Knowing the Unknown

Knowing the Unknowable

SAC

College

Astronomy

June 13, 2012

In terms of the work we’ve done so far in this course, Hubble’s Law is extremely strong evidence for the existence of dark energy. Hubble had first proven that galaxies existed and were light years away. He also proved that they were receding with his observations of the red shifts, and found that the further a galaxy was, the more pronounced its red shirt. He found that the further away a galaxy was, the redder its light compared to what was expected.  We use the term "redder" because the low energy, long wavelength end of the visible spectrum is red. The Doppler Effect causes waves emitted by a receding source to have longer wavelengths, so the red-shifted light emitted by galaxies is evidence that they are receding from us. We now refer to this phenomenon as Hubble’s Law.

Hubble's law states that the velocity with which a galaxy is receding is directly proportional to its distance from the observer.  The further away a galaxy is, the faster it is moving away from us, since the red shifts of light from these galaxies become more pronounced with greater distance. The observations confirm that the expansion of the universe is not slowing down; it is in fact speeding up.

When astronomers calculate how much energy is required to cause the outward acceleration of all of the galaxies in the universe (i.e., the expansion of the universe) they find that this energy would constitute most of the matter-energy extant in the known universe.  This energy is referred to as the dark energy. An even distribution of this dark energy throughout the universe would cause space itself to expand, evenly, in all directions. Our observations of the universe and understanding of the nature of light, energy, and acceleration require the existence of dark energy, especially since we consider Hubble’s discovery a Law. One of the primary reasons we accept the idea of dark energy is because all of the...