In the previous parts, we traced the path leading to the currently most favored model for the Universe, now we present how cosmologists are trying to make sense of it.
The accelerating expansion may be explained with a dark energy component, but what is the physical meaning or origins for this "dark energy" is entirely another matter.
In fact, physicists are not really comfortable with dark energy, and dark matter for that matter. No fundamentalist theoretician should accept natural constants as such without further justification. Einstein himself worked hard to the last day, seeking some form of geometric basis from which to extract each physical constant from Nature, the way number Pi follows naturally from the geometric ratio of a circumference to its diameter.
Beyond General Relativity
General Relativity has been the most successful theory to account for the interaction of matter and space-time. It has survived stringent tests including fine measurements of transmission frequency shifts of the Cassini probe - scheduled for the Saturn campaign beginning June/2004 - in its 2002 conjunction with the Sun. These frequency shifts are caused by the Sun's gravity well and fall in excellent agreement with predictions.
The same holds true for equally precise measurements of the orbital precession for a recently discovered fast rotating binary neutron star system. This effect was one of the first predictions of GR and explained the puzzling precession of Mercury's orbit.
Precession is the spatial shift of the normally elliptical orbit, which according to Kepler laws should remain fixed in the absence of external influences. The relativistic precession effect for the neutron star binary system is the most precisely measured one so far, and observation matches theory exquisitely well.
But, as happened before with Newtonian Mechanics, General Relativity necessarily represents an accurate yet not ultimate model for the Universe. For one, it does not marry with Quantum Mechanics, and on the other hand it does not provide but an assumption for the origin of space and time in the Big Bang theory.
Quantum Mechanics for its part, is the other immensely successful theory but in the ream of the very small. As much as General Relativity shines at the very large scales, Quantum Mechanics does so at the opposite extreme, providing the theoretical framework that supports nuclear physics and makes possible solid state electronics for example. Tragically, both theories clash irreconcilably.
For General Relativity, space and time are stately dimensions extending continuously all the way from zero to infinity, and reality is strictly causal severing any connection between observer and observed.
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