So, this last poll had some of the best results, with 24 votes. I was kind of hoping to do asteroids, but the expansion of the Universe won with 9, which is totally cool. There were also a couple of good suggestions that will be up in the new poll, which should be up, so go vote!
The expansion of the Universe is an intricate topic, not exactly easy to dump a massive load of information and have it be understandable. In order to help present this on as basic a level as possible, I am busting out some of my old astronomy books for reference. The most helpful will probably be my old Astronomy 111/113 textbook: Universe, 7th edition. I managed to dig it out of a box in my closet. I'll also provide some Wikipedia links if you want to read more. So, lets begin!
I guess a little bit of history is a good way to start. The expanding Universe is a relatively young concept in human understanding, developed in the late 1920s. The establishment that some 'nebula' where in fact galaxies only became conclusive by 1925.
These are concepts that have been around for less then a hundred years, but in that time, we have built such an understanding of the Universe. One of the primary contributors to this discovery, and famous telescope namesake, was Edwin Hubble.
First he helped to determine that these great groupings of stars were not part of our own galaxy, the Milky Way, but distant galaxies in their own right. Galaxies a term meaning 'island universe.' This was done by using special sorts of stars called Cepheid variables. A Cepheid variable has a well determined relationship between luminosity and pulsation period, making them great standard candles in determining the massive distances between galaxies.
|A Cepheid Variable as seen by Hubble Space Telescope.|
The red end of the electromagnetic spectrum has wide wavelengths. What happens in a redshift is that when an object is moving away from us, the wavelength gets stretched out, making objects appear redder. The opposite side of Doppler effect is blue shift, were waves are compressed as they move towards us. The best example is that of an ambulance, when it is heading towards you it will have a higher frequency then when it is heading away from you.
|Objects further from us move away quicker and have higher redshifts.|
Hubble's law describes the expansion of space. So, not only are other galaxies moving away from the Milky Way, they are moving away from each other. The only way to allow for that is some sort of expansion in the spaces between the galaxies. My favorite analogy (and the most delicious analogy) is that of a chocolate chip cookie. Before the cookie bakes, it is small and the chocolate chips (representing galaxies) are closer together. As it bakes and the cookie dough expands, the chips spread apart but stay the same size.
|A Universe of Delicious. Google Image Search.|
It's an issue that will continue to have its fine points debated. The Hubble space telescope, besides taking pretty pictures, is one of the main tools in studying the fine points of galaxies to get precise observations. Several other things, including the cosmic microwave background and other telescopes are focus points of this research too. Ultimately, it is hoped to get an idea of where the Universe is heading, unending expansion or a 'Big Crunch.'
|Hubble Deep Field. NASA|
|Inflating Universe. University of Wisconsin-Madison.|
In addition to the Wikipedia links, I found this page to be informative: http://csep10.phys.utk.edu/astr162/lect/cosmology/expansion.html