Astrophysics Cosmology - the study of the nature of the universe

1. The cosmological principle has a profound role in the development of cosmology. Describe the principles of homogeneity and isotropy.  Homogeneity - the universe looks uniform - uniform  Isotropy - no one direction is special in comparison with any other direction

2. How would the principles of homogeneity and isotropy be violated in the universe had an edge or a center?  Edge - part near the edge would look different  Center - looking from the center would show a different picture than from any other point

3. In your own words, describe Olber’s Paradox.  The stars are on a series of shells. Each shell contributes the same amount of energy (closer shells have fewer stars but appear brighter).  The are an infinite number of shells, so total energy is infinite. The night sky should be infinitely bright, but it is not.

4. What are three reasons why the radiation received by an observer is small and finite?  1. There are a finite number of stars with a finite life - the total radiation is small and finite compared to an infinite number of stars with an infinite life.  2. Some stars are so far away that the light hasn’t had enough time to reach us.  3. Received radiation is red-shifted and contains less energy (remember E=hf?)

5. Describe how Red Shift supports the Big Bang Theory of an expanding universe.  The dark lines in absorption spectra are red-shifted (shifted toward the red end of the spectrum)  Red light has a longer and a lower f, indicating that the universe is expanding outward (remember the Doppler Effect?)  Emission spectra are also red-shifted, but they are usually too faint to see

6. Explain the statement: The universe is not expanding into empty space.  The galaxies moving away from us aren’t moving into empty space- space is being created between the galaxies  The increasing distance between the galaxies creates an illusion of galaxies moving relative to each other

7. According to Penzias and Wilson, there is background radiation in the universe, filling all space. Where do they believe that it comes from?  It is uniform in all directions  It is the remnant of the hot explosion at the beginning of time  It is the afterglow of an enormous temperature from an early universe - temperature of space has fallen to 2.7K

8. How old is the universe? Describe the Big Bang Theory.  The universe is 14 billion years old.  It started as infinitesimally small (single point) with enormous heat and pressure  At t=0 a gigantic explosion set matter moving outward - distant galaxies are receding

9. What is the main experimental evidence in support of the Big Bang Theory?  1. Expanding universe  2. Cosmic background radiation  3. Helium abundance - 25% of the universe by mass

10. Distinguish between a closed, open,and flat universe. How are they related to critical density? GeometryDensityTypeVolumeExpansion hyperbolicp<p c openinfiniteforever Euclideanp=p c flat (open)infiniteforever, but slows down sphericalp>p c closedfinite stops, followed by collapse

11. How is critical density estimated?  Estimated using Hubble’s Law, energy, and approximated as a sphere  Critical density of the universe kg-m -3

12. How do WIMPS and MACHOS make it difficult to estimate the mass density of the universe?  WIMPS - weakly interactive massive particles - include neutrinos and other particles  Neutrino masses are not yet determined  MACHOS - mass compact halo objects, include objects like black and brown dwarfs  Dwarfs are dark matter - too cold to radiate - can’t see them