Vacuum Energy Density
In 1917, just one year after the publication of his general theory of relativity, German American physicist Albert Einstein wrote a short paper titled Cosmological Considerations on the General Theory of Relativity. For the first time the universe was treated as a four-dimensional (three spatial and one temporal) manifold termed space-time. The geometry of the model universe was determined by solving the Einstein field equations, which relate the curvature of space-time to matter distribution. Modern cosmology was born. And so was the physics of the vacuum.
Einstein initially argued for a static universe and a dozen years would pass before Milton Humason and American astronomer Edwin Hubble would discover the universal expansion of the cosmos. Although Einstein's own field equations dictated that the universe could not be static, Einstein attempted to reconcile general relativity with a static universe by adding a new term to his field equations, termed the cosmological constant. Using this artificial constant, Einstein was able to model a static, matter-filled universe.
If Einstein's modified equations were applied to an empty universe, the cosmological constant essentially fills the empty universe (a vacuum) with an energy density and a (negative) pressure. And, of course, it adds to the energy density and pressure in a matter-filled universe. It is this negative vacuum pressure, in fact, that kept Einstein's model universe static.
After Hubble's work demonstrated that the universe continues to expand, Einstein determined there was no need for his ad hoc term in his field equations, and he renounced it. In doing so he also renounced the concept of the vacuum energy that it predicted. Subsequently Einstein would characterize his introduction of the cosmological constant as the greatest blunder of his. But the cosmological constant was not an absolute blunder. Half a century after Einstein's abjuration, quantum field theory tells us that there must be (or at least must have been) a large vacuum energy density.
The most current models of the fundamental forces, the Weinberg-Salam electroweak theory and quantum chromodynamics, the gauge theory of the strong interaction, are examples of spontaneously broken gauge theories (as would be any Grand Unified Theory uniting all three interactions). Models of this type predict the production, at the very high temperatures that existed in the early universe, of regions of space-time possessing very large vacuum energy densities. If the idea of spontaneous symmetry breaking is correct, there were, at a very early time in the universe's evolution, space-time regions with very large effective cosmological constants. The vacuum energy density in a region drives an enormously rapid (though quite brief) period of expansion, known as inflation. One such tiny region can, in this manner, inflate enough to become the entire observable universe known today.
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