1.3. Modern Cosmological Puzzles

We close this first chapter by describing the topics we will pursue in detail throughout the rest of this book. These topics are all relevant to the determination of H, and which are needed to fully specify our cosmology as previously discussed. Our approach will be to show how observations have been used to constrain these cosmological parameters. In addition to this topic, we will also focus attention on the dark matter content of the Universe and the nature of the dark matter as well as the formation of structure (galaxies and clusters) in the Universe. In general, galaxies and clusters of galaxies are used as test particles or probes to determine the cosmological parameters. However, most of these determinations are complicated by the unknown distribution and nature of the dark matter. The recognition of the importance of dark matter is the single biggest difference between current cosmological models and those that were popular a mere 20 years ago.

We devote a chapter to each of the following issues and use the most modern observations available to characterize our current state of knowledge:

1. What is the age of the universe as determined from the observed expansion rate and cosmological distance scale? Is there a need to invoke the cosmological constant to reconcile the ages of the oldest stars with the value of the Hubble constant?

2. What is the nature of the large-scale distribution of matter in the universe as traced by the three-dimensional galaxy distribution and do we have a sample that accurately characterizes it?

3. What is the evidence for the existence of dark matter and what is its overall contribution to the total mass density of the Universe? Do we really live in a Universe dominated by an exotic form of matter?

4. How did structure form in the Universe and what formation scenarios are consistent with the current observational data?

5. Where do the baryons reside? Are they predominately inside or outside of galaxies? How efficient was the process of galaxy formation?

6. What is the true nature of the galaxy population? Do we have a representative survey of galaxies in the nearby universe from which coherent arguments about galaxy formation and evolution can be made?

Our theme in this book will be to go where the data leads, even if the data leads us towards scenarios that challenge our physical understanding. Along the way, we will encounter acrimonious debates over the distance scale and new but controversial methods of measuring distances. We will show how our knowledge of galaxy clustering is still rudimentary since we have not yet sampled a "fair" volume of the Universe and how this impacts on our ability to measure H. We will see how theoretical prejudice shapes our thinking of what the dark matter content of the Universe is and how this thinking is in apparent conflict with the observations. This conflict means that remains rather unconstrained as its value is uncertain by a factor of 10. We will examine how most structure formation models fail to pass the simultaneous constraints of matching the observed power spectrum of the galaxy distribution on small scales with the large scale anisotropy determined by the COBE data. Finally, we will examine where the normal matter in the Universe resides. That is, how much of the baryonic content of the Universe is conveniently located in bright, easy to discover galaxies compared to that which is hidden either in very diffuse galaxies that are difficult to discover or in still more diffuse backgrounds that faintly glow at different wavelengths. So let's ride the data and see where we arrive.