General Relativity does treat gravity as being simply an aspect of
curved spacetime (not a force), but that curvature responds to the
distribution of matter and energy. On a smale scale, you can pretend
that spacetime is flat and that matter creates a gravitational field
(force), and you will get nearly the right predictions. These
predictions, using Newton's model of gravitation are quite accurate for
nearly all practical purposes, from engineering design of objects which
must bear gravitational weight, to predicting the trajectories of
spacecraft sent to the moon and the planets.
It was only the introduction of Special Relativity at the
beginning of the 20th century (needed to reconcile electricity and
magnetism with the idea that all physical laws are the same in all
reference frames) which gave an indication that Newton's gravitational
model was insufficient. General relativity was a tremendous achievement
of Einstein's because it was formulated to reconcile special relativity
with gravity, and not to explain any anomaly in the currently available
experimental data. Only later were its predictions found to be true.
In the process of reconciling special relativity with gravity,
Einstein did away with "spooky action at a distance", a common
criticism of Newton's model. The motion of a piece of matter only
depends on the properties of space nearby, and not on what's happening
far away at that instant. Gravitational disturbances propagate at a
finite speed, the speed of light. If two black holes collide for
example, the matter distribution changes rapidly enough to emit
enormous amount of propagating gravitational waves.
So is there any evidence of those propagating waves? I can think
of two pieces of evidence, but an astrophysicist would probably know of
more. One is that some spinning objects slow down by just the amount
predicted if they were losing energy to gravitational waves. Another is
that the shape of the ripples in the early universe, which we see as
small ripples in the density of the microwave background radiation,
matches predictions obtained using the propagating gravity of General
In other words, the geometric results of curved spacetime are not static, but do radiate.
As for your question about the early universe, both matter and
energy contribute to the curvature of spacetime. So even at the very
beginning, when matter particles were constantly being created in pairs
with antimatter particles and annihilated again, the energy density was
enormous and the spacetime was very curved.
Mike W. and Tom J.
(republished on 07/23/06)