## ASTR/PHYS 328/428 - HW 6

This HW set consists solely of integrative essays. Each essay should be ~ 1.5 single-spaced pages long, and include good physical descriptions that answer the questions being posed. No equations should be used. Sketches are fine, but don't count towards the length requirements.

1. Describe in detail three pieces of evidence discussed in class that tell us that *nonbaryonic* dark matter is needed in LCDM. Note: you *cannot* use galaxy rotation curves or galaxy cluster kinematics to make these arguments.

2. Describe in detail three pieces of evidence that tell us that dark energy is needed in LCDM.

3. Describe three pieces of evidence that the population of galaxies in the universe has evolved strongly from high redshift (say, z=3) to today. Explain how this evolution may or may not be consistent with the conceptual model of hierarchical growth of galaxies. Describe the fundamental difference between the evolution of a galaxy destined to grow up as a spiral like the Milky Way versus one that turns out to be an elliptical galaxy like, say, M87.

4. On one single plot, make a sketch of how the scale factor changes with time for the following cosmological models:

1. OM=1, OL=0, H0=70 km/s/Mpc
2. OM=0.3, OL=0, H0=70 km/s/Mpc
3. OM=0.04, OL=0, H0=70 km/s/Mpc
4. OM=0.3, OL=0.7, H0=70 km/s/Mpc
5. OM=0.3, OL=0, H0=50 km/s/Mpc
6. OM=2, OL=0, H0=70 km/s/Mpc
Your curves should have the x-axis as t-t0, and at t-t0=0 they should all have R=R0=1. In other words, all curves should go through the point (t-t0=0, R=1). Your sketches need not be perfectly precise, but should be qualitatively correct relative to one another in shape and in what time they show R=0. Make sure your sketch shows not only the past, but the future evolution of these universes. And please take the time to sketch this carefully and neatly, in a way such that the curves can be differentiated from one another.

Then for each universe, explain physically why the curves have the shape they do, how they depend on the cosmological parameters, and why they give different values for t(R=0). Also describe and explain physically how structure evolves differently in each universe, including on into the future.