|
| Cosmology |
 |
1. | We often say loosely that the Universe is expanding and thus getting larger. How can this have meaning if (as is possible) the Universe is of infinite extent, since doubling infinity still leaves us with infinity? |
 |
2. | Gravity is the weakest force, yet it completely determines the structure of the Universe on large scales. Why? |
|
3. | The distant galaxies all appear to be flying away from the Milky Way, implying that we are the center of the Hubble expansion. So if I view from another distant galaxy, won't some distant galaxies be blueshifted since that galaxy will not be at the center? |
|
4. | There is evidence that the Small Magellanic Cloud passed through the Milky Way at some time in the recent past. Doesn't this contradict the Hubble Law, since if all galaxies are receding from us none of them could collide with us? |
|
5. | The Compton Gamma Ray Observatory observes in some gamma-ray bursts (which typically last some number of seconds) a "flickering" in the intensity on a millisecond timescale (1/1000 of a second). What does this millisecond flickering tell us about the maximum extent of the energy source for the burst? |
 |
1. | As the Universe expands, the temperature and density both drop but the density decreases faster than the temperature. How does this effect the Jeans mass (the minimum mass of a cloud that leads to gravitational instability)? |
|
2. | Estimate the Jeans mass at photon-matter decoupling. Take an average matter density of 10-18 kg/m3, assume the gas to be hydrogen, and assume the decoupling temperature to be 3000 K. |
|
3. | What temperature is required in the early Universe so that an average photon has enough energy to create an electron-positron pair? The rest mass of the electron and positron are each 0.511 MeV, and you may assume that the radiation is described by the blackbody formula, E = kT, where E is the average energy, k the Boltzmann constant, and T the temperature. |
|
4. | Repeat question 3 but replace electrons and positrons with protons and antiprotons, which have a rest mass of a little less than 1 GeV. |
|
5. | We cited evidence that in the early Universe there were many collisions and interactions between galaxies. Does the Hubble Law allow such collisions, since according to it all galaxies should be receding from any one galaxy? |