|MadSci Network: Astronomy|
First, Newton's laws fall underneath Einstein's relativity. They may be derived from relativistic equations of motion, but are valid only where the speeds of objects under discussion are much less than the speed of light and gravitational acceleration (or other acceleration) is low. But Newton's laws were important in the historical development of cosmology, as was relativity. So your real question may take one of two flavors, historical and scientific. 1) Historical... Many books on the history of science exist which chronicle the development of physics. Newton developed laws of motion which gave us a much more accurate framework in which to analyze observations of the stars. Eventually, accurate measurements led to the discovery of small inconsistencies between Newton's theories and measurements. Einstein's theory of relatvitiy resolves these discrepancies, although that wasn't necessarily what he was trying to do when he first came up with relativity. Astronomer Edwin Hubble, observing that galaxies which were further away were moving away faster than galaxies closer to us (by looking at this very red-shift) was able to determine that the universe was expanding. What caused the expansion, and where was the universe 15 billion years ago? Projecting the expanding universe back billions of years, we can calculate that the universe must have been hotter and denser in the past. This Big Bang theory was created to explain the evolution of the universe from that point to what we see today. A google search, or any book on the history of science in the library would tell you a more exhaustively detailed story. Page 500-501 of the book "An Incomplete Education" tells a shorter version, and any astronomy book should have a thorough account. 2) Scientific... There are an equally dizzying number of references to approach this from, and the above references are a good place to start, as is Brian Greene's "Elegant Universe" if you want to know more about more modern physics, cosmology, and how they are all related. As I said before, Newton's laws (scientifically) are a special case of Einstein's relativity. And Einstein's relativity is the language and framework in which we can get an overall picture of the universe, including the theory of the Big Bang, and how we derive its existence from current observations. There are other theories involved, you should particularly look up information on blackbody radiation and the cosmic microwave background...Gingio Segre's book "A Matter of Degrees" is an excellent popular reference on this subject, as well as a good book on how science depends on and advances which are driven by the measurement of temperature and our understanding of thermal physics.
Try the links in the MadSci Library for more information on Astronomy.