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What do we know about Dark Matter and Dark Energy?

An overview of our current understanding of dark matter and dark energy, exploring their properties, classification, and potential candidates. Includes evidence for dark matter and alternative explanations.

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What do we know about Dark Matter and Dark Energy?

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  1. What do we know about Dark Matter and Dark Energy? CAS Meeting, 14 January 2006 Dr. Uwe Trittmann Otterbein College

  2. Starting Point • Before we can say anything about the “dark side”, we have to answer the following questions: • What is “bright” matter? • What do we know about “bright” matter?

  3. “Bright” Matter • All normal or “bright” matter can be “seen” in some way • Stars emit light, or other forms of electromagnetic radiation • All macroscopic matter emits EM radiation characteristic for its temperature • Microscopic matter (particles) interact via the Standard Model forces and can be detected this way

  4. Atom:Nucleus and Electrons The Structure of Matter Nucleus: Protons and Neutrons (Nucleons) Nucleon: 3 Quarks | 10-10m | | 10-14m | |10-15m|

  5. Elementary Particles All ordinary nuclear matter is made out of quarks: Up-Quark Down-Quark (charge +2/3) (charge -1/3) In particular: Proton uud charge +1 Nucleons Neutron udd charge 0 (composite particles)

  6. Force (wave) Gravity: couples to mass Electromagnetic force: couples to charge Weak force: responsible for radioactive decay Strong force: couples to quarks Carrier (particle) graviton (?) photon W+, W-, Z0 8 gluons The Forces of the Standard Model massless carriers  long ranged massive carriers  short ranged

  7. The particles of the Standard Model Matter particles have half-integer spin (fermions) Force carriers have integer spin (bosons)

  8. Conclusion • We know a lot about the structure of matter! • We know a lot about the forces between matter particles • We know al lot about the theory that describes all of this (the Standard Model)  Great News !

  9. Pie in the Sky: Content of the Universe 5% We know almost everything about almost nothing! Dark Energy Dark Matter SM Matter 25% 70%

  10. What is the dark stuff? Dark Matter is the stuff we know nothing about (but we have some ideas) • Dark Energy is the stuff we have absolutely • no idea about

  11. Conclusion • If we don’t know anything about it, it is boring, and there is nothing to talk about. •  End of lecture!

  12. Alternate Conclusion • If we don’t know anything about it, it is interesting because there is a lot to be discovered, learned, explored,… •  beginning of lecture!

  13. So what do we know? Is it real? • It is real in the sense that it has specific properties • The universe as a whole and its parts behave differently when different amounts of the “dark stuff” is in it • Let’s have a look!

  14. First evidence for dark matter: The missing mass problem • Showed up when measuring rotation curves of galaxies

  15. The Mass of the Galaxy • Can be determined using Kepler’s 3rd Law • Solar System: the orbital velocities of planets determined by mass of Sun • Galaxy: orbital velocities of stars are determined by total mass of the galaxy contained within that star’s orbit • Two key results: • large mass contained in a very small volume at center of our Galaxy • Much of the mass of the Galaxy is not observed • consists neither of stars, nor of gas or dust • extends far beyond visible part of our galaxy (“dark halo”)

  16. Properties of Dark Matter • Dark Matter is dark at all wavelengths, not just visible light • We can’t see it (can’t detect it) • Only effect is has: it acts gravitationally like an additional mass • Found in galaxies, galaxies clusters, large scale structure of the universe • Necessary to explain structure formation in the universe at large scales

  17. What is Dark Matter? • More precise: What does Dark matter consist of? • Brown dwarfs? • Black dwarfs? • Black holes? • Neutrinos? • Other exotic subatomic particles?

  18. Classification of Dark Matter • Classify the possibilities • Hot Dark Matter • Warm Dark Matter • Cold Dark Matter • Baryonic Dark Matter You could have come up with this, huh?!

  19. Hot Dark Matter • Fast, relativistic matter • Example: neutrino • Pro: • interact very weakly, hard to detect  dark! • Con: • Existing boundaries limit contribution to missing mass • Hot Dark matter cannot explain how galaxies formed • Microwave background (WMAP) indicates that mastter clumped early on • Hot dark matter does not clump (it’s simply too fast)

  20. Baryonic Dark Matter • “Normal” matter • Brown Dwarfs • Dense regions of heavy elements • MACHOs: massive compact halo objects • Big Bang nucleosynthesis limits contribution

  21. Cold Dark Matter • Slow, non-relativistic particles • Most attractive possibility • Large masses (BH, etc) ruled out by grav. lensing data • Major candidates: • Axions • Sterile neutrinos • SIMPs (strongly interacting massive particles) • WIMPs (weakly …), e.g. neutralinos • All of the above are “exotic”, i.e. outside the SM

  22. Alternatives • Maybe missing mass, etc. can be explained by something else? • Incomplete understanding of gravitation • Modified Newtonian Dynamics (MOND) • Nonsymmetric gravity • General relativity

  23. The silent majority: Dark Energy 70%

  24. Aside: Standard Cosmology • Based on Einstein’s theory of Gravity, aka General Relativity • Assumes isotropic, homogeneous universe • This “smeared out mass” property is approximately valid if we average over large distances in the universe

  25. General Relativity ?! That’s easy! (Actually, it took Prof. Einstein 10 years to come up with that!) Rμν -1/2 gμνR = 8πG/c4 Tμν OK, fine, but what does that mean?

  26. The Idea behind General Relativity • In modern physics, we view space and time as a whole, we call it four-dimensional space-time. • Space-time is warped by the presence of masses like the sun, so “Mass tells space how to bend” • Objects (like planets) travel in “straight” lines through this curved space (we see this as orbits), so “Space tells matter how to move”

  27. Still too complicated? • Here is a picture: Sun Planet’s orbit

  28. Effects of General Relativity • Bending of starlight by the Sun's gravitational field (and other gravitational lensing effects)

  29. What General Relativity tells us • The more mass there is in the universe, the more “braking” of expansion there is • So the game is: Mass vs. Expansion And we can even calculate who wins!

  30. The “size” of the Universe – depends on time! Expansion wins! It’s a tie! Mass wins! Time

  31. The Universe expands! • Where was the origin of the expansion? Everywhere! • Every galaxy sees the others receding from it – there is no center

  32. Big Bang • The universe expandsnow, so looking back in time it actually shrinks until…? Big Bang model: The universe is born out of a hot dense medium 13.7 billion years ago.

  33. The Fate of the Universe – determined by a single number! • Critical density is the density required to just barely stop the expansion • We’ll use 0 = actual density/critical density: • 0 = 1 means it’s a tie • 0 > 1means the universe will recollapse (Big Crunch) Mass wins! • 0 < 1means gravity not strong enough to halt the expansion Expansion wins! • And the number is: 0 = 1 (probably…)

  34. The Shape of the Universe • In the basic scenario there is a simple relation between the density and the shape of space-time: DensityCurvature2-D exampleUniverseTime & Space 0>1 positive sphere closed, bound finite 0=1 zero (flat) plane open, marginal infinite 0<1 negative saddle open, unbound infinite

  35. Expansion of the Universe • Either it grows forever • Or it comes to a standstill • Or it falls back and collapses (“Big crunch”) • In any case: Expansion slows down! Surprise of the year 1998 (Birthday of Dark Energy): All wrong! It accelerates!

  36. Enter: The Cosmological Constant • Usually denoted 0, it represents a uniform pressure which either helps or retards the expansion (depending on its sign) • Physical origin of 0is unclear • Einstein’s biggest blunder – or not ! • Appears to be small but not quite zero! • Particle Physics’ biggest failure

  37. Effects of the “Cosmological Constant” • Introduced by Einstein, not necessary • Repulsive  accelerates expansion of universe Hard to distinguish today

  38. Triple evidence for Dark Energy • Supernova data • Large scale structure of the cosmos • Microwave background

  39. Microwave Background:Signal from the Big Bang • Heat from the Big Bang should still be around, although red-shifted by the subsequent expansion • Predicted to be a blackbody spectrum with a characteristic temperature of 3Kelvin by George Gamow (1948) Cosmic Microwave Background Radiation (CMB)

  40. Discovery of Cosmic Microwave Background Radiation (CMB) • Penzias and Wilson (1964) • Tried to “debug” their horn antenna • Couldn’t get rid of “background noise”  Signal from Big Bang • Very, very isotropic (1 part in 100,000)

  41. CMB: Here’s how it looks like! Peak as expected from 3 Kelvin warm object Shape as expected from black body

  42. Maybe pigeons? • Proposed error: pigeon crap in antenna • Real reason: a signal from the Big Bang Pigeon trap  Horn antenna

  43. Latest Results: WMAP(Wilkinson Microwave Anisotropy Probe) • Measure fluctuations in microwave background • Expect typical size of fluctuation of one degree if universe is flat • Result: Universe is flat !

  44. Experiment and Theory Expect “accoustic peak” at l=200  There it is!

  45. Supernova Data • Type Ia Supernovae are • standard candles • Can calculate distance • from brightness • Can measure redshift • General relativity gives us distance as a • function of redshift for a given universe • Supernovae are further away than expected for any decelerating (“standard”) universe

  46. Supernova Data magnitude redshift

  47. Redshift: Everything is moving away from us! • Measure spectrum of galaxies and compare to laboratory measurement • lines are shifted towards red • This is the Doppler effect: Red-shifted objects are moving away from us

  48. Example: Spectrum of a Quasar Highly redshifted spectrum  the quasar is very far away –and keeps going! Quasar Lab

  49. Large Scale Structure of the Cosmos • Large scale structure of the universe can be explained only by models which include Dark Matter and Dark Energy Experiments: 2dF GRS, SDSS

  50. Properties of Dark Energy • Should be able to explain acceleration of cosmic expansion  acts like a negative pressure • Must not mess up structure formation or nucleosynthesis • Should not dilute as the universe expands  will be different % of content of universe as time goes by

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