Announcements

1
Announcements

• Final homework assignment
• Fate of the Universe tutorial
• Strict deadline, due Weds May 11 5pm

As noted on www.jca.umbc.edu/turner/course105.ht
ml Final Exam 30 Homework 30 Quizzes plus
Telescope Night 20 Mid-term Exam 20
A gt 85 B 70-84 C 55-69 D 45-54 F lt 45
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Final Exam
As noted on www.jca.umbc.edu/turner/course105.ht
ml

• Monday May 23 1-3pm LH1
• -there will be plenty of spare time
• Covers chapters 15-23

3
An Aside
Planet 'seen' around distant sun

• April 30 2005 press release, ESO scientists using
  the Very Large Telescope facility in Chile
• The planet, 2M1207b, is x5 the size of Jupiter
  orbiting at a distance nearly twice as far as
  Neptune is from our Sun.
• Star and planet are more than 200 light-years
  away near the southern constellation of Hydra.
• Star which is brown dwarf

An artist's impression of the planet around the
brown dwarf
4
DM

• Recap last time
• DM is a generic term for any matter that is
• not radiating or reflecting light, but is
  exerting grav influence
• some of it (tiny bit) is due to MACHOs,
• tiny bit due to neutrinos,
• but vast majority thought to be WIMPs...

5
Large Scale Structure of Universe
slice of the Universe out to 7 x 108 ly
slice of the Universe out to 4 x 109 ly

• On scales of 108 ly galaxies distributed in
  gigantic chains sheets surrounding great
  voids.
• Chainssheets from initial regions of density
  enhancement
• Voids from initial regions of density depletion
• On scales of several x 109 ly galaxies appear
  evenly distributed.

6
Simulation Different Times
brown color represents neutral Hydrogen
Movie. Click to play.
simulation courtesy of Prof. Nickolay Gnedin
(Univ. Colorado)
7
The Critical Density

• So gravitational attraction between galaxies can
  overcome expansion of Universe in localized
  regions
• How strong must gravity be
• to stop the entire Universe from expanding?!?
• - it depends on the total mass density of the
  Universe
• mass density required for this gravitational pull
  to equal the kinetic energy of the Universe as
  the critical density
• mass lt critical density, Universe will expand
  forever
• mass gt critical density, Universe will stop
  expanding and then contract

8
The Critical Density

• To calculate the current kinetic energy of the
  Universe
• need to knows its speed
• this can be obtained from value of Ho
• (see Hubbles Law v Ho d)
• this being known, critical density is 1029 g /
  cm3 (v.small density!)
• All luminous matter we observe
• accounts for lt 1 of critical density
  (!!!!!)
• for dark matter to stop Universal expansion,
• average M/L of Universe would have to be 1,000
  M?/ L?
• a few times greater than clusters
• This line of argument suggests Universe will
  expand forever

9
Mass Density Expansion
10
Is Gravity alone ?

• Recent observations of white dwarf supernovae in
  very distant galaxies have yielded unexpected
  results.
• (remember, white dwarf supernovae make very good
  standard candles)

these supernovae are apparently fainter than
predicted for their redshifts!

• At a given cosmological redshift
• galaxies should be closer to us (i.e.
  shorter lookback time)
• for greater Universal mass densities
• these supernova are farther back in time than
  even the
• models for an ever-expanding (coasting) Universe
  predict
• This implies that the Universal expansion is
  accelerating!

11
Dark Energy ?!?
must be an as yet unknown force which repels
galaxies !!!! (aka dark energy)
12
Mass Density and Dark Energy
13
4 Models for the Future

• 1) Recollapsing Universe expansion will halt
  reverse
• 2) Critical Universe will not collapse, but
  expands more
• slowly with time
• 3) Coasting Universe will expand forever with
  little slowdown
• 4) Accelerating Universe expansion will
  accelerate with time

currently favored
14
4 Models for the Future
15
What have we learned?

• Does dark matter really exist?
• Yes, inferred from its gravitational effects on
  visible matter
• evidence for its existence is overwhelming
• if we correctly understand theory of gravity
  
• (can never be 100 sure of course)
• How does the distribution of dark matter compare
  to the distribution of luminous matter in spiral
  galaxies?
• luminous matter is concentrated in the disk
• dark matter is distributed throughout the
  spherical halo beyond
• How do we determine the distribution of mass in
  distant galaxies?
• spiral galaxy rotation curve - flat at large
  distances from center
• ellipticals orbital speeds of its stars at
  different distances from center,
• (measured from broadening of spectral lines)

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What have we learned?

• How does a galaxys M/L ratio tell us how much
  dark matter it contains?
• What have we learned about galaxies from their
  mass-to-light ratios?
• M/L ratio gives how many solar masses of matter
  galaxy contains
• for each solar luminosity of light output.
• can estimate M/L ratio if galaxy were made only
  of stars, any excess mass must be dark matter
• Find indeed M/L of galaxies is much higher than
  stars-only estimate
• Describe 3 (independent) ways to measure total
  mass of a cluster of galaxies.
• (1) orbital speeds positions of the galaxies
• (2) temperature distribution of its hot,
  intracluster medium
• (3) distortion of more distant galaxies by
  gravitational lensing

17
What have we learned?

• What have we learned about dark matter in galaxy
  clusters?
• All three methods agree
• large amounts of dark matter in clusters
• What do we mean when we ask whether dark matter
  is ordinary or extraordinary matter?
• Ordinary matter is made from protons, neutrons,
  and electrons
• referred to as baryonic matter since protons
  neutrons are both baryons.
• But baryonic matter does not account for all the
  dark matter
• Possible that most dark matter is made of
  nonbaryonic particles
• (yet to be discovered).
• i.e. form of matter very different from that
  encountered in daily life (so
  extraordinary)

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What have we learned?

• What are MACHOs, and can they account for dark
  matter?
• MAssive Compact Halo Objects
• ordinary objects (dim stars, brown dwarfs,
  planet-size bodies)
• populating galactic halo without being
  visible to our telescopes MACHOs exist (seen
  via gravitational lensing)
• but probably not in large enough numbers
• to account for all dark matter
• What are WIMPs, and can they account for dark
  matter?
• Weakly Interacting Massive Particles
• undiscovered particles of extraordinary
  (nonbaryonic) matter
• do not interact with light
• leading candidate for dark matter
• Why cant neutrinos account for dark matter in
  galaxies?
• are weakly interacting particles but too light
  travel too fast to be gravitationally bound
  in galaxies.

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What have we learned?

• How does structure appear to be growing in the
  universe?
• grew from regions of slightly enhanced density in
  early universe.
• Gravity in these higher density regions
• drew matter together to form galaxies,
• drew those galaxies together to form clusters,
• currently superclusters are just beginning to
  form.
• What does the universe look like on very large
  scales?
• Galaxies distributed in gigantic chains sheets
  surrounding voids.
• What is the critical density?
• average matter density universe must have in
  order for strength of gravity to be enough to
  someday halt the expansion of universe
  (assuming todays expansion rate).
• Although may be dark matter unaccounted for,
• appears that overall matter density is only
• about 30 of critical density.

20
What have we learned?

• Describe the four general models for the future
  expansion of the universe. Which model is
  currently favored?
• (1) Recollapsing universe expansion will someday
  halt and reverse.
• (2) Critical universe universe will never
  collapse but will expand
• more and more slowly with time.
• (3) Coasting universe universe will continue to
  expand forever,
• with little change in rate of expansion.
• (4) Accelerating universe expansion of universe
  will accelerate with time.
• Recent observations favor the accelerating
  universe.
• Do we know what might be causing the universe to
  accelerate?
• No!!! although people give names to the
  mysterious force that could be causing
  acceleration (dark energy, quintessence, or
  cosmological constant)
• no one (yet)
  really knows what it is !!!!