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Chapter 1. Cosmological Model Building

1.1. Overview of Historical Progess in Understanding the Cosmos

A measure of the advancement of a particular civilization may well lie in its ability to provide comprehensible answers to fundamental questions. Two examples of such fundamental questions are 1) What is the Nature of the Universe and 2) What is the Universe made of? Seeking the answers to these questions defines the discipline of Physical Cosmology. While this discipline is a convolution of elementary particle theory, General Relativity and astronomical observations, there is still room for elements of mysticism and imagination in our cosmological models. Indeed, history has taught us that humans have an insatiable appetite for grand ideas about the nature of the World. Furthermore, at any given time in history, everyone always thinks their world model is correct. Since the answers to our two posed questions continue to elude us, it is safe to say that no cosmological theory has yet proven to be entirely satisfactory. Hence one should expect, and even demand, continual revisions and challenges to any existing cosmological paradigm.

This book will focus on recent challenges and observations in Cosmology. Theory today is bombarded by a vast array of observational data but there remains no clear and preferred model for the origin and evolution of structure in the Universe. In this book we outline some of the most outstanding observational problems associated with our modern cosmological model. While much mathematical formalism is essential in order to convey the context of these problems, this book is not about exploring the physics of the early Universe or about the details of General Relativity. Many excellent reviews of those areas are available (see references in Appendix A). Rather, this book will focus on issues not normally addressed in one collection. In brief, we focus on the current (e.g., as of 1996) status of the following issues:

bullet Our observational attempts to determine the expansion rate (H0) and density (Omega) of the Universe.

bullet Our characterization of large scale structure and the discovery of deviations from pure expansion motions.

bullet Observational evidence for the the presence of dark matter, its distribution in the Universe and its overall nature (e.g., baryonic vs. non-baryonic)

bullet A detailed investigation of the gravitational instability paradigm for structure formation and the current set of observational constraints that need to be satisfied by structure formation models.

bullet How many baryons are contained in bright, easy to detect galaxies compared to those that might constitute a diffuse background or are contained in very dim, diffuse galaxies that generally escape detection.

The resolution of these issues has profound influences on our cosmological model and they form the basis for intense observational inquiry utilizing the most sophisticated telescopes and detectors which are available. Indeed, with a refurbished Hubble Space Telescope, the working 10-meter Keck Telescope and the coming of the general 8-10 meter class telescope, one can look forward to an extremely data-rich era in cosmology. It is the theme of this book to go where the data leads and to use good data as a strong constraint on various cosmological models. As we shall see, when this is done, no existing model is able to explain the observations in full and this situation is likely to be exacerbated in this new era. This, however, is not disheartening but instead is an indication that we are merely in the preliminary stages of cosmological inquiry as we continue to gather and identify fundamental data. Indeed, this has been the case throughout our history of cosmological model making.

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