Chapter 22 Dark Matter, Dark Energy and the Fate of the Universe

 

Dark Matter -- the matter we see is just the tip of the iceberg; the rest is unseen

 

Galaxies:
         Calculate a spiral galaxy's mass from rotation rate: Massenclosed = r v2 / G
         Use Doppler shifts to find the velocities: Diagram
         Resulting rotation curves require unseen (dark) matter
         Dark matter distributed further out from center than the luminous matter: Diagram
         Calculate elliptical galaxy's mass from velocity dispersion: Diagram
         Typical galaxy (averaged over galaxy) = 90% dark matter

 

Clusters of Galaxies:
         Calculate cluster's mass from velocities of galaxies
         Typical cluster (averaged over cluster): much more than 90% dark matter

         Intracluster gas: can see if look in X-rays (x-ray image versus visible image)
                 Intracluster medium doesn't account for missing mass
                 Calculate total cluster mass from velocity of gas from temperature of gas
                 Confirm cluster mass is many times greater than "luminous mass"

         Clusters act like gravitational lenses: Diagram
                 Hubble Space Telescope image of cluster lensing
                 HST image of Abell 2218
                 Also confirm cluster mass is many times greater than "luminous mass"

 

Vocabulary: Mass/Light Ratio

 

What is Dark Matter made of?

 

Could it be dim, ordinary matter (called baryonic matter)?
         Search for MACHOs (Massive Compact Halo Objects): Example     light curve
                 Spiffy "movie" of Baltimore's harbor being lensed
         <20% of dark matter

Could it be extraordinary matter (Weakly Interacting Massive Particles = WIMPS)?
         Possibly "Cold Dark Matter"
         Possibly "Hot Dark Matter" (example = neutrinos)
         Don't know yet

 

Structure

 

Local Geography
         Local Group    another image of Local Group
         Neighbors, incl Virgo Cluster
         Further out
         Clusters and groups pull each other => superclusters
         and superclusters pull on other superclusters:
                  Velocity Field Map

Large Scale Structure
         Soap bubble foam
         Existing figure shows nearby clusters, superclusters, voids   
         CfA survey shows walls and voids
         More extensive Map
         More normal view from our perspective
         Cosmological Principle

Growth of "Structure"
         Galaxies grew where the dark matter density was larger than average
                  These regions attracted normal matter
         Clusters grew where the dark matter density was larger than average
                  The regions attracted normal matter
                  and are still attracting matter today

         Computer simulations show that lumps grow as time elapses
                 Evolution
                 Now

 

The Fate of the Universe: Will the universe stop expanding?

 

Is the potential energy (gravity) great enough to counter the kinetic energy (expansion)?
         Re-phrased: Is the density great enough to counteract the expansion velocity?
         "critical density" = 10-29g/cm3
Historical List of Possibilities
         "Recollapsing" (universe's density > critical density) universe recontracts
         "Critical" (universe's density = critical density) universe eventually stops expanding
         "Coasting" (universe's density < critical density) universe expands forever
         Sketch
Mass tally:
         Luminous matter < 1% of critical density
         Dark matter < 30% of critical density

Suprising Data -> Universe's acceleration is expanding! Diagram
         There must be a mysterious repulsive force ("dark energy") pushing out the universe

 

 

 

Supplemental Material:
         dark energy movie
         Omega plot
         Virgo Cluster