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R. Barrett and P. P. Delsanto
The driving force in the Cosmos is gravity. This may seem a little strange,
because gravity is by far the weakest of the four fundamental forces that
operate in nature. So what is going on here? Why is a force that has only a
strength of about one billion trillion trillionth that of the weak nuclear force
so important, both on earth (in holding us on the ground) and in outer space?
The answer lies in the range of the forces, and the enormously large masses
of the objects involved (compared with the masses of fundamental particles).
The two nuclear forces have extremely short ranges and as a consequence do
not extend beyond the radius of the nucleus.
Both the gravitational and electromagnetic forces are long range. However,
the electromagnetic force can be either attractive or repulsive. Positive and
negative electrical charges both exist in nature, and the force between similar
charges (i.e. either both positive or both negative) is repulsive, and the force
between dissimilar charges (i.e. one positive and one negative) is attractive.
As the number of positive and negative charges are on the average equal and
evenly distributed, the attractive and repulsive forces tend to cancel each other
out. The gravitational force has no repulsive component, and so the forces
between objects reinforce each other. As a result, when bodies are the size
of stars and planets, the gravitational attraction between them is powerful
enough to keep the planets in orbits about their suns, and to influence the
creation and motion of galaxies.
One of the major achievements of Sir Isaac Newton was his Theory of
Gravity. As we saw in Chap. 7, Einstein in his General Theory of Relativity,
proposed a geometric interpretation, in which the curvature of space–time
produces the effects of gravity. However, Einstein’s theory is formulated with
equations that are horrendously difficult to solve. Only a few special cases
lend themselves to an analytical solution. One of these, which we encountered in Chap. 7, is the FLRW model of a universe filled uniformly with a
dust cloud.
The aspect of the solution of this model relevant to our discussion here is
that such a universe must either expand or contract: no steady state is allowed.
The universe can be pictured to be analogous to the surface of an inflating
(or deflating) balloon. The two dimensional surface of the balloon represents space (which in reality is actually three-dimensional), with time along
the radial direction. The dust grains, although locally at rest on the surface,
drift apart (or get closer to one another) as the expansion (or contraction)
proceeds. The steady state solution, where the radius of the balloon does not
change, is not allowed by the equations.
Although this result was recognized to be mathematically correct by
Einstein, he did not like it because in an expanding universe, going back in
R. Barrett and P. P. Delsanto
The driving force in the Cosmos is gravity. This may seem a little strange,
because gravity is by far the weakest of the four fundamental forces that
operate in nature. So what is going on here? Why is a force that has only a
strength of about one billion trillion trillionth that of the weak nuclear force
so important, both on earth (in holding us on the ground) and in outer space?
The answer lies in the range of the forces, and the enormously large masses
of the objects involved (compared with the masses of fundamental particles).
The two nuclear forces have extremely short ranges and as a consequence do
not extend beyond the radius of the nucleus.
Both the gravitational and electromagnetic forces are long range. However,
the electromagnetic force can be either attractive or repulsive. Positive and
negative electrical charges both exist in nature, and the force between similar
charges (i.e. either both positive or both negative) is repulsive, and the force
between dissimilar charges (i.e. one positive and one negative) is attractive.
As the number of positive and negative charges are on the average equal and
evenly distributed, the attractive and repulsive forces tend to cancel each other
out. The gravitational force has no repulsive component, and so the forces
between objects reinforce each other. As a result, when bodies are the size
of stars and planets, the gravitational attraction between them is powerful
enough to keep the planets in orbits about their suns, and to influence the
creation and motion of galaxies.
One of the major achievements of Sir Isaac Newton was his Theory of
Gravity. As we saw in Chap. 7, Einstein in his General Theory of Relativity,
proposed a geometric interpretation, in which the curvature of space–time
produces the effects of gravity. However, Einstein’s theory is formulated with
equations that are horrendously difficult to solve. Only a few special cases
lend themselves to an analytical solution. One of these, which we encountered in Chap. 7, is the FLRW model of a universe filled uniformly with a
dust cloud.
The aspect of the solution of this model relevant to our discussion here is
that such a universe must either expand or contract: no steady state is allowed.
The universe can be pictured to be analogous to the surface of an inflating
(or deflating) balloon. The two dimensional surface of the balloon represents space (which in reality is actually three-dimensional), with time along
the radial direction. The dust grains, although locally at rest on the surface,
drift apart (or get closer to one another) as the expansion (or contraction)
proceeds. The steady state solution, where the radius of the balloon does not
change, is not allowed by the equations.
Although this result was recognized to be mathematically correct by
Einstein, he did not like it because in an expanding universe, going back in