Chapter 16 Notes
Dark Matter: An undetected form of mass that emits little or
no light but whose existence we infer from its gravitational influence
Dark Energy: An unknown form of energy that seems to be the
source of a repulsive force causing the expansion of the universe to accelerate.
We measure the mass of the solar system using the orbits of
planets
Orb. Period, Avg. Distance: Or for circles: Orb. Velocity, Orbital
Radius
A plot of orbital velocity versus orbital radius Solar
system’s rotation curve declines because Sun has almost all the mass
The visible portion of a galaxy lies deep in the heart of a large
halo of dark matter
We can measure rotation curves of other spiral galaxies using
the Doppler shift of the 21-cm line of atomic H
Spiral galaxies all tend to have flat rotation curves indicating
large amounts of dark matter
Broadening of spectral lines in elliptical galaxies tells us
how fast the stars are Orbiting These galaxies also have dark matter.
We can measure the velocities of galaxies in a cluster from
their Doppler shifts
The mass we find from galaxy motions in a cluster is about
50 times larger than the mass in stars!
Clusters contain large amounts of Xray emitting hot gas Temperature
of hot gas (particle motions) tells us cluster mass: 85% dark matter 13% hot
gas 2% stars
Gravitational lensing, the bending of light rays by gravity,
can also tell us a cluster’s mass.
All three methods of measuring cluster mass indicate similar
amounts of dark matter.
Ordinary Dark Matter (MACHOS): Massive Compact Halo Objects:
dead or failed stars in halos of galaxies
Extraordinary Dark Matter (WIMPS): Weakly Interacting
Massive Particles: mysterious neutrino-like particles
Ordinary Dark Matter (MACHOS): Massive Compact Halo Objects:
dead or failed stars in halos of galaxies
MACHOs: Occasionally make other stars appear brighter
through lensing
MACHOs occasionally make other stars appear Brighter through
lensing but not enough lensing events to explain all the dark matter
Gravity of dark matter is what caused protogalactic clouds
to contract early in time
WIMPs can’t contract to center because they don’t radiate
away their orbital energy
Dark matter is still pulling things together After
correcting for Hubble’s Law, we can see that galaxies are flowing toward the
densest regions of space
Fate of Universe depends on
the amount of dark
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