Amber Reed's Quiz

Chapter 16: Dark matter, dark energy and the fate of universe

16.1:unseen influences in the cosmos

what do we mean by dark matter and dark energy?
Content of Universe:
dark matter: an undetected for 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 
normal matter ~4.6%
normal matter inside stars ~.7%
normal matter outside stars~3.9%
dark matter~23%
dark energy ~72%

16.2 evidence for dark matter

what is the evidence for dark matter in galaxies?
we measure the mass of the solar system using the orbits of planets
orbital period
average distance
or for circles:
orbital velocity
orbital radius
rotation curve: a plot of orbital speed versus orbital radius
solar system's rotation curve declines because Sun has almost all the mass
the rotation curve of the Milky Way stays flat with distance
mass must be more spread out than in the solar system
the mass in the Milky Way is spread out over a larger region than the stars
most of the Milky Way's mass seems to be dark matter!
the visible portion of a galaxy lies deep in the heat of a large halo of dark matter
we can measure orbital velocities in other spiral galaxies using the Doppler shift of the 21-cm line of atomic H
the broadening of spectral lines in elliptical galaxies tells us how fast the stars are orbiting
theses galaxies also have dark matter

what is the evidence for dark matter in clusters of galaxies?
the mass we find from galaxy motions in a cluster is about 50 times larger than the ass in stars!
clusters contain large amounts of X-rays -emitting hot gas
the temperature of 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
a gravitational lens distorts our view of things behind it
all three methods of measuring cluster ass indicate similar amounts of dark matter
does dark matter rely exist?
dark matter really exists and we are observing the effects of its gravitational attraction
something is wrong with out understanding of gravity, causing us to mistakenly infer the existence

what might dark matter be made of?
ordinary matter(MACHOs)
massive compact Halo Objects: dead r failed stars in halos of galaxies
exotic particles(WIMPs)
weakly interacting massive particles:mysterious neutrino-like particles
compact starlike objects occasionally make other stars appear brighter through lensing...
Why WIMPs?
theres not enough ordinary matter
WIMPs could be leftover from the Big Bang
Models involving WIMPs explain how galaxy formation works

16.3 structure formation

what is the role of dark matter in galaxy formation?
gravity of dark matter is what caused protogalactic clouds to contract early in time 
WIMPs can contract to the center because they don't radiate away their orbital energy 
dark matter is still pulling things together
after correcting fro Hubble's law, we can see that galaxies are flowing toward the densest regions of space

what are the largest structures in the universe?
maps of galaxy positions reveal extremely large structures:superclusters and voids
models show that the gravity of dark matter pulls mass into denser regions-the universe grows lumpier with time
structures in galaxy maps look very similar to the ones found in models in which dark matter is WIMPs

16.4 the fate of the universe

Will the universe continue expanding forever?
fate of universe depends on the amount of dark matter
amount of matter is ~25% of the critical density, suggesting fate is eternal expansion 
but expansion appears to be speeding up! (dark energy??)
estimated age depends on both dark matter and dark energy

Is the expansion of the universe accelerating?
the brightness of  distant white dwarf supernovae tells us how much the universe has expanded since they exploded
an accelerating universe is the best fit to supernova data