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 has 5 times mass of cluster's galaxies 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
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
Forming the structures of the universe Galaxies grew from slight density enhancements Gravitationally bound, gravity overcame universe's expansion Galaxies attracted to each other => galaxy clusters (and groups) Example: Virgo Also, gravitationally bound, gravity overcame universe's expansion Clusters (and groups) attracted to each other => superclusters We can see them moving: Velocity Field Map Net effect: "Structure" Map shows nearby clusters, superclusters, walls, voids Map More extensive map shows Map More normal view from our perspective Computer simulations show that lumps grow Evolution Now
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
