Exam #3 Study Guide:


Exam 3 will have 40 multiple choice questions based on material covered in lectures from 11-15 to 12-1. Material from HW9-11 may be on the exam. A few of the questions will involve simple calculations, so you should bring your calculator. You will not need your clicker. You may bring one double-sided 8.5x11 inch sheet of paper with notes to the exam.


I String Theory

  1. Know that science is trying to find one theory that describes everything.
  2. Know that String Theory tries to describe everything in terms of vibrating strings.
  3. The size of the strings is 10-35m.
  4. String theory requires at least 10 dimensions to work.The minimum number of dimensions for M-theory to work is 11.
  5. We experience only 4 dimensions. The others are too small to be detected.
  6. The problem with String Theory is that it theory as a whole has not yet made falsifiable predictions that would allow it to be experimentally tested. So far all the explanations it provides could be explained in other ways.

II The Big Bang

  1. All evidence is consistent with the Universe beginning in a very hot (1035K) fireball 13.7 billion years ago. We call this event the Big Bang.
  2. Know the three main pieces of evidence for the Big Bang
    1. The cosmic microwave background radiation
    2. The hydrogen, helium and lithium produced (this is called Big Bang Nucleosynthesis)
    3. The expansion of the Universe. Beyond our nearest neighbors all galaxies appear to be moving away from us.
  3. Looking out from Earth is like looking back in time. It takes about 3 million years for light from the Andromeda Galaxy to reach us.
  4. The structure of the Universe
    1. There are approximately 200 billion other galaxies
    2. We are part of a local group of about 20 galaxies, which includes the Andromeda galaxy
    3. Galaxies are clumped into groups called clusters.
    4. The large scale structure of the Universe is clusters and voids
  5. The first person to discover the size of the Universe was Edwin Hubble. He did this by studying variable stars. Variable stars work because it is possible to know their luminosity accurately. A measurement of brightness and a know luminosity can be used to determine a distance.
  6. Be able to use the inverse square law to guess a distance.
  7. Hubble also discovered that on average galaxies are moving away from us and the farther away the galaxy, the faster it is moving. This is called the Hubble law.
  8. Know the steps in determining distances in the universe (many of the steps involved the use of standard candles)
    1. Radar – Sun
    2. Parallax – nearby stars
    3. Spectroscopic parallax – stars in our galaxy
    4. Variable stars – distances to nearby galaxies
    5. Supernova – to nearby clusters of galaxies
    6. Hubble Law – to farther galaxies and quasars
    7. Brightness of bright galaxies - to the farthest galaxy clusters..
  9. The furthest view of the Universe is of the cosmic microwave background radiation. This was formed 300,000 years after the Big Bang.
  10. Know the timeline for the Big Bang.
  11. Know that at about 10100 years all the stars will have burned out, and at about 101000 years the universe will be at maximum entropy and nothing else will happen. This is of course based on what we know now. It is very likely there is more to the story. (The last two sentences will not be on the test, but had to be said anyway.)

III Black holes and Worm holes

  1. Know what is meant by the conservation of energy
    1. Energy is conserved
    2. Kinetic energy is energy of motion. Potential energy is energy of position.
    3. Gain in kinetic energy = loss in potential energy
  2. Be able to use conservation of energy to find a velocity, or from a velocity a position.
  3. What is escape velocity? What happens to escape velocity if the mass of the planet is changed? What happens if the radius is changed?
  4. Black holes are a large mass in a small region, so that the escape velocity becomes greater than the speed of light.
  5. Once inside the event horizon (aka the Schwarzschild radius) not even light can escape.
  6. Black holes
    1. Black holes are formed in the collapse of massive stars at the end of their life
    2. Black holes also form at the centers of galaxies. Some galaxies have supermassive black holes with 109 solar masses.
    3. Black holes at the centers of galaxies explain active galaxies and quasars.
  7. Worm holes are a possible solution of Einstein’s equations, but to be stable something would have to keep them open. Know that no corresponding white holes have been observed. White holes violate the second law of thermodynamics.
  8. Know that the second law of thermodynamics says that entropy always increases in any process
  9. Know the two formulas for entropy.
  10. The entropy of the early Universe was far too low, hence the probability of any process increasing entropy is vanishingly small. This gives time its direction since to go back in time would allow entropy to be decreased.

IV Dark Matter and Dark Energy

  1. We are made of atoms, atoms are made of nuclei and electrons, nuclei are made of … Know all of these steps down to strings at 10-35m.
  2. This type of luminous matter only makes up 4% of the universe.
  3. Know the evidence for dark matter
    1. Gravitational lensing
    2. Rotation curves of galaxies
    3. Cosmic microwave background radiation
  4. WMAP indicates that 23% of the universe must be in the form of some type of cold dark matter. These may be particles that interact by the weak force. They are called WIMPS.
  5. Know that 73% of the Universe is made of Dark Energy. We don’t know what this is, but it acts like an antigravity that is pushing the Universe apart.
  6. Know about WMAP and the things it has discovered (LONCAPA problem).
    1. Within a 1% accuracy the Universe is 13.7 billion years old.
    2. We don't know what 96% of the Universe is made of.
    3. The first stars formed about 200 million years after the Big Bang.
    4. The picture of the background microwave radiation is from 379,000 years after the Big Bang.
    5. At the present it appears the Universe will expand forever, but since we don't know what dark energy is, this conclusion could change.