Astronomy 490 Homework #7 Spring 2007
Due date: Monday, April 2 at start of class.
Please make a copy of your assignment to refer to during the class discussion.
1. Read the Physics Today article.
2. Read the first 3 sections (pp. 1-8) of the Astrophysical Journal article.
3. In Fig. 5 of the Physics Today article, draw a best fit straight line by eye through the
blue region defined by the SN Ia measurements. Then come up with an equation for
WΛ as a function of Wm that represents the line you drew. Finally, compare the
equation you get from your eyeball fit to the equation given in the abstract of the
Astrophysical Journal (ApJ) paper.
4. Take a close look at Fig. 2a of the ApJ paper.
(i) For each of the flat-Universe models (blue dashed lines), count how many points
fall above vs. below the line. Use only points for which z > 0.3. Judging from
your results, which of the dashed blue lines gives the best fit to the data?
(ii) For the best-fit dashed line (or two closest ones, if one doesn’t stand out as the
clear best), count how many of the points’ error bars touch the line that defines the
model (i.e. have values within 1s of the model). Then count the number of points
that land within 2s and 3s (as best you can judge this by eye). Do the data seem
to be reasonably well described by Gaussian random statistics (68% of points
within 1s, 95% within 2s, 99.7% within 3s?). If not, what reasons can you think
of that might explain why not?