1. Introduction

You have two weeks to complete this unit. Below I give a syllabus for the unit, together with guidance as to where you can find the relevant material. More detailed advice on how to approach the work is given in the introduction to Unit 1 , and will not be repeated here. You should aim to spend about 12 hours on the unit, plus a further 4 hours for the assessed exercise.

2. Syllabus & sources

This unit is about the matter which makes up the Universe. Dynamical evidence strongly indicates that the Universe is dominated by dark matter, which may take several different forms. Baryonic matter may only constitute about 10% of the total, but it is the part in which most of the action takes place. We examine our current knowledge of how much baryonic and dark matter is present in the Universe, and the forms it may take. A "logical map" of the topic, which has been constructed to help you grasp the big picture and to summarise some of the main results, can be found here. Note that your understanding of some of the observational results (e.g. constraints from high redshift SNIa) will be improved by Unit 5.

A lot of the material you are encountering here is close to the cutting edge of observational cosmology. This means that it can change rapidly, and much of it is still under active debate. You will therefore find disagreements between different authors. This can be a little confusing, but it represents the way science works. You should flag up such areas of dispute in your notes when you spot them. Remember that things written even a few years ago can be out of date.  Bothun's book contains a lot of useful material, but gives much more than you need on some of his pet topics.
 

Topic	Sources	Comments
Types of matter: baryons, photons, neutrinos, dark matter	L(2.5)  L2(2.5)
Evidence for dark matter
- Introduction and M/L	B(4.1, 4.3.3-4.3.4), Van den Bergh paper
- Dark matter near the sun	B(4.2.2)	Don't worry about the detail here.
- Dark matter in galaxies	L(8.1.3), RR(6.4), L2(9.1.3), B(4.2.3, 4.3.1)
- Dark matter in clusters	RR(6.4), L2(9.1.4), B(4.3.5)	Only bother with the first part of the section in Bothun, and note that his equation 4.20 is 1000 times too large! Revise the V.T. if necessary.
Forms of dark matter
(a) Baryonic dark matter  - searches for baryonic dark matter  - limits on baryonic dark matter	L(8.2), RR(6.6), L2(9.2), B(4.6.1)
(b) Non-baryonic dark matter - Hot and cold dark matter - Detection of non-baryonic dark matter	L(8.2, 8.3), Berk, RR(6.8), L2(9.2,9.3), B(4.6.4, 5.2.1)	Look here for some amusement on this subject (sorry about the image quality).
Observed baryonic matter
- stars - intergalactic gas - Ly alpha forest clouds - baryon census and "missing baryons"	Fukugita paper, RR(6.5-6.7)	Aim to understand where (& in what form) most of the observed baryons are. Some of the numbers in the Fukugita paper are now out of date, but the basic argument is still good.
Omega and dark energy
- CMB fluctuations and the value of Omega - High z SNIa and the case for accelerating expansion	L2(A5.4, A2.3), RR(5.5 and p.134-5), SCP, Unit 4 lecture	You should understand this better after Unit 5. Concentrate on the results themselves for the present. Liddle is good on the CMB.
Best-buy (concordance) cosmology	Krauss paper , Lahav & Liddle, Unit4 lecture	Krauss is a fine example of  the cautious approach one should take to cosmological data, but is getting "old". Lahav & Liddle is up to date, but more advanced.

Notes
1.  Key: RR=Rowan-Robinson, L=Liddle (L2=2nd edition), B=Bothun, Berk=Berkeley d.m. pages, SCP=Supernova Cosmology project website - relevant sections are given in brackets.
2. The topics listed are not of equal size.
3. References given are not by any means the only ones (e.g. check out some of the links and references on the Home Page).
4. For the more complex topics it pays to consult several sources and to synthesise the results. This takes longer, but should result in a better understanding.

3. Self-test problems

Use these questions as you proceed through the unit, to judge whether your coverage of the material and level of understanding are adequate. Answers are just a click away, via the      button, but you will greatly reduce the diagnostic value of the questions if you look at the solutions before making a serious attempt to answer the question yourself.

1. What is the M/L ratio ? What units are usually used?  What would be the M/L ratio of a galaxy composed of 0.2 solar mass stars? 
2. Which single constraint on the amount of baryonic matters shows that (a) dark baryonic matter is required and (b) the dark baryonic matter density cannot equal the critical density? 
3. What key observation shows that there is dark matter in spiral galaxy halos?  In clusters of galaxies? 
4. If the mean blue luminosity density of the Universe is j_B =2.4x10⁸h  L_solMpc^-3 (where h=H₀/100 km/s/Mpc), calculate the mass-to-light ratio required to close the Universe if H₀=70 km/s/Mpc.
Why is the estimated value of j_B proportional to the assumed value of H₀? 
5. What are typical M/L values for the visible regions of galaxies ?  Using the answer from the previous question, what fraction of the closure density is in the visible regions of galaxies ? 
6. Why is the total M/L value for galaxies still subject to considerable uncertainty? 
7. How do estimates of the masses of galaxy clusters derived from (a) virial analyses, and (b) X-ray analyses assuming hydrostatic gas, scale with H₀? 
8. Stellar remnants such as neutron stars and black holes have extremely high M/L ratios. Why can these not provide the bulk of the dark matter? 
9. What is the essential difference between hot and cold dark matter? 
10. In what form are most of the observed baryons around us ? What is the total contribution to Omega of the observed baryons ? 


11. When a virial analysis is applied to a galaxy cluster to obtain M_v, what exactly has one derived the mass of?
    A similar approach can be applied to an elliptical galaxy. Would this give the mass of the whole galaxy? 
12. Combining Lyman alpha absorption studies with Big Bang nucleosynthesis results, what picture emerges about the likely state of the baryonic matter at z~3? 

a) Lecture:  A Census of the Universe

This lecture will present a summary of current estimates of the different types of matter which constitute the Universe, and  outline some of the techniques used to derive these estimates.

b)  Discussion class:  Data analysis exercise

In this class we will complete our discussion of the Robertson-Walker metric, and how to use it. We will also discuss issues arising from the first assessed exercise.

c) Assessed exercise:

The third assessed exercise for Year 3 students is now available here.
It must be completed and returned to the Teaching Office by 4pm on Friday Dec 12th.