ISP205 Lecture #28, April 19, 2001

1. Announcements:    
   1. Exam #3: Next Tuesday, 4-24 
              
2. The Universe
   1. Age: 1/H = 15 billion years
      from globular clusters: older than ~13 billion years
   2. Origin in big bang. Evidence for the big bang is:
      1. Expansion of the Universe determined from redshift
         and stretching of supernova light curves
      2. Predicts the correct Hydrogen/Helium ratio (1:12)
         and correct deuterium and lithium trace abundances
      3. The cosmic microwave background (especially its smoothness,
         and black body character, and exactly the right temperature)
      4. other evidence
   3. The future of the universe
      1. Gravity between objects in universe tends to slow down
         expansion.
      2. Possible space time geometries: closed, flat, open (picture)
      3. Ignoring dark energy, the nature and future of the universe
         depend on the matter density.
         1. less than critical density (6 atoms per cubic centimeter)
            • Universe expands forever
            • Space weakly bend - open universe
         2. equal critical density
            • Universe slows down to a halt in infinite future
            • flat universe
         3. larger than critical density
            • Universe will slow to halt and then collapse again in 
              a big crunch
            • close universe (the balloon)
      4. What do we know ? 
         (1) Inventory of the Universe: (in % of critical density)
         

         visible matter (stars, HI, HII, ...)	0.5 %
         dark, ordinary matter (brown dwarfs,gas,dust)	4.5 %
         dark, not ordinary matter (WHIMPS, neutrinos,..)	35 %
         Total	40 %

          

      5. (2) The universe is flat (or very close to flat)
         From recent cosmic microwave background experiments
         (picture)
         
      6. This would cause 2 problems:
         1. As far as we know matter in the universe makes only 
            40% of the critical density (INCLUDING dark matter)
            Where is the rest that would make the universe flat ???
         2. A critcial density, flat universe would deaccelerate too much,
            the age of the universe would be smaller than the Hubble age
            - the universe would be younger than the oldest stars
            
      7. The accelerating universe comes to the rescue !!!
         1. 60% of the critical density is "dark energy" - the 
            energy of "nothing", that has mass but ...
         2. dark energy has repulsive gravity that accelerates the 
            expansion (as it dominates over normal matter that 
            slows it down). 
            An accelerating universe is older than the hubble age
            and older than the oldest stars - good !
            
      8. The good news: there is evidence for an accelerating universe
         from high redshift type Ia supernova observations
            
      9. Summary:
         The universe is flat, expanding faster and faster, and is mostly 
         made of nothing (which has dark energy and therefore mass, so 
         it is really something)
                 
3. Why is the universe the way it is ?
   1. There are a lot of "accidents" that allow humans to exist in this universe:
      1. Universe does expand just right (fast enough to prevent everything
         from forming a giant black hole, but slow enough to allow stars
         to form)
      2. Hydrogen and Helium cannot fuse into iron during the big bang
         because there is no stable nucleus that can be made of 2 helium
         nuclei. Therefore stars can shine.
      3. Helium can fuse into carbon at the conditions in stars, but not
         quickly enough into oxygen, so that some carbon is left
      4. ... many more ...
   2. The Anthropic Principle: The Universe is the way it is, BECAUSE
      only then humans exists that can discover its properties
   3. There are theories proposing that countless universes exist with
      different physical laws and properties.
4. Exam Review