Newton envisioned an infinitely large universe, in which God had placed the stars at just the right distances so their attractions cancelled, as precisely as balancing needles on their points..
NEWTONIAN COSMOLOGY.
The equations that describe the time evolution of an expanding universe which is homogeneous and isotropic can be deduced from Newtonian dy- namics and gravitation..
Isaac Newton believed gravity demands that the Universe be without a centre or an edge, and of infinite extent in all directions.
According to Newton, a finite and bound Universe would 'fall down into the middle of the whole space, and there compose one great spherical mass'..
According to the Newtonian theory of gravitation, the orbit of a planet around the Sun is a closed ellipse with the Sun at one of the two foci..
According to Newton, a finite and bound Universe would 'fall down into the middle of the whole space, and there compose one great spherical mass'.
In an infinite Universe, he believed, 'the fixed stars, being equally spread out in all points of the heavens, cancel out their mutual pulls by opposite attractions. '.
However, a good deal of cosmology can be understood strictly in terms of Newtonian gravity, and in these notes we will explore cosmology in that context.
Even the gravitational e ects of electromagnetic radiation can be inferred correctly by using Newtonian physics combined with some well-motivated guesses.
Eqs. (3.24) and (3.31) are the two key results of Newtonian cosmology.
As long as the mass density is dominated by nonrelativistic matter, as has been the case for most of the history of our universe, these equations are both identical to the corresponding equations obtained from general relativity.
It is traditional in big bang cosmology to de ne the zero of time to be the moment when the scale factor a(t) vanishes | sometimes regarded as the instant of the big bang.
One then has the following important result which holds for a at, matter-dominated universe:.
Newton assumed that since the mass distribution is symmetric about any point, the gravitational eld at any point must vanish, since there is no preferred direction in which it could point.
He therefore believed that a static con guration of xed stars" could exist in equilibrium.
