# 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