The next experiment in the Top Eleven is a set of observations, not an experiment.
Who: Edwin Hubble (1889-1953), an American astronomer, and the guy the Hubble Space Telescope is named after.
When: He was nominated for two related but different discoveries which were announced in 1924 and 1929.
What: Hubble's most famous work concerns galaxies: first, he proved that they were, well, distant galaxies, and then he showed that they were receding from us with a velocity proportional to their distance, which is the first piece of evidence leading to the Big Bang model of the universe. (More after the cut...)
In the early part of the 20th Century, astronomers had made a number of observations of other galaxies, which were then termed "spiral nebulae," but nobody was quite sure what they were. There was considerable debate over whether they were odd collections of gas relatively close to us (i.e., within our own galaxy), or whether they were tremendously huge collections of stars an extremely large distance away. This is a difficult question to resolve because it's fiendishly difficult to measure the distance to astronomical objects. If you know how bright the thing you're looking at is, you can easily determine how far away it is, but it's very difficult to distinguish between a faint nearby object and a bright object a long way away.
Hubble proved that galaxies are extremely distant collections of stars by using a type of star called a Cepheid Variable, whose brightness varies over a period of days. The nice thing about Cepheid variables is that the period of the variation in brightness is closely related to the intrinsic brightness of the star-- the brighter the star, the faster the variation. Once you measure the period, you've measured the brightness, and can use the apparent brightness to determine the distance to the star. Hubble identified Cepheid variables in other galaxies (most notably Andromeda), and showed that they were tremendously far away.
He followed that up by combining a bunch of measurements of the distances to various galaxies with measurements of the Doppler shift of spectral lines from those galaxies. The direction and magnitude of the Doppler shift depends on the velocity of the object emitting light: the spectra of a galaxy that's moving away from us will have all the lines shifted to the red, and the faster it's moving, the bigger the shift.
Hubble plotted the red shift of distant galaxies as a function of the distance, and found a roughly linear relationship between the two (I say "roughly linear" because I'm an atomic physicist, and I'm used to having linear data fall on a line. Astronomers are prone to fitting lines to collections of data points that are nearly randomly distributed, so by the standards of astronomy, Hubble's data are absolutely fantastic...). All the galaxies in the universe appear to be moving away from us, and the farther away they are, the faster they're moving. The simplest explanation for this result is that the entire universe is expanding isotropically in all directions.
Why It's Important: Hubble's discoveries required two major revisions to the general view of the universe. First, the discovery that spiral nebulae are actually distant galaxies forces us to accept that the universe is really mind-bogglingly huge, much bigger than had previously been expected. On top of that, the discovery that distant galaxies are receding shows that the universe is evolving in time, not static and unchanging. These are both major blows to conventional thinking.
The discovery of Hubble's Law was the first of a great many pieces of evidence supporting the Big Bang model of the creation of the universe (which, Fred Hoyle and George Deutsch notwithstanding, is really overwhelmingly supported by facts). The velocity-versus-distance plot Hubble used to demonstrate the expansion of the universe is now a standard prop for cosmology talks, called a "Hubble Plot," and pretty much all subsequent discoveries regarding the evolution of the universe have been made by looking at Hubble plots. They've also been made using all sorts of different techniques for determining distance, all the way up to the recent version made with gamma-ray bursts.
Reasons to Vote for Him:: His discoveries forced radical revisions of astronomy and cosmology, and led to our current understanding of how the universe came to be. These discoveries are tremendously upsetting to all manner of religious cranks, or would be if they could get past the whole icky "descended from apes" thing and consider the bigger picture.
Reasons to Vote Against Him: A lot of the observations he used were actually made by other people, and he just put them together in the right way.
Another reason to vote against him: He was surprisingly timid about the conclusions he drew (or perhaps one might say, was pushed to draw) from his redshift observations. To the end of his days as I understand it, he refused to use the word galaxy and was far from enthusiastic about expansion models.
As far as I am aware, Hubble was actually very doubtful about the interpretation of the redshift as an actual recessional velocity and has expressed this in several of his publications and talks (see http://home.pacbell.net/skeptica/edwinhubble.html for more).
For an alternative explanation of the galactic redshifts see my own page Plasma Theory of Hubble Redshift of Galaxies.