General Relativity: Predictions and Observations

General Relativity: Predictions and Observations

The Russian scientist Alexander Friedman did not see any need for a cosmological constant, and
chose to take Einstein’s more basic equations of general relativity and solve them for the entire
universe. By assuming the universe is relatively uniform (homogeneous and isotropic) the
equations can be solved, even without the aid of a computer. Although it now appears that the
universe is not very uniform today (we see densely populated regions of galaxies interspersed
between large voids), the assumption is still a reasonable one for two reasons. First of all, the
concentrations of mass we see today were not so unevenly clustered during much of the initial
expansion, as we shall discuss later. Secondly, the growing development of anisotropies in the
universe does not significantly alter the overall solutions, which predict a uniformly expanding
universe. His solutions were published in 1922 [15]. In contrast to Einstein’s static solution, all
of his solutions involve a beginning point reached by extrapolating backwards in time. Although
his solutions were derived from the more basic equations of general relativity without the
cosmological constant, this did not necessarily imply that they were a more correct description of
our universe. General relativity itself had not even been confirmed by experiment.
In 1919, astrophysicist Arthur Eddington led a team of scientists to Africa to observe a solar
eclipse. Although solar eclipses had been observed many times before this, this occasion was to
provide a very important test of general relativity. Einstein realized that for a theory to be
successful, it needed experimental verification. He had provided a bold prediction that easily
allowed his theory to be confirmed or be proven incorrect. The prediction was that starlight
would be bent by strong gravitational fields, such as light from a distant star passing near our
Sun. Although the actual angle was very slight, the experimental measurement would reveal
whether general relativity or Newtonian gravity was correct. Eddington’s team showed
convincingly that starlight is indeed bent by gravity, and that general relativity was the correct
description of gravity. Additionally, the orbital irregularities of planet Mercury proved to be in
excellent agreement with the predictions of general relativity. In the following years, additional
tests would continue to confirm the accuracy of general relativity [16]. But while the theory of
general relativity was vindicated, the question of whether the universe was static or expanding
remained unanswered.