age depends on dark matter and dark energy