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1.1 Astronomy and cosmology
No branch of science can claim to have a bigger area of interest than cosmology, for cosmology is the study of the universe, and the universe by definition contains everything. Although, because of its profound implications, cosmology had traditionally excited the imaginations of poets, philosophers and religious thinkers, our approach to the subject will be through the science of astronomy. Astronomy started as a study of the properties of planets and stars, and gradually reached out to include the limits of the Milky Way System, which is our Galaxy. Modern astronomical techniques have taken the subject beyond the Galaxy to distant objects from which light may take billions of years to reach us.
Cosmology is concerned mainly with this extragalactic world. It is a study of the large-scale structure of the universe extending to distances of billions of light-years - a study of the overall dynamic and physical behaviour of billions of galaxies spread across vast distances and of the evolution of this enormous system over several billion years.
At first such a study may appear an ambitious task. Are our tools of observation good enough to provide sufficient scientific information about the large-scale structure of the universe? Is our knowledge of the laws of nature sufficiently advanced and mature to interpret this information? We may answer these questions with a remark of Albert Einstein: ``The most incomprehensible thing about the universe is that it is comprehensible.'' Although our observing techniques are far from perfect and our knowledge of physical laws still leaves considerable room for improvement, we are now in position to make some sense out of what we observe about the universe. We can begin to study cosmology as a branch of science, just as we study the structure of the universe. This is what this book is all about.
We will begin with a brief survey of some of the features of the universe that are pertinent to the subject of cosmology.